The International Vacuum Electronics Conference for 2002 was held in Monterey, California from May 21 - 23. Nearly 400 people attended the conference, which had over 200 presented papers broken into a plenary session, two poster sessions, and 24 oral sessions. After the conference, the chairpersons for each session were asked to write a brief summary of the session they chaired, and this is a compilation of those summaries. The IVEC 2002 committee would like to heartily thank each of the chairpersons who submitted a summary for their contribution. Please enjoy these summaries.
Will Menninger, Dave Whaley, and Monica Blank
IVEC 2002 Publicity and Publications Chairpersons
The first three plenary sessions of the conference described three important applications of vacuum electron devices – space observational instruments and downlinks, digital communications, and high power heating for particle accelerators and thermonuclear fusion systems. In the first plenary session, Dr. Wayne Harvey of the Jet Propulsion Laboratory discussed the history and future requirements of downlink TWTAs for missions such as the Mars Reconnaissance Orbiter. As well, observational systems for applications such as cloud profiling radar, oceanic altimeters, and atmospheric ozone detection were described requiring high power, high frequency sources available only through vacuum electronics technology. Dr. Harvey predicted an increased use of vacuum devices in NASA space systems as the interest and stringent source requirements for such applications grow in future years. The second plenary session was given by Dr. Dan Goebel of Boeing Electron Dynamic Devices on requirements for future high data rate communication applications. Higher-order modulation schemes such as 8PSK and 64QAM were described as well as TWT development efforts needed to realize amplifier performance required of systems utilizing these techniques. Streaming digital video from space missions, high-definition Internet television, and digital satellite radio are a few of the myriad of applications which will benefit from the development of this technology. The third speaker, Dr. Manfred Thumm of Forschungszentrum Karlsruhe described current state-of-the-art high-power vacuum devices used for both particle accelerators and fusion systems in Europe. Particle accelerators use klystrons operating up to 65 MW pulsed at S-Band frequencies and up to 1MW CW at 500 MHz. In addition, fusion systems using vacuum devices for ion cyclotron, electron cyclotron, and lower hybrid resonant heating exist throughout Europe. These RF vacuum devices, including gyrotrons, klystrons and tetrodes, generate RF power at frequencies between 20 MHz and 170 GHz at power levels of 1 MW CW to 100 MW in microsecond pulses.
The fourth plenary talk, presented by Dr. James Dayton, Jr. from GENVAC Aerospace in Cleveland, Ohio, described the results of an informal survey circulated among young professionals in the vacuum electronics field designed to gain insight into the issues surrounding the training of the next generation in our field. By and large, professionals in the field are drawn by the quality people in the vacuum electronics industry and by the challenge of a science that covers so many disciplines. Next, Dr. Kyu-Suk Chang from Samsumg Electronics in Suwon City, Korea presented a history of the development and mass production of magnetrons for microwave ovens. No doubt many in the audience were envious of Samsung's ability to produce sophisticated magnetrons at a per-item item cost of $7. Dr. Bruce Miller of SureBeam Corporation in San Diego, California then described the technology of food irradiation using electron accelerators. He focused on key issues such as how the volume and type of food that must be processed dictate the energy of the radiation and the type of device necessary to adequately irradiate the food. He emphasized the large amount of research which has concluded that food irradiation is not only safe, but has significantly benefited public health. In the final plenary talk, presented by Dr. John Booske from the University of Wisconsin in Madison, Wisconsin, the new opportunities for microfabricated vacuum electronic devices were presented. The new opportunities he mentioned were primarily focused on precise, reliable, high-yield fabrication techniques. He mentioned the impressive progress on field emitter arrays and the growing research in microfabricated vacuum electronic devices, or "mVED's." Research is underway to develop high power W-band klystron amplifiers using LIGA, which involves a combination of xray lithography and electrodeposition.
The first session at IVEC 2002 included several interesting papers demonstrating the ever-growing power and accuracy of modern computational codes in the area of vacuum electronics. The session began with a keynote talk by C. L. Chang from SAIC in collaboration with CPI and NRL, which described a validation of the suite of TWT design codes developed under ONR-NRL using a 750 W, Ku-Band, communications TWT manufactured by CPI as a model. The suite of codes includes Michelle - a 3D gun and collector code; CTLSS - a 3D cold test code; CHRISTINE - 1D and 3D large signal codes; and mPPM/lesPPM – a magnet design code. Simulation results were presented, and the challenge of developing a proper interface between codes for accurate computations was discussed.
C. T. Chevalier from Analex Corp. in collaboration with NASA Glenn Research Center, described a comparison of simulated cold test characteristics using two computer codes from Computer Simulation Technology (CST), MAFIA and Microwave Studio (MWS). The dispersion, interaction impedance and attenuation were compared for three TWT slow-wave circuits: a coupled-cavity circuit, a novel finned-ladder circuit and a folded waveguide circuit. It was shown that MWS provided increased accuracy in dispersion compared to experiment over MAFIA, with an average decrease in computation time by a factor of four.
R. Harper from Triton Services, Electron Technology Division spoke of work being done with NRL on the optimization of a dynamic velocity taper for a wide-band TWT using CHRISTINE1D. The optimization tool uses a modified steepest descent method to search for increased efficiency as TWT parameters are varied. For this application, the helical pitch was varied using the optimizer, resulting in an increased predicted efficiency by a factor of 1.5 across a three-to-one bandwidth.
D. Smithe from Mission Research Corporation presented physics studies with Northop Grumman Electron Devices on hollow-beam helix TWTs using the 3D particle-in-cell (PIC) code, MAGIC3D. Traditional dispersion and small signal gain studies were discussed to provide initial benchmarking and confidence in the model.
To conclude the session, C. B. Wilsen (Northop Grumman Electron Devices) presented a paper in association with SAIC, University of Maryland and Mission Research Corporation on a simulation study of beam loading in a cavity using the 2D version of the PIC code, MAGIC. The concept of beam loading is important in high and medium power microwave sources where the intense electron beam alters resonant frequency, quality factor and gap transit time factor of cavity structures. Wilsen et al. observed that the beam loading is a function of perveance. Simulations were compared to two theoretical approaches.
This session examined progress in developing and analyzing a number of novel RF sources over a wide frequency and power spectrum. Larry Ludeking of Mission Research Corporation presented results of MAGIC 3-D simulations of amplitrons (backward wave cross field amplifiers). Simulation results were presented for an S-Band tube currently used by the Navy and a new, experimental, ultra-high-average power, UHF amplitron. The S-Band device is being analyzed to investigate spurious output, which is believed to be caused by impedance mismatches within the tube. Comparison of simulation results with cold test measurements were in very good agreement, and hot test simulations agree very well with observed behavior. The simulations are identifying mode boundaries and providing information for optimizing gain and frequency performance. Analysis of the UHF device is providing information for development and performance improvement.
Julian Robinson of Communications and Power Industries, Inc. – Beverly Microwave Division presented measured performance data for a 32 MW, S-Band magnetron. The device operates with 45% efficiency in the current magnet. A life test experiment successfully demonstrated 4 x 107 pulses at 1.2 microseconds. Output power actually improved during the life test due to improved surface emission. Molybdenum tips on the anode structure successfully dissipated 418 kW/cm2 power densities. The current magnetron operates with a 1200 pound magnet and has three outputs. CPI has proposed an upgrade to reduce the magnet weight to 200 pounds and provide a single output. Extensive computer analysis should result in a first-pass design success for the upgrade program.
S. Bhattacharjee from the University of Wisconsin presented simulations of a microfabricated, folded waveguide traveling wave tube (TWT). These are proposed for Terahertz frequency RF sources and would be manufactured using lithographic techniques as for semiconductors. Simulated performance for a 56 mW, 560 GHz TWT and a 370-420 GHz TWT were presented. By providing a controlled amount of feedback to the 560 GHz device, it can be made into a single frequency oscillator. Small signal gain of ~ 10 dB was predicted by MAFIA and a code called TWA3. The 400 GHz device would be an amplifier generating approximately 175 mW at 12 kV with 3 mA of beam current. Gains around 10 dB are predicted when conservative values of RF loss are included in the calculations. A 50 GHz scale model of the circuit is currently under test.
A different type circuit is being developed at the Jet Propulsion Laboratory for operation in the submillimeter-wave regime. Harish Manohara described a "Nanoklystron," a high frequency version of a reflex klystron. Micron range features are manufactured monolithically in silicon using standard micro-electro-mechanical systems (MEMS) techniques. Circuits were fabricated using multi-step lithography and deep-reactive ion etching and consist of a resonant circuit, iris coupler, ridged waveguide transformer and coupling holes for the electron beam. There is a separate repeller and cathode. Developers are currently exploring cathode options for the first prototype device.
The session closed with presentation by Mike Lopez from the University of Michigan on relativistic magnetron experiments and a new theory for limiting current in a relativistic, magnetically-insulated diode. The magnetron has generated over 200 MW in 500 nsec pulses at 1 GHz. Two types of cathodes were employed (1) a non end-capped cathode with an aluminum explosive emission region in the center of the vanes, and (2) an end-capped cathode with a spherical knob on the end beyond the vanes with the emission region within the vanes. Results showed that end loss of electrons from the cathode was reduced for the end-capped cathode. Also described were some novel concepts on limiting current theory for a time-independent cycloidal flow in a relativistic, magnetically-insulated diode. This was motivated by previous particle-in-cell simulations indicating the that maximum emission current density was not given by the space charge limiting condition in the deeply nonrelativistic regime. Researchers at Wisconsin extended the existing theory by relaxing the space charge limiting assumption and were able to predict current under space charge limiting conditions and in the deeply non-relativistic regime.
The first session on Materials featured four papers describing recent progress in the development of vacuum-compatible, high thermal conductivity ceramics based on aluminum nitride (AlN) as alternatives to beryllia (BeO) based materials and a paper on process optimization for the manufacture of rare-earth magnet materials.
To start the session, Mikijelj et al. of Ceradyne, Inc. presented several new classes of AlN-based lossy dielectrics with dielectric properties comparable to the 60%-BeO 40%-SiC composites commonly used in high average power tube designs. The new AlN-based dielectrics feature thermal conductivities up to 110 W/m-K (compared with 130 W/m-K for BeO-SiC) and relatively high electrical conductivities at low frequencies to facilitate the dissipation of surface charge.
Abe (U.S. Naval Research Laboratory) presented a paper by Savrun et al. (Sienna Technologies, Inc.) describing the development of pressureless-sintered high purity AlN for rod and window applications with thermal conductivities ranging from 185-200 W/m-K, as well as two new classes of AlN-based lossy dielectrics with dielectric properties comparable to 60%-BeO 40%-SiC. The thermal conductivities of the lossy ceramics ranged from 110 to 150 W/m-K.
The results of RF cold- and hot-test evaluations of the AlN-based lossy dielectrics developed by Ceradyne and Sienna Technologies was presented by Kirshner et al. (Northrop Grumman Electronic Systems). The new materials were shown to have characteristics compatible with high peak and average power applications that were previously the exclusive domain of BeO-SiC composites. In a series of X-Band termination wedge experiments, a Sienna material (ST-100C-58B) was observed to outperform BeO-SiC, successfully handling >1050 Watts of average power. In S-band klystron cavity experiments, Ceradyne loss buttons (137CD-1) demonstrated excellent performance, handling a maximum of 333 kilowatts peak power and an average power of 1164 Watts.
As an alternative to conventional hot press and pressureless sintering techniques, Carmel et al. of the University of Maryland presented an interesting paper on the use of microwave energy at 2.45 GHz to produce fully densified ceramics in which the microstructures (and materials properties) could be controlled by the processing history. Using this technique, several high thermal conductivity materials were produced: single-phase polycrystalline AlN (224 W/m-K) as well as lossy dielectrics based on AlN-TiB2 and AlN-SiC (128 W/m-K).
The Materials I session concluded with a paper given by Willhite et al. (University of Kentucky/Semicon Associates) on the development of optimum milling process parameters for the production of rare-earth samarium cobalt magnets. The work resulted in an improved yield of target Sm-Co grain sizes coupled with a minimization of the required milling time.
The first TWT session of the conference focused on design and performance of several different TWT types used for varied applications. Both broadband and narrowband TWTs were discussed operating at frequencies between 4 GHz and 100 GHz. The first paper by Dr. Komm of Boeing Electron Dynamic Devices showed results of a K-Band TWT designed specifically for applications requiring low-noise and low phase distortion, typically digital communications. Through proper design, a 20 dB improvement of carrier-to-noise-power-density ratio was achieved while simultaneously reducing the noise figure from 36 dB to 30 dB. Low AM/PM conversion of 2.4 degrees/dB was also demonstrated.
Dr. Wintucky of NASA Glenn Research Center presented work underway to develop a 32 GHz 20 W high-efficiency TWT required for future NASA Enterprise missions. This TWT uses a finned-ladder circuit and has been successfully modeled for cold circuit dispersion and impedance with MAFIA and Microwave Studio. Several different methods for fabrication of the intricate high frequency circuit are being considered including photochemical machining, ultra precision laser micromachining and electric discharge machining.
The third paper, given by Dr. Borgne of Boeing Electron Dynamic Devices, described development of a 250 W CW Ka-Band TWT operating between 27.5 GHz and 31 GHz. Experimental data shows only 1 dB variation in output power across the entire frequency band for constant input drive power. Gain ripple is also seen to be small showing 1.7 dB maximum variation over frequency in the small signal region where gain variation is typically the highest.
Life testing of TWTs is important to evaluate performance changes over long operating periods. Dr. Weekley of Boeing Electron Dynamic Devices described ongoing lifetests of over 133 TWTAs over the past 27 years. In some cases, more than 100,000 hours were logged on a single TWT. Measurements of cathode current, saturated power and gain, helix current and compensating anode voltage give evidence of remarkable stability in the operation and performance of the TWT over many years of service.
Wideband TWT design and experimental development was presented by Dr. Joo of the Vacuum Electrophysics Laboratory of Seoul National University. Dr. Joo discussed methods of dispersion control and harmonic power reduction in the design of a 20-100 W 6-18 GHz heavily loaded helix TWT. Asymmetric ceramic rods and positive phase velocity tapering were employed resulting in increased fundamental output power and reduced harmonic levels. Good correlation with experiment was also obtained.
The final paper in this session by Dr. Shin of the School of Physics at Seoul National University focused on the design of a W-Band coupled-cavity TWT operating between 95 GHz and 108 GHz. The goal of the program is to provide 20 kW of output power with 1 GHz bandwidth. The program is developing LIGA fabrication techniques for the manufacture of the coupled-cavity circuit with sub-mm features. MAGIC3D simulations of the output section of the circuit show 5 percent efficiency and 23 dB gain in the device.
Among five papers scheduled for this Session, there were 2 papers submitted by Russian and 1 by Ukranian scientists who were unable to attend the Conference. This fact by itself indicates that in the future the Conference organizers should pay more attention to the ability of scientists from Eastern Europe to attend conferences.
Two papers, which were presented at the Session, were quite impressive. The first one presented by Yuval Carmel from the University of Maryland describes the progress in studies of plasma-assisted slow-wave oscillators (PASOTRONs). These plasma-filled high-power microwave sources were initially developed at Hughes Research Laboratories (Malibu, CA). Later, starting from the late 1990’s, AFOSR started to support the investigation of various physical processes in these tubes at the University of Maryland. Recall that in these devices an electron beam is focused by plasma ions (Bennett pinch) instead of the guiding magnetic field used in other microwave tubes. A thorough analysis showed that in the absence of guiding magnetic field the beam electrons may experience under the action of the axial and radial components of the wave electric field a two-dimensional motion. The radial motion can be very beneficial for those electrons, which are injected near the waveguide axis, because it moves the electrons from the region of a weak slow-wave field towards the slow-wave structure surface where the field is strong. This not only increases the beam coupling to the wave, but also can greatly enhance the efficiency. In accordance with this concept developed in previous theoretical studies, an experimental configuration of the pasotron was modified. This resulted in the efficiency increase from about 20% to more than 50%.
The second paper was presented by Lawrence Ives from Calabazas Creek Research (CCR), Inc. As follows from the list of authors given in the advance program, this work was done in collaboration with the Russian company "Istok", which is known as a manufacturer of submillimeter-wave backward-wave oscillators (BWOs) for spectroscopy studies. Authors reported very impressive results of the development of Terahertz Backward-Wave Oscillators (THz BWOs). This work is sponsored by NASA because there is a strong interest in the development of frequency-tunable, light-weight local oscillators for heterodyne receivers in a THz frequency range. Such devices are needed for numerous radioastronomy observations, which include the spectroscopy of Earth and other planetary atmospheres and investigations of the interstellar medium. The authors decided to use an advanced manufacturing technology (LIGA instead of the EDM) for fabricating the tiny microwave circuits at frequencies up to 1.5 THz. To develop such a process, CCR collaborates with Sandia National Laboratory. Presently, the circuits, which were manufactured already, operate at frequencies in the range from 600 GHz to 700 GHz. The authors proposed an interesting method of scraping an initially generated sheet electron beam into five parallel beamlets, each of which propagates between two parallel rows of pintles forming a slow-wave structure. Since the interaction efficiency should be less than 5%, they also plan to use a single-stage depressed collector. Simulations indicate that by using this technique more than 80% of the energy in a spent beam can be recovered. This would greatly reduce the power consumption from 270 W to less than 80 W and eliminate the water cooling. This CCR initiative seems to be extremely promising for numerous applications of tunable sources in the THz spectroscopy.
There is no argument that vacuum microelectronics has the power to transform the field of vacuum electronics. As operating frequency increases, tolerance limits in standard manufacturing techniques will force the move to photolihtographic definition and chemical machining of interaction structures. Moreover, field-emission or cold cathodes hold the promise of low-power, fast-turn-on, and high-frequency modulation of the electron beam at the cathode. The people presenting papers in this session are part of the community that will bring about this transformation.
The first session paper, given by Dr. Charles Hunt with the University of California at Davis, presented an overview of the 14th International Vacuum Microelectronics Conference (IVMC-2001). This paper showcased the progress of the field of vacuum microelectronics from its initial focus on microwave amplifiers, through a period of development in flat-panel displays, to the wide-ranging and sometimes novel application areas of current interest. Summaries of papers on spacecraft propulsion systems and other space applications, terabit mass storage devices, high-resolution high-throughput lithography systems, lighting, power control, and applications in analytical instrumentation were shown. The technologies represented included carbon-based emitters (nanotubes, micro- and nano-crystalline surfaces, and structured diamond-like tips) and Spindt-type emitters made from various materials including carbides and nitrides.
The second paper, given by Dr. Takiguchi of NHK Science and Technical Research Laboratories in Japan, presented work on a High-gain Avalanche Rushing amorphous Photoconductor, or HARP, camera tube. In the presented device, icident light generates electric charges that are multiplied by the HARP target. These charges recombine with electrons emitted from the underlying field emitter array to produce an output signal current. The technology is under development at NHK and Futaba as a high-sensitivity, high-resolution replacement for current CCD image sensors. Several example monochrome images were shown in the paper, including a short movie.
The third paper, given by Dr. Capp Spindt of SRI International, explained a method of heat treatment to achieve more uniform emission from individual tips and thus a higher total current output for a given array. Cathodes fabricated at SRI and conditioned using this technique generated significant RF output power using cathode current modulation in a Northrup-Grumman TWT as reported in paper number 11.1 of this conference: "Characterization of Field Emitter Arrays Operating In A Traveling Wave Tube Amplifier" by Whaley et al.
The fourth paper, given by Dr. Robert Espinosa of Microwave Power Technology in Campbell CA, showed a commercial hand-held X-ray spectroscopy instrument that uses a nanostructured carbon cold cathode. The instrument is currently available from Oxford X-Ray Technology in Scotts Valley CA. The cold cathode yielded a reduction in size, weight, and power requirements for the unit, in addition to eliminating the complications associated with radioactive isotope sources.
The fifth and final paper of the session presented work done jointly by Saratov State University in Russia and Seoul National University in Korea on micromachined microwave devices using cold cathodes. The speaker, Dr. Ryskin of Saratov State University, gave results from numerical simulations of micromachined analogs to the traveling-wave klystron, klystrode, and crossed-field amplifier structures, in addition to a novel proposed structure with a distributed field emission cathode.
The abundance of commercial applications and the range of new ideas presented in the Vacuum Microelectronics session of IVEC 2002 reinforces the view that the field of vacuum microelectronics continues to grow both in participation and technical diversity.
The session opened with a keynote paper presented by Dr. Baruch Levush from the Naval Research Laboratory, Washington DC. Dr. Levush reviewed the results obtained using codes which had been developed under a program supported by the Office for Naval Research. He showed examples of comparisons between measured and simulated results for cold and hot tests and for automatic optimization of tube parameters. The agreement between the computed and measured results was generally very good. Dr. Levush went on to point out the uncertainties in the data input to these codes and to show that small changes to the input data can have big effects on the results. For example in a helix TWT: a 10% error in the relative permittivity of the support rods produces a 1.5 dB change in the gain, manufacturing tolerances of 3% lead to 0.5 dB change in gain, and uncertainty about the profile of the attenuator can also lead to uncertainties in the gain of several dB. His conclusion was that reliable computer simulation requires accurate data and focussed validation of the codes being used.
The four other papers delivered in this session reported advances in some of the codes whose development is being sponsored by the Office for Naval Research. Dr. Henry Freund described GATOR, a non-linear, time domain, analysis of coupled-cavity TWTs, which is able to model the initial transient signals in a tube. He showed excellent agreement with measured results for the dependence of output power on input power and also the gain ripples resulting from changes in the termination matches caused by the electron beam. It was expected that the model would be useful for studying tube stability and operation within the cold stop-band of the slow-wave structure. Dr David Chernin described the use of the CHRISTINE code to investigate regenerative, backward wave and band edge instabilities in helix TWTs. New developments in space-charge modelling in CHRISTINE were described by Dr Simon Cooke. Most current models of TWTs assume that the space-charge fields are bounded by a conducting tube at the radius of the helix. The errors in the space-charge forces arising from this assumption have been examined and an improved model was described. Finally Dr. Alexander Vlasov showed how the code MAGY, which had originally been developed to model gyro-devices, was being adapted to model other types of tubes with cylindrical symmetry. Promising results were presented for a Ka-band klystron.
Three types of fast-wave amplifiers were reported in this session. The gyrotron traveling-wave amplifier (gyro-TWT) features broad bandwidth with moderate output power while the gyroklystron amplifier is capable of high power amplification over a relatively narrow bandwidth. Sirigiri of MIT reported the first operation of their 140 GHz quasi-optical gyro-TWT, demonstrating 9 kW of power at a gain of 25 dB. The University of Maryland group is continuing the development of frequency-doubling coaxial gyroklystron amplifiers as drivers of linear colliders. Tens of MW of output power were reported by Lawson in the Ku-band with 4 cavities.
CPI and the University of Maryland have recently teamed up to extend the frontiers of the gyroklystron amplifier into the Ka-Band for testing and conditioning the CERN Compact Linear Collider. It is currently well into the design stage aimed at 50 MW peak power at 30 GHz, as is described by Blank. Calabazas Creek Research, Inc., in collaboration with the Maryland group, is developing a 10 MW, 91 GHz gyroklystron. As discussed by Neilson in their progress update, all major subassemblies have now been fabricated and are being cold tested. Hot test is planned for July, 2002.
The relative merits of single-frequency and frequency-multiplying gyroklystrons are analyzed in a theory talk by Nusinovich on behalf of the Maryland and UC Davis collaboration. The third type of gyro-amplifiers, reported by Whyte of the University of Strathclyde, is based on the Raman free electron maser operation in a reversed guide magnetic field. It features high efficiency (35%) and broad bandwidth (up to 70%) at MW power levels.
Seventeen posters were presented during Poster Session I, covering a wide range of topics including slow-wave circuit design, electron gun development, and fast-wave device experiments.
Hargreaves (Northrop Grumman), et al., presented work on the optimized design of helix traveling wave tubes (TWTs). Various designs were compared with respect to output power, gain, and third order intermodulation products.
Han (Seoul National University), et al., described the design of a folded waveguide TWT. The design was done with analytic theory and the three dimensional codes HFSS and MAGIC3D. Another geometry, the coaxial inverted helical groove slow-wave structure, was presented by Wei (Seoul National University), et al. The cold circuit parameters were calculated via analytic theory and checked with HFSS for the structure comprised of a smooth outer wall and a coaxial inner conductor with a helical groove.
Kageyama (NEC Corporation) detailed a methodology to design a low VSWR match from a coupled cavity circuit to waveguide. The methodology uses a modified Curnow equivalent circuit model and requires a transition region in the circuit where cavity and coupling slot geometry are gradually changed from period to period to provide a broadband match. Another folded waveguide design was shown by Na (Kwangwoon University), et al. The two stage circuit design was centered at Q-band (44Ghz) and is predicted to produce 200 watts of saturated power.
Wilson (NASA Glenn Research Center), et al., described the implementation of discrete-state simulated annealing methodology and compared it to the more traditional use of simulated annealing. The two methods were compared to the optimization of a TWT circuit showing much shorter optimization times for discrete-state simulated annealing.
Ives (Calabazas Creek Research, Inc.), et al., showed work done on the design, fabrication and testing of a magnetron injection gun. The testing utilized a segmented anode that enabled the measurement of the gun current at 60 locations, allowing direct measurements of the azimuthal asymmetry of the cathode emission. A tetrode gridded gun having diode gun beam properties was described by Theiss and True. This gun is useful for high power millimeter wave TWTs. A unique beam concentricity measurement scheme was shown.
With presentations from IVEC 2000 two years ago clearly indicating that three dimensional (3-D) TWT modeling had arrived, the IVEC 2002 presentations showed that 3-D modeling continues to reach more into the mainstream. Three of the six presented papers in this session covered the progress of the Naval Research Laboratory's fully three-dimensional electron gun and collector modeling tool, named MICHELLE. John Petillo started the session with a MICHELLE overview. The code is just about ready for a full U.S. industry release, with both 3-D and 2-D steady-state versions being available with both structured and unstructured gridding capability. A very complete secondary model for MICHELLE was presented by Norm Dionne (fourth talk). The model includes low energy secondaries as well as backscattered primaries. Eric Nelson rounded out the MICHELLE discussions by showing modeling results on a gridded gun in full 3-D (last talk). The vastly different grid sizes required throughout the model made this problem particularly challenging, yet MICHELLE converged well if given enough mesh points.
Thuc Bui from Calabazas Creek Research (CCR) presented a progress report on CCR's beam optics analysis (BOA) code (third talk) which has many features similar to MICHELLE and also includes adaptive meshing. Some test validation cases were presented, but a full 3-D gun model is still forthcoming. Karl Vaden and Tushar Ghosh (winner of the IEEE EDS 2002 Graduate Student Fellowship) each presented initial results on optimizing multistage depressed collector geometries. Ghosh (second talk) used a genetic algorithm, while Vaden (fifth talk) used a simulated annealing algorithm. Ghosh used the 3-D simulator LKOBRA for his collector solves and pushed the collector efficiency from 80% to over 90% on a particular case. Vaden used the 2-D simulator EGUN for his collector analysis and was also able to optimize a collector efficiency to beyond 90%. Both analyses are still in the early stages, and more complete results are expected.
Session 10 included three presentations on the state-of-the-art R&D of high power electron guns. The first two presentations focused on beam forming systems for multiple-beam amplifiers, and the last presentation discussed X-band RF gun. Dr. Khanh Nguyen (KN Research) et al. delivered the first talk on the design of an 8-beam electron gun for a S-band multiple beam klystron. The presentation also demonstrated the utility of the new 3D gun code MICHELLE to produce a convincing design of a well behaved beams for multiple-beam amplifiers. The second talk was presented by Dr. L. Ives et al. from Calabazas Creek Research on the design of multiple beam guns for X-band and S-band klystrons. Both Drs. Nguyen and Ives described the issues associated with the electron beams rotation due to the shear in the magnetic field. Dr. Guharay et. al. gave a compelling talk, which described the R&D in X-band RF gun development for accelerators. This talk also detailed the initial and very promising performance of this gun.
Session: TWT II, chaired by Carter Armstrong of Northrop Grumman Electron Devices, reported on exciting work being carried out on novel RF-gated and electrostatically-focused TWTs, miniature microwave and millimeter-wave TWTs, basic interaction studies, and state of the art high-efficiency Space TWTs. The session began with an invited keynote presentation by Dave Whaley of Northrop Grumman Electronic Systems on the investigation of the noise and reliability characteristics of a cold-cathode, field emitter array (FEA) TWT. The FEA-TWT is a revolutionary advance in RF vacuum device technology providing instant turn-on capability with high efficiency in an ultra-miniature package. The use of cold cathode technology allows for device operation without any inherent cathode wear out mechanisms. Excellent reliability was demonstrated on a C-Band FEA-TWT with over 38 million shots accumulated without a failure. The measured noise figure of the FEA-TWT, while higher than a thermionic helix C-Band power booster TWT used for comparison, was found to be quite acceptable at 35-36 dB. Close-in pulsed carrier measurements on the FEA-TWT showed no evidence of spectral broadening from flicker noise or other low frequency noise source. Noise measurements at higher beam power and lifetime studies of the device are continuing.
The keynote talk was followed by a presentation by Alain Laurent of Thales Electron Devices on recent development of compact, broadband microwave and millimeter wave helix TWTs. Work reported included the development of a low gain 100W, 4.5 to 18 GHz miniature TWT. Excellent mid-band efficiency of greater than 40% was reported with the use of a three stage depressed collector. Work at millimeter wave reported included the development of a broadband mini-TWT operating from 18 to 40 GHz providing 80 W at mid band and 40 W over the full frequency band. Recent results on a novel dual Ka-EHF TWT was also reported showing greater than 40W over the 26.5-31 GHz and 43.5-45.5 GHz frequency bands.
Following the mini-TWT discussion a presentation was given by Bernard Vancil of FDE Inc. on a new look at a prior concept: electrostatically focussed TWTs. The thrust of the talk was on the inherent cost advantages possible through the lack of a magnet focussing structure and through the application of low cost construction techniques developed by the CRT industry. Application for wireless communication was highlighted in the talk with a design presented for a 100-200W linear TWT at 2.5 GHz. Cost in production at quantity was estimated to be under $1000.
A study of the fundamental beam-wave interaction processes in a helix TWT was the subject of the next talk by Ph.D. student Mark Converse of the University of Wisconsin-Madison. Using a novel experimental booster TWT with internal sensors along the device in-situ measurements of the amplified wave phase velocity were performed. This work is being carried out under the DUSD (S&T) Innovative Microwave Vacuum Electronics Multidisciplinary University Research Initiative program managed by Robert Barker at the US Air Force Office of Scientific Research.
The last two talks of the session described recent advances in high efficiency space TWT performance. The first talk by Will Menninger of Boeing Electron Dynamic Devices discussed upgrades to 135 W S-band and 50W C-band designs for satellite communications. Through improvements to the multi-stage collector, beam optics and circuit design overall TWT efficiencies of greater than 65% at S-band and 67% at C-band were realized. The measured performance was in excellent agreement with device simulations.
In the last talk of the session, Ernst Bosch of Thales Electron Devices described efficiency enhancements for low power C-Band and Ku-Band TWTs for satellite applications. Recent work at Thales concentrated on high power TWT development resulting in production tubes with powers of greater than 120 W at C-band and Ku-band with efficiencies of around 70% operating under normal space requirements. In the last two years however Thales has seen an interest in lower power TWTs for local broadcast service and as replacements for solid state power amplifiers used in older satellites. In response Thales developed 20 to 40 W C-band TWTs and 30 to 50 W Ku-band TWTs with efficiencies from 65 to 68% at C-Band and 70% at Ku-band.
Both Boeing and Thales noted in their talks that future TWT development would concentrate on optimizing performance for linearized amplifier operation.
The afternoon poster session included over twenty exhibitors presenting their work on important issues in vacuum electronics. There was an eclectic array of posters covering everything from the peniotron to frequency multiplier grid arrays. Industry, academia and government institutions were all well represented. There was excellent participation from the Far East with representatives traveling from Korea and Taiwan to present posters on PIC codes for magnetron analysis and properties of high-pressure discharge in plasma-panel displays.
Several posters were presented on electron devices including a tunable klystron, a multiple beam klystron, peniotron, magnetron, and microwave power module. Several microwave components were discussed in papers on electron guns for sheet beam klystrons, low back flow collector design, and rf window designs for wideband and high power applications.
Posters related to systems included the work presented by the group from DTI on solid-state high voltage modulators used to operate high power vacuum devices. The switching power supply and solid-state modulator used in the AN/SPG-60, the 50 kilovolt switches used in the test stand for the AN/SPS-49 klystrons and the replacement power supply and arc detection system for the C-band TWT driven MIR radar were all mentioned in the poster. The aptly named "magic modulator" which produces 65 kilovolts switching at up to 400 kilohertz was discussed in this poster as well.
The modulator for the Next Linear Collider, which will operate at 500 kV and 265 amps with 3.2 microsecond pulses at 180 hertz was presented in a separate poster. This hybrid modulator utilizes a solid state switch and a pulse transformer. Two other design approaches, one utilizing a transformerless hard switch and another using a Marx Bank were also presented.
Several posters relating to the modeling and performance of electron guns were displayed. These included the poster of the sheet beam klystron gun that is being developed for the Stanford Linear Accelerator by Calabazas Creek Research. The objective is to develop a sheet beam cathode that will operate at 415 kilovolts and 250 amps. This design is required to alleviate the space charge issues that arise and which limit the current that can be generated at high voltages. Posters were presented on the negative grid gun and on predicting electron gun life expectancy, both by representatives of CPI, Inc. Semicon hosted a presentation of acceptance and workmanship issues related to cathode manufacturing.
Device modeling was a theme that was well covered in the poster session. There were posters covering CFA noise, and modeling of a rising-sun magnetron oscillator. There was also a late entry poster describing recent successful results obtained in modeling a helix device using HFSS.
The diverse expertise represented in the material presented at the afternoon poster session was impressive. Individuals from the Stanford Linear Accelerator Center, University of California Davis, Fermi National Accelerator Laboratory, Kwangwoon University, Ajou University Suwon, and Seoul University, Seoul, Korea, National Chiao University and Chung-Shan Institute of Science and Technology, Taiwan were contributors to the session. Industry representation was lead by Calabazas Creek Research, Communications and Power Industries, KMW of Kyungki-do, Korea, Semicon and Diversified Technologies. Other participants represented Defense Science and Technology, Portsmouth, U.K. and Microwave and Antenna Systems, Malvern, U.K.
Judging from the interactions that occurred during the afternoon poster session, the exhibit was a success. Access to the researchers was excellent and a dissemination of the current state of the art was achieved. We look forward to the next IVEC with anticipation of advances to be made in the coming year.
Accurate modeling and measurement of collector electron dynamics are important for efficient device design. This session contained four papers focusing on modeling and experimental validation of collector design tools applied to TWT collectors, and electron backscatter models in both a high power gyro device and an X-ray tube. The first paper, presented by Dr. Thouvenin of Thales Electron Devices, described collector simulation using a proprietary three-dimensional time dependent computer code COLLECT3D. This code allows for electron bunching, true secondaries, backscattered primaries and rereflected primaries. The case of a truly three dimensional problem was shown which included a non-axisymmetric magnetic field to reduce backstreaming. The amazing result was given that run times of only 5 minutes are required even with problems allowing for multiple generations of secondary electrons.
The second paper on 3D collector design using MICHELLE, a newly-developed NRL/SAIC electron gun and collector design tool, was given by Dr. Zhai of Boeing Electron Dynamic Devices. This paper represents the first use of MICHELLE for practical TWT collector design. The design process was described including the CAD interface and 3D magnetic field import. Useful information regarding setup and run times was given showing that fully relevant 3D problems are tractable with desktop UNIX or PC platforms. Memory requirements of 500 MB and run times of 30 minutes were shown to be typical. Comparison of predicted current distributions to experimental data was also given showing good correlation.
Dr. Valfells of The Institute for Research in Electronics and Applied Physics of the University of Maryland presented the model for backscattered secondary electrons and its implementation in depressed collector design. The model uses a Monte Carlo and ray coalescing approach to accurately treat both emission coefficients and angular distribution of the emitted electrons while limiting the number of traced rays. This model was implemented for a 100 GHz 1.5 MW gyro device and includes calculation of collector heat dissipation profiles which are important in such high power devices.
The final paper of the session was presented by Dr. Salasoo of the GE Global Research Center. This paper described the modeling and measurement of backscattered electrons from the target of a 100 keV 0.250 A X-ray tube. A Monte Carlo methodology is used to model the backscatter process and predict backscatter distributions. A 2D segmented collector was built and tested and data was presented which shows good general correlation of the backscattered distribution. Fine scale correlation showed discrepancy between experiment and prediction and will be investigated further. After the initial target impact, as much as 40 percent of the original current can be backscattered and collected on surrounding interior surfaces of the tube. This predictive modeling is therefore important in target-collector design of X-ray tubes at these high beam powers.
The first paper in this session was delivered by Paul Tallerico and dealt with the RF system for the Spallation Neutron Source (SNS). Los Alamos is responsible for the RF system that will drive the SNS proton accelerator. A superconducting cavity design was chosen after evaluating both normal and superconducting cavity accelerator designs. Paul described the performance objectives and current status of the three klystron designs that will be used in the accelerator. These consist of seven, 2.5 MW, 402.5 MHz klystrons, six, 5 MW, 805 MHz klystrons and eighty-one, 550 kW, 805 MHz klystrons. All the devices must support a 1ms pulse at 60 Hz.
Steve Lenci of CPI reported on the development of a 805 MHz, 550 kW klystron for SNS. An overview of the design and some initial RF data were presented.
The third paper of the session was presented by Tom Hargreaves on an earlier klystron design for SNS. The 805 MHz klystron has a peak output power of 2.75 MW. Tom presented data on operation into a 1.2 to 1 mismatch.
The final paper of the session was presented by Nikita Ryskin from the Saratov State University in Russia. He discussed both theory and hot test results of chaotic behavior in a multicavity klystron. By adjusting the amount of feedback or the beam current, the klystron could be made to oscillate. Further increases in feedback or beam current produced period doubling and then chaos. One application for the device would be as a wide band noise source for modern communication systems.
This session is highlighted with two reports on the axis-encircling electron gun(or cusp gun) and two reports on the magnetron-injection-gun(MIG). Jeon et al. from Seoul National University, Korea reported the analysis of axis-encircling electron beam using a single cusp magnetic field where the rotation of electron beam before the magnetic cusp is included. From the analysis, the minimum relative position between the cathode and magnetic cusp is found for the low-velocity-spread operation. The analysis was compared with the egun simulation showing a good agreement. Also the measurement using a pinhole was in a reasonable agreement with the estimation. The beam voltage and current used are 30kV and 1A respectively. Gallagher et al. from Northrop Grumman Corporation reported a high power cusp electron gun operated at the beam voltage of 70kV and beam current of 8A. Their gun design is unique where the beam passes through the field reversal while it is still converging. A reduced beam scalloping and less sensitivity to dimensional tolerance could be achieved. The estimated axial velocity spread of the higher power cusp gun was estimated less than 10% with the velocity ratio of 1.5. Anderson et al. from MIT and Felch from CPI reported the emission uniformity study of gyrotron MIG cathode. Their concern of mode competition in gyrotrons due to nonuniform emission of electrons from the thermionic cathode led them to investigate the uniformity of the cathode. Here they described the uniformity in terms of the work function of the emitter. They measured the work function spread by measuring the electron beam current by assuming a distribution for the work function. Yeh et al. from Southern Taiwan University of Technology, Taiwan reported a mechanically tunable MIG gun. This tunable gun is controlled by adjusting the axial position of the center electrode. The center electrode consists of fixed and movable electrode. This gun was developed for the gyro-TWT experiment operated at the beam voltage of 93.6kV and beam current of 3A. The beam current and the velocity ratio are simulated and measured while various parameters such as magnetic field, compression ration, cathode temperature and relative position of the center electrode.
For the first time this topic covered two sessions , thus outlining the increasing importance of linearity requirements in modern Digital Communications. The new challenges for telecommunication VEDs designers had been well introduced by Dan Goebel during the opening session , through a review of different modulation schemes used in high data rate communications and corresponding RF amplifier linearity specifications. The purpose of this session was to review different techniques allowing to reduce the intermodulation products at microwave tube or RF amplifier level.
The first talk , given by Dr Yehuda Goren of Teledyne Electronic Technologies , presented a new method to linearize TWTAs based on phase and amplitude compensations through adequate variations of helix and grid voltages versus RF input level. A simple circuitry has been developed and tested on a 800W S band TWT , showing a two tone IM3 improvement up to 23 dB at 6 dB O.B.O..
The following paper by John G. Wöhlbier -PhD Student at University of Wisconsin - reviewed different simulation studies performed with a 1-D non linear multifrequency TWT model S-MUSE to analyze the Physics of Harmonic Injection in a TWT. The influence of injected harmonic amplitude and phase has been studied on a broadband ECM TWT , showing that the optimum phases for 2 tone IM3 or H2 level reduction differ completely from the value allowing to maximize the fundamental RF output power. This topic was further explored by John Scharer – professor at University of Wisconsin – who presented the corresponding experiments performed on the XWING TWT , a research version of a Northrop Grumman 2-6 GHz TWT. The tests were performed with 1.95 and 2.00 GHz fundamental drive tones and 4.00 GHz harmonic tone injection , yielding an upper IM3 reduction by 24 dB in optimum conditions.
Craig Wilsen of Northrop Grumman reported on the IM3 simulations and experiments performed to validate a new code allowing to compute intermodulation products and harmonics in multi-cavity klystrons. Preliminary 3 tone simulations show the possibility to substantially reduce an IM3 level by injecting a weak signal at the same frequency with appropriate amplitude and phase.
The last paper of the session , presented by Richard Jenkings PhD student at Lancaster University , was devoted to a parametric study of efficiency and linearity of an helix TWT with respect to frequency and beam voltage. One conclusion was that the optimum performance trade-off is obtained for a lower beam voltage in Back-off operation , as compared with saturation.
The papers in this session were concerned with either L-Band or X-Band klystrons, all intended for relatively high average power operation in different kinds of applications, with an emphasis on particle accelerators. A trend into multi-beam klystron (MBK) operation became obvious.
The session was opened with a paper by Al Mizuhara, CPI, describing the development of a 100 kW CW L-Band klystron intended for FEL application at Jefferson Lab, VA. Since the 1497 MHz cryogenic cavities of the FEL require very accurate and fast feedback, the klystron has relatively high bandwidth (14 MHz at – 1 dB points) and exceptionally high gain (60 dB). Three klystrons have been built and delivered.
Part of the effort to develop a suitable 75 MW short-pulse, PPM focused 11.4 GHz klystron for SLAC’s Next Linear Collider (NLC) was described in a joint CPI/SLAC paper, presented by Edward Eisen, CPI. The main issue in this development section was the improvement of the mechanical design of the klystron body via a Design for Assembly/Design for Manufacturing (DFA/DFM) effort. This resulted in improved thermal stability and reduced part count.
Adam Balkcum, CPI, presented the current status of the development of a 1.3 GHz long-pulse, multi-beam klystron for the TESLA linear collider project of DESY, Germany. The device, which is intended to produce 10 MW peak power pulses of 1.5 ms duration at a duty cycle of 1.5 %, employing a beam voltage of only 114 kV, has finished its simulation phase. Most effort was spent on the issue of focusing its six off-axis beams. The prototype is expected to be in test by end of the year.
The next paper, a joint effort of KEK and Toshiba, Japan, and presented by Yong Ho Chin, KEK, led back to X-Band and 11.4 GHz linear colliders. The PPM-2 version of the KEK/Toshiba klystron for the Japan Linear Collider (JLC) has reached its goal of 75 MW at 1.5µs pulse length with the remarkable efficiency of 56 %. The next goal is the increase of the repetition rate from 25 to 150 pps.
The closing paper of the session was the account of Ding Yaogen, Chinese Academy of Science (IECAS), on another L-Band MBK. I this case the emphasis was on broadband application. The described 18-beam device, operating at a voltage of 18 kV, has achieved a bandwidth of 9.2 % around a center frequency of 1.3 GHz, with peak output power levels around 200 to 250 kW and efficiencies between 35 and 47 %. Problems have been encountered with spurious noise through returning electrons, and with the evaporation of cathode material.
The session opened with talk describing measurements of the emission and work function uniformity of large thermionic cathodes, presented by C. Fortang from Los Alamos National Laboratory. A sophisticated test facility was developed and used in the analysis of 6.5 inch and 8 inch diameter cathodes for 2 kA and 4 kA Pierce-type electron guns. Next T. Grant from CPI presented an improved life prediction model for barium calcium aluminate impregnated tungsten cathodes. Predictions from the model, which is valid for both M-type and B-type cathodes, are consistent with previously presented experimental data. In the third talk of the session, R. Longo described the Boeing EDD integrated cathode test facility, which includes the capabilities for Auger surface analysis, cathode activity tests, cathode current stability tests, barium evaporation rate measurements, and analysis of robustness to poisons. S. Gold from the Stanford Linear Accelerator Center described a method for the evaluation of cathodes for high-power pulsed and CW klystron electron guns at low voltage and low duty. In the fifth talk of the session, K. Jensen presented a summary of theoretical work carried out at the University of Maryland on a generalized current density model for field, thermal and photo-emission. This general theory, which is more realistic in certain regimes than the Fowler Nordheim model or the RLD model, can be applied to photocathodes producing high-brightness beams for free electron lasers. In the final talk of the session, J. Tartar from Semicon Associates described recent work on the design and evaluation of fast warm dispenser cathodes for TWT applications.
The second session on intermodulation distortion started with a keynote presentation by Carol Kory of Analex and NASA Glenn Research Center. Dr. Kory described the use of MAFIA (a commercially-available 3D particle-in-cell or PIC code) to model intersymbol interference effects during TWT amplification of digital modulations of a single frequency carrier. The motivation was to study internal-reflection memory effects on high-order-modulations (e.g., 16 or 64 QAM). The advantages of the 3D PIC code included its time-domain representation (necessary for this problem) and its exact representation of all dispersive effects through a physically exact three-dimensional model of the helix circuit. Through the use of constellation and eye diagrams, the degradation of high-order-modulations due to internal reflections was dramatically illustrated. A disadvantage of the 3D PIC code was also revealed: using current high-end desktop PC’s (excess of 1 GHz clock speed, dual processor), a single simulation required 20 days to complete. Until computer speeds and memory technologies are significantly increased, it was concluded that time domain block models that incorporate physical characterization "data" (from computer TWT models or experiments) would be the recommended approach.
Joe Qiu of the U.S. Naval Research Laboratory (NRL) described a new TWT characterization system suitable for wide bandwidth (approaching 1 GHz), large-record-length testing of TWTs for amplifying digital signal modulations. He also identified and illustrated the value of "power margin in a digital communications link" as a system-level figure of merit for designing linear TWT amplifiers. Power margin is defined as the ratio of the available power to the power needed to sustain a given symbol error rate given a particular noise level in a communications link. Dave Abe, also from NRL, described the optimized helix circuit design and subsequent fabrication of linear C-Band TWT for digital communication experiments. Optimizations that were investigated included maximized efficiency, minimized AM/PM distortion, and a compromise between AM/PM and AM/AM distortions. It was discovered that the third choice (also called "complex gain" optimization) produced the best overall combination of performance, especially with regard to third order intermodulation distortions. Preliminary drive curve measurements indicate the tube is working as designed, with more extensive measurements and model comparisons in progress.
The next two papers described complementary methods to realize time-domain block models of TWTs as predictors of digital modulation distortions, including memory effects. The first paper (fourth paper overall in the session) presented by Christopher Silva of the Aerospace Corporation, described a block model approach based on a simplified version of Volterra series. This work represented a cross-over application of prior similar work in nonlinear mechanical systems modeling. A principle motivation for developing this predictive model was the recent successful development (by Aerospace Corporation) of a patented, highly-resolving, time-domain characterization instrument. This instrument makes it possible to experimentally measure the time-domain based parameters needed to implement the Volterra type block models for the TWT. The following paper, presented by Pedro Safier (SAIC) described latest results developing a time-domain block model built around frequency-domain characteristics of a TWT. These frequency-domain inputs can be acquired from steady state simulation models (such as CHRISTINE) or from laboratory measurements of experimental devices. The recent advances include the incorporation of frequency dependence into the nonlinear gain, i.e., nonlinear gain with memory.
The final talk in the session, presented by Juliette Plouin, a Ph.D. student at Ecole Polytechnique in Paris, described a theoretical re-examination of mechanisms for saturation in a traveling wave tube. The first determination was that inertial bunching dominates any space charge force effects, for parameters typical of many helix TWTs. This insight led to interesting speculative proposals that a departure from the sinusoidal carrier wave paradigm might be better suited to get more uniform bunching and thus more linear TWT amplification near saturation. An example cited was to consider sawtooth carriers, rather than sinusoids. A first order approximation to the sawtooth waveform would be a combination of fundamental and second harmonic signal. This prompted the observation that this proposed approach and TWT linearization by second harmonic injection (described in talks in a prior session) might be the same idea, but approached from two complementary perspectives.
(Session summary not available yet.)
The session opened with a keynote presentation on the 140 GHz, 1 MW gyrotrons developed for the Wendelstein 7-X stellarator in Germany. The talk was presented by G. Dammertz from FZK on behalf of a large team from a number of universities, laboratories, and companies throughout Europe. In recent tests, the "maquette" gyrotron demonstrated 1 MW output power at 10 second pulse lengths; 0.9 MW output power for 45 second pulses; 0.74 MW for 100 second pulses; and 0.64 MW for 140 seconds. A second gyrotron, with several improvements over the first, has been constructed and is currently in test at FZK. In the second talk, K. Felch detailed the recent progress on CW and long pulse gyrotrons for fusion applications at CPI. Experimental results for three 110 GHz, 1 MW power level gyrotrons; one 140 GHz, 1 MW power level gyrotron; and one 84 GHz, 500 kW gyrotron were discussed. Also, a summary of CPI’s experience with CVD diamond windows for fusion gyrotrons was presented. G.G. Denisov from Gycom presented results from a recent demonstration of a 1 MW 170 GHz gyrotron developed for ITER. Pulse lengths of up to 0.4 seconds were demonstrated at 950 kW peak output power and 44% efficiency with a boron nitride window. The boron nitride window was later replaced with a CVD diamond window for pulse width extension. K. Sakamoto from the Japan Atomic Energy Research Institute discussed the development of 1 MW power level 170 GHz and 110 GHz gyrotrons for fusion applications. At 170 GHz, 900 kW peak output power for 9.2 seconds has been demonstrated and at 110 GHz 1.2 MW for 4 second pulses was achieved. In the final talk of the session, G. Nusinovich from the University of Maryland presented a theoretical analysis of the effect of the radial thickness of electron beams on the stability in gyrotron oscillators.
Yehuda Goren of Teledyne reported phase noise reduction in a TWT, by up to 10dB, employing a feedback circuit and utilizing the TWT as a fast response phase shifter and a fast response variable-gain amplifier. K. B. Mitsdarffer of NSWCC (Crane) compared the accuracy of various pulsed measurements of the output phase in a PPM focused, coupled-cavity TWT. The measurement techniques included vector network analyzer, vector demodulator, microwave component analyzer, and an intermediate frequency technique. A. Choffrut of U of Wisconsin (Madison) studied the out-of-band emission due to imperfect components and amplifier mismatch in the LINC (Linear Amplification with Nonlinear Component) approach on TWT, and found that this approach greatly relaxes the demand on the linearity of the amplifier. Dean Thelen (TSC and NSWCC, Crane) reported on the attempts to reduce the ion noise and body current in a continuous wave TWT sample, primarily with a modified design that lowers beam scalloping and lowers potential barriers to ion flow. Bill Tighe of Boeing EDD reported on the identification of the jitter mechanism on TWT in terms of the trapped and untrapped ions in specific locations in the tube, and addressed the need for jitter reduction in advanced communication techniques.
The first three papers of Session 22 were about the microwave power module (MPM). Only two of these papers were actually presented. The first paper was presented by R.F. Watkins of Northrup Grummens on Higher Power, Low Cost Mini TWT’s. With only small changes to the gun and collector, the "new mini TWT" has been upgraded 2 to 3 dB, to provide between 150 to 270 watts from 6 to 18 GHz. J. Tucek from Northrop Grumman was to present a paper on the Millimeter Vacuum Power Booster Development, but it was withdrawn. The MMPM was to operate over the frequency band 18 to 40 GHz. The development tube provides 100 watts from 18 to 35 GHz, however, the power fell off rapidly above 35 GHz requiring a redesign of the tube. J. Kennedy from Northrop Grumman presented a paper titled Low Voltage Power Booster TWT. This paper described a Ka-Band Prototype, which produces 18 watts minimum and 80 watts maximum across the frequency band 24 to 40 GHz. Redesign will be necessary to center over the 18 to 40 GHz band.
D. R. Whaley from Northrop Grumman presented a paper on High Perveance TWT Modeling and Experimental Verification. A modified version of CHRISTINE3D was implemented with a self-consistent Poisson solution. The computer simulated results of this highly scalloping, high perveance (10 mP) annular electron beam were very in close agreement to actual experimental data. T. Machida from NEC Electron Devices presented a paper entitled Development of Ka-Band 250 Watt Peak Power Helix TWT. The data on a TWT 27.5 to 30 GHz TWT providing 150 watts average and 250 watts peak was presented. This tube achieved 48% efficiency at 30 GHz. R. Dionisio from Alelco SPA Microwave Tubes submitted a paper entitled High Power X-Band Helix TWT for Airborne Radar Applications, which was not presented since the authors were not able to participate in the conference. The paper involved an 8 kW helix tube operating over the frequency 7.5 to 10 GHz at a duty of 5% with a shadow gridded gun attached directly to the cathode and an electronic efficiency of 25%.
The second session of Fast-Wave Amplifiers began with a Keynote paper presented by G.G. Denisov from the Institute of Applied Physics in Nizhny Novgorod, Russia. The paper described the experimental demonstration of a novel gyro-TWT that makes use of a helically-grooved waveguide to create a more favorable dispersion characteristic with a non-zero group velocity in the region near zero axial wave number. In a demonstration at Ka-band, the device achieved 27% efficiency at 180 kW peak output power with a saturated gain of 23-25 dB and a bandwidth, which was drive source limited, of greater than 5%. Next, B.G. Danly from NRL gave an overview of the long- standing program in the Vacuum Electronics Branch aimed at developing millimeter-wave amplifiers for radar applications. Results of experimental demonstrations of gyro-klystrons and gyro-TWTs in W-band and Ka-band were described. In a related paper, M.T. Ngo presented a detailed description of the transmitter components and operation of the NRL W-band WARLOC radar, which is driven with a high-power W-band gyroklystron amplifier driver. Data from the operating radar was presented. C.W. Baik from Seoul National University described the design and status of a Ka-band two-stage tapered harmonic-multiplying gyro-TWT, which is predicted to achieve greater than 3% bandwidth. J.J. Choi from Kwangwoon University presented initial hot-test results for a Ka-band five-cavity gyroklystron amplifier. Amplified rf signals were observed at short pulses, but an failure of the modulator occurred ending the experiments before detailed measurements of gain or bandwidth could be made. Repairs of the modulator and gyroklystron are underway. The session ended with a talk presented by N.C. Luhmann, Jr. from UC Davis on the initial demonstration of a high-power heavily-loaded TE01 mode gyro-TWT. In the initial experiments, 100 kW peak power was observed in the BWO mode. The amplifier was tuned to parameters where it was zero-drive stable in preparation for exploration of the amplified rf performance.
The second session on materials began with a talk by Thumm, et al. (University of Karlsruhe and Forschungszentrum Karlsruhe, Germany) on the current state of materials used in K-band devices. Thumm first reviewed the advantages materials can provide millimeter-wave device developers. He discussed the physical and technological advantages of producing millimeter-wave versus microwave devices. Thumm finished with a comparison of efficiencies and power versus size, weight and accelerating voltages for a 24.15 GHz magnetron, klystron, extended interaction oscillator and gyrotron. Next Choi et al (Kwangwoon University, Korea) presented data on recent dielectric measurements of beryllia, Teflon and CVD diamond using two Fabry-Perot open resonators developed for low-loss materials in Ka-band (26-40 GHz), V-band (50-75 GHz) and W-band (75-110 GHz). Gamble et al. (DSTL, MAAS and DeBeers, England) discussed the use of CVD diamond as a waveguide window in a high-powered, wide-bandwidth, free-electron maser. Gamble described the loss requirements the window must maintain under CW operation and the predicted window thickness from HFSS modeling. Finally, he presented experimental results on CVD diamond window, built to the model thickness and bonded into a waveguide. Both insertion and transmission losses measured were in agreement with model predictions. The final talk of the session was presented by Bosman et al. (University of Michigan, USA) describing a theoretical model of diamond window failure. The model evaluated proposed that the failure mechanism may be based on formation of graphite from amorphous carbon at grain boundaries. Stresses induced by increasing graphite formation would then be the cause of destructive failure. However Bosman, et al.’s results showed that the temperatures required to cause the reaction leading to the failure are thermodynamically improbable and that a new model should be sought.