Session chairperson and summary author: R. Lawrence Ives
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.