Session chairperson and summary author: Carter Armstrong
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.