Session chairperson: Yehuda Goren
Summary author: David Whaley
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.