Richard True

High-Average-Power W-band TWT Development

A PPM-focused TWT for airborne applications producing over 400 W of peak RF output power at 90.6 GHz and average-power levels between 150 and 180 W is described. Design goals of 1% bandwidth, 250 W of peak power, and 30 dB of gain have all been met despite the invariable fabrication errors in this nascent type of tube. The TWT utilizes a novel integral-pole-piece ferruleless-coupled-cavity-class folded-waveguide circuit that features thick iron pole pieces with hollowed out centers, and a gridded gun for fast beam modulation. This thermally robust circuit plus the tetrode gun represent two key enabling technologies for compact high-average-power TWTs in W-band.

The deformable relaxation mesh technique for solution of electron optics problems

This paper describes the principles of a method for the solution of electron optics problems which has led to significant design improvements in traveling wave tubes over the past few years. In contrast to Conventional procedures which solve Poissons equation on square meshes, the space within the problem boundaries is filled with a network of irregular triangles resembling a stretched fishnet laid out by the computer itself. The technique has found its greatest use in the design of high-performance electron guns. The high accuracy and resolution of this method has enabled numerous better guns. The approach has also been used to analyze beam focussing arrangements, multistage depressed collectors, CRT guns and lenses, and problems obeying Laplaces equation. The procedure can be used for calculation of ion beam optics as well.

A theory for coupling gridded gun design with PPM focussing

This paper presents theoretical and experimental methods for the design and optimization of gridded Pierce electron guns. The design of periodic permanent magnet (PPM) focussing systems to control the nonlaminar beams originating from such grided guns is discussed. Simplified relationships are developed which directly couple the, heretofore separate, gun and PPM focusing design problems.

Emittance and the design of beam formation, transport, and collection systems in periodically focussed TWT's

This paper describes how emittance can be used to advantage in the design of beam optical systems for TWTs and other devices, and presents progress on the quantitative intercoupling of the beam formation, capture, focussing, transport, and collection problems via a parameter called tunnel emittance (T-emittance). First, a thermal electron beam model for gun-design computer codes is presented. It is necessary to employ a thermal beam model for accurate determination of T-emittance at the beam waist (especially in diode and low-perveance guns). Next, the paper considers the beam capture problem. How the match between the beam and the periodic permanant magnet (PPM) focussing field affects T-emittance growth downstream is described. After this, the effect of tube length on beam transmission is considered. It is shown that T-emittance can grow downstream due to the transfer of nonlinear field energy to transverse beam motion (explaining the observed degradation in transmission in very long tubes). Finally, at the exit of the tube, the grown value of T-emittance can be used as the basis for a spent beam model useful for preliminary single and multistage depressed collector (MDC) simulations. Appendices are presented which relate the quantities used in this paper to those used within the high-energy physics community.

A 100 Watt W-Band MPM TWT

L-3 EDD has developed a W-Band TWT with 100 W RF power over 4 GHz of bandwidth around 94 GHz suitable for MPM integration. The TWT employs an aperture grid modulated electron gun with mod-anode current control, a serpentine waveguide interaction circuit, and a single-stage depressed collector. Two TWTs have been built and tested to over 100 W pulsed output power. The first unit has been operated at high duty, producing 65 W CW output power and 75 W average pulsed power with reduced beam current. The TWT is designed for ease of manufacture, and is suitable for MPM integration, relying on conduction cooling and capable of operation to an altitude of 50k feet.

An ultra-laminar tetrode gun for high duty cycle applications

This paper describes a high convergence Pierce gun having beam laminarity essentially equal to that of ungridded guns and which is capable of duty cycles well in excess of triode intercepting gridded guns. Since current is drawn more uniformly from 100% of the open area between the grid wires in the tetrode gun, cathode loading is significantly less per total unit area than either standard or contiguous shadow gridded guns. Theoretical and beam analyzer studies, and hot testing of a high power pulsed IJ band helix TWT have confirmed that there is a profound performance improvement as the two control grid voltages in the tetrode gun are trimmed. The adjustable grid wire lens elements enable optimization of beam diameter and laminarity, and account for variations in tolerances. A simple self-biasing network can be included within the tube package for operation by a conventional grid pulse modulator.

High average power W-band TWT development

A second full-gain W-Band folded-waveguide TWT is in test. The TWT largely duplicates the design reported on in 2008 [1] and maintains the goals of one-percent-bandwidth, 250 watts of peak power, 100 watts of average power, and small-signal gain near 30 dB.

Experimental Investigation of a Novel Circuit for Millimeter-Wave TWTs

A novel folded waveguide circuit that features thick iron pole pieces with hollow centers was built as part of a periodic-permanent-magnet-focused W-band single-stage test-vehicle traveling-wave tube (TWT). These hollow centers, which comprise part of the slow wave circuit, increase the rms axial field and significantly reduce the unwanted transverse field imbalance. For this TWT, a tetrode gun that creates an ultralaminar 20-kV 0.25-A nominal electron beam was used. It was demonstrated that this gun and magnetic structure can provide greater than 97% beam transmission for peak beam power levels as high as 9.25 kW (25 kV, 0.37 A). The unplated circuit, operating around 91 GHz on the edge of a passband, exhibits between 10 dB and 12 dB gain that compares favorably with results of device modeling utilizing the 3D particle-in-cell code Magic3D. Using a feedback approach to characterize large-signal operation, the tube generated 40 W of regenerative oscillator power. Design-optimized versions of this circuit show promise of enabling W-band TWT amplifiers that provide up to 300 W of peak RF output power

Experimental investigation of a tetrode gun for MM-wave TWTs

A tetrode-type electron gun suited for millimeter-wave TWTs has been investigated experimentally. The gun creates ultra-laminar 18 kV, 0.1 microperveance beams utilizing voltage swings between -100 V and +50 V on the inner grid and between -300 V and +200 V on the outer grid. This gun was fabricated by joining tight-tolerance parts into assemblies that each exhibited both non-concentricity and axial-position errors of about 0.001 inches.

Beam optics calculations in very high power microwave tubes

The author outlines advances in computer simulation, computer-aided design procedures, and technology related to the formation, focusing, and collection of very high power beams used in superpower klystrons for advanced linear colliders, free-electron lasers, and other such high-power microwave (HPM) devices. Grading electrodes can be included in the beam formation region of Pierce guns and between the ceramic insulators to increase the high-voltage standoff limit (as in Van de Graaff accelerators). A theoretical basis for their use is established here. Recent improvements in the fully relativistic version of the deformable triangular mesh beam optics code written by the author are described. Selected test cases and examples of its use are presented. A high-perveance 1 MV Pierce gun having three grading electrodes is described which can provide a 1.83 GW 9.13 kJ./pulse beam.<<ETX>>