Q.S. Wang

High-power harmonic gyro-TWT's. I. Linear theory and oscillation study

A linear theory using Laplace transforms which is applicable to both gyrotron traveling wave amplifiers (gyro-TWTs) and gyrotron backward-wave oscillators (gyro-BWOs) is presented. The validity of the linear theory is verified by comparing it with an existing nonlinear self-consistent theory based on a different approach. In conjunction with a time-dependent multimode particle simulation code, the linear theory is applied to study the stability of harmonic gyro-TWTs. It is shown that a harmonic gyro-TWT can be made stable to all forms of spontaneous oscillations by employing a multistage interaction structure and that it can generate power levels far in excess of those possible for a fundamental gyro-TWT. The linear bandwidth of a second-harmonic gyro-TWT amplifier is also calculated. >

High-power harmonic gyro-TWT's. II. Nonlinear theory and design

For pt.I, see ibid., vol.20, no.3, p.155-162 (1992). Based on an analytical study of the stability problems of gyrotron traveling wave amplifiers (gyro-TWTs), an extremely high power second-harmonic gyro-TWT has been designed, evaluated and optimized with a self-consistent nonlinear numerical simulation code. The design, which is based on the magnetron-injection-gun (MIG)-type beam, is presented. Using a 100 kV, 25 A MIG beam with alpha =1 and an axial velocity spread of 5%, nonlinear self-consistent analysis of a three-stage second-harmonic gyro-TWT amplifier predicts a peak output power of 533 kW, peak efficiency of 21.3% and a 7.4% saturated bandwidth, which verifies the theoretical predictions that a stable harmonic gyro-TWT can generate power levels an order of magnitude higher than those possible from a fundamental gyro-TWT. It is shown that the positioning of the electron beam is very important. A multistage structure is used to recover the loss in gain resulting from shortening the interaction sections to ensure stability. >

Design of a W-band second-harmonic TE/sub 02/ gyro-TWT amplifier

A harmonic gyrotron traveling-wave tube (gyro-TWT) amplifier is described that can stably deliver high peak and average power in the low-loss TE/sub 02/-mode at 91.4 GHz. The single-stage second-harmonic TE/sub 02/ gyro-TWT is predicted to produce a peak power of 600 kW with an efficiency of 24%, a saturated gain of 30 dB, and a 3-dB bandwidth of 2.7%. The amplifier employs a 100-kV, 25-A electron beam emitted from a magnetron injection gun with v/sub /spl perp///v/sub /spl par//=1.2 and /spl Delta/v/sub /spl par///v/sub /spl par//=8%. The device is based on the proven concept that the electron beam current can be much higher in a stable harmonic gyro-TWT amplifier than at the fundamental due to the relatively weaker strength of the harmonic interaction. The TE/sub 02/ overmoded interaction waveguide is sufficiently large to handle an average power of 60 kW and provides considerable clearance for the high current electron beam. An innovative mode-selective interaction circuit prevents the amplifier from oscillating in undesired modes.

Stability of a 95-GHz slotted third-harmonic gyro-TWT amplifier

A low-magnetic-field moderate-voltage gyrotron amplifier has been designed for stable high-performance operation at 95 GHz. A slotted interaction circuit is utilized to achieve strong amplification near the third cyclotron harmonic frequency. The start-oscillation conditions were determined by an analytical theory and confirmed by a multimode particle-in cell simulation code. The dominant threat to the amplifiers stability is from a third-harmonic peniotron backward-wave interaction. A slow-timescale particle-tracing simulation code predicts the three-section slotted third-harmonic gyro-TWT, which utilizes an 11.6-kG magnet and a 50-kV 3-A /spl upsi//sub /spl perp////spl upsi//sub z/=1.4 axis-encircling electron beam with an axial velocity spread of 6% will yield an output power of 30 kW with an efficiency of 20%, a saturated gain of 40 dB, and a constant-drive bandwidth of 2%.

Stable 2 MW, 35 GHz, third-harmonic TE/sub 41/ gyro-TWT amplifier

A third-harmonic gyro-TWT amplifier operating in the TE/sub 41/ mode is described that is predicted to generate 2 MW at 35 GHz. The device is based on the proven concept that the electron beam current can be significantly higher in a stable harmonic gyro-TWT amplifier than at the fundamental due to the comparatively weaker harmonic interaction. For stability, the circuit is sliced to suppress electromagnetic modes without the desired symmetry and the 100 kV, 100 A electron beam from a magnetron injection gun is placed at the null of the strongest remaining gyro-BWO mode. The single-stage gyro-TWT is predicted to produce 2 MW with 20% efficiency, 30 dB saturated gain and 3.5% constant-drive bandwidth.

High power harmonic gyro-TWT amplifiers in mode-selective circuits

A second-harmonic gyro-TWT amplifier operating in the circular TE31 mode is described that is predicted to generate 400 kW at 35 GHz with 20% efficiency, 35 dB large-signal gain and 4.5% saturated bandwidth. The mode-selective interaction circuit is sliced to suppress electromagnetic modes without m=3 symmetry.

CARM AMPLIFIER DESIGNS FOR HIGH POWER

The cyclotron autoresonance maser (CARM) is shown to be capable of extremely high power efficient operation with relatively weak magnetic field requirements. A general description of the CARM traveling wave amplifier is presented. The linear theory is applied to study the stability of the device and a self-consistent numerical simulation code is used to predict the performance. It is found that grazing intersection often gives superior performance, especially for high current electron beams. The designs of two CARM amplifiers for the applications of a high gradient rf linac and electron cyclotron resonance plasma heating as well as a proof of principle experiment are presented.

High-power CARM for high-gradient RF linac

A CARM amplifier can produce the power required to feed a high gradient 1 TeV linear collider. The CARM is shown to be capable of extremely high power highly efficient and requires a relatively weak magnetic field. The designs of two proposed CARM amplifiers and a proof of principle experiment are presented. 1.

Development of high-power second-harmonic gyro-TWT

A Ka-band second-harmonic TE21 gyro-TWT amplifier, capable of generating extremely high power, has been designed with a self-consistent nonlinear simulation code. To demonstrate that a harmonic gyro-TWT can generate significantly higher output power with better stability than fundamental gyro-TWTs due to the higher electron beam currents allowed for the weaker harmonic interaction, a Ku-band proof-of-principle experiment is being built at UCLA and described in this paper. An output power of 400 kW, with an efficiency of 20% and a constant-drive bandwidth of 6% have been predicted for this device. A single anode 100 kV, 20 A MIG is being built to generate the (alpha) equals (upsilon) (perpendicular)/(upsilon) (parallel) equals 1 electron beam with (Delta) (upsilon) PLL/(upsilon) (parallel) equals 8%. A wide- bandwidth 0 dB two-mode phase-velocity coupler and a TE21/TE11 mode converter have been designed to couple the rf power into and out of the amplifier.

Magnetically tapered CARM (cyclotron autoresonance maser) for high power

A cyclotron autoresonance maser (CARM) inherently demonstrates high efficiency due to its autoresonant feature. Tapering the axial magnetic field can further enhance the CARMs already attractive efficiency. A self-consistent, three-dimensional, nonlinear numerical simulation code has been developed and used to optimize the design of CARM amplifiers, including an axial magnetic field taper. There are usually two free parameters involved in tapering the magnetic field linearly, namely, the tapering rate and taper-starting position. It will be shown in the simulation results that these parameters can be reduced to only one, that is, the tapering rate. By starting the magnetic field taper directly before the first electron crossover occurs, both optimum efficiency and growth rate can be obtained.