M. I. Petelin

Scenario for output pulse shortening in microwave generators driven by relativistic electron beams

At the present time, microwave generators driven by high current relativistic electron beams are not baked and sealed, so their inner surfaces are densely covered with molecules of gas and oil. This allows the production of microwave pulses of 10/sup -8/ s to 10/sup -7/ s duration, but not longer. A microwave pulse termination scenario is speculated as follows: (1) Electrons oscillating in the strong RF field near the metallic surfaces multiply owing to the secondary emission (the multipactor effect); (2) the multipactor electron bombardment stimulates desorption of gas molecules from the metallic surfaces; (3) the gas undergoes avalanche RF breakdown; and (4) the resultant plasma stops microwave generation and, since electron-ion recombination is slow, does not allow the RF field to revive. At the gigawatt power level, the characteristic time of such a scenario is much shorter than that of the cathode and collector plasma expansion and electron beam instabilities. The energy output parameters of relativistic electron microwave generators can be (and usually are) improved at high pulse repetition rates. A more radical improvement is possible using the technology typical for high vacuum tubes, i.e., baking and sealing.

Nonstationary generation in free electron lasers

Abstract Evolution of the alternating field structure in the high- Q multimode cavity excited by the beam of relativistic electron oscillators is described by a self-consistent set of equations. Stationary and pulse injection regimes are investigated. The injection current increasing over the threshold, three consecutive stages take place: a) stationary single-mode generation (or, correspondingly, generation of identical pulses), b) periodic and c) stochastic self-modulation.

One century of cyclotron radiation

During the one century after the theoretical prediction of the cyclotron radiation, this effect was found to contribute to many phenomena in nature and now delivers the highest powers at the shortest microwaves. The author surveys the evolution of the main ideas in this field and considers some of the devices which are based on this effect.

Diplexers for Power Combination and Switching in High Power ECRH Systems

Electron Cyclotron Resonance Heating (ECRH) systems for next step large fusion-devices operate at a Continuous Wave (CW) power well beyond 10 MW generated by a large number of gyrotrons with typically 1 MW power per unit. The combination of the power of two (or more) gyrotrons and switching of the power between different launchers for different physics applications is an attractive feature for such systems. The combination of beams from different gyrotrons would reduce the number of transmission lines and the requirements on port space. Fast switching between two antennas synchronously with the Magneto-Hydro Dynamic (MHD) modes frequency would increase the efficiency of mode stabilization. Both combination and switching as well as power sharing between different ports can be performed with high-power four-port diplexers using small frequency differences or small frequency-shift keying of the gyrotrons, respectively. Fast directional switches (FADIS) and beam combiners (BC) can be designed on the basis of different physical mechanisms: some selected design variants were investigated and the results are presented. Considerations on the integration of FADIS/BCs into large ECRH systems and their use in test arrangements are presented.

Quasi‐Optical Microwave Pulse Compressor at 34 GHz

We describe a 34.272 GHz pulse compressor based on a three‐mirror traveling‐wave resonator, which creates pulse compression using linear frequency modulation of the input pulse, rather than step‐wise phase modulation used traditionally. The results of testing the compressor prototype at a low power level under different modulation methods and different widths of the input pulse are discussed. We present also the results of calculations and tests of an electrically controlled diffraction grating, which can serve as the active switch for such a three‐mirror resonator.

Pulse compressor based on electrically switched Bragg reflectors

A novel switched energy storage (SES) pulse compressor is described with the apparent capability for high efficiency compression of high power 11.4 GHz pulses in the pulse energy range of interest for future electron-positron collider applications.

Ka band /spl sim/10 MW gyro-devices: an experiment and a project

Summary form only given. The international program for development of electron-positron colliders needs high-power microwave sources at frequencies up to the millimeter-wave band. At the shortest microwaves, the highest powers are produced with fast-wave devices. Among them, the gyroklystron seems especially robust and reliable. As an introduction, a 30 GHz free running gyrotron operating at the H/sub 53/ mode and driven with 300 kV/80 A electron beam was manufactured and tested: 12 MW/200 ns pulses were produces with 50% efficiency. The gyrotron is planned to be converted into a gyroklystron amplifier; for better coupling to the electron beam and better suppression of parasitic modes, the modulating and intermediate cavities will operate at the H/sub 51/ mode. For further experiments, a test stand with a 500 kV/ 300 A/1 /spl mu/s modulator is being designed and a relevant electron gun is being manufactured.

Mode selection in high power microwave sources

For microwave sources based on the stimulated radiation of electron beams, an attractive method to enhance the output power is expansion of the electron/RF-field interaction space. However the expansion densifies the spectrum of the system oscillation modes, originates their competition, thus aggravating the problem to keep the RF radiation coherent, In the RF electronics, the multi-mode problem was thoroughly studied, beginning with the papers of Van der Pol, for almost a century. An outstanding example of success based on a spurious mode suppression method is implementation of straps into the multi-cavity magnetron. For the same purpose, most of the classical electron microwave generators and amplifiers (TWTs, klystrons, EIAs etc) have a sectioned interaction space partially filled with absorbers. The innovations appearing in the field during recent decades are reviewed.

High-power microwave pulse compressors: passive, active, and combined

Microwave pulse compressors are of interest for plasma experiments, particle accelerators and, hopefully, radar. At the shortest microwaves (at frequencies higher than 10 GHz), high power compressors must be based on use of oversized, quasi-optical, mode selective structures.

On the mechanism on self-modulation onset in free electron lasers

Abstract The self-modulation in FELs (arising currents sufficiently large compared to the starting values I / T st ≳ 2.2) is treated as four-photon decay. The latter is provided by overbunching of the electron beam and by slipping of electons relative to the wave envelope. The frequency dispersion of the wave group velocity does not suppress the decay completely and only shifts its threshold to larger I / I st ≳ 4.1?.