N. I. Zaitsev

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 processes in an X-band relativistic gyrotron with delayed feedback

This paper describes experimental and theoretical studies of nonstationary processes in a relativistic X-band gyrotron with delayed feedback. For the first time, the chaotic self-modulation regimes were realized with the power level exceeding 2 MW and efficiency up to 17%. The full particle-in-cell simulations of the self-modulation regimes have been carried out. The period-doubling pattern of the transition from the periodic to chaotic self-modulation regimes have been observed both in experiment and simulations.

X-band high-efficiency relativistic gyrotron

As a result of the theoretical and experimental studies, a relativistic gyrotron with efficiency 50% at power 7 MW or with power 11 MW at efficiency 30% was developed. The full microwave pulse duration was about 6 /spl mu/s, which corresponded to the duration of the voltage-pulse flat top. The regime of periodical, almost 100% depth self-modulation with period 17 ns was observed in the gyrotron with the delayed feedback formed by an additional reflector in the output waveguide.

Relativistic carcinotron with a thermionic injector of electrons

An experimental study directed to the determination of a mechanism for microwave breakdown in a relativistic X-band carcinatron is presented. An electron beam was generated using a thermionic cathode, which provided a stable beam geometry. The use of this cathode decreased the probability of breakdown caused by electron bombardment of the slow-wave structure. An important part played in microwave breakdown are molecules absorbed on the slow-wave structure surface. It is shown that the presence of these molecules, an implementation of conditions for secondary-electron resonant discharge (SERD), can result in a very fast (during 10-20 ns) limitation of the radiation pulsewidth. To remove the adsorbed molecules, heat degassing of the slow-wave structure and a collector of electrons was applied, going on continuously, during a working day of the device. By degassing and choice of slow-wave structure material, output radiation power of the device was increased by a factor of ten (up to 5 MW at the pulsewidth of 10 /spl mu/s).

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.

Self-modulated generation observed in a delayed feedback relativistic microwave gyrotron

Self-modulated generation regimes were studied in a delayed feedback relativistic gyrotron operating on the H01 mode with a central frequency of 9.2 GHz. At a fixed electron beam energy of 230 keV, an increase in the electron beam current from 10 to 45 A led to the transition from a stationary to periodic self-modulated generation regime. The modulation period was about 16 ns, while the relative amplitude of the modulation increased in proportion to the beam current, reaching up to 90%. The microwave pulse duration exceeded 6 μs at an average power of up to 1 MW. The experimental data obtained agree well with the results of simulation using the PIC code KARAT.

Experimental study of a multimegawatt pulsed gyroklystron

A pulsed gyroklystron with the TE5,3 output mode is studied. A gain of 30 dB is obtained at a frequency 30 GHz, an output power of 15 MW, an efficiency of 40%, a pulse duration of 0.35 μs, and a gain bandwidth of 50 MHz.

Development of a multipactor discharge in the output channel of a powerful pulsed gyroklystron

Reasons for microwave discharge initiation in the vacuum section of the output waveguide of a powerful gyroklystron are discussed. It is found that the discharge is located near the range of an electron cyclotron resonance. A one-sided multipactor discharge is numerically simulated in wide ranges of tilt angles and permanent magnetic fields. It is shown that the cyclotron resonance may considerably lower the multipactor discharge threshold in contrast to earlier concepts.

Microwave components for 30 GHz high-power gyroklystron

A 30 GHz high-power gyroklystron is under construction now in IAP (Russia). In the first experiments, carried out at a single pulse regime, RF power 6.5 MW, power gain 30 dB, pulse duration 0.5 mcs, and efficiency 25% were achieved. Next experiments with a new cryogenic magnet and a new electron gun are planned at the second half of 2007 and must demonstrate a repetition rate regime. The gyroklystron has classical three-cavity (TE52/TE52/TE53) scheme and requires a lot of external quasi-optical components: input and output mode converters, a window, and others. It is planned to feed the gyroklystron, using 10 kW magnetron, by means of TE10 (rectangular) - TE52 (circular) mode converter connected with the first cavity. The output mode converter is a straight TE53- TE04-TE01 mode transformer with built-in beam collector based on expansion of oversized waveguide. Results of calculations and low power tests of all gyroklystrons components are reported.

Controllable spectrum of an axial-mode gyrotron with external reflections

The feasibility of controlling the spectrum of multifrequency oscillations in a gyrotron by means of external reflections is studied. It is shown that, in self-modulation oscillation modes of operation, the radiation spectrum lines may come close to the resonance frequencies of a combined electromagnetic system including the reflector-limited part of the output waveguide. Under these conditions, the frequency separation of modes and, accordingly, the self-modulation period can be controlled by varying the position of the reflector. Theoretical results are corroborated by experiments with a 30-GHz relativistic gyrotron with an external reflector.