A. P. Fokin

Experimental tests of a 263 GHz gyrotron for spectroscopic applications and diagnostics of various media

A 263 GHz continuous-wave (CW) gyrotron was developed at the IAP RAS for future applications as a microwave power source in Dynamic Nuclear Polarization / Nuclear magnetic resonance (DNP/NMR) spectrometers. A new experimental facility with a computerized control was built to test this and subsequent gyrotrons. We obtained the maximum CW power up to 1 kW in the 15 kV/0.4 A operation regime. The power about 10 W, which is sufficient for many spectroscopic applications, was realized in the low current 14 kV/0.02 A regime. The possibility of frequency tuning by variation of the coolant temperature about 4 MHz/1 °C was demonstrated. The spectral width of the gyrotron radiation was about 10(-6).

Effect of ion compensation of the beam space charge on gyrotron operation

In gyrotrons, the coherent radiation of electromagnetic waves takes place when the cyclotron resonance condition between the wave frequency and the electron cyclotron frequency or its harmonic holds. The voltage depression caused by the beam space charge field changes the relativistic cyclotron frequency and, hence, can play an important role in the beam-wave interaction process. In long pulse and continuous-wave regimes, the beam space charge field can be partially compensated by the ions, which appear due to the beam impact ionization of neutral molecules of residual gases in the interaction space. In the present paper, the role of this ion compensation of the beam space charge on the interaction efficiency is analyzed. We also analyze the effect of the electron velocity spread on the limiting currents and discuss some effects restricting the ion-to-beam electron density ratio in the saturation stage. It is shown that the effect of the ion compensation on the voltage depression caused by the beam space charge field can cause significant changes in the efficiency of gyrotron operation and, in some cases, even result in the break of oscillations.

Plasma density in discharge sustained in inhomogeneous gas flow by high-power radiation in the terahertz frequency range

We have measured the density of plasma (electron concentration) in discharge maintained in inhomogeneous argon flow under the action of high-power pulsed radiation of gyrotron (frequency, 0.67 THz; power 40 kW; pulse duration, 20–30 μs) in a range of background gas pressures in the discharge chamber from 10–3 to 300 Torr. The electron concentration at low pressures (10–3 to 7 Torr) was determined using Starkeffect induced broadening of the Hα atomic emission line (656.3 nm) of hydrogen present in discharge as a small impurity in residual gases. The maximum observed Stark broadening of the Hα line corresponded to a plasma density on the order of 2 × 1016 cm–3, which exceeded the critical value for the given frequency of radiation sustaining the discharge. At background pressures above 7 Torr, the plasma density was estimated from analysis of the spatiotemporal patterns and waveforms of discharge glow in the visible spectral range. These estimations gave electron concentrations on the level of (1–2) × 1015 cm–3.

Measurement of plasma density in the discharge maintained in a nonuniform gas flow by a high-power terahertz-wave gyrotron

We performed measurements of plasma density in a “point-like” discharge, which is generated in a nonuniform flow of a gas (argon) under the action of high-power terahertz gyrotron radiation with a frequency of 0.67 THz, a power of 40 kW, and a pulse duration of 20 μs. The nonuniform flow was produced by injecting the gas to the vacuum chamber through a small hole (0.14 mm in diameter) under a background pressure at a level of 0.01 Torr. The discharge developed and was localized only in a small region of space (about 1 mm) near the gas injection hole, where the pressure was high (close to the atmospheric one) and the breakdown conditions (the “point-like” discharge) were fulfilled. The density of electrons in the discharge was measured by observing the Stark broadening of the atomic radiation line Hα of hydrogen (656.3 nm) which was present in the discharge as a minor admixture. The plasma density in the discharge was equal to about 2 × 1016 cm−3, which exceeds the cut-off density for a frequency of 0.67 T...

Optimization of terahertz range gyrotron self-excitation conditions by increasing the lifetime of cyclotron oscillators in low-voltage interaction space

We propose a method of reducing the starting and operating currents of short-wavelength gyrotrons that is based on the application of a decelerating voltage directly to the resonator. The desired effect is achieved due to increasing electron lifetime in the interaction space. At a preset injection current, this circumstance ensures gyrotron self-excitation at a relatively low diffraction Q-factor, which leads to significant reduction in ohmic losses.

A Magnetron Injection Gun with a Reduced Filament Temperature and Elongated Cathode Lifetime

An optoelectronic gyrotron system that makes it possible to efficiently heat a cathode by electrons reflected from a magnetic mirror has been developed. The parameters of the primary electron beam are retained at a level that is acceptable for efficient microwave power generation. The results of a trajectory analysis of electrons and the cathode temperature conditions are reported. The corresponding decrease in power and temperature of the cathode filament suggests a significant (severalfold) increase in the filament lifetime, which is one of the main parameters determining the device lifetime.

First experimental tests of powerful 250 GHz gyrotron for future fusion research and collective Thomson scattering diagnostics

A 250 GHz continuous-wave (CW) gyrotron has been developed at the IAP RAS jointly with GYCOM Ltd., as a prototype of the microwave source for the envisaged prospective nuclear fusion power plants (DEMO). The main applications of such a tube are electron cyclotron resonance heating and electron cyclotron resonance current drive of magnetically confined plasma as well as its diagnostics based on collective Thomson scattering in various reactors for controlled thermonuclear fusion (e.g., tokamaks and stellarators). The results of the preliminary experimental tests in a pulsed mode of operation are presented. The microwave power of up to 330 kW with an efficiency of 30% without collector depression was obtained. At an accelerating voltage of 55 kV and an electron beam current of 12.5 A (which corresponds to the design parameters for CW operation), the measured output power was about 200 kW. The TEM00 mode content evaluated at the tube output is not less than 98.6%.

Operation of a sub-terahertz CW gyrotron with an extremely low voltage

Decreasing the operating voltage for medium-power sub-terahertz gyrotrons aimed at industrial and scientific applications is highly attractive, since it allows size and cost reduction of the tubes and power supply units. In this paper, we examine such an opportunity both numerically and experimentally for the fundamental cyclotron resonance operation of an existing gyrotron initially designed for operation at the second cyclotron harmonic with a relatively high voltage. Simulations predict that output power higher than 10 W can be produced at the fundamental harmonic at voltages less than 2 kV. To form a low-voltage helical electron beam with a sufficiently large pitch-factor, a positive voltage was applied to the first anode of the gyrotron three-electrode magnetron-injection gun with a negative voltage at the cathode. CW gyrotron operation at voltages down to 1.5 kV has been demonstrated at a frequency about of 256 GHz.

Frequency Tuning in the Gyrotron Oscillator With a Klystronlike Sectioned Cavity

Several spectroscopy applications of high-frequency gyrotrons require relatively broadband (~1%) frequency tuning. In this paper, the use of a sectioned klystronlike cavity with mechanically changing parameters is proposed as a way to solve this problem.

Multiparametric gyrotron power control during microwave processing of materials

Optimization of the control of gyrotron generation regime through consistent variation of the cathode voltage and solenoid magnetic field makes possible a significant (up to threefold) decrease in power consumption during microwave processing of materials. It is experimentally established that, for this purpose, it is possible to use fast (with a characteristic time on the order of a fraction of millisecond) modulation of microwave power through variation of the working magnetic field by the current of an auxiliary coil arranged near the cavity.