Showing papers by "G. G. Denisov published in 1998"

First mm-wave generation in the FOM free electron maser

Abstract: A free electron maser (FEM) has been built as a pilot experiment for a mm-wave source for applications on future fusion research devices such as ITER, the International Tokamak Experimental Reactor. A unique feature of the Dutch Fusion-FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the first phase of the project, the so-called inverse set-up is used. The electron gun is mounted inside the high-voltage terminal. The entire beam line was tested successfully with extremely low loss current, lower than 0.05%. First generation of mm-waves was achieved in October 1997. Up to now the highest peak power measured is 700 kW at 200 GHz. This was achieved with a beam current of 8 A and an accelerator voltage of 1.77 MV. Output power, start-up time and frequency correspond well with simulation results. Parameter scans for the longitudinal undulator gap, accelerator voltage and reflection coefficient have given a wide range of interesting data of which a few highlights are given.

First microwave generation in the FOM free-electron maser

Abstract: A free-electron maser (FEM) has been built as a pilot experiment for a millimetre-wave source for applications on future fusion research devices such as ITER, the International Tokamak Experimental Reactor. A unique feature of the Dutch fusion FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the first phase of the project, the so-called inverse set-up is used. The electron gun is mounted inside the high-voltage terminal. The entire beam line was tested successfully with extremely low loss current, lower than 0.05%. The first generation of millimetre waves was achieved in October 1997. The highest peak power measured so far is 700 kW at 200 GHz. This was achieved with a beam current of 8 A and an acceleration voltage of 1.77 MV. The output power, start-up time and frequency correspond well with the simulation results. The parameter scans for the longitudinal undulator gap, acceleration voltage and reflection coefficient have given a wide range of interesting data of which a few highlights are given.

Development of 1 mw output power level gyrotron for ITER

Abstract: The report presents the results of calculation, design and experimental study of the 170 GHz/1 MW and other ITER related gyrotrons in Russia. The main problems of powerful gyrotrons at present are discussed. The dependence of gyrotron parameters on the beam current, accelerating voltage and magnetic field is analysed. ITER – related 110 GHz and 140 GHz gyrotrons are also investigated including a single-stage depressed collector version (CPD). The possibility to make high – efficient 170 GHz/1 MW gyrotron for the ITER is proved by calculations and experiments.

Simulations of the build-up of transverse and longitudinal structures of the microwave field in the Fusion FEM

Abstract: The interaction of longitudinal and transverse modes in the Dutch Fusion FEM is studied, taking the characteristics of the feedback system, provided by quasi-optical stepped reflectors, into account. The simulations predict that the stable single-frequency operation is possible basically due to the frequency dispersion of the feedback system. As for the transverse mode interaction, the selective properties of the feedback system, as well as the effect of the suppression of parasitic modes by the operating mode at the output of the interaction region lead to a very pure transverse structure of the output microwave radiation.

First high power experiments with the Dutch free electron maser

Abstract: A free electron maser (FEM) has been built as a mm-wave source for applications on future fusion research devices such as ITER, the international tokamak experimental reactor [M. A. Makowski, F. Elio, and D. Loeser, April 97, Proc. 10th Workshop on ECE and ECRH, EC10, 549-559. World Scientific (1998)]. A unique feature of the Dutch fusion-FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the first phase of the project, a so-called inverse setup is used. The electron gun is mounted inside the high-voltage terminal. The entire beam line was tested successfully with extremely low loss current, lower than 0.05%. This included the accelerating structure up to 2 MV level and the transport through the undulator. First generation of mm-waves was achieved in October 1997. With an electron beam current around 8 A and an accelerator voltage of 1.76 MV the mm-wave pulse starts after 3 μs and lasts for 3 μs, reaching a maximum saturated peak power level of more than 500 kW at a frequency of 200 GHz. Output power, start-up time, and frequency correspond well with simulation results.

The Dutch free-electron maser: 730 kW, 200 GHz

Abstract: At the FOM Institute for Plasma Physics "Rijnhuizen", The Netherlands, an electrostatic high power free-electron maser is operated at various frequencies. An output power of 730 kW at 206 GHz is generated by a 7.2 A, 1.77 MeV electron beam, and 360 kW at 167 GHz is generated by a 7.0 A, 1.61 MeV electron beam. The lower output power at the latter frequency is due to relatively high losses in the present mm-wave cavity. It is shown experimentally and by simulations that, depending on the electron beam energy, the FEM can operate in single-frequency regime. So far, the pulse length was limited to some 12 /spl mu/S. Nevertheless, many aspects of generation of mm-wave power have been explored, such as the dependency on electron beam current and energy and settings of the mm-wave cavity.

A gyro-TWT with a weak sensitivity to electron velocity spread

Abstract: A new microwave system for a gyro-TWT in the form of a circular waveguide with a helically grooved inner surface is proposed and studied. The corrugation radically changes the wave dispersion in the region of small axial wavenumbers: at "nearly infinite" phase velocity the group velocity of the wave is finite and constant in a broad frequency band. According to the theory, this allows significant reduction in the sensitivity of the amplifier to the axial velocity spread and an increase in its efficiency and frequency bandwidth. The helical gyro-TWT is realized for the first time experimentally. In the regime of single-frequency amplification, the gyro-TWT operating at the second cyclotron harmonic with a 200 keV/25 A electron beam having a large axial velocity spread of 30% and moving in a relatively weak magnetic field of 2.2 kG yields in the X-band, a high output power of 1 MW, a gain of 23 dB and an efficiency of 20%.

Efficient schemes of millimeter wave FEM-oscillator and amplifier

Abstract: The paper is devoted to progress in the experimental study of FEM with a guide magnetic field based on a linear induction accelerator (1 MeV/200 A/200 ns). In a Bragg FEM-oscillator a mode pattern in the resonators for different Q-factors was studied both theoretically and experimentally. In accordance with theoretical predictions single-frequency operation in a low Q-factor resonator and multifrequency operation in a high Q-factor resonator were observed. Under an optimal resonator configuration at a frequency of 31 GHz a single-mode generation with a power of 37 MW and an efficiency of 26% was obtained. A new wideband scheme of RF-input based on the effect of microwave beam multiplication was studied in the FEM-amplifier experiment. At the present stage at a frequency of 36.4 GHz amplification of 20 dB was obtained.