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During the first few seconds of longer pulses in high-power gyrotrons, the main mode frequency decreases over a few hundred MHz, until a stable operation is reached. This is due to the combination of two effects, namely the thermo-mechanical expansion of the cavity and a shift in the effective electron energy through ionization of residual gas in the tube. Most investigations focus on the quasi-stationary...
During tests of a megawatt-class 140 GHz gyrotron for the fusion experiment Wendelstein 7-X, a window RF arc was documented with a high-resolution spectral measurement system. The arc event coincided with strong perturbations of the nominal cavity mode TE-28,8 and a final mode switch to a neighbor mode. Simulations using a self-consistent interaction code (SELFT) have reproduced the main features...
The stellarator Wendelstein 7-X, currently in its final construction phase at Greifswald, Germany, will employ a 10 MW, 140 GHz, continuous wave (CW) gyrotron-based electron-cyclotron resonance heating (ECRH) and-current drive (ECCD) system. To fulfill this demand for CW millimeter-wave power, Karlsruhe Institute of Technology (KIT) and Thales Electron Devices (TED, France) have been maintaining a...
In this work a measurement system for detection of time-dependent effects in broad-band spectra of high-power millimeter-wave sources is demonstrated. The key feature is an unambiguous reconstruction of the RF spectrum from the intermediate frequency range at which the final spectral domain measurements are performed. This is done by evaluating the upper and lower mixer sidebands of two parallel heterodyne...
At KIT, high power gyrotrons of conventional (1 MW CW at 140 GHz for the stellarator Wendelstein 7-X) and coaxial type (2 MW short-pulse at 170 GHz for ITER) for fusion applications are developed and verified experimentally.
ECH & CD is widely used in fusion plasmas. Gyrotrons are applied as power sources as they offer continuous wave operation at MW power level. This contribution reports on the status and plans of major projects of KIT towards high power gyrotrons, i.e. results from series production of the W7-X gyrotron, the development of the EU pre-prototype coaxial-cavity gyrotron, the 2 MW, 170 GHz CW coaxial...
At KIT, high power millimeterwave oscillators (gyrotrons) for plasma physics heating and current drive experiments are developed and tested. This paper describes the introduction and application of a time-based measurement system which allows broadband high-dynamic range spectrum analysis of the gyrotron output signal in the range 110–170 GHz.
At high-power operation points, in the improved W7-X SN4R series gyrotron spurious parasitic oscillations could be detected experimentally. This paper describes high-dynamic RF spectrum measurements in short and long pulse operation and corresponding simulations with a self-consistent gyrotron interaction code. Preliminary simulation results show good agreement with the measured spectra and suggest...
The SELFT code has been modified to study the influence of B=B(z) on spurious oscillations. An initial attempt has been made to simulate the spurious oscillations in the cavity for an example (W7X 140GHz, TE−28, 8 mode gyrotron). The radio frequency (RF) spectrum at the end of the simulation (using B=B(z)) clearly indicates the presence of spurious oscillations.
In recent years, the so called after cavity interaction (ACI) in high power gyrotrons operating in the 100-200 GHz range gained attention as an influence factor on overall efficiency. While investigations concentrated on ACI as a stationary effect until now, recent simulations show that an undesired interaction in the uptaper region can also result in additional parasitic oscillations. In this paper,...
Recently megawatt gyrotrons are found to suffer from various parasitic oscillations, particularly RF oscillations in the beam tunnel induced by the electron beam prior to the desired interaction zone (cavity). The paper describes experimental and theoretical investigations on both high and low frequency parasitic oscillations, employing a special measurement system as well as a novel analysis method...
A 2 MW, CW, 170 GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within an European Gyrotron Consortium (EGYC*). To support the development of the industrial prototype of a CW gyrotron, a short pulse tube (pre-prototype) is used at KIT (former FZK) for experimental verification of the design...
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