Theoretical investigation on possible operation of a 140 GHz 1 MW gyrotron at 175 GHz for CTS plasma diagnostics at W7-X

Abstract

Collective Thomson scattering is a common diagnostic technique for ion temperature measurements in experimental fusion plasma reactors. Such a system was successfully installed and commissioned at the Wendelstein 7-X stellarator. For this purpose, a 140 GHz gyrotron of the Electron Cyclotron Resonance Heating system was used as a source of the required probing millimeter (mm)-wave beam. However, accurate measurements in the plasma core were not possible at this heating frequency due to the absorption of the mm-waves and the high electron cyclotron emission background. To suppress these effects and to enhance the accuracy of the measurements, it is required to increase the frequency of the probing beam. In this work, the possibility to operate the same gyrotron, which has its nominal operation at 140 GHz, at a higher frequency is comprehensively investigated.

Commissioning and first results of the 174 GHz collective Thomson scattering diagnostic at Wendelstein 7-X

“…Indeed, this range is located between the second and third harmonic of ECE coming from the plasma [23]. Theoretical investigation showed that existing gyrotrons could emit microwave pulses of several ms particularly at a frequency of 174 GHz, should their magnets be replaced to those which can deliver around 7.1 T instead of currently used 5.6 T [24]. The prediction was proven experimentally.…”

“…The conversion efficiency of other modes is lower, as discussed in ref. [24]. High fluxes of stray radiation inside the gyrotron can lead to damages.…”

Collective Thomson Scattering Diagnostic for Wendelstein 7-X at 175 GHz

“…It was shown theoretically that the current probing gyrotron is able to emit at the frequency of 173.92 GHz using a TE 34,10 cavity mode instead of the current TE 28,8 cavity mode. For this mode less than 5% of the microwave power is converted into stray radiation [34]. The output power will be at the level of 60% of the output power at 140 GHz, and new phase-correcting mirrors will be necessary.…”

Systematic Design & Analysis of a 42 GHz Gyrotron and the Effects of Structure & Beam Parameters on Its RF Performance

“…Gyrotrons producing moderate level of power in millimeter to sub-millimeter wave have applications as in high resolution radars ranging and imaging in atmospheric and planetary science, ceramic sintering, plasma scattering measurements, electron spin resonance (ESR) experiments, and nuclear magnetic resonance (NMR) signal enhancement by dynamic nuclear polarization (DNP), etc. [1][2][3][4][5][6]. However, the high power gyrotrons at high frequency are playing important role in the area of plasma fusion research.…”

“…However, the high power gyrotrons at high frequency are playing important role in the area of plasma fusion research. Powerful gyrotrons, providing more than MW power level, are mainly used for electron cyclotron resonance plasma heating (ECRH) in both tokamaks and stellarators, controlling of local current density profile by non-inductive electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for generation of energy by controlled thermonuclear fusion [1][2][3][4][5][6].…”