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

Abstract

A: The Collective Thomson Scattering (CTS) diagnostic measures the scattering spectrum of incident radiation off collective fluctuations in plasmas. In Wendelstein 7-X (W7-X) the diagnostic uses a 140 GHz heating gyrotron as a source of the probing radiation. At this frequency, the CTS spectra are heavily affected by the electron cyclotron emission, and the microwave beam propagation is restricted at typical W7-X plasma parameters. The diagnostic was successfully commissioned in the last experimental campaign and demonstrated ion temperature measurements. However, the signal-to-noise ratio was too low for measuring other quantities such as the fast-ion velocity distribution function or the fuel ion ratio. Currently, the W7-X CTS diagnostic is undergoing an upgrade to a frequency of 175 GHz. This will increase the sensitivity of the diagnostic, since the noise due to electron cyclotron emission will be reduced, and it will relax the constraints on 1Corresponding author.

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

“…We showed, that the optimal frequency range is 171-177 GHz [22]. Indeed, this range is located between the second and third harmonic of ECE coming from the plasma [23].…”

“…Initially, the CTS diagnostic at W7-X was designed for ion temperature (𝑇 𝑖 ) measurements using one of the heating gyrotrons as a source of probing radiation [21,22]. The diagnostic delivered ion temperature, however poor signal-to-noise ratio (SNR) due to strong electron cyclotron emission (ECE) in the order of several keV required long averaging.…”

Investigation of gyrotron-based collective Thomson scattering for fast ion diagnostics in a compact high-field tokamak

“…In high-density plasmas, the use of CTS is limited by the refraction effect of microwaves which makes it difficult to set the location of the scattering volume accurately. To broaden the operation range in high-density plasmas, a 300 GHz gyrotron has been developed for the LHD CTS diagnostic [12], and the CTS diagnostic on W7-X is undergoing an upgrade to a probing frequency of 175 GHz [13].…”

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

“…4 For the probing beam of this CTS diagnostic, a gyrotron 5 of the Electron Cyclotron Resonance Heating (ECRH) system of W7-X, 6 which operates at 140 GHz, has been used. However, measurements in the plasma core at this frequency cannot be adequately accurate due to the absorption of the probe mm-wave beam and due to the high electron cyclotron emission (ECE) background, 7 which both deteriorate signal-to-noise ratio. For optimal CTS measurements in the plasma core, the frequency of the probing beam should be in the range in which ECE of the plasma is minimal.…”

“…It was determined that a change to a higher frequency range between 170 GHz and 185 GHz is more beneficial for the CTS diagnostic. 7 In contrast to tokamaks that have a Greenwald density limit and operate at relatively low densities and high temperatures, stellarators work at a different regime. Due to 1/ transport and absence of the Greenwald density limit, stellarators operate at low plasma temperatures and high densities.…”