Collective Thomson scattering with 77, 154, and 300 GHz sources in LHD

Abstract

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A megawatt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ~10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co-and counter-fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

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

“…However, the scattered spectrum is affected by many factors, such as the frequency of the incident wave, the scattering geometry and the plasma parameters. A decrease in the Salpeter parameter α would make the ion measurement difficult [12,19]. Figure 3(b) shows the CTS spectrum under different scattering angles.…”

“…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].…”