Measuring Electron Temperature in the Tandem Mirror GAMMA 10 Plasma Using a Yttrium-Aluminum-Garnet Thomson Scattering System

Abstract

A yttrium-aluminum-garnet Thomson scattering (TS) system was constructed and applied to the tandem mirror GAMMA 10 plasma to measure the electron temperature. A large solid-angle TS light-collection system was set using a spherical mirror system and a large numerical aperture of bundled optical fiber. A five-channel polychromator with avalanche silicon photo diodes was employed after being calibrated with standard light. Calibration was performed by Rayleigh and Raman scattering. An electron temperature increase from 40 eV to 80 eV was observed with application of electron cyclotron heating to plug/barrier cells. GAMMA 10 is the world largest tandem mirror machine with which plasma confinement is achieved not only by a magnetic mirror configuration but also high potentials at both end regions [1][2][3]. The main plasma is produced and heated by ion-cyclotron range of frequency waves. The confinement potentials are produced by plug and barrier electron cyclotron heating (P/B-ECH) at the plug/barrier regions. During the formation of confinement potential, the typical electron density, electron temperature, and ion temperature are about 2 × 10 18 m −3 , 0.1 keV, and 5 keV, respectively. Thomson scattering (TS) is the most reliable diagnostic for measuring electron temperature and electron density. In GAMMA 10, a ruby-laser TS system was installed to measure the electron temperature. However, the system experienced problems. Previously, electron temperature has been measured by soft X-ray measurement. Thus far, direct electron heating by ECH experiments have been conducted in the central cell. In addition, density and potential fluctuation suppressions during the formation of confinement potential have been studied using a gold neutral beam probe system [1][2][3]. After the aforementioned problems with the ruby-laser TS system, we planned to install a neodymiumdoped, yttrium-aluminum-garnet laser (Nd:YAG) TS system to measure electron temperature directly in the central cell of GAMMA 10. In comparison with the ruby-laser author's e-mail: yosikawa@prc.tsukuba.ac.jp system, operation of the YAG laser system is easier and the repetition rate of such a system is much larger. The YAG TS system is normally used in higher electron density plasmas, over 10 19 m −3 . Moreover, an efficient TS system is necessary to measure low-density plasmas in the region of less than 10 19 m −3 , such as the GAMMA 10 plasma and the peripheral plasma of high-density fusion plasmas. To obtain a TS signal with a good signal-to-noise ratio, we developed an optical-collection system with a large solid angle. In this paper, we describe the newly installed YAG TS system and the first results of electron temperature and density measurements in the tandem mirror GAMMA 10. The YAG TS system is constructed with laser, incident optics, light-collection optics, signal-detection electronics, and a data-recording system. A 10-Hz Nd:YAG laser (Continuum, Powerlite 9010) with an energy per pulse of 2 J, a pulse width of about 10 ns, and operating at a fund...

Design of the polarization multi-pass Thomson scattering system

“…The system is based on the GAMMA 10 TS system, which successfully observed the electron temperature of the GAMMA 10 plasma in 2010. 1 Horizontally polarized laser light of 2 J from a 10 Hz Nd:YAG laser (Continuum, Powerlite 9010) is focused into the plasma by the first convex lens. After interaction with the plasma, laser light is emitted from the port window, which has an anti-reflection coating, and is collimated by the second convex lens.…”

“…The typical electron density and electron and ion temperatures of GAMMA 10 are about 2 × 10 18 m −3 , 0.1 keV, and 5 keV, respectively, during application of the plug and barrier electron cyclotron heating. 1 To increase the scattering probability at low densities, a multipass TS scheme is effective, which allows a laser pulse to be focused several times into the scattering volume, thus increasing the scattering photon number. In addition to improving the signal-to-noise ratio, a multi-pass TS system can be used to improve the time resolution and expand the measurable range of Te.…”

Radial Profile Measurements of Electron Temperature and Density Using the Thomson Scattering System in GAMMA 10/PDX

“…Thomson scattering (TS) is the most useful diagnostic available to measure the electron temperature and density in fusion devices [1][2][3][4][5][6][7][8][9][10]. To apply a TS system in lower electron density plasmas, such as GAMMA 10/PDX and the spherical region of fusion plasmas, large numerical aperture collection optics and large scattering lengths are required.…”

“…To apply a TS system in lower electron density plasmas, such as GAMMA 10/PDX and the spherical region of fusion plasmas, large numerical aperture collection optics and large scattering lengths are required. In GAMMA 10/PDX, we installed an yttriumaluminum-garnet (YAG)-TS system with large angle collection optics and a large scattering length optical system [8,9]. The YAG-TS system can measure radial electron temperatures and densities at seven fixed positions in a single laser shot at 10 Hz.…”

First Results of Electron Temperature Measurements with a Multi-Pass Thomson Scattering System in the Tandem Mirror GAMMA 10

“…The Thomson scattering (TS) diagnostic is one of the most useful methods for measuring electron temperatures and radial density profiles in plasmas. For low-electrondensity plasmas, such as the GAMMA 10 plasma and peripheral plasmas in fusion devices, an effective TS system must be developed [1,2]. The GAMMA 10 TS system can measure the radial profiles of electron density and temperature in the electron-density range above 5×10 17 m −3 .…”