Injection and Confinement of a Laser Pulse in an Optical Cavity for Multi-Pass Thomson Scattering Diagnostics in the TST-2 Spherical Tokamak Device

Abstract

A multi-pass Thomson scattering (TS) system based on confining laser pulses in an optical cavity was constructed for measuring very low-density plasma in the TST-2 spherical tokamak device. This paper describes the setup of the optical system, injection of the laser pulse into the cavity, and properties of the confined laser pulse. A combination of Pockels cell plus polarizer, which serves as an optical shutter, allows us to inject and then confine intense laser pulses in the cavity. A photodiode signal monitoring the very weak light leaking from the cavity mirrors demonstrated that the laser pulse makes many round trips, with a round-trip efficiency of approximately 0.73. The effective number of round trips (i.e., the signal enhancement factor) is approximately 3.7. For an injection efficiency of approximately 0.69, a cavity-confined laser pulse, applied to Thomson scattering, will yield a scattered signal that is five times larger than that from a single-pass laser pulse. In fusion plasma experiments, electron temperature T e and density n e profiles are indispensable data for analyzing the experiments. Most reliable T e and n e data are obtained by Thomson scattering (TS). Usually, the laser energy per pulse is a few joules, which is enough to yield high quality data for contemporary fusion plasma experiments, where n e > 10 19 m −3 . However, there are very low n e plasmas (n e 10 19 m −3 ) such as plasmas started-up and sustained by RF in the TST-2 that give us interesting subjects to study, but for which the laser energy is not enough to yield high-quality data. Adopting a 10-J laser is not realistic. One conceivable way to alleviate this difficulty is to reuse (recycle) the dumped laser energy. This concept, though slightly different, was first realized as a multi-pass intracavity TS system on TEXTOR [1]. It was demonstrated that more than 40 laser pulses at 5 kHz repletion frequency generated 900 J of total probing energy. It seemed easy to adopt a similar multi-pass optical system for TST-2, but a few drawbacks were encountered; e.g., at each pass, the laser beam path is slightly different, and the number of passes is limited by the drift of the laser beam spot on the mirror surface; therefore, the spot eventually reaches the entrance hole. A more elegant scheme is to inject and con-

YAG-Thomson Scattering System in GAMMA 10/PDX Central and End Cells

“…Thomson scattering (TS) is one of the most useful diagnostic methods for measuring electron temperature and density in plasmas and fusion devices [1][2][3][4][5][6][7][8][9][10]. However, lower electron density areas, such as in GAMMA 10/PDX and spherical region of fusion plasmas, require large numerical aperture collection optics, longer scattering lengths, and high-sensitivity detection systems.…”

Development of a laser amplification system for the multi-pass Thomson scattering system for GAMMA 10/PDX

“…MPTS systems have been developed in several fusion plasma devices. [9][10][11][12][13] In GAMMA 10/PDX (Potential-control and Divertor-simulator eXperiments), the MPTS system of the polarization-based system with imagerelaying optics has been developed. [14][15][16][17][18][19] GAMMA 10/PDX is an effectively axisymmetric minimum-B anchored tandem mirror with a thermal barrier at both end mirrors.…”

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.…”