Design and operation of the multipulse Thomson scattering diagnostic on DIII-D (invited)

TLDR

The paper presents the design and operation of a 40‑channel Thomson scattering system using multiple 20‑Hz Nd:YAG lasers to measure electron temperature and density profiles throughout a plasma discharge. The system employs up to eight 20‑Hz Nd:YAG lasers fired alternately for an average 160‑Hz measurement rate or in rapid bursts (<10 kHz), with scattered light dispersed by interference‑filter polychromators and detected by silicon avalanche photodiodes, while laser control, data acquisition, and real‑time analysis are handled by a VME‑based microcomputer and a MicroVAX 3400, providing full statistical error treatment and beam‑quality monitoring. The system’s 1.3‑cm spatial resolution and 10 eV–20 keV dynamic range allow it to resolve steep electron density and temperature gradients at the plasma edge and scrape‑off layer during H‑mode, making it a powerful tool for investigating L–H transitions, ELMs, beta limits, transport, and disruptions, as demonstrated by component testing, calibration, plasma operation, and error analysis.

Abstract

This paper describes the design and operation of a 40 spatial channel Thomson scattering system that uses multiple 20-Hz Nd:YAG lasers to measure the electron temperature and density profiles periodically throughout an entire plasma discharge. As many as eight lasers may be fired alternately for an average measurement frequency of 160 Hz, or they may be fired in rapid succession (&amp;lt;10 kHz), producing a burst of pulses for measuring transient events. The high spatial resolution (1.3 cm) and wide dynamic range (10 eV–20 keV) enable this system to resolve large electron density and temperature gradients formed at the plasma edge and in the scrape-off layer during H-mode operation. These features provide a formidable tool for studying L–H transitions, edge localized modes (ELMs), beta limits, transport, and disruptions in an efficient manner suitable for large tokamak operation where shot-to-shot scans are impractical. The scattered light is dispersed by interference filter polychromators and detected by silicon avalanche photodiodes. Laser control and data acquisition are performed in real time by a VME-based microcomputer. Data analysis is performed by a MicroVAX 3400. Additional features of this system include real-time analysis capability, full statistical treatment of error bars based on the measured background light, and laser beam quality and alignment monitoring during plasma operation. Results of component testing, calibration, plasma operation, and error analysis are presented.