Invited Review Article: Gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion devices

Gas puff imaging (GPI) is a diagnostic of plasma turbulence which uses a puff of neutral gas at the plasma edge to increase the local visible light emission for improved space-time resolution of plasma fluctuations. This paper reviews gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion research, with a focus on the instrumentation, diagnostic cross-checks, and interpretation issues. The gas puff imaging hardware, optics, and detectors are described for about 10 GPI systems implemented over the past ∼15 years. Comparison of GPI results with other edge turbulence diagnostic results is described, and many common features are observed. Several issues in the interpretation of GPI measurements are discussed, and potential improvements in hardware and modeling are suggested.

Diagnostics for the biased electrode experiment on NSTX

A linear array of four small biased electrodes was installed in NSTX in an attempt to control the width of the scrape-off layer by creating a strong local poloidal electric field. The set of electrodes was separated poloidally by a 1 cm gap between electrodes and were located slightly below the midplane of NSTX, 1 cm behind the rf antenna, and oriented so that each electrode is facing approximately normal to the magnetic field. Each electrode can be independently biased to +/-100 V. Present power supplies limit the current on two electrodes to 30 A and the other two to 10 A each. The effect of local biasing was measured with a set of Langmuir probes placed between the electrodes and another set extending radially outward from the electrodes, and also by the gas puff imaging diagnostic located 1 m away along the magnetic field lines intersecting the electrodes. Two fast cameras were also aimed directly at the electrode array. The hardware and controls of the biasing experiment will be presented and the initial effects on local plasma parameters will be discussed.

Characteristics of the first H-mode discharges in the national spherical torus experiment

We report observations of the first low-to-high ( L-H) confinement mode transitions in the National Spherical Torus Experiment. The H-mode energy confinement time increased over reference discharges transiently by 100-200%, as high as approximately 100 ms. This confinement time is approximately 2 times higher than predicted by a multimachine scaling. Thus the confinement time of spherical tori has been extended to a record high value, leading to an eventual revision of confinement scalings. Finally, the power threshold for H-mode access is >10x higher than predicted by an international scaling from conventional aspect-ratio tokamaks, which could lead to new understanding of H-mode transition dynamics.