A photodiode array and Langmuir probe for characterizing plasma in GLAST-III tokamak device

Abstract

A newly developed spectroscopic system based on linear photodiodes array has been installed on upgraded Glass Spherical Tokamak (GLAST-III) for spatial as well as temporal characterization of hydrogen discharge through light emission. The Spectral range of each silicon photodiode is from 300 nm to 1100 nm with response time of 10 ns and active area of 5 mm2(circular). The light from the plasma is collected through holes along 4 lines of sight channels with spatial resolution of about 5 cm passing from entire poloidal cross section. The photodiode's signals located at position of 10 and 14 cm from inboard side show fluctuations in the central plasma region. Moreover, the sequence of plasma lighting shows that plasma instigates from the central resonant field region and then expands outwards. At lower pressure, outboard movement of the plasma is slower suggesting better plasma confinement. In addition to photodiode array, an Ocean spectrometer (HR2000+) has been used to record the visible spectrum over the selected range (597–703 nm) with a spectral resolution of 0.15 nm. The studies have been conducted during initial phase of plasma formation for two different hydrogen gas fill pressures. Langmuir triple probe (LTP) is used to get time-resolved information on plasma parameters in the edge region. The time evolution of whole discharge including microwave pre-ionization phase and current formation phase has been demonstrated by temporal profiles of light emission and plasma floating potential.