Effect of Nitrogen on Heat Flux to a Tungsten Target in He Plasma

Abstract

Nitrogen (N2) gas was injected into the helium (He) plasma with a LaB6 cathode as an impurity to observe the variations of heat flux and the SOL heat flux width in the Divertor Plasma Simulator-2 (DiPS-2) device. The injection rate of N2 impurities was varied from 2.5% to 30% with a constant neutral pressure (~9 mTorr). To evaluate the plasma heat flux to a tungsten target, three diagnostic methods were used: (1) three thermocouples (TCs) were installed within the tungsten target to estimate the inner temperature of the tungsten, (2) an infrared (IR) camera detected the surface temperature of the tungsten; (3) a triple (electric) probe (TP) was used to determine the electron temperature (Te), plasma density (n0), and heat flux (q) of He plasma. With 30% N2 injection, a 20% decrease in Te and a 60% decrease in n0 were observed at the center of the plasma. The target temperature and heat flux also decreased with N2 seeding rate. The heat fluxes to the tungsten target deduced from the IR and TC methods produce higher values than those of the TP. The width of the heat flux transported from the main plasma to the scrape-off layer (SOL) heat flux width (λq) decreases with the impurity ratio; hence, the heat flux to the tungsten target.