Z eff Measurement in Ohmic, L- and H-Mode Plasmas on HL-2A Tokamak

This is the authors' abstract. We don't add key points for this paper.

Zeffis measured by two visible bremsstrahlung diagnostics on HL-2A tokamak, one of which uses a high spectral resolution spectrometer coupled with an electron multiplied CCD (temporal resolution of 100 Hz), and the other uses a filterscope system (temporal resolution of 1 MHz). The bremsstrahlung brightness measured by them coincides with each other very well. Z̅effis systematically measured under different discharge conditions, such as divertor and limiter configurations, ohmic and auxiliary heating phases, before and after siliconization, and L-H transition. Under ohmic phase, Z̅effis higher in the limiter configuration than that in the divertor configuration by a factor of ~1.5 under high-density regime. Z̅effincreases in all cases of auxiliary heating scenarios due to enhanced plasma-wall interaction, accompanied by influx increases of low ionization states of impurities such as C2+and Fe1+in plasma edge. Siliconization for wall conditioning is prominently effective in lowering the main impurity influxes in plasma edge and thus the impurity level, and a concentration of ~4% for carbon and ~0.2% for iron is roughly evaluated for a typical nonsiliconized plasma. A simple relationship that Zeff is positively correlated with Φinand τpis verified from the Land H-mode features, where Φin is the impurity influx and τpthe particle confinement time. A nearly constant Z̅effevolution is observed during L-H and H-L transition and it seems that Z̅effis insensitive to L-H and H-L transition under some Zeff(0) threshold. The Zeffprofiles are flat/weakly hollow in plasma core region (rhoeff(0) and tend to be peaked when Zeff(0) is larger than 3, which agrees with the neoclassical theory prediction.