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

Z _eff is 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. Z̅ _eff is 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, Z̅ _eff is higher in the limiter configuration than that in the divertor configuration by a factor of ~1.5 under high-density regime. Z̅ _eff increases 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 C ²⁺ and Fe ¹⁺ 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 Φ _in and τ _p is verified from the Land H-mode features, where Φin is the impurity influx and τ _p the particle confinement time. A nearly constant Z̅ _eff evolution is observed during L-H and H-L transition and it seems that Z̅ _eff is insensitive to L-H and H-L transition under some Z _eff (0) threshold. The Z _eff profiles are flat/weakly hollow in plasma core region (rho <; 0.7) in case of low Z _eff (0) and tend to be peaked when Z _eff (0) is larger than 3, which agrees with the neoclassical theory prediction.