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

<inline-formula> <tex-math notation="LaTeX">$Z_{\mathrm {eff}}$ </tex-math></inline-formula> 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. <inline-formula> <tex-math notation="LaTeX">$\bar {Z}_{\mathbf {eff}}$ </tex-math></inline-formula> is systematically measured under different discharge conditions, such as divertor and limiter configurations, ohmic and auxiliary heating phases, before and after siliconization, and <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> transition. Under ohmic phase, <inline-formula> <tex-math notation="LaTeX">$\bar {Z}_{\mathbf {eff}}$ </tex-math></inline-formula> is higher in the limiter configuration than that in the divertor configuration by a factor of ~1.5 under high-density regime. <inline-formula> <tex-math notation="LaTeX">$\bar {Z}_{\mathbf {eff}}$ </tex-math></inline-formula> 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<sup>2+</sup> and Fe<sup>1+</sup> 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 <inline-formula> <tex-math notation="LaTeX">$Z_{\mathrm {eff}}$ </tex-math></inline-formula> is positively correlated with <inline-formula> <tex-math notation="LaTeX">$\Phi _{\mathrm {in}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\tau _{\mathrm {p}}$ </tex-math></inline-formula> is verified from the <italic>L</italic>- and <inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula>-mode features, where <inline-formula> <tex-math notation="LaTeX">$\Phi _{\mathrm {in}}$ </tex-math></inline-formula> is the impurity influx and <inline-formula> <tex-math notation="LaTeX">$\tau _{\mathrm {p}}$ </tex-math></inline-formula> the particle confinement time. A nearly constant <inline-formula> <tex-math notation="LaTeX">$\bar {Z}_{\mathbf {eff}}$ </tex-math></inline-formula> evolution is observed during <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> transition and it seems that <inline-formula> <tex-math notation="LaTeX">$\bar {Z}_{\mathbf {eff}}$ </tex-math></inline-formula> is insensitive to <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> transition under some <inline-formula> <tex-math notation="LaTeX">$Z_{\mathrm {eff}}$ </tex-math></inline-formula>(0) threshold. The <inline-formula> <tex-math notation="LaTeX">$Z_{\mathrm {eff}}$ </tex-math></inline-formula> profiles are flat/weakly hollow in plasma core region (rho < 0.7) in case of low <inline-formula> <tex-math notation="LaTeX">$Z_{\mathrm {eff}}$ </tex-math></inline-formula>(0) and tend to be peaked when <inline-formula> <tex-math notation="LaTeX">$Z_{\mathrm {eff}}$ </tex-math></inline-formula>(0) is larger than 3, which agrees with the neoclassical theory prediction.