Assessment of Impurity Content and Radiated Power in LHI discharges on the <emph type="smallcap">Pegasus</emph> ST

Local Helicity Injection (LHI) is routinely used in the <emph type="smallcap">Pegasus</emph> ST to produce high-performance discharges with I_p <= 0 . 24 MA, n_e 10¹⁹ m^{- 3} and T_e 100 eV. Recent experiments show that the impedance of the plasma is independent of the helicity drive as I_p scales linearly with V_LHI. One possible explanation of this behavior would be that Z_eff and the plasma resistivity increases with I_p. To examine this possibility an assessment of radiated power and impurity concentration is being done in <emph type="smallcap">Pegasus</emph> using a tangential bolometer array, VB spectroscopy and a SPRED VUV spectrometer. Discrete line radiation from N and O are at least 10 × higher during LHI than Ohmic. A broadband spectrum at λ <= 35 nm is observed during LHI but absent during Ohmic discharges. Radiated power increases rapidly with V_LHI, suggesting an increase in Z_eff as I_p increases. Initial observations suggest Bremsstrahlung radiation measurements are infeasible during LHI but may be measured during an Ohmic phase in an LHI-Ohmic handoff scenario. To quantify estimates of impurity content, a new multichannel diode bolometer array with full coverage across the plasma is being developed.

Work supported by US DOE Grants DE-FG02-96ER54375 and DE-SC0019008.