Helium line emission measurements in PISCES-B as a tool for Te-profile determinations in tokamak boundary plasmas

Abstract

The potential of helium spectroscopy for quasicontinuous electron temperature profile measurements in the boundary layer of a tokamak plasma is tested. Particularly thermal atomic helium beams are easy to produce and can penetrate relatively far into typical tokamak boundary plasmas because of their high ionization energy. However, in the case of helium and for the range of densities and temperatures characteristic for tokamak boundaries the validity of the corona model cannot be assumed. Therefore it is vital to measure Hel-line intensities in well diagnosed plasmas with tokamak boundary like conditions, which may be compared with calculated ones. For the production of a plasma with parameters close to that of a tokamak boundary layer the PISCES-B facility was used, where plasmas with up to T e ≈ 30 eV simultaneously with n e ≈ 5 × 10 12/cm 3 could be obtained. T e- and n e-profiles were determined both by fixed and reciprocating Langmuir probes. The helium was injected into the central part of an argon plasma, and its spectral line emission in the visible was measured by means of an OMA attached to an optical spectrometer. Six triplet and six singlet emission lines of HeI were measured and the ratio of these — which is assumed to be a function of T e — evaluated for a range of temperatures and densities. From their absolute intensities also the individual line excitation rates could be derived as a function of electron temperature. It was found that the majority of the experimental values both for the line excitation rate and the line intensity ratio do not agree with the calculated values, which take only excitation from the ground state into account. On the other hand, experimental intensity ratios were found, which do not show a density dependence, and are recommended to be used for an experimental technique, which would allow a quasicontinuous T e-profile measurement in the plasma boundary region of tokamaks.