Electron temperature measurement by a helium line intensity ratio method in helicon plasmas

Electron temperature measurements in helicon plasmas are difficult. The pre sence of intense rf fields in the plasma complicates the interpretation of Langmuir probe measurements. Furthermore, the non-negligible ion temperatur e in the plasma considerably shortens the lifetime of conventional Langmuir probes. A spectroscopic technique based on the relative intensities of neu tral helium lines is used to measure the electron temperature in the HELIX (Hot hELicon eXperiment) plasma [P. A. Keiter , Phys. Plasmas 4, 2741 (1997 )]. This nonintrusive diagnostic is based on the fact that electron impact excitation rate coefficients for helium singlet and triplet states differ a s a function of the electron temperature. The different aspects related to the validity of this technique to measure the electron temperature in rf ge nerated plasmas are discussed in this paper. At low plasma density (n(e)les s than or equal to 10(11) cm(-3)), this diagnostic is believed to be very r eliable since the population of the emitting level can be easily estimated with reasonable accuracy by assuming that all excitation originates from th e ground state (steady-state corona model). At higher density, secondary pr ocesses (excitation transfer, excitation from metastable, cascading) become more important and a more complex collisional radiative model must be used to predict the electron temperature. In this work, different helium transi tions are examined and a suitable transition pair is identified. For an ele ctron temperature of 10 eV, the line ratio is measured as a function of pla sma density and compared to values predicted by models. The measured line r atio function is in good agreement with theory and the data suggest that th e excitation transfer is the dominant secondary process in high-density pla smas. (C) 2001 American Institute of Physics.