Ion energy measurements on a reversed-field pinch experiment using Doppler broadening

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Abstract
Ion energies have been measured on the ZT‐40M reversed‐field pinch experiment using Doppler broadening of various impurity lines in the vacuum UV and ultraviolet range. The impurity ion temperatures are expected to relax quickly to the deuteron temperature according to the classical relaxation times. These ion temperatures as measured using O+6 (1623 Å) exceed the electron temperatures for plasma currents greater than 300 kA and fill pressures ≤2.0 mTorr. These investigations were carried out over a wide range of peak plasma currents 120 ≤Iφne≤1.2×1014 cm−3 yielding ion temperatures up to ∼500 eV. A zero‐dimensional model using an approximation for the ion heating rate by the current driven electrostatic ion‐cyclotron instability has been used to make comparison with the measured ion temperatures. This mechanism appears plausible but other mechanisms may be equally likely.