Determination of the deuterium plasma density ratio n̑d/n̑e through neutron measurements

JET has been operating with deuterium plasmas for the past two years, during which time neutron spectrometry has been applied as a diagnostic technique for determining the ion temperature for suitable discharges. Combining the 'neutron temperature' with the neutron yield determination made with a fission counter diagnostic permits the central deuteron to electron density ratio (n̑d/n̑e) to be determined. It is found that this ratio, for ohmically heated discharges, appears to fall with ion temperature from about 70% at 2 keV to 40% at 3 keV. The addition of ICRF heating does not alter the density ratio. These findings are consistent with the observation that high temperatures achieved with Ohmic heating in tokamaks are correlated with high impurity levels.