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Abstract
Measurements of d(d,p)t and d(d,n)$\sp3$He thermonuclear-fusion-reaction products are used to study ion energy confinement in the PLT tokamak during $\sp3$He-minority ion-cyclotron-resonance heating of a deuterium plasma. For low ($\sim$5%) minority concentrations and moderate ($\sim$1 MW) ICRH power levels, collisional energy exchange between the resonant minority and the bulk deuterium is the dominant source term in the power balance, raising the ion temperature to the level of the electron temperature and thereby decoupling the ohmic power input from the ion energy balance. The higher ion temperature also enhances radial thermal conduction by raising the absolute temperature gradient. The ion energy containment process is thus effectively reduced to a simple balance between the RF input power and the thermal conduction loss to the cold plasma edge region. The ion temperature profiles and local temperature gradients are inferred from 3 MeV proton flux measurements in order to determine the magnitude of the ion thermal conductivity. Neutron flux diagnostics provide a detailed study of ion behavior during sawtooth oscillations; the observed time evolution of the neutron signal suggests a radial redistribution and deuterium and a subsequent enhancement of the radial thermal conduction for 2-5 ms following the sawtooth crash, resulting in a $\sim$10% drop in neutron emission. The combined neutron and proton data suggest ion thermal conduction during the sawtooth recovery phase which is approximately neoclassical near $r = a/4,$ but about twice neoclassical near $r = a/2,$ implying that the spatial dependence of the thermal diffusivity coefficient $\chi\sb{i}$ differs from neoclassical theory. The ratio of the measured ion temperature- and density-gradient scale-lengths exceeds the critical value for the onset of so-called "$\eta\sb{i}$" ion mixing modes, providing a plausible explanation for the experimental $\chi\sb{i}$ profile.
