Growth Rate, Saturation, and Radial Transport for the Trapped Ion Instability.
This dissipative trapped ion instability (DTII) is observed as an m = 1 mode localized to the mirror cell of the Columbia Linear Machine (CLM). A gated feedback diagnostic has been installed on CLM to allow the direct measurement of the DTII linear growth rate. The growth rate is measured by suppressing the DTII mode amplitude with feedback, abruptly gating the feedback loop open, and observing the free growth of the density fluctuations until non-linear saturation is reached. This gated feedback experiment is typically performed 400 times in a few minutes, and the density fluctuation signals are recorded by a computer controlled data acquisition system. A numerical signal envelope detection scheme is used to simultaneously improve the signal-to-noise ratio and detect the growth envelope of the DTII induced density fluctuations. Growth rates are determined by curve fitting to an exponential function over the linear growth phases of the density fluctuation envelopes. The DTII growth rate is measured as a function of trapped fraction, keeping all other parameters fixed. The measured growth rates clearly display the scaling with trapped fraction that is predicted by the radially local linear dispersion relation, including ion Landau damping. The measurements are generally about 30% less than the predictions. Saturated DTII mode amplitudes are also measured from the density fluctuation envelopes. The saturated amplitudes display a nearly linear dependence on the measured growth rates, normalized to the DTII real frequency, and are nearly proportional to the square-root of the measured growth rates. A striped particle collector is used to nonperturbatively measure the ambipolar radial plasma flux as the trapped fraction is varied. The radial particle flux is measured under plasma conditions identical to those used in the growth rate study. The flux is observed to increase over the background flux by a factor of (TURN)20 as the saturated DTII induced density fluctuation level increased from 0% to (TURN)13%. The radial diffusion coefficients measured from the radial plasma fluxes display a scaling intermediate between the Kadomtsev weak and strong turbulence predictions.
- Pub Date:
- Physics: Fluid and Plasma