The dependence of the drift cyclotron loss cone instability on the radial density gradient

Abstract

Large-amplitude fluctuations (eφrms/kTe≲2) observed near the ion cyclotron frequency and its harmonics in a magnetic mirror are identified as the drift-cyclotron loss-cone (DCLC) instability through measurements of the frequency, wavenumber, propagation direction, and dependence on the loss cone ion velocity distribution and the radial density gradient. The mode amplitude is reduced when the plasma density and radial density gradient length are near the linear stability boundary, dropping by two orders of magnitude when the density gradient scale length is increased from 2 to 100 ion gyroradii. The mode is localized radially to a region of one ion gyrodiameter width near the location of the steepest density gradient. The bursting appearance of mode amplitude is accompanied by changes in its perpendicular wavelength as the plasma parameters change with time during its decay. Mode amplitude modulation which occurs simultaneously throughout certain flux volumes indicates that the fluctuations arise from an absolute instability. While oscillations at the fundamental of the ion-cyclotron frequency occur most frequently with the largest amplitude, a dominant mode is observed near the third harmonic of the ion cyclotron frequency localized at a steep gradient near the axis. One possible interpretation is that the wavelengths of lower harmonic DCLC modes are too long to satisfy the azimuthal quantization condition.