Magnetohydrodynamic Stability, the Drift Cyclotron Loss Cone Mode, and Confinement of Collisional Plasma in AN Axisymmetric, Large Diameter Magnetic Mirror.
An axisymmetric magnetic mirror is stabilized against MHD interchange modes by adding an average minimum B field region to the surface of a simple magnetic mirror. The device has large diameter (radius/ion gyroradius < 20), and mirror ratio variable over a wide range (2 < R < 20). It is shown that the presence of the average minimum B field region is essential to stable confinement. Pressure profiles in the central region ranging from a steep positive gradient to a gently negative gradient are stable in agreement with the predictions of the Rosenbluth-Longmire minimum energy criterion. A flat density profile with local radial density gradient scale length larger than the device diameter can be produced. Large amplitude fluctuations near the ion cyclotron frequency are observed at R = 2 and identified as the drift cyclotron loss cone mode through measurements of the frequency, wavenumber, propagation direction and dependence on the loss cone ion velocity distribution and the radial density gradient. The fluctuation amplitude is reduced 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 shown to be due to changes in the mode characteristics which satisfy the required resonance conditions as the plasma parameters change with time during a shot. An additional large amplitude mode is observed near the third harmonic of the ion cyclotron frequency, localized where the gradient is too steep for a mode at the cyclotron frequency to be present. The scaling of collisional plasma confinement as the mirror ratio is varied (4 < R < 20) is described. For strongly collisional conditions (ion mean free path << mirror to mirror length) the confinement time increases linearly with the mirror ratio as predicted for collisional flow and, in addition, increases linearly with the ion mean free path. A new parameter regime is identified where the ion mean free path is approximately equal to the device length. In this regime the confinement time increases as the square of the mirror ratio due to the presence of an axial density gradient which increases linearly with mirror ratio, is independent of density and is approximately inversely proportional to the square root of the ion temperature.
- Pub Date:
- Physics: Fluid and Plasma