Density Measurement Using a Lithium Beam Probe at the Inner Mirror Throat of the Tandem Mirror GAMMA 10

An inner mirror throat of the plug/barrier cell is one of the noticeable locations in the tandem mirror GAMMA10, because the location is the most suitable for a measurement of the ions bounced by the plug potential, which are essential for the tandem mirror confinement. A lithium beam probe was designed as a main part of the diagnostic system to measure the radial profiles of the electron and neutral particle density at the inner mirror throat. A neutral lithium beam is injected into the plasma and the light emitted from the beam is detected. We estimated the upper limit of the plasma density measurable by the lithium beam probe and discussed validity of the reconstruction for various types of radial profiles. We adopted, at first, a Gaussian type of radial profile of the density with the radius of 2.5 cm for the estimation of the upper limit of the density. It was found that the profile reconstruction was carried out well up to the peak density of 5 × 10¹³ cm^-3, and also well even in the non-axisymmetric radial profile. This method is quite appropriate for the measurement of the density profile at the inner mirror throat.