In order to improve the axial confinement, in the tandem mirror machine, the electrostatic potentials are created on both sides of the machine. Usually, thermal barrier potential is created at the midplane of the plug/barrier cell. We note that the electrostatic potential at the Inner Mirror Throats (IMT) is able to play an important role acting as a barrier of the electron flow from the central cell to the plug cell. We examined the influence of the IMT potential on the electron flow by calculation, and it was found that the IMT potential acted effectively as the thermal barrier. In consideration of the undesirable condition at the IMT region, that is the tight accessibility and the strong magnetic field, we adopted a newly designed gold neutral beam probe system including a new type of MCP detector. The characteristics of the MCP detector were obtained successfully using a teststand with strong magnetic field created by the nuclear magnetic resonance (NMR) magnets.

This beam probe system was applied to the tandem mirror GAMMA 10, and the electrostatic potential was measured successfully by the beam probe system located in the neighborhood of the IMT region.