Suppression of longitudinal losses in a gas-dynamic trap by using an ambipolar mirror

Abstract

The efficiency of utilizing ambipolar mirrors for suppression of longitudinal losses of particles and energy in a gas-dynamic trap (GDT) was investigated. An additional relatively small axisymmetric mirror cell was installed in one of the facility ends. Hydrogen or deuterium atomic beams with an energy of 22 keV and equivalent current density of up to 1 A/cm2 were injected into the additional cell at an angle of 90° to the facility axis. Trapping of the beams with a total power of 800 kW by the plasma in the additional cell leads to the formation of a hot ion population with an anisotropic velocity distribution, a mean energy of 13 keV, and a density of up to 4.5 × 1013 cm−3. It is shown that the confinement of hot ions in the additional cell is determined by classical processes, such as charge exchange on the beam atoms and collisional deceleration by electrons, in spite of the onset of Alfven ion-cyclotron instability at fast ion densities higher than 2.5 × 1013 cm−3. The effect of ambipolar confinement manifests itself in that, at hot ion densities higher than 3 × 1013 cm−3, the flux density of ions escaping from the trap in the mode with beam injection decreases fivefold as compared to that without injection. In this case, the density of the Maxwellian plasma component in the central cell is about 2.5 × 1013 cm−3. The efficiency of suppression of longitudinal particle losses by the ambipolar mirror substantially exceeds estimates obtained for both collisional (gas-dynamic) and collisionless (adiabatic) confinement modes. Qualitatively, this is because, in the GDT experiments, the mode of warm plasma confinement is transitional between the gas-dynamic and adiabatic modes and the use of an ambipolar mirror facilitates a transition from the lossy gas-dynamic mode into a nearly adiabatic one.