Kinetic instabilities in two-isotopic plasma in the gas-dynamic trap magnetic mirror

A fusion neutron source (FNS) based on the gas-dynamic trap (GDT, Budker Institute, Novosibirsk) is considered for confinement of two-species plasma heated by neutral beam injection in a regime where the fast ion distribution function is far from Maxwellian. Kinetic instabilities are expected to develop in this regime, and in this paper we investigate the ion-cyclotron instability evolving in moderate densities of pure hydrogen and mixed deuterium-hydrogen target plasmas. The properties of the studied unstable mode, such as its azimuthal wavenumbers, propagation direction and its being affected by changes in the bulk plasma density and composition, allow us to identify it as the drift cyclotron loss cone (DCLC) instability. This mode scatters fast ions and thereby leads to drops in diamagnetic flux signals and increases longitudinal energy and particle losses, with the average energy of the lost ions estimated to be far above the temperature of warm Maxwellian ions. Our interpretation is that the unstable wave grows due to interaction with the fast ions located near the loss cone in the velocity space and scatters them. Applying the method of suppressing the DCLC instability by filling the loss cone with warm plasma, we have determined the values of plasma density and deuterium percentage that allow us to suppress the DCLC instability in the GDT. These findings justify using mixed bulk plasmas in fusion neutron source operation.