Investigation of ion heating on a relativistic electron beam injection into a plasma

Results are presented of an experimental investigation of the distribution function and amount of heated ions produced on injection of a relativistic electron beam into a plasma. An approximately 1 MeV beam with a current 7-10 kA and duration approximately 70 nsec has been injected into a plasma with a density 3 x 1012-4 x 1014 cm-3 located in a 6-12 kOe magnetic field. Ion heating has been investigated by analysing the fast neutral charge-exchange particles emitted from the plasma across the magnetic field. The form of the distribution function and number of heated ions are found to depend in a pronounced manner on the density of the initial plasma. For a plasma density n > 1014 cm-3 the ion distribution function in the energy range studied is close to the Maxwellian and the heated ion density is 10-3-10-4 of the total number of ions. With decrease of plasma concentration (n < 1014 cm-3) the distribution function begins to differ from the equilibrium function and the number of heated ions increase, comprising up 10-20% of the total amount of ion. At low densities the distribution function may be interpreted as a 'two-temperature' function; however the distribution of all plasma ions under all conditions differs from the equilibrium form. At high densities (n > 1014 cm-3) ion heating occurs in the space occupied by the electron beam. With decrease of the plasma density ( n < 5 x 1013 cm-3) heating of the ions is observed throughout the plasma column. Possible mechanisms of ion heating are discussed