Double charge exchange from helium neutral beams in a tokamak plasma

Helium neutral beams of 29.5 keV amu−1 have been injected into helium plasmas on the JET tokamak. Thermal neutral helium in its ground state forms along the beam trajectory due to resonant double charge exchange. The helium neutrals form a halo about the neutral beam axis, and the latter is observed spectroscopically. The cross section for double charge exchange is derived relative to the cross section for beam emission. The double charge exchange spectrum is used to measure the plasma ion temperature and the frequency of toroidal rotation and can be used to infer the helium ion density. The potential for a He ash diagnostic based on observation of the helium halo is discussed.