Neutral‐beam diagnostic for fast confined alpha particles in a burning plasma: Application on CIT

Measurement of the slowing‐down distribution for fast confined alpha particles is essential for a D–T burning‐plasma experiment. Quantitative estimates of signal level for a fast neutral‐beam probe using a two‐electron‐capture method of measuring the alpha‐particle velocity distribution are presented as applied to the proposed tokamak CIT (Compact Ignition Tokamak). The best probe is found to be a fast ground‐state helium‐atom beam because of its relatively good penetration into a dense plasma and because of the large cross section for two‐electron capture; fast He0 can be produced in useful quantities from fast HeH+, which is sufficient to allow time‐resolved measurements of the alpha‐particle velocity distribution in CIT.