Hα studies on the Tokamak Fusion Test Reactor

Hα emission is a useful indicator of hydrogen ionization since the energy required to excite hydrogen to the n=3 level is approximately the same as to ionize the hydrogen atom. Other aspects of recycling, such as plasma scrapeoff parameters, wall particle fluxes, and wall retention rates can be inferred from spatial distributions of Hα emission. There are two Hα diagnostics on the Tokamak Fusion Test Reactor (TFTR) observing the inner limiter: a relatively calibrated wide‐angle television camera, and the absolutely calibrated Hα interference filter array (HAIFA) diagnostic, both viewing the inner limiter. Hα observations by these instruments have been analyzed using the degas neutral transport code. The core recycling rate, and ion and neutral particle limiter fluxes, have been deduced from the calculations. The results are very sensitive to the ion flux distribution, and therefore also provide information on the plasma scrapeoff conditions. A survey was made of over 500 TFTR discharges. Hα emission appears to increase in proportion to n̄2e. The ratio of the core ionization to average Hα emission was calculated to be roughly constant over a large range of discharges.