Experimental evidence of charge-exchange recombination of highly ionized iron and titanium in Princeton large torus

The observed behavior of the emissivitives of boronlike Fe xxii, lithiumlike Fe xxiv and Ti xx, and the heliumlike Fe xxv ions in the Princeton large torus tokamak during high-power neutral (${\mathrm{H}}^0$ or ${\mathrm{D}}^0$) beam heating is described. A substantial lowering of the dominant ionization state in the center of the discharge, while the electron temperature is rising, is attributed primarily to increased recombination rate of the ions through charge exchange with neutral hydrogen. This interpretation is supported by the different space and time behavior or the lithiumlike and boronlike ions of comparable ionization potentials, and by comparisons of neutral beam heating of the plasma with ion cyclotron resonance heating, which does not appreciably change the neutral hydrogen concentration. The observations are compared with approximate zero-dimensional model calculations, using experimental plasma conditions and estimated charge-exchange rates.