Observations of giant recombination edges on the Princeton Large Torus tokamak induced by particle transport

In this paper we report on the observation of characteristic "steps" in the continuum spectrum of high-temperature tokamak plasmas associated with recombination radiation from impurity ions. During special argon-seeded discharges on the Princeton Large Torus tokamak the x-ray spectrum exhibited large enhancements over the bremsstrahlung continuum beginning with energies of 4.1 keV. This corresponds to the radiative capture of free electrons by hydrogenlike argon into the ground state of heliumlike argon. The size of these edges increased to unexpectedly large values with minor radius (decreasing electron temperature), consistent with a departure of the hydrogenlike species from the predictions of corona equilibrium. Hence, the coronal equilibrium equations must be modified to account for the radial transport of argon. A simple particle diffusion model is proposed, with the Ar XVIII radial profiles evaluated from the size of the recombination edges. For the case of moderate density ($〈n_e〉\sim 3\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$) and temperature ($T_e\mathrm{}\left(0\right)\mathrm{}\sim 1.5$ keV) discharges the outward radial transport velocity is found to be approximately 10 m/sec.