Determination of plasma-ion velocity distribution via charge-exchange recombination spectroscopy

TLDR

Charge‑exchange recombination spectroscopy of impurity ions in tokamak plasmas is a powerful diagnostic that can measure ion temperature, bulk motion, impurity transport, and fast alpha‑particle distributions. The authors calculated cascade‑corrected excitation rates for Δn=1 transitions of He⁺, C⁵⁺, and O⁷⁺ at 5–100 keV/amu, modeled line‑intensity effects such as fine structure, l‑mixing, Stark shifts, and beam‑produced ions, and applied a fiber‑optical spectrometer on PDX to record He⁺ emission. The technique yields space‑ and time‑resolved ion temperature and toroidal rotation profiles, with PDX measurements showing central temperatures up to 2.4 keV and rotation speeds up to 1.5×10⁷ cm s⁻¹, and the authors provide estimates of fine‑structure, l‑mixing, Stark, and beam‑ion effects for PDX and TFTR.

Abstract

Spectroscopy of line radiation from plasma impurity ions excited by charge-exchange recombination reactions with energetic neutral-beam atoms is rapidly becoming recognized as a powerful diagnostic for magnetically confined tokamak plasmas. Ion temperature, bulk plasma motion, impurity transport, and more exotic phenomena such as fast alpha-particle distributions can all be measured with this technique. In particular, it offers the capability of obtaining space- and time-resolved ion temperature and toroidal plasma rotation profiles with relatively simple optical systems. Cascade-corrected excitation rate coefficients for use in both fully stripped impurity density studies and ion-temperature measurements have been calculated for the principal $\ensuremath{\Delta}n=1$ transitions of ${\mathrm{He}}^{+}$, ${\mathrm{C}}^{5+}$, and ${\mathrm{O}}^{7+}$ with neutral-beam energies of 5-100 keV/amu. Line intensities and profiles can be affected by atomic fine structure, $l$-mixing collisions, motional Stark effects, and product ions created in the neutral-beam region which drift into the viewing sightline. General estimates of the importance of these effects for the transitions of interest are provided, along with specific examples calculated for the PDX (Poloidal Divertor Experiment) and TFTR (Tokamak Fusion Test Reactor) tokamaks. A fiber optically coupled spectrometer system has been used on PDX to measure visible ${\mathrm{He}}^{+}$ radiation excited by charge exchange to illustrate some of these points. Central ion temperatures up to 2.4 keV and toroidal rotation speeds up to 1.5\ifmmode\times\else\texttimes\fi{}${10}^{7}$ cm/s were observed.