Measurement of M-shell iron ionization balance in a tokamak plasma

We present the △n = 0 (n = 3 to n = 3) EUV spectra of M-shell iron ions from a tokamak plasma obtained during the "ramp-down" phase of the discharge. The fractional ion abundances for K-like Fe⁺⁷ to Si-like Fe⁺¹² are found from fitting model spectra to the measured spectrum. The plasma temperature is measured as a function of position by Thomson scattering of laser light and Langmuir probes. Collisional-radiative simulations have been computed for the above ions, using atomic data generated with the HULLAC suite of codes. The blended M-shell spectrum is strongly sensitive to temperature; this makes it a potentially useful diagnostic for observations of stellar coronae where the differential emission measure is dominated by temperatures around 1 MK.