EBIT EUV MEASUREMENTS OF KR XXI – KR XXXV AND THE EFFECT OF A FORBIDDEN LINE ON ALLOWED TRANSITIONS
Yuri A. Podpaly, John D. Gillaspy, Yuri Ralchenko, Joseph Reader
Atomic emissions from plasma impurities provide data required for tokamak plasma purity, rotation, and ion temperature measurements. Krypton is one of the elements proposed for use with the ITER tokamak, the fusion reactor under construction in Cadarache, France. Krypton n=2 X-ray emission may be used for deducing rotation and ion temperature profiles. The presence of this impurity in the ITER experiment, as well as other currently operating tokamaks, increases the interest in improving the measurements of its various transitions and energy levels.
In this work, helium-like through argon-like krypton emissions are measured on an Electron Beam Ion Trap (EBIT), a tunable electron beam ionizing a small volume of gas trapped in an electric and magnetic well. The electron energies scanned were between 1.1 keV and 30 keV, which cover the range of interest to fusion reactors. The emission wavelength range studied was approximately 4 nm - 18 nm, and a grating spectrometer for Extreme Ultraviolet measurements was used. Calibration of the spectrum was achieved with well-known emission lines of carbon, oxygen, xenon, and barium. Overall, forty-four krypton lines were identified and measured with careful accounting of the statistical and systematic uncertainties, the total of which ranged from approximately 0.001 nm to 0.004 nm. The measurements agreed well with previous experimental work. Theoretical modeling of the spectra using the FAC and NOMAD codes was also performed. It was found that the intensity ratios of some of the aluminum-like krypton lines varied drastically from their theoretical values based on statistical distribution of populations, with the supposed strongest transitions reduced by a factor of ten relative to the next intensity neighbor. The simulations showed that this effect was due to the depopulation of a near ground state level, from which one of the excited levels would be normally created by electron impact excitation, by a magnetic dipole transition. In this way, a magnetic dipole transition was significantly affecting the electric dipole line ratios, highlighting the large effect that can occur due to relaxing the assumption of thermal equilibrium in a plasma.