Spectroscopic measurements of ion temperature and rotation of two ion components in C-2 FRC plasma

Ion temperature and rotational velocity were measured by means of spectroscopy for both main and major impurity ions in the C-2 FRC¹ plasma. Charge-exchange recombination spectroscopy was used to measure the main ion temperature and rotational velocity for two radial locations with space resolution of ~15 cm and time integration over ~10 μs during the entire plasma lifetime. Parameters of the dominant impurity ions were obtained with higher spatial resolution of ~2 cm and lower time resolution ~100 μs, by implementing conventional spectroscopic methods. Both ion components were found to be nearly thermalized already at early times. The ion temperature decreases during the experiment from ~700 eV to ~200 eV, but it remains significantly higher than that of electrons ≲100 eV thoroughout the plasma lifetime. Main ions were observed to rotate in the ion-diamagnetic direction, quickly spinning up to ~60 km/s and to stop rotating by the end of experiment. The impurity ions were found to counter-rotate with main ions, except at latest stage of the experiment. A plausible explanation of the early counter rotation is a reversed pressure gradient of the impurity ions and a stronger effect of the electric field for ions with higher charge. Later in the shot, both ion components are rotationally coupled either due to collisions or changing in time electric field. More detailed experiments with higher spatial resolution are required for better understanding of the ion rotation driving forces.