Experimental study of single vs. two-close-frequency heating impact on confinement and losses dynamics in ECR Ion Sources plasmas by means of X-ray spectroscopy and imaging

We hereby report the study of confinement and electron losses dynamics in the magnetic trap of an Electron Cyclotron Resonance Ion Source (ECRIS) using a special multi-diagnostic setup that has allowed the simultaneous collection of plasma radio-self-emission and X-ray images in the range 500 eV - 20 keV. Argon plasmas were generated in single and two close frequency heating (TCFH) modes. Evidences of turbulent regimes have been found: for stable and unstable configurations quantitative characterizations of the plasma radio self-emission have been carried out, then compared with local measurement of plasma energy content evaluated by X-ray imaging. This imaging method is the only one able to clearly separate X-ray radiation coming from the plasma from the one coming from the plasma chamber walls. X-ray imaging has been also supported and benchmarked by volumetric spectroscopy performed via SDD and HPGe detectors. The obtained results in terms of X-ray intensity signal coming from the plasma core and from the plasma chamber walls have permitted to estimate the average ratio: plasma vs. walls (i.e., plasma losses) as a function of input RF power and pumping wave frequency, showing an evident increase (above the experimental errors) of the intensity in the 2-20 keV energy range due to the plasma losses in case of unstable plasma. This ratio was well correlated with the strength of the instabilities, in single frequency heating (SFH) operation mode; in TCFH mode, under specific power balance conditions and frequency combinations, it was possible to damp the instabilities, thus the plasma losses were observed to decrease and a general reconfiguration of the spatial plasma structure occurred (the X-ray emission was more concentrated in the center of the plasma chamber). In the end, a simplified model has been used to simulate electron heating under different pumping frequencies, discussing the impact of velocity anisotropy vs. the onset of the instability, and the mechanism of particles diffusion in the velocity space in stable and unstable regimes.