Experimental study of single- vs two-close-frequency heating impact on confinement and loss dynamics in ECR ion source plasmas by means of x-ray spectroscopy and imaging

Abstract

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.

Metal evaporation dynamics in electron cyclotron resonance ion sources: plasma role in the atom diffusion, ionisation, and transport

“…The position and orientation of the lines of sight for the HPGe array, as shown in figure 1, have been chosen so as to prevent the magnetic branches within their field of detection. This because electrons escaping the trap through the branches produce larger fluxes of background signal from bremsstrahlung x-rays emission [49].…”

First numerical evidence of the two-close frequency heating effect on electron cyclotron resonance ion sources

“…The experiment described in [11] was carried out using the ECRIS installed at ATOMKI-Debrecen [12]. It is a 2 nd generation source, normally operating at a microwave frequency of 14.25 GHz amplified by a Klystron, with a cylindrical plasma chamber of 210 mm in length and 29 mm in radius.…”

“…It basically consists in heating the plasma with two frequencies (between 14 and 18 GHz for the 2 nd ECRIS) that differ for few hundreds of MHz. In particular, during an experiment carried out on the ECRIS operating at ATOMKI-Debrecen with an argon plasma it has been possible to characterize the onset of such instabilities and the effect on the plasma played but the TCFH, by acquiring the plasma self-emitted radiations in the microwave and X-ray range in different experimental conditions [11]. This contribution presents the first attempt to simulate numerically the TCFH and its effect on the plasma, taking as physics case the above mentioned experiment.…”

Laboratory magnetoplasmas as an ideal experimental environment for nuclear astrophysics β-decay studies

“…Figure 2.b) shows a SPhC soft X-ray energy-filtered image, corresponding to X-rays coming from plasma due to ionized Kα Argon lines (red) and X-rays coming from plasma chamber wall material, due to excited Ti (green) lines. Characteristic peaks associated to the emission from each material allow to investigate the spatial structure of the plasma [16] and confinement dynamics (plasma vs. losses X-ray emission) [17]. A model to link the experimental information to local plasma parameters is under development [12].…”