Measurements of the energy distribution of electrons lost from the minimum B-field—The effect of instabilities and two-frequency heating

Abstract

Further progress in the development of ECR ion sources (ECRIS) requires deeper understanding of the underlying physics. One of the topics that remains obscure, though being crucial for the performance of the ECRIS, is the electron energy distribution (EED). A well-developed technique of measuring the EED of electrons escaping axially from the magnetically confined plasma of an ECRIS was used for the study of EED in unstable mode of plasma confinement, i.e. in the presence of kinetic instabilities. The experimental data were recorded for pulsed and CW discharges with a room-temperature 14 GHz ECRIS at the JYFL accelerator laboratory. The measurements were focused on observing differences between the EED escaping from a stable and unstable plasmas. It was found that nonlinear phenomena alter the EED noticeably. The electron losses are enhanced in both unstable regime and with two-frequency heating suppressing the instabilities. It has been shown earlier that two-frequency heating boosts the ECRIS performance presumably owing to the suppression of instabilities. We report the observed changes in EED introduced by the secondary frequency in different regimes, including an off-resonance condition where the secondary frequency is lower than the minimum frequency satisfying the resonance condition for cold electrons at the magnetic field minimum. Finally, we suggest an experimental method of qualitative evaluation of the energy distribution of electrons confined in the magnetic trap using a method of measuring energy distribution of lost electrons during the plasma decay in pulsed operation of the ion source.

Plasma diagnostic tools for ECR ion sources—What can we learn from these experiments for the next generation sources

“…However, the exact mechanism for the instability suppression by multiple frequency heating remains unclear and is actively investigated. 34,55 • The experiments probing the ion temperature, 18 the discrepancy between the confined ion population and the extracted beam, 53 and ion confinement time 23,31 support the view that the ions are electrostatically confined and need to acquire a certain energy to overcome the ΔΦ potential barrier and become extracted. Such diagnostics methods could now be used for probing the effects of various techniques such as the biased disc, multiple frequency heating, and, perhaps most importantly, gas mixing on the ion temperature and confinement time.…”

“…The EED of the axially escaping electrons was measured with the JYFL 14 GHz ECRIS using the bending magnet as a spectrometer. 33,34 It was discovered that the EED is non-Maxwellian and depends strongly on the magnetic field together with the absolute electron loss rate in the stable operation regime. These experiments 34 have (i) yielded direct evidence on RF-induced pitch angle scattering causing electron losses, (ii) enabled measuring the EED of the electrons expelled by kinetic instabilities, and (iii) allowed detecting the change in the EED when the kinetic instabilities limiting ECRIS performances, and being the most likely cause for the Bmin = 0.8BECR-scaling, 5 are suppressed using double frequency heating.…”

“…33,34 It was discovered that the EED is non-Maxwellian and depends strongly on the magnetic field together with the absolute electron loss rate in the stable operation regime. These experiments 34 have (i) yielded direct evidence on RF-induced pitch angle scattering causing electron losses, (ii) enabled measuring the EED of the electrons expelled by kinetic instabilities, and (iii) allowed detecting the change in the EED when the kinetic instabilities limiting ECRIS performances, and being the most likely cause for the Bmin = 0.8BECR-scaling, 5 are suppressed using double frequency heating. 35 The apparent limitation of the technique is that it measures the EED of the escaping electron, whereas it is the confined electron population that produces high charge state ions and confines them electrostatically.…”

“…A novel diagnostics method to evaluate the EED of the confined electrons from the EED of the escaping electrons, utilizing pulsed operation of the ion source, and the comparison of the results with particle-in-cell simulations are presented elsewhere in these proceedings. 34 The detection method of the escaping electron flux sets a limit for the minimum accessible electron energy. The cold electron temperature can be obtained with OES using the line-ratio method, yielding an estimate of 20-40 eV.…”

Towards explanation of the two-frequency heating effect in electron cyclotron ion sources

“…Such electrons, accelerated due to interaction with externally generated waves, originate from a relatively cold and dense background plasma. The energy gained by a typical electron is much larger than its initial energy: from tens keV to MeVs compared to the ionization energy of few tens eV [17]. In the first approximation, one can assume zero initial energy of accelerated electrons.…”