1-6 September 2019
Lanzhou
Asia/Shanghai timezone
The deadline for the ICIS 2019 manuscript submission is extended to September 30. All oral presentations have been uploaded on the page of 'timetable'.

ECRIS Plasma Spectroscopy with a High Resolution Spectrometer

4 Sep 2019, 09:00
30m
Lanzhou

Lanzhou

No. 1 Beibinhe East Road, Chengguan District, Lanzhou
Invited oral

Speaker

Mr Risto Kronholm (Department of Physics, University of Jyväskylä)

Description

Electron cyclotron resonance ion source (ECRIS) plasmas contain high-energy electrons and highly charged ions. This kind of plasmas are very sensitive to outside disturbance, which means that only non-invasive methods are reliable in their characterization. One of the spearheads of the JYFL ion source group has been to utilize the spontaneous de-excitation of electronic states of atoms, ions and molecules for diagnostics. This enables studying multiple plasma parameters non-invasively through optical emission spectroscopy (OES) of weak emission lines characteristic to ECRIS plasmas. A high-resolution spectrometer (10 pm FWHM at 632 nm) coupled with a lock-in data acquisition setup has been developed at JYFL specifically for this purpose.
Densities of ions, neutral atoms and the temperature of the cold electron population play a major role in determining the different reaction rates such as ionization of neutrals and low charge state ions, excitation to radiative and metastable states, and charge exchange. Methods to study these plasma parameters with high resolution OES and results from measurements with the JYFL 14 GHz ECRIS will be presented. The temperature of the cold electron population can be studied using a line-ratio method. For example, it has been observed that the cold electron temperature drops from 40 eV to 20 eV when the extraction voltage of the ion source is switched off, accompanied by almost two orders of magnitude decrease in Ar$^{9+}$ optical emission intensity [1]. This suggests that diagnostics results of ECRIS plasmas obtained without the extraction voltage are not depicting the plasma conditions during normal ECRIS operation. The relative changes of both the plasma optical emission and the ion beam current have been measured in CW and amplitude modulation (AM) operation mode of microwave injection. It is concluded that in the normal CW operation mode the ion currents could be limited by diffusion transport and electrostatic confinement of the ions rather than beam formation in the extraction region and subsequent transport [2]. The study also revealed discrepancies between the parametric dependencies of high charge state ion densities in the core plasma and their extracted beam currents.
The high resolution of the spectrometer also allows to study the ion temperature by measuring the Doppler broadening of the emission lines after subtracting the wavelength dependent instrumental broadening. The measured ion temperatures in the JYFL 14 GHz ECRIS are between 5–28 eV, depending on the plasma species and charge state [3]. These values are significantly higher than has been generally accepted for ECR plasmas. The effect of gas mixing on the measured ion temperature will be presented when oxygen is injected to pure argon plasma. It was found that 1+ ions reach temperatures on the order of 10 eV, which cannot be explained by ion heating via electron drag and therefore other possible heating mechanisms will be discussed.
Finally, future plans to upgrade the spectroscopic setup to enable time-resolved measurements of the optical emission line broadening with millisecond resolution will be presented and the prospects of such experiments discussed.

References
[1] R. Kronholm et al., Review of Scientific Instruments 89, p. 043506, 10.1063/1.5023434, (2018)
[2] R. Kronholm et al., Proceedings of the 17th International Conference on Ion Sources, p. 040014,
10.1063/1.5053288, (2018)
[3] R. Kronholm et al. (accepted manuscript), Plasma Sources Science and Technology, 10.1088/1361-6595/ab27a1, (2019)

Category of your contribution Fundamental processes in ion source, plasma

Primary author

Mr Risto Kronholm (Department of Physics, University of Jyväskylä)

Co-authors

Dr Taneli Kalvas (Department of Physics, University of Jyväskylä) Hannu Koivisto (Department of Physics, University of Jyväskylä) Sami Kosonen (Department of Physics, University of Jyväskylä) Miha Marttinen (University of Jyvaskyla) Mr Derek Neben (National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI, USA) Dr Muneer Sakildien (iThemba LABS) Olli Tarvainen (STFC Rutherford Appleton Laboratory) Dr Ville Toivanen (Department of Physics, University of Jyväskylä)

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