Limitations of electron cyclotron resonance ion source performances set by kinetic plasma instabilities.
Electron cyclotron resonance ion source (ECRIS) plasmas are prone to kinetic instabilities due to anisotropy of the electron energy distribution function stemming from the resonant nature of the electron heating process. Electron cyclotron plasma instabilities are related to non-linear interaction between plasma waves and energetic electrons resulting to strong microwave emission and a burst of energetic electrons escaping the plasma, and explain the periodic oscillations of the extracted beam currents observed in several laboratories. It is demonstrated with a minimum-B 14 GHz ECRIS operating on helium, oxygen, and argon plasmas that kinetic instabilities restrict the parameter space available for the optimization of high charge state ion currents. The most critical parameter in terms of plasma stability is the strength of the solenoid magnetic field. It is demonstrated that due to the instabilities the optimum Bmin-field in single frequency heating mode is often ≤0.8BECR, which is the value suggested by the semiempirical scaling laws guiding the design of modern ECRISs. It is argued that the effect can be attributed not only to the absolute magnitude of the magnetic field but also to the variation of the average magnetic field gradient on the resonance surface.
Department of Physics, University of Jyväskylä, 40500 Jyväskylä, Finland.
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O
Tarvainen
Department of Physics, University of Jyväskylä, 40500 Jyväskylä, Finland.
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J
Laulainen
Department of Physics, University of Jyväskylä, 40500 Jyväskylä, Finland.
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J
Komppula
Department of Physics, University of Jyväskylä, 40500 Jyväskylä, Finland.
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R
Kronholm
Department of Physics, University of Jyväskylä, 40500 Jyväskylä, Finland.
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T
Kalvas
Department of Physics, University of Jyväskylä, 40500 Jyväskylä, Finland.
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H
Koivisto
Institute of Applied Physics, RAS, 46 Ul'yanova St., 603950 Nizhny Novgorod, Russian Federation.
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I
Izotov
Institute of Applied Physics, RAS, 46 Ul'yanova St., 603950 Nizhny Novgorod, Russian Federation.
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D
Mansfeld
Institute of Applied Physics, RAS, 46 Ul'yanova St., 603950 Nizhny Novgorod, Russian Federation.
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V
Skalyga