Greifswald 2024 – scientific programme

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P: Fachverband Plasmaphysik

P 17: Magnetic Confinement V/HEPP VI

P 17.4: Talk

Wednesday, February 28, 2024, 17:30–17:55, ELP 6: HS 3

Electron cyclotron resonance during plasma formation in nonuniform magnetic fields — •Albert Johansson and Pavel Aleynikov — Wendelsteinstraße 1, 17491 Greifswald

Electron cyclotron resonance is used to start up various fusion experiment devices. In Wendelstein 7-X (W7-X), the second harmonic extraordinary mode (X2) is used for breakdown. Third harmonic extraordinary mode (X3) breakdown is of particular interest, as some future experiments intend to investigate the effects of a lower magnetic field strength. Presently, no experiment at W7-X has successfully used X3 for breakdown. Because the resonance depends on the gyrofrequency, proportional to magnetic field strength over Lorentz factor B/γ, an energy increase is associated with an increase in γ and the resonance condition breaks, causing a finite resonance width. It has been shown that for a uniform magnetic field this resonance width is not enough to ensure a breakdown process [1].

However, the magnetic field of a stellarator is not homogeneous. This opens the possibility of field gradients along the electron trajectory. When the magnetic field strength is increasing in step with the electron energy, the “width” of the resonance is extended considerably. In addition, when several beams are used, a “resonance overlap” can be constructed such that electrons gain almost hundred times the ionization energy of 13.6 eV. We discuss the effect of magnetic field inhomogeneity on energy gain and show how the composition of multiple beams can be optimised.

[1] D Farina. Nuclear Fusion 2018, 58(6):066012

Keywords: Waves; X3; Magnetic Field Inhomogeneity; Ignition; W7X