Electron Kinetics Inferred from Observations of Microwave Bursts During Edge Localized Modes in the Mega-Amp Spherical Tokamak

Recent measurements of microwave and x-ray emission during edge localized mode (ELM) activity in tokamak plasmas provide a fresh perspective on ELM physics. It is evident that electron kinetics, which are not incorporated in standard (fluid) models for the instability that drives ELMs, play a key role in the new observations. These effects should be included in future models for ELMs and the ELM cycle. The observed radiative effects paradoxically imply acceleration of electrons parallel to the magnetic field combined with rapid acquisition of perpendicular momentum. It is shown that this paradox can be resolved by the action of the anomalous Doppler instability which enables fast collective radiative relaxation, in the perpendicular direction, of electrons accelerated in the parallel direction by inductive electric fields generated by the initial ELM instability.

Figure 1

Evolution of microwave intensity (solid line) and $D_{\alpha }$ (dashed line) emission in a typical ELM.

Figure 2

Evolution of soft x-ray (SXR) intensity (top) and $D_{\alpha }$ emission (bottom) in a typical ELM.

Figure 3

Microwave intensity of a BME versus time recorded using a high time resolution radiometer.

Figure 4

Poloidal snapshots of parallel electric field (left plot) and plasma density normalized to the magnetic axis value (right plot) in a jorek simulation of an ELM in MAST [23]. The white curve in the left plot shows the separatrix.

Figure 5

Electrostatic field amplitude (on the logarithmic scale) versus wave number and frequency in PIC simulation with initial electron density $2\times {10}^{18}{\mathrm{m}}^{-3}$. Branch 1 is a whistler mode, 2 is a generalized Langmuir mode, 3 is an $O$ mode, and 4 is an $X$ mode.

Figure 6

Contours of (a) initial and (b) final electron velocity distributions in PIC simulation with initial electron density $2\times {10}^{18}{\mathrm{m}}^{-3}$ and a nonthermal electron tail fraction of 5%. The color bar has a logarithmic scale.