The state of the art in the field of electron cyclotron emission and absorption of fusion plasmas confined by a magnetic field is reviewed. The general theory is described concisely, explicit results being given mainly for plasmas in local thermodynamic equilibrium. Moreover, the practical implications of these effects with respect to cyclotron radiation losses, the use of cyclotron emission as a diagnostic tool, and electron cyclotron heating and current drive are discussed with emphasis on tokamak plasmas.

http://iopscience.iop.org/article/10.1088/0029-5515/23/9/005/pdf

Electron cyclotron emission and absorption in fusion plasmas

Experimental and theoretical studies of runaway electrons in toroidal devices are reviewed here, with particular reference to tokamaks. The complex phenomenology of runaway effects, which have been the subject of research for the past twenty years, is organized within the framework of a number of physical models. The mechanisms and rates for runaway production are discussed first, followed by sections on runaway-driven kinetic relaxation processes and runaway orbit confinement. Next, the equilibrium and stability of runaway-dominated discharges are reviewed. Models for runaway production at early times in the discharge and the scaling of runaway phenomena to larger devices are also discussed. Finally, detection techniques and possible applications of runaways are mentioned.

https://iopscience.iop.org/article/10.1088/0029-5515/19/6/008/pdf

Runaway electrons in toroidal discharges

Basic principles of heterodyne techniques are introduced and the various components of a heterodyne system are summarized. Special applications in ECE, interferometry and reflectometry are discussed after introducing the diagnostic principles. Realized systems as described in the literature are briefly outlined. Ordering principles are radiometer types in the case of ECE, mixing scheme and generation and stabilization of local oscillator and intermediate frequency signals in the case of interferometry and reflectometry. Special techniques and their impact on the performance of the diagnostic instruments are discussed.

https://iopscience.iop.org/article/10.1088/0741-3335/39/11/001/pdf

Heterodyne methods in millimetre wave plasma diagnostics with applications to ECE, interferometry and reflectometry

/pdf/physics-of-runaway-electrons-in-tokamaks-53zgw20f64.pdf

Physics of runaway electrons in tokamaks

DIII-D experiments on rapid shutdown runaway electron (RE) beams have improved the understanding of the processes involved in RE beam control and dissipation. Improvements in RE beam feedback control have enabled stable confinement of RE beams out to the volt-second limit of the ohmic coil, as well as enabling a ramp down to zero current. Spectroscopic studies of the RE beam have shown that neutrals tend to be excluded from the RE beam centre. Measurements of the RE energy distribution function indicate a broad distribution with mean energy of order several MeV and peak energies of order 30?40?MeV. The distribution function appears more skewed towards low energies than expected from avalanche theory. The RE pitch angle appears fairly directed (????0.2) at high energies and more isotropic at lower energies (??<?100?keV). Collisional dissipation of RE beam current has been studied by massive gas injection of different impurities into RE beams; the equilibrium assimilation of these injected impurities appears to be reasonably well described by radial pressure balance between neutrals and ions. RE current dissipation following massive impurity injection is shown to be more rapid than expected from avalanche theory?this anomalous dissipation may be linked to enhanced radial diffusion caused by the significant quantity of high-Z impurities (typically argon) in the plasma. The final loss of RE beams to the wall has been studied: it was found that conversion of magnetic to kinetic energy is small for RE loss times smaller than the background plasma ohmic decay time of order 1?2?ms.

http://iopscience.iop.org/article/10.1088/0029-5515/53/8/083004/pdf

Control and dissipation of runaway electron beams created during rapid shutdown experiments in DIII-D

A code has been developed which calculates the cyclotron radiation spectrum emitted by a slab model tokamak plasma in a direction outward along a major radius. The calculation assumes both a thermal and a suprathermal electron component for the plasma, and includes the effects of self-absorption of the radiation by the thermal plasma. Various methods are described by which the cyclotron radiation spectrum can be unfolded to obtain parameters of the electron population. A procedure is described by which the electron temperature profile can be obtained from spectral measurements of the cyclotron emission at optically thin frequencies. No absolute calibration of the detection equipment is needed for this method. The code can be used to find parameters of the suprathermal distribution function from its synchrotron emission spectrum, and for the special case of suprathermal electrons at a constant major radius, analytical results are obtained. Finally, the code has been used to calculate cyclotron spectra for various tokamaks: CLEO, TFR, ATC, PLT, and Alcator.

https://iopscience.iop.org/article/10.1088/0029-5515/17/4/009/pdf

C. M. Celata

Papers

Cyclotron radiation as a diagnostic tool for tokamak plasmas