|approved||bb_sherwood_2016_talk.pdf||2016-04-13 18:33:40||Boris Breizman|
Author: Boris N. Breizman
Requested Type: Pre-Selected Invited
Submitted: 2016-02-14 14:44:22
Co-authors: P. B. Aleynikov
Institute for Fusion Studies, The University of Te
2515 Speedway C1500
Austin, TX 78712
The need to control the runaway electron (RE) behavior in ITER calls for better understanding of the underlying physics. We present an advanced description of electron kinetics during impurity dominated thermal quenches and a rigorous kinetic theory for relativistic RE in the electric field that is close to the avalanche onset threshold. Although the avalanche mechanism of RE production is anticipated to be the dominant mechanism in ITER, multiplication of the REs still requires a seed current. We demonstrate that injection of impurities provides a substantial seed current up to prompt conversion of the total pre-quench current into the RE current. We also find that the seed current tends to be restrictively small in plasmas with high pre-quench temperatures.
Our analysis of the avalanche mechanism shows that the electric field for avalanche onset is higher and the avalanche growth rate is lower than previous predictions. The theory also provides a distribution function of the REs and describes a mechanism for hysteresis in the avalanche development. A noteworthy feature of the avalanche-produced RE current is a self-sustained regime of marginal criticality. This feature determines the time scale of toroidal current decay during RE mitigation.
A strongly anisotropic distribution of the REs is generally prone to high-frequency kinetic instabilities that may cause beneficial enhancement of the RE energy losses. The relevant instabilities are in the frequency range of whistlers and electron plasma waves. The instability thresholds reported by other authors have now been revised considerably to reflect strong dependence of collisional damping on the wave frequency and the role of plasma non-uniformity, including radial trapping of the excited waves in the plasma. Our recently developed ray-tracing code incorporates these improvements and thereby enables fast stability assessment for any given configuration of the plasma and any distribution of the REs.