RUNAWAY RIPPLE INTERACTIONS IN TOKAMAKS
In tokamak discharges, a fraction of the electrons (the runaway electrons) can be accelerated up to energies of the order of several tenths of an MeV until synchrotron losses equalise the loop voltage acceleration. In large tokamaks such electrons are created mainly at the beginning of the discharge when the collisionality is low, or during disruptions when large loop voltages are generated. For such electrons it is possible to find two integers N(t), N(c) such that N(t) w(t) - N(c) w(c) = 0 where w(t), w(c) are the transit and cyclotron frequency. When this condition is fulfilled the pitch angle of electrons alters considerably. The resonant interaction may affect drastically the energy spectrum of runaway electrons. This report contains two approaches to the study of these resonances. In the first, the resonance effect is considered as an intense cyclotron heating of the electrons by the ripple field in the guiding centre frame of the fast particles. This method provides a good understanding of the runaway ripple interaction. Although the decorrelation time of the cyclotron phase is not handled rigorously, quasilinear diffusion coefficients have been obtained. A simple criterion is used to take into account the radiative cooling by cyclotron radiation of electrons. In the second approach, a Hamiltonian formalism is used. A criterion for the onset of chaotic behaviour is given, and the results of the first study are confirmed. The interactions between cyclotron radiation and pitch angle scattering are treated more accurately.
Bibliographic Reference: REPORT: EUR-CEA-FC-1374 EN (1989) AVAILABLE FROM CEA
Record Number: 1989128017700 / Last updated on: 1990-11-09
Available languages: en