Investigation of the dynamics of runaway electrons in the magnetic field of a tokamak

The dynamics of runaway electrons in tokamaks are investigated theoretically by taking into account the acceleration in the toroidal electric field, the collisions with the plasma particles, the deceleration due to synchrotron radiation and the resonance of their gyromotion with the ripple of the toridal magnetic field. With this ripple resonance mechanism the momentum parallel to the magnetic field is converted into momentum of the gyromotion. As a consequence the synchrotron radiation losses are increased and can balance the energy gain in the electric field such, that the energy of the runaways stays constant. So the maximum energy, which is finally reached is much lower, than the maximum possible energy of about 65 MeV, which is obtained if they move on a circle centered around the torus axis. Experimentally, the energy blocking is deduced by measuring the Bremsstrahlung spectrum of runaways, yielding runaway energies of 10-15 MeV, depending on the plasma parameters, in agreement with the described ripple resonance mechanism. The runaway electrons, which are captured in a ripple resonance, form a monoenergetic electron beam with an energy spread of #DELTA#E/E<10"-"2. These extremely monoenergetic runaways fulfill the conditions necessary for a free-electron maser. The maser radiation of the runaways, which lies in the microwave region, is clearly detected on ASDEX Upgrade. (orig.)67 refs.Available from TIB Hannover: RA 71(1/287) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman