Dynamics of high energy runaway electrons in the Frascati Tokamak Upgrade

Abstract

The dynamics of high energy (up to 20 MeV) runaway electrons in the Frascati Tokamak Upgrade (FTU) is investigated using a gamma-ray spectrometer system which detects photons produced when runaway electrons interact with the plasma facing components [B. Esposito et al., Nucl. Instrum. Methods 476, 522 (2002)]. Runaway electrons are usually generated during the plasma current ramp-up, accelerated to MeV energies, and contained stably during the whole discharge time, which lasts for more than one second. This time is long enough for them to reach the limiting energy that results from the balance between acceleration in the electric field, collisions with the plasma particles and synchrotron radiation losses. The maximum energy inferred from the gamma spectra is shown to be in agreement with the runaway limiting energy predicted by a test particle description of the runaway dynamics [J. R. Martı́n-Solı́s et al., Phys. Plasmas 5, 2370 (1998)]. It is found that the runaway energy behavior during the discharge is determined by the time evolution of the plasma parameters only (mainly E∥, ne and Zeff) and that the synchrotron radiation losses associated with the electron gyromotion around the magnetic field lines can explain the measured limiting runaway energy in FTU.