An energy cap of runaway electrons is found experimentally by observing their bremsstrahlung spectra in the ASDEX tokamak. This observation is explained by analyzing the dynamics of runaway electrons, including acceleration in the toroidal electric field, deceleration due to synchrotron radiation losses, collisions with plasma particles, and a resonance between gyromotion and magnetic field ripple of the tokamak. For the dynamics of runaway electrons a Fokker-Planck equation in momentum space is developed.

• Received 6 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.4626