Runaway electron measurements in the JET tokamak

The perpendicular x-ray emission up a to few MeV of runaway electrons has been measured in JET low-density ohmic discharges by means of the fast electron bremsstrahlung profile monitor. A diffusion model simulating the temporal evolution of the line-integrated x-ray signals is used to determine the runaway radial transport coefficient in the central region of the plasma (D-r similar or equal to 0.2 m(2) s(-1) for r/a < 0.5); a comparison is made with the predictions of magnetic and electrostatic turbulent transport theories and limits on the level of radial magnetic field fluctuations are found.