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 ( 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.
Influence of Bt on the magnetic turbulence and on the runaway transport in low-density discharges