During TJ-II discharges, hard X-rays are generated by fast electrons during the ramp-up, flattop and ramp-down phases of the coil currents. In order to reduce the generation of runaway electrons, a mechanical shutter has been installed. It consists of a pneumatically operated paddle that is swept through the poloidal magnetic surfaces in a few milliseconds both before and after the flattop, thus intercepting electrons associated with the magnetic field lines over the whole cross section. Initial experiments have shown efficient suppression of these electrons. [1][2]

Left: sketch of the mechanical paddle. Right: photo. Bottom: detail of the same photo showing damage by repeated exposure to runaway electrons


  1. L. Rodríguez-Rodrigo, A. López-Fraguas, and A. Gabriel. Runaway electrons control during ramp-up and ramp-down of the magnetic fields in the coils of the TJ-II flexible heliac, Rev. Sci. Instrum. 70 (1999) 645
  2. . Alejaldre et al., First plasmas in the TJ-II flexible Heliac, Plasma Phys. Control. Fusion 41 (1999) A539