High energy runaway electron transport deduced from photonuclear activation of the PLT limiter

In large tokamaks, runaway electrons may be accelerated up to energies over 20 MeV by the toroidal electric field. When these particles leave the plasma, they impact the material limiters and emit bremsstrahlung hard x-rays. This bremsstrahlung may cause photonuclear reactions in the limiter, leaving radioisotopes behind. Both the amount of activation and its spatial distribution on the limiter provide information on the confinement of high energy electrons. The stainless steel poloidal ring limiter from the Princeton Large Torus has been analyzed and six observable photonuclear reactions with energy thresholds have been found for the reactions varying from 8 to 23 MeV. The amount of activation from each reaction determines the average number of runaway electrons above each activation threshold energy. The inferred runaway electron population decreases exponentially with energy as exp (-E/3.2 MeV) implying an energetic runaway electron confinement time of 50 to 80 msec. The poloidal variation of the activation can be described by a 0.02 cm scrape-off layer step size which implies about a 90 msec confinement time