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Fig. 2-7 Orbits of relativistic electrons in a perturbed tokamak magnetic field Orbits of relativistic electrons accelerated to 99% of the speed of light in a stochastically perturbed tokamak magnetic field are shown. Theoretically it was thought that the confinement of relativistic electrons would not deteriorate in such a perturbed field. Our present study revealed for the first time that macro-scale magnetic perturbations can destroy the confinement of relativistic electrons by deviating their motion along the magnetic field lines from inside of the plasma, that leads to a rapid loss of electrons from the plasma surface.

A technique of killer pellet injection (KPI) has been proposed for fast plasma shutdown in case of emergency of a fusion reactor. The point of this technique is to cool and terminate hot plasmas rapidly by the high speed injection of ice pellets (killer pellets) of an impurity gas such as neon into the reactor core. The KPI method has already been tested on JT-60. The experimental result has shown that the KPI is an effective technique of fast plasma shutdown in many respects, and in particular it can favorably avoid or suppress the generation of runaway electrons, contrary to theoretical predictions that indicate an enhancement of runaway electrons in a perturbed magnetic field resulting spontaneously from the pellet injection. Runaway electrons are generated in a tokamak and are accelerated to ultra-high energies and often damage structures and components in the reactor vessel. To make clear the discrepancy between the experimental results and the theoretical predictions, we have followed the orbital motion of relativistic electrons in a perturbed tokamak magnetic field with a 3-dimensional computer simulation and analyzed a possible loss mechanism of relativistic electrons. The simulations have shown that in the fast shutdown experiment on JT-60 with the KPI, magnetic perturbations induced by the KPI grow into overlapped "magnetic islands" with a macro-scale size of several centimeters, and the relativistic electrons are scattered as they transit the islands; the accumulation of the scattering deviates the motion of the relativistic electrons from their confinement orbits, eventually leading to the loss of relativistic electrons. The estimated loss rate for relativistic electrons due to this loss mechanism was found to agree closely with the experimental result on JT-60. This study suggests that KPI can be used as an effective method for fast shutdown of fusion plasmas preventing the generation of runaway electrons. Results of simulation are shown in Fig. 2-7.

Reference S. Tokuda et al., Simulation Study on Avoiding Runaway Electron Generation by Magnetic Perturbations, Nucl. Fusion, 39, 1123 (1999).

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