Stochastic Acceleration using Non-Resonant Microwave Launch in Heliotron-Type Magnetic Configuration Device Heliotron J

Here, we report on experimental study of the stochastic acceleration in a Heliotron-Type magnetic configuration device, Heliotron J. When a non-resonant 2.45 GHz microwave was launched in the vacuum magnetic field of Heliotron J, the relativistic electrons exceeding 2 MeV were observed from the X-ray spectrum. Under the conditions, normalized vector potential was 0.01, which was quite low compared with laser plasma acceleration experiments. A synthetic diagnostic of the X-ray energy spectrum with a Monte-Carlo simulation reveals that the high energy electrons have a power-law energy distribution. The power-law exponent depends on the direction of the electron trajectory and the energy distribution before the microwave launch, which is controlled by pre-filled gas and additionally applied vertical magnetic field during the current ramp-up of the Helical coil. The electron acceleration simulation reveals the formation of the power-law spectrum consistent to the experimental results and the characteristics of the heating timescale (~ms) longer than the torus circulating period (~100ns). The acceleration simulation also shows that the strong dependence of the energy dissipation on the electron energy is a cause of the formation of the power-law spectrum. Exhaustive search using Gaussian regression process to the simulation shows that not only the electron position and phase relation to the microwave field, but the toroidal period is also important to determine the acceleration/deceleration.