Measurement of x-ray bremsstrahlung radiation from high energy electrons by stochastic acceleration in Heliotron J

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Abstract
A synthetic diagnostic of x-ray bremsstrahlung radiation and a Monte Carlo radiation transport simulation were carried out to obtain the electron energy distribution in stochastic acceleration experiments in Heliotron J, a mid-sized heliotron-type magnetic confinement device. Three sets of LaBr3(Ce) scintillator and photomultiplier tubes were installed in Heliotron J in three directions relative to the magnetic field line, co, counter, and perpendicular, to determine the velocity distribution of the high-energy electrons. These systems are positioned about 5 m away from the vacuum chamber and shielded by lead blocks and magnetic shields to reduce the influence of stray radiation and magnetic fields. The vacuum chamber of Heliotron J is made of stainless steel with a 3D helical shape. Since its x-ray shielding effect is not negligible when obtaining the x-ray energy distribution in the vacuum chamber, the Monte Carlo radiation transport code particle and heavy ion transport code system was applied to Heliotron J to clarify the shielding effect. Given the vacuum chamber and coil shape information and x-ray energy distribution expected in the vacuum chamber, this code can calculate x-ray energy distribution considering the shielding effect of Heliotron J. This simulation model was based on computer aided design data of Heliotron J devices. The x-ray energy distribution in the vacuum vessel was adapted until the simulated and measured x-ray spectra outside the vacuum vessel match with each other. The shapes of resultant distribution have two types of shape: power-law distribution at more than 450 keV and Maxwell distribution at less than 450 keV. At a higher energy range, x-ray bremsstrahlung energy distribution is consistent with the characteristic of stochastic acceleration.
Funding Information
  • Japan Society for the Promotion of Science (19H01875)
  • National Institute for Fusion Science (NIFS23KUHL117, NIFS23KUHL121)

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