Simulation study of relativistic dynamics of MeV alpha particles in magnetized plasmas for explaining an experimental anomaly

In a Test Fusion Tokamak Reactor [R. J. Hawryluk, Phys. Plasmas 5, 1577 (1998)] experiment, the measured energy spectrum of the deeply trapped alpha particles is found to be 1 MeV too broad to be explained by classical collisions and the peak energy similarly off by 450 keV. The relativistic effect is proposed as an explanation. Here, we report high-resolution Monte Carlo (MC) and particle-in-cell (PIC) simulation studies in detail, under the assumption of a uniform magnetic field, for the identification of the cause of the observed anomaly. The 3.5 MeV alpha particles produced by thermonuclear fusion reaction are broadened due to Doppler effect. The relativistic alpha particle dynamics are followed with the PIC code. The relativistic ion cyclotron instability grows to saturation on a time scale (10-5 s) much shorter than the experimental time scale of 0.1 s. The MC code is then used to follow, in real time, the collisional slowing down of the gyrobroadened alphas, including the effect of the time delay in diagnostic pellet releasing and flight. Relativistic gyrobroadening is shown to be crucial in shaping the birth and slowed-down spectra. The resultant alpha particle energy spectrum fits well with that of the measurement, with a reduced chi square of unity.