A fast method for simulating α-particle orbits in tokamaks

The simulation of alpha-particle trajectories, to determine their losses out off the plasma in tokamak reactors, by means of orbit-following Monte-Carlo (OFMC) codes, consumes vast amounts of computer CPU time. The order of magnitude is roughly 100 hrs Cray-CPU time for 1000 alpha particles launched with birth energy of 3.52 MeV, followed for about a slowing-down time. Since of those launched particles, only 25-30% are non-passing particles (i.e., banana blocked and ripple trapped), which are the ones followed without time enhancement, we are talking about 300 particles in 100 hrs or 20 mins per particle (K. Tani, private communication). To perform simulations for reactor-design purposes, it is necessary to develop faster codes. In this paper, we report on a speed-up technique, based on reasonable approximations of the physics involved, that enables us to gain roughly a factor 3 on a scalar computer. In combination with vectorization, we have obtained an additional gain factor of 6-7, resulting in an overall speed-up factor of about 20. In Section 2, we give a short overview of the physics-based approximation technique employed in the code Alphanet. An evaluation of the results produced by Alphanet is presented in Section 3. Finally, Section 4 summarizes the paper. (author) 2 refs., 4 figs