Investigation of fast ion behavior using orbit following Monte–Carlo code in magnetic perturbed field in KSTAR

Abstract

The fast ion dynamics and the associated heat load on the plasma facing components in the KSTAR tokamak were investigated with the orbit following Monte-Carlo (OFMC) code in several magnetic field configurations and realistic wall geometry. In particular, attention was paid to the effect of resonant magnetic perturbation (RMP) fields. Both the vacuum field approximation as well as the self-consistent field that includes the response of a stationary plasma were considered. In both cases, the magnetic perturbation (MP) is dominated by the toroidal mode number n = 1, but otherwise its structure is strongly affected by the plasma response. The loss of fast ions increased significantly when the MP field was applied. Most loss particles hit the poloidal limiter structure around the outer mid-plane on the low field side, but the distribution of heat loads across the three limiters varied with the form of the MP. Short-timescale loss of supposedly well-confined co-passing fast ions was also observed. These losses started within a few poloidal transits after the fast ion was born deep inside the plasma on the high-field side of the magnetic axis. In the configuration studied, these losses are facilitated by the combination of two factors: (i) the large magnetic drift of fast ions across a wide range of magnetic surfaces due to a low plasma current, and (ii) resonant interactions between the fast ions and magnetic islands that were induced inside the plasma by the external RMP field. These effects are expected to play an important role in present-day tokamaks.

Efficient estimation of drift orbit island width for passing ions in a shaped tokamak plasma with a static magnetic perturbation

“…In this section, we compare our estimates of the magnetic and orbit island width against the island widths obtained from Poincaré maps of the magnetic field and drift orbit trajectories. The magnetic perturbation used in this example is based on the RMP field in KSTAR as described in reference [19], with dominant toroidal mode number n = 1. The toroidal components of both the magnetic field B and the plasma current density vector…”

“…Now we proceed to the case where the plasma response is taken into account. The resulting self-consistent magnetic perturbation has been calculated with the HINT code [37], 5), showing a quantitative comparison between the estimated orbit island widths w( Pζ,res ) EST defined in equation ( 28) and the actual width w( Pζ,res ) POIN obtained from the Poincaré maps of the drift orbits in figure 6 and a few additional cases with parameters from table 1 which is a 3D resistive MHD equilibrium code and has been applied here to the same coil current configuration as described in [19]. HINT evaluates the non-linear 3D equilibrium MHD response by evolving the parallel plasma current along the perturbed field lines in three dimensions.…”

“…The present work is motivated by the need to develop a computationally efficient reduced model for the transport of fast ions subject to externally applied static magnetic field perturbations. In an earlier study using the Orbit Following Monte-Carlo (OFMC) code, it was shown that the prompt fast ion losses increase significantly when resonant magnetic perturbations (RMPs) are applied in the KSTAR tokamak [19]. Such losses can occur within a few poloidal transits after the fast ions are born deep inside the plasma, and they are facilitated by the combination of two effects: (i) the large magnetic drift due to a low plasma current, and (ii) the resonant interaction of the fast ions with the magnetic perturbations.…”

Performance of the BGSDC integrator for computing fast ion trajectories in nuclear fusion reactors

“…Simulating a full ensemble of fast ions in JET until thermalisation takes several days, despite making use of modern GPU clusters. Some models, for example NUBEAM [3,4] or OFMC [5], use a guiding centre approximation where gyro effects are neglected or only included once a particle is near the walls. This allows to take larger time step and thus reduces computational cost.…”

“…There seems to be agreement that due to its significant energy drift, RK4 (5) is less efficient than Boris and so the latter is typically used. Although Boris is surprisingly efficient [11], long solution times remain an issue.…”