Energetic particle transport in compact quasi-axisymmetric stellarators

Abstract

Hamiltonian coordinate, guiding center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the atractiveness of compact configurations which are optimized for ballooning stability. A new stellarator particle following code is used to predict ion loss rates and particle confinement for thermal and neutral beam ions in a small experiment with R=145 cm, B= 1-2 T and for alpha particles in a reactor size device. In contrast to tokamaks, it is found that high edge poloidal flux has limited value in improving ion confinement in QAS, since collisional pitch angle scattering drives ions into ripple wells and stochastic field regions, where they are quickly lost. The necessity for reduced stellarator ripple fields is emphasized. The high neutral beam ion loss predicted for these configurations suggests that more interesting physics could be explored with an experiment of less constrained size and magnetic field geometry.PACS numbers 52.20.Dq, 52.55.Hc,52.65.Cc,52.65.-y 2

Robustness and flexibility in compact quasiaxial stellarators: Global ideal magnetohydrodynamic stability and energetic particle transport

“…15 shows the time evolution of the fraction of beam ion loss in one energy slowing down time for configuration P01/I00. The concave structure of the loss evolution with time is characteristic of QAS [19], and is unlike the convex time evolution plots more typically seen in tokamak beam ion orbit simulations [21,22]. Over time, more and more ions find their way into loss orbits in the QAS stellarator, while in tokamaks, the incremental losses decrease with time.…”

“…We have recently shown that in QAS the thermal and energetic particle transport behavior are correlated [19], so that we expect these plasmas will also not differ greatly in their thermal ion confinement. While good transport and MHD stability are not anticorrelated in these thirty equilibria, stability only results from a delicate balance of the pressure and shear stabilization forces.…”

“…It was found that for a three field period, compact stellarator similar to QAS3_C82 (called QAS3_53 (1T) in Ref. 19), a neutral beam of deuterium ions at 40 keV, injected parallel to the magnetic axis, would be depleted in energy by 41% after one slowing down time. We have calculated the effect of the various pressure and ι profiles described above on such a beam of heating ions.…”

“…In recent work with the ORBITMN code [19] we have surveyed a variety of quasiaxial stellarators and examined both thermal and energetic particle transport. It was found that for a three field period, compact stellarator similar to QAS3_C82 (called QAS3_53 (1T) in Ref.…”

Recent advances in the design of quasiaxisymmetric stellarator plasma configurations

“…In addition to the thermal confinement times, we have also calculated the energy losses of 40 keV H-neutral beam ions at B=2T using Monte Carlo simulations. 23,24 In addition to our physics targets, we have also compared candidate configurations with respect to two quantities that provide a measure of the attractiveness of the associated coils. For this purpose, a set of saddle coils has been constructed for each candidate configuration.…”