Neoclassical transport in quasi-axially symmetric stellarators

The author presents a numerical and analytic assessment of the transport in two quasi-axially symmetric stellarators, including one variant of the MHH2 class of such devices, and a configuration they refer to as NHH2, closely related to MHH2. Monte Carlo simulation results are compared with expectations from established stellarator neoclassical theory, and with some empirical stellarator scalings, used as an estimate of the turbulent transport which might be expected. From the standpoint of transport, these may be viewed as either tokamaks with large ({delta} {approximately} 1%) but low-n ripple, or as stellarators with small ripple. For NHH2, numerical results are reasonably well explained by analytic neoclassical theory. MHH2 adheres less to assumptions made in most analytic theory, and its numerical results agree less well with theory than those for NHH2. However, for both, the non-axisymmetric contribution to the heat flux is comparable with the symmetric neoclassical contribution, and also falls into the range of the expected anomalous (turbulent) contribution. Thus, it appears effort to further optimize the thermal transport beyond the particular incarnations studied here would be of at most modest utility. However, the favorable thermal confinement relies heavily on the radial electric field. Thus, the present configurations will have a loss cone for trapped energetic ions, so that further optimization may be indicated for large devices of this type

Fast ion transport in quasisymmetric equilibria in the presence of a resonant Alfv\'{e}nic perturbation

Significant progress has been made in designing magnetic fields that provide excellent confinement of the guiding enter trajectories of alpha particles using quasisymmetry (QS). Given the reduction in this transport channel, we assess the impact of resonant Alfv\'{e}n eigenmodes (AEs) on the guiding center motion. The AE amplitudes are chosen to be consistent with experimental measurements and large-scale simulations. We evaluate the drift resonance condition, phase-space island width, and island overlap criterion for quasisymmetric configurations. Kinetic Poincar\'{e} plots elucidate features of the transport, including stiff transport above a critical perturbation amplitude. Our analysis highlights key departures from the AE-driven transport in tokamaks, such as the avoidance of phase-space island overlap in quasihelical configurations and the enhanced transport due to wide phase-space islands in low magnetic shear configurations. In configurations that are closer to QS, with QS deviations $\delta B/B_0 \lesssim 10^{-3}$, the transport is primarily driven by the AE, while configurations that are further from QS, $\delta B/B_0 \sim 10^{-2}$, experience significant transport due to the QS-breaking fields in addition to the AE

Effect of magnetic islands on the impurity flows in NCSX geometry

A new physics idea to protect a plasma from impurity ions with the use of magnetic islands can be examined on the new National Compact Stellarator Experiment device (NCSX), which is under construction at Princeton Plasma Physics Laboratory, USA. On the basis of the MHD approach the impurity ions flows are studied in the configuration with the parameters of NCSX with the magnetic islands m / n=5/3 and m / n=6/3 , which can be excited with the trim coils in NCSX. It is shown that solving the flow trajectory equations dr × uα =0 can lead us to the conclusion that ion flow trajectories are concentrated in the region of the magnetic islands.Нову фізичну ідею захисту плазми від проникнення іонів домішки за допомогою магнітних островів можна перевірити в експериментах на новому пристрої National Compact Stellarator Experiment (NCSX), який споруджується в Princeton Plasma Physics Laboratory, USA. На основі МГД наближення потоки іонів домішки вивчаються для конфігурації з параметрами NCSX з магнітними островами з «хвильовими» числами m / n=5/3 and m / n=6/3, які можна збудити спеціальними токовыми катушками (trim coils) у NCSX. Рішення рівнянь траєкторій потоків dr × uα =0 доводить, що потоки іонів можуть концентруватися в межах магнітних островів.Новая физическая идея - защитить плазму от проникновения примесных ионов c использованием магнитных островов - может быть проверена в экспериментах на новой установке National Compact Stellarator Experiment (NCSX), строящейся в Princeton Plasma Physics Laboratory, USA. На основе МГД приближения потоки ионов примеси изучаются для конфигурации с параметрами NCSX с магнитными островами с «волновыми» числами m / n=5/3 и m / n=6/3, которые могут быть возбуждены специальными токовыми катушками (trim coils) в NCSX. В результате решения уравнений траекторий потоков dr × uα =0 показано, что потоки ионов могут концентрироваться в области магнитных островов

A symplectic, symmetric algorithm for spatial evolution of particles in a time-dependent field

A symplectic, symmetric, second-order scheme is constructed for particle evolution in a time-dependent field with a fixed spatial step. The scheme is implemented in one space dimension and tested, showing excellent adequacy to experiment analysis.Comment: version 2; 16 p