R. H. Fowler

Monte Carlo studies of transport in stellarators

Transport is studied in toroidal geometry by integrating the guiding‐center equations in magnetic coordinates and simulating collisions with a Monte Carlo collision operator. The effects of the ambipolar electric field on diffusion losses are determined for model magnetic fields and the correct magnetic field of the Advanced Toroidal Facility (ATF‐1) stellarator. Comparisons are made of the computed diffusion coefficients and the theoretically predicted values.

Numerical evaluation of magnetic coordinates for particle transport studies in asymmetric plasmas

A numerical procedure is described for the evaluation of magnetic coordinates given a toroidal, scalar pressure plasma with an arbitrary magnetic field. The accurate representation of magnetic field strength in this way is invaluable for the calculation of drift orbits and transport in asymmetric plasmas.

Neutral beam injection benchmark studies for stellarators/heliotrons

Neutral beam injection in stellarators/heliotrons is studied with Monte Carlo codes that treat the initial beam deposition and the fast-ion thermalization process. The birth deposition model carefully treats the geometry of the vacuum vessel and includes beam divergence, focusing, and aperture losses. The thermalization process is determined by integrating the guiding centre equations of the fast ions and simulating collisions with the plasma by Monte Carlo collision operators. This process may include charge exchange and neutral reabsorption. For the purposes of this benchmark, we review the different formulations of the guiding centre equations and the Monte Carlo collision operators. We studied perpendicular injection into Heliotron-E, which is located at the Plasma Physics Laboratory at Kyoto University. The magnetic fields of Heliotron-E are computed using the Biot-Savart law with realistic filament models. The sensitivity of the computed heating efficiency to the modelling of the particle loss boundary and to the numerical procedures is examined. The results of three different codes were compared. When the codes solve the same problem, the answers agree quite well. However, changing some of the modelling assumptions (such as the loss boundary location) can create significant differences in the results.

Mean spherical model for the structure of Lennard‐Jones fluids

The mean spherical model (MSM) has been solved for the 12/6 fluid in four different states which correspond to those of neon, argon, krypton, and xenon at relatively high densities. Structure and correlation functions calculated from the model are in good agreement with results derived from diffraction experiments and molecular dynamics calculations.

Three‐Body Dipole and Quadrupole Interactions in Neon and Argon

Contributions from three‐body dipole and quadrupole interactions to the third virial coefficients of neon and argon and to the cohesive energy of solid argon are evaluated. In both cases it is found that the triple‐dipole term dominates the nonadditive effect while the dipole—dipole—quadrupole, dipole—quadrupole—quadrupole, and triple‐quadrupole terms are of successively decreasing importance.

Characterization of fast ion behaviour during tangential neutral beam injection in the Advanced Toroidal Facility

The confinement characteristics and behaviour of energetic ions injected during tangential neutral beam injection (NBI) experiments on the Advanced Toroidal Facility (ATF) are examined using several experimental and computational methods. Measurements by a two dimensional scanning neutral particle analyser of the fast ion, slowing down spectra of the injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (ne ≤ 1.0 × 1019 m-3). The energetic ions are found to behave in a manner consistent with the classical slowing down process embodied in the fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (ne ≈ 7.5 × 1019 m-3). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consistent with classical predictions; however, there are indications in some ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fast ions in these configurations. In addition, computational studies suggest that the effectiveness of tangential NBI in certain operating regimes in ATF may be limited owing to the presence of loss regions at the trapping boundary

Reply to Comments of Mynick and Hitchon

A reply to the comment on charged particle transport associated with the collisionless detrapping/retrapping orbits in a nonaxisymmetric torus is presented. (AIP)

Structure of liquid sodium from Monte Carlo calculations

The structure function of liquid Na was determined using the Monte Carlo method and the SST (Shyu-Singwi--Tosi) potential. Results agree with molecular dynamics calculations and experimental values. (DLC)