Neutral-beam injection into a tokamak, part I: fast-ion spatial distribution for tangential injection

Abstract

The production processes and spatial distribution of fast ions resulting from tangential injection of a diffuse neutral beam into a tokamak are discussed. The spatial distribution of fast ions for various injection trajectories and absorption mean free paths are calculated and discussed in detail. Maximum beam absorption for a parabolic density profile is shown to occur for injection roughly halfway between the inner wall of the torus and the magnetic axis; however, since this maximum is near unity and only weakly dependent on the injection trajectory, this is not the most important possible optimization. Since the drift orbit surface area over which the fast ions are distributed is roughly proportional to the distance from the magnetic axis, the fast ion density is found to be strongly peaked at the magnetic axis for present experiments where the absorption mean free path λ is comparable to the plasma radius a. This geometric peaking effect is strong enough to overcome the exponential beam attenuation and cause the fast-ion density and consequent beam energy deposition to be peaked at the plasma centre as long as λ0 ≳ a/4. Charge exchange of the fast ions with neutrals in the plasma can deplete the fast-ion population, particularly near the plasma edge. When charge exchange is an important loss mechanism, beam injection nearly tangent to the magnetic axis is found to maximize the beam effectiveness in heating.

Comparison of toroidal viscosity with neoclassical theory

“…Since the deposition of the fast ion energy declines due to orbit loss and charge exchange loss, their effects have been involved in the model to match the result of the 3-D Monte Carlo code HELIOS. 9 The major source of error in the effective viscosity is the toroidal force, while the error bar of toroidal rotation velocity itself is 5%-20%, as seen in Fig. 3.…”

Co-counter asymmetry in fast wave heating and current drive

“…2(c), both up-down and co-counter asymmetries are eliminated because the Hall terms in Eq. (1) are proportional to density gradients. These results are obvious in the full-wave calculations of Fig.2, but they would not be seen in ray tracing calculations unless the rays were launched with a spectrum which models the updown asymmetry.…”

The role of the neutral beam fueling profile in the performance of the Tokamak Fusion Test Reactor and other tokamak plasmas

“…This definition is similar to the density peakedness factor Fne =ne(0) /<ne >, where Pte(0) is the central electron density and <ne > is volume-averaged electron density. Note that Hne is similar in definition to H(0) established in the earliest days of neutral beam research [21]. When the beam has single energy component, Hne = H(0).…”