Beam-ion transport in tokamaks: Thermalization in large orbit-width regime

Neoclassical transport of neutral-beam-injected fast ions is investigated. A new method is developed where the orbit-width parameter {dollar}rhosb{lcub}theta{rcub}/L{dollar} is not restricted to be a small parameter and a linearized collision operator that retains effects of pitch-angle scattering, and electron and ion drag is used. The resulting ion drift kinetic equation is specialized to facilitate a computationally efficient numerical implementation.; An asymptotic expansion is constructed in the banana regime collisionality parameter {dollar}nusb{lcub}*i{rcub} = nusb{lcub}C{rcub}/omegasb{lcub}B{rcub} ll 1.{dollar} The lowest-order component {dollar}fsp0{dollar} is determined by solving an orbit-averaged drift kinetic equation. The equation is a second-order, non-self-adjoint, inhomogeneous partial differential equation in a three-dimensional, non-orthogonal coordinate system. A step model approximation of the large aspect ratio, circular cross-section tokamak magnetic field is employed to facilitate calculation of the orbit-averages and illustrate the method. The Galerkin variational method is used to determine to {dollar}fsp0.{dollar}; Parametric surveys are performed over the initial source pitch-angle, orbit-width parameter {dollar}rhosb{lcub}theta{rcub}/L{dollar}, and pitch-angle scattering frequency. Moments of the distribution are compared with a "convective" model that neglects pitck-angle scattering. Parameter ranges are identified where effects of large orbit-width {dollar}rhosb{lcub}theta{rcub}/L{dollar} and pitch-angle scattering are significant and should not be neglected.; Comparison with the present results shows that the small orbit-width approximation introduces {dollar}ge{dollar}5% errors in the density, parallel momentum, and energy moments when {dollar}rhosb{lcub}theta{rcub}/L = 0.86{dollar} in low-{dollar}Zsb{lcub}rm eff{rcub}{dollar} plasmas. These results are observed for all source pitch-angles studied except the deeply passing sources, which are in good agreement ({dollar}le{dollar}5%) with the small orbit-width model. A non-zero current, due to the large orbit-width of the trapped fast ions, is identified and related to, the superthermal alpha particle bootstrap "seed" current. In low-{dollar}Zsb{lcub}rm eff{rcub}{dollar} plasmas, pitch-angle scattering is only important for marginally passing ion sources. Scattering into the trapping region causes substantial inward-directed transport that is not predicted in the convective model. Parallel momentum is reduced by {dollar}sim{dollar}70%. For cases with marginally passing sources in higher {dollar}Zsb{lcub}rm eff{rcub}{dollar} plasmas, transport into the trapping region scales linearly with scattering frequency {dollar}nusb{lcub}perp{rcub}.{dollar} The results are sensitive to source localization in a layer at the trapping/passing boundary. Significant broadening of profiles is observed for cases with deeply trapped sources in higher {dollar}Zsb{lcub}rm eff{rcub}{dollar} plasmas.