Benchmarking of Neutral Beam Current Drive Codes as a Basis for the Integrated Modeling for ITER

This paper discusses the results of a benchmark study in which the predictions of numerical codes for neutral beam current drive and heating were compared using the parameters of the reference ITER steady state scenario, as a collaboration work of in the frame of the ITPA-SSO group. The models employed in the benchmarked codes for each physics related to NB heating and current drive, such as the beam model, beam stopping cross section, fast ion solver, orbit effects and electron shielding, are reviewed and examined through comparison. 1 Introduction Neutral beam injection (NBI) is a robust method for heating and current drive (CD) because it does not depend on any resonance conditions or coupling conditions at the edge. High-energy neutral beam current drive (NBCD) was experimentally validated for central current drive in JT-60U [1], giving a further confidence in ITER predictions. These features make NBCD a dominant non-inductive current drive source in ITER [2]. However, discrepancy from theoretical predictions has been reported for an off-axis NBCD case in ASDEX-UG [3], where the off-axis NBCD capability could have a substantial impact on prospected ITER hybrid [4] and steady-state scenarios [5]. Recent progress in diagnostics, equilibrium solvers and analysis techniques enable rather detailed comparisons with NBCD codes. However, different codes give somewhat different results. Thus, we need to clarify physics implementations in NBCD codes, such as the beam model, ionization process, fast ion diffusion in the velocity space, orbit effects and electron shielding. Also from an integrated modeling viewpoint, an NBCD code benchmark is needed to establish a more solid basis for ITER operations. 2 The NB codes benchmarked This NB code benchmark study were done with three orbit following Monte-Carlo (MC) codes OFMC [6], ONETWO [7]/NUBEAM [8] and NEMO/SPOT [9], and two Fokker-Planck (FP) codes ACCOME [10] and ASTRA [11]. OFMC : The MC code OFMC follows fast particle guiding center orbits in an arbitrary axi-symmetric geometry. The fast ion source is calculated by the MC technique using the parallel beam model with a bi-gaussian intensity profile and Janev's [13] or Suzuki's [14] fitting formula for the beam stopping cross-section incorporating the multi-step ionization process. ACCOME : Used is the same models for beam and ionization as OFMC. The fast ion source