On ray stochasticity during lower‐hybrid current drive in tokamaks

Using a combined ray‐tracing and Fokker–Planck code, a comprehensive and detailed analysis is presented on the importance of toroidally induced ray stochasticity for the modeling of lower‐hybrid (LH) current drive in tokamaks and for the dynamics of the launched power spectrum. The injected LH power distribution in poloidal angle and in parallel wave index is accurately represented by taking into account the poloidal extent of the antenna and by efficiently covering the full range of its radiated spectrum. The influence of the balance between the wave damping and the exponential divergence of nearby ray trajectories in determining the shape and robustness of the predicted LH power deposition profiles is emphasized. When stochastic effects are important, code predictions are shown to be stable with respect to small changes in plasma parameters and initial conditions, and to be consistent with experimental data, provided a sufficiently large number of rays is used. Sensitivity studies indicate that the component of the launched power spectrum that is not affected by stochastic effects is well described by a grid in parallel wave index whose spacing may be as large as 10−1, whereas the component that is affected by such effects suffers strong randomization and needs a grid whose spacing must not exceed 10−3. Ray stochasticity tends to broaden the launched power spectrum, to increase the LH power deposition in the inner half of the plasma, and to favor power deposition profiles that are spread over most of the plasma cross section and whose dependence on the injected LH power distribution in poloidal angle and in parallel wave index is weak. It is found that stochastic effects may be effectively reduced by using bottom launch schemes. The fact that it is, in general, misleading to divide a priori the launched power spectrum into ‘‘accessible’’ and ‘‘inaccessible’’ parts is stressed.