In this work, we have compared different linear methods to estimate the electron Bernstein current drive (EBCD). The expressions for the current drive efficiency have been plugged into the ray tracing code TRUBA, which was used in previous works for electron Bernstein wave (EBW) heating studies in the TJ-II stellarator. This device is taken here as an example for this comparison.The driven current is calculated for different densities and temperatures, as well as launching directions of the heating beam, which is a critical issue in the O–X–B mode conversion scenario considered in TJ-II. The range of applicability of each model is discussed. The influence of the Ohkawa, relativistic and frictional trapping effects on the total current generated is studied by comparing the results obtained by pairs of models that include and neglect those effects. The Ohkawa effect has resulted in being the least important. Although the relativistic effects are not negligible, the main disagreement between the results arises from including or not momentum conservation and neglecting frictional trapping effects. The total EBCD current drive efficiency calculated is in all cases greater than the experimental ECCD one, previously measured in TJ-II. The results presented in this work are the guideline for future experiments in this device.
Confinement studies on the Wendelstein VII-AS stellarator