Pedestal electron collisionality and toroidal rotation during ELM-crash suppression phase under n=1 RMP in KSTAR

Abstract

Excellent reproducibility of KSTAR resonant magnetic perturbation (RMP)-driven, edge-localized mode (ELM)-crash suppression enables us to construct a database reliably for the study of ELM-crash suppression conditions. To establish a high-fidelity database, we have selected one of the frequently used RMP configurations in KSTAR, n=1, + 90 degrees phasing RMP-coil configuration. A series of fitting curves based on edge profile diagnostics data is used for normalized electron collisionality ( nu e *</mml:msubsup>) and plasma toroidal rotation ( V tor) at pedestal top. Since ITER is expected to employ slowly rotating, low-collisionality, high-density plasmas whose conditions are not readily accessible in the existing devices, the exploration and understanding of these two parameters would be important not only for RMP-driven, ELM-crash-suppression physics but also for the success of ITER. The data points for the ELM-crash suppression phase are in the range of 0.2 < <nu> e , ped *</mml:msubsup> <1.1 with Z eff=2 assumption and V tor , ped >40km/s. Suppression thresholds or boundaries in nu e , ped *</mml:msubsup> or V tor , ped are not confirmed in the investigated parameter ranges so far. The KSTAR database still needs additional experimental datasets in ITER-relevant conditions ( nu e , ped *</mml:msubsup> similar to 0.1 and low-torque low-rotation) to confirm the boundary of the ELM-crash suppression window in KSTAR and reduce the uncertainties of the RMP ELM-crash control technique in ITER. In both nu e , ped * and <mml:msub> V tor , ped parameters, the phase-space distribution of the ELM-crash suppression has no clear distinction from that of the ELM-crash mitigation. Linear discriminant analysis provides a linear combination of parameters relevant to ELM-crash suppression, best separating two data categories. Recursive feature elimination indicates that <mml:msub> n e , ped and <mml:msub> I RMP, as well as nu e , ped * and <mml:msub> V tor , ped, are critical variables in the separation of the data groups.