Abstract
Abstract
Electron Bernstein current drive (EBCD) systems in spherical tokamaks are sensitive to plasma and launch conditions, and therefore require large parametric scans to optimise their design. One particular bottleneck in the simulation workflow is quasilinear modelling of current drive efficiency. Linear adjoint models are an attractive alternative, offering a
∼
10
3
×
speed-up compared to quasilinear codes. While linear models are well-tested and commonly used for electron cyclotron current drive (ECCD), they have seen little use in EBCD modelling. In this work, variants of the linear model are applied to EBCD and compared to quasilinear results in a reactor-relevant plasma, i.e. Spherical Tokamak for Energy Production (STEP). This comparison reveals it is important to accurately model the collision operator and finite Larmor radius effects in the linear model. When done properly, good agreement is found with quasilinear calculations, at least for normalised minor radii ρ < 0.7 and at low power densities. The power density threshold for quasilinear effects during EBCD is found to be significantly lower than that of ECCD. This is attributed to the much lower group velocity of the electron Bernstein wave (EBW). Thus, the linear model is only valid for EBCD modelling at low power densities (e.g.
≲
1 MW launched EBW power in STEP). This may be satisfied in present-day experimental devices, but certainly not in reactors targeting non-inductive operation.
Subject
Condensed Matter Physics,Nuclear and High Energy Physics
Reference43 articles.
1. Electron Bernstein wave heating and diagnostic;Laqua;Plasma Phys. Control. Fusion,2007
2. Electron Bernstein wave (EBW) current drive profiles and efficiency for STEP;Wilson;EPJ Web Conf.,2023
3. Microwave current drive for STEP and MAST Upgrade;Freethy;EPJ Web Conf.,2023
4. The GENRAY ray tracing code;Smirnov,2001
5. The CQL3D Fokker-Planck Code (reconstituted in editable form from General Atomics Report GA-A20978, 1992);Harvey,2005