Bodhi Biswas

Fully-relativistic electron Bernstein wave current drive predictions for STEP

Electron Bernstein waves (EBWs) are theorised to efficiently drive current in reactor-grade spherical tokamaks, e.g. STEP. At high temperatures (Te ≳ 5 keV), relativistic effects can significantly impact wave propagation (and surely impacts wave damping). This work presents relativistic calculations of EBW wave propagation, damping, and current drive (CD) in a conceptual STEP plasma. Kramers-Kronig relations are exploited to efficiently evaluate the fully-relativistic dispersion relation for arbitrary wave-vectors, leading to a >50x speed-up compared to previous efforts. A recently verified linear adjoint model is used to estimate CD efficiency. Thus, for the first time, large parametric scans of fully-relativistic EBW CD simulations are performed through ray-tracing. In STEP, three main classes of rays are identified. The first class propogate deep into the core (ρ < 0.5), but only exist if relativistic effects are accounted for. They damp strongly at the 1st harmonic on nearly-thermal electrons and thus drive little current. A second class of rays propagate to intermediate depths (ρ ≈ 0.3−0.7) before damping at the 2nd harmonic. Their CD efficiencies are significantly altered due to relativistic changes to trajectory and polarisation. The third class of rays damp strongly far off-axis (ρ > 0.7), predominantly at the first harmonic. These ray trajectories are sufficiently short and “cold” such that relativistic effects are unimportant. Presently, the EBW antennas for STEP are being optimised around this third class of rays due to their large CD efficiencies. This suggests that non-relativistic simulations are adequate for this particular operating point.

Sid Leigh

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Yorick Enters

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Adam Dearling

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Michael Kryjak

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Theo Gheorghiu

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Ben Pritchard

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Koki Imada

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Daniel Greenhouse

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Christina Ingleby

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Calum Freeman

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Steven Thomas

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Leo Richardson

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Luke Thompson

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Ryan Magee

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Chris Arran

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