Role of Non-Maxwellian Distribution Function in the Electron Temperature Measurements in Alcator C-Mod and SPARC

Future fusion devices aim to operate with high performance, achieving temperatures around 25 keV. Measurements of electron temperature with ECE and Thomson scattering are fundamental systems, planned to be utilized in SPARC. However, despite decades of theoretical developments and extensive experience with both diagnostics, there remain unresolved records of discrepancy, prevalent especially at high temperatures. Non-Maxwellian electron distribution functions are proposed to lead to the Thomson-ECE discrepancy. Perturbed distribution functions will be shown to explain the discrepancy observed on Alcator C-Mod, and the same physical mechanism could cause up to 50% temperature deviation on SPARC. We apply a heuristic model of bipolar perturbation to Maxwellian, which mimics a plateau in the electron distribution function [1]. We introduce a tool for modelling ECE and show how non-Maxwellian electron distribution function could impact ECE measurements. The simulation is based on an established code ECESIM [2,3] but makes further enhancement to account for perturbation to the equilibrated distribution [4]. Performance of the code is evaluated by comparison of the old and the new formulation on three different machines. The response of Thomson T_e measurement to the identical perturbation have been modeled and will be presented. Conclusions from existing ECE-Thomson data collected in Alcator C-Mod are used to guide implications for the generated fusion power in SPARC.



[1] M. Fontana et al 2023 Phys. Plasmas 30 122503

[2] M.E. Austin, U. Texas FRC Report #534

[3] M. Bornatici et al 1983 Nucl. Fusion 23 1153

[4] G. Giruzzi 1988 Nucl. Fusion 28 1413