Electromagnetic gyrokinetic instabilities in STEP

We present herein the results of a linear gyrokinetic analysis of electromagnetic microinstabilites in the conceptual high -beta, reactor-scale, tight-aspect-ratio tokamak Spherical Tokamak for Energy Production, https://step.ukaea.uk. We examine a range of flux surfaces between the deep core and the pedestal top for two candidate flat-top operating points of the prototype device. Local linear gyrokinetic analysis is performed to determine the type of microinstabilities that arise under these reactor-relevant conditions. We find that the equilibria are dominated at ion binormal scales by a hybrid version of the kinetic ballooning mode (KBM) instability that has significant linear drive contributions from the ion temperature gradient and from trapped electrons, while collisional microtearing modes (MTMs) are sub-dominantly also unstable at similar binormal scales. The hybrid-KBM and MTM exhibit very different radial scales. We study the sensitivity of these instabilities to physics parameters, and discuss potential mechanisms for mitigating them. The results of this investigation are compared to a small set of similar conceptual reactor designs in the literature. A detailed benchmark of the linear results is performed using three gyrokinetic codes; alongside extensive resolution testing and sensitivity to numerical parameters providing confidence in the results of our calculations, and paving the way for detailed nonlinear studies in a companion article.

Automated summary

In this study, a linear gyrokinetic analysis is conducted to investigate the electromagnetic microinstabilities in a high-beta, reactor-scale, tight-aspect-ratio tokamak called Spherical Tokamak for Energy Production. The results reveal that the dominant instabilities at ion binormal scales are a hybrid version of the kinetic ballooning mode (KBM) instability, with contributions from the ion temperature gradient and trapped electrons, while collisional microtearing modes (MTMs) are sub-dominantly unstable at similar scales. The different radial scales of the hybrid-KBM and MTM are also observed. The study examines the sensitivity of these instabilities to physics parameters and discusses potential mitigation mechanisms. Benchmarking and testing with multiple gyrokinetic codes provide confidence in the accuracy of the results, setting the stage for further nonlinear studies in future research.