Self-consistent modeling of STEP flat top scenario with realistic electron cyclotron heating and current drive

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Abstract
The spherical tokamak for energy production (STEP) is the UK’s flagship program to design a prototype fusion power plant capable of delivering net electricity to the grid. Due to geometric constraints in spherical tokamaks, STEP is being developed for fully non-inductive operation, relying primarily on microwave-based electron cyclotron (EC) and electron Bernstein wave systems for plasma heating and current drive (HCD). A key requirement for achieving this operation is the optimization of these systems to ensure high wall-plug efficiency and reliable current profile control. This paper presents a self-consistent modeling approach using the JINTRAC framework, where the JETTO transport code is coupled with the GRAY quasi-optical beam-tracing code. Previous scenario development relied on simplified, prescribed EC current drive (ECCD) profiles. In this work, these were replaced with the profiles derived from detailed parametric GRAY scans, optimized for launcher geometry, frequency, and polarization. These realistic ECCD profiles are integrated into a fully self-consistent JETTO/GRAY simulation, capturing the dynamic interplay between plasma transport, heating, and current evolution. The methodology developed in this study offers a more accurate representation of HCD dynamics and is applicable to future STEP scenario development.

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