Prediction of ELM-free Operation in Spherical Tokamaks With High Plasma Squareness

TL;DR

This paper predicts that increasing plasma squareness in spherical tokamaks can lead to ELM-free H-mode operation by affecting stability boundaries, supported by gyrokinetic calculations and machine learning models.

Contribution

It introduces the idea that higher plasma squareness can naturally produce ELM-free discharges, supported by gyrokinetic analysis and RF machine learning predictions.

Findings

Higher squareness destabilizes ballooning modes in first stability.

ELM stability boundary remains unchanged with increased squareness.

RF models accurately predict proximity to stability boundaries.

Abstract

We predict that high plasma squareness in spherical tokamaks (STs) could result in edge-localized-mode (ELM)-free H-mode. The effect of squareness on gyrokinetic and peeling-ballooning-mode width-height pedestal scalings is calculated for STs. Because STs can sustain H-mode in first ballooning stability, first-stable pedestals with lower gradients may be further from the peeling-ballooning-mode boundary and therefore naturally free of Type 1 ELMs. We show that while higher squareness destabilizes ballooning modes in first stability, the ELM stability boundary is essentially unchanged. Therefore, higher squareness could result in ELM-free discharges. Random Forest (RF) machine learning models for the gyrokinetic growth rate and distance from first stability are used to predict how squareness affects stability. A RF model with only three easily obtainable geometric inputs predicts proximity to the gyrokinetic width-height scaling on a test dataset with high accuracy, $R^2 = 0.965$.