Kinetic-Ballooning-Limited Pedestals in Spherical Tokamak Plasmas

TL;DR

This paper introduces a theoretical model that accurately predicts the pedestal width-height scaling in spherical tokamaks by incorporating kinetic-ballooning effects, revealing a new limit on plasma confinement.

Contribution

It presents the first model matching experimental pedestal measurements in NSTX and identifies kinetic-ballooning as the key limiting instability, unlike ideal-ballooning.

Findings

Kinetic-ballooning limits pedestal pressure in spherical tokamaks.

Derived gyrokinetic width-height scaling laws for NSTX.

Discovered significant differences from conventional tokamak scalings.

Abstract

A theoretical model is presented that for the first time matches experimental measurements of the pedestal width-height Diallo scaling in the low-aspect-ratio high-$\beta$ tokamak NSTX. Combining linear gyrokinetics with self-consistent pedestal equilibrium variation, kinetic-ballooning, rather than ideal-ballooning plasma instability, is shown to limit achievable confinement in spherical tokamak pedestals. Simulations are used to find the novel Gyrokinetic Critical Pedestal constraint, which determines the steepest pressure profile a pedestal can sustain subject to gyrokinetic instability. Gyrokinetic width-height scaling expressions for NSTX pedestals with varying density and temperature profiles are obtained. These scalings for spherical tokamaks depart significantly from that of conventional aspect ratio tokamaks.