H-Mode Power Threshold Studies on MAST

Yasmin Andrew,Tomas Jirman,Jan-Peter Bähner,Ronan Battle

doi:10.3390/plasma2030024

Abstract

Analysis of the L–H and H–L transition power thresholds (Pth) and pedestal parameters are presented for the mega ampere spherical tokamak (MAST). The dependencies of Pth on the average, core plasma electron density, X-point height, and plasma current are described. Increasing X-point distance from the divertor floor over 10–12 cm is found to increase Pth by a factor of three, while X-point heights greater than this have no further influence. The X-point height dependence of Pth is also observed to be sensitive to the plasma current. An Ip decrease from 0.77 MA to 0.65 MA, is observed to lower Pth by a factor of three across the X-point height scan and increases the maximum X-point height at which Pth stops increasing by 3 cm. Finally, a comparison of the experimental results with the predictions by the finite beta drift wave model is made, which provides a reasonable condition for the transition into and out of the H-mode.

Highlights

The high confinement or H-mode [1] is the operational scenario for the next-step device ITER [2].The edge, radial plasma temperature, and density profiles steepen following the transition fromL-mode to H-mode, leading to a reduced core plasma pressure peaking factor, which allows higher stored energy limits [3,4]
While enhanced stored energy and the bootstrap current are advantages of H-mode, the steep H-mode pressure gradients are characterized by periodic pedestal collapse known as edge localized modes (ELMs), which release high energy plasma particles to the plasma facing materials
The very weak of Te considered on both core and ne alsolinear confirms that branch the density range In the absence of spatially resolved pedestal ion temperature and rotation velocity measurements, it is considered is in the high density, linear H-mode branch [19,21,22]

Summary

Introduction

The high confinement or H-mode [1] is the operational scenario for the next-step device ITER [2]. Where, ne is the plasma line average density (×1020 m−3 ), Bt is the toroidal magnetic field at the magnetic axis (T), and SA is the plasma boundary surface area (m2 ) In addition to these global parameters, it is known that each device H-mode access Pth has other dependences, which are referred to as “hidden variables”. These include variations in the plasma boundary shape (number of X-points, magnetic balance, radial, poloidal, vertical location of the X-points, and plasma elongation), plasma ion species, applied 3D non-axisymmetric fields, wall conditioning.

Density

General

X-Point Height Dependence

Comparison with Theory

Findings

Conclusions