Two-fluid simulations of driven reconnection in the mega-ampere spherical tokamak

Journal Article · Sun Dec 15 00:00:00 EST 2013 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4830104· OSTI ID:22218330

Stanier, A.; Browning, P.; Gordovskyy, M. ^[1]; McClements, K. G.; Gryaznevich, M. P. ^[2]; Lukin, V. S. ^[3]

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1. Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL (United Kingdom)
2. EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB (United Kingdom)
3. Space Science Division, Naval Research Laboratory, Washington, DC 20375 (United States)

In the merging-compression method of plasma start-up, two flux-ropes with parallel toroidal current are formed around in-vessel poloidal field coils, before merging to form a spherical tokamak plasma. This start-up method, used in the Mega-Ampere Spherical Tokamak (MAST), is studied as a high Lundquist number and low plasma-beta magnetic reconnection experiment. In this paper, 2D fluid simulations are presented of this merging process in order to understand the underlying physics, and better interpret the experimental data. These simulations examine the individual and combined effects of tight-aspect ratio geometry and two-fluid physics on the merging. The ideal self-driven flux-rope dynamics are coupled to the diffusion layer physics, resulting in a large range of phenomena. For resistive MHD simulations, the flux-ropes enter the sloshing regime for normalised resistivity η≲10{sup −5}. In Hall-MHD, three regimes are found for the qualitative behaviour of the current sheet, depending on the ratio of the current sheet width to the ion-sound radius. These are a stable collisional regime, an open X-point regime, and an intermediate regime that is highly unstable to tearing-type instabilities. In toroidal axisymmetric geometry, the final state after merging is a MAST-like spherical tokamak with nested flux-surfaces. It is also shown that the evolution of simulated 1D radial density profiles closely resembles the Thomson scattering electron density measurements in MAST. An intuitive explanation for the origin of the measured density structures is proposed, based upon the results of the toroidal Hall-MHD simulations.

• OSTI ID: 22218330
• Journal Information: Physics of Plasmas, Vol. 20, Issue 12; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ASPECT RATIO
AXIAL SYMMETRY
DIFFUSION
ELECTRIC CURRENTS
ELECTRON DENSITY
MAGNETIC RECONNECTION
MAGNETIC SURFACES
MAGNETOHYDRODYNAMICS
MAST TOKAMAK
PLASMA DENSITY
PLASMA FLUID EQUATIONS
PLASMA SIMULATION
SOUND WAVES
TEARING INSTABILITY
THOMSON SCATTERING
WALL EFFECTS