5-9 September 2016
Prague Congress Centre
Europe/Prague timezone

P1.047 Equilibrium modelling of merging compression startup in a spherical tokamak

5 Sep 2016, 14:20
1h 40m
Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Foyer 2A (2nd floor), 3A (3rd floor)

Prague Congress Centre

5. května 65, Prague, Czech Republic
Board: 47
Poster C. Plasma Engineering and Control P1 Poster session

Speaker

Peter Buxton (Tokamak Energy Ltd)

Description

Merging compression startup, pioneered on START, is a successful and robust method for plasma breakdown and plasma current startup which does not involve a solenoid. Tokamak Energy is currently constructing a relatively small (R~0.4m) high toroidal field (BT>2T) spherical tokamak (aspect ratio ~ 1.8) called ST40 which will have ~2MA of plasma current. A consequence of the ambitiously high toroidal field is that the maximum current in the solenoid is limited to ensure that the twisting force on the toroidal field coil assembly is manageable. Consequently, solenoid startup will not be sufficient to produce ~2MA of plasma current. The intention therefore is to use merging compression for breakdown and to produce the operating plasma current. Merging compression on START and MAST produced a maximum plasma current of 155kA and 470kA. Currently, a complete theoretical model for the merging process does not exist. We therefore rely on an experimentally derived scaling law, created using START and MAST data, to extrapolate the required in vessel PF coil current required to produce ~2MA of plasma current (note: ST40’s in vessel PF coils are equivalent to MAST’s P3 coils; about which plasma rings form prior to merging). Using a free boundary MHD equilibrium solver we modelled the plasma as a series of snapshots in time; before and after merging. Unexpectedly we found that eddy currents in the vessel play an important role in the plasma equilibrium. Neglecting eddy currents we find that the equilibrium is extremely sensitive to the vertical field produced by PF coils outside the vessel, which is in contradiction to the experience on START and MAST which showed merging compression to be an extremely robust and repeatable technique. We therefore conclude that it is essential to take vessel eddy currents into account when modelling merging compression in tokamaks.

Co-authors

A. Sykes (Tokamak Energy Ltd, Milton Park, United Kingdom) Mikhail P. Gryaznevich (Tokamak Energy Ltd, Milton Park, United Kingdom) Peter Buxton (Tokamak Energy Ltd, Milton Park, United Kingdom)

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