News from MAST - June 2004

MAST Status

An earth fault between a toroidal field coil and the vacuum vessel delayed the start of the 2004 experimental campaign (M4). The fault was detected during daily high voltage tests, after initial machine commissioning had been completed. The earth fault has now been fixed. The location of the fault was traced, using a combination of electrical resistance measurements and analysis, and an 'in-situ' repair was successfully carried out by Garry Voss and his team. Special tooling was developed by the Special Techniques Group and a series of trials was carried out to develop a repair technique. Plasma operation is now scheduled to start during the week beginning 19th July.

Error Field Correction Coil System

Small inaccuracies in the magnetic field configuration can have a significant effect on plasma performance. During the recent engineering break, the error resulting from asymmetry in the main MAST poloidal coils was measured, with a view to possible re-alignment of the coils to minimise this error. The static measurements were obtained using a precision magnetometer mounted inside the vessel on a precise but moveable jig. This allowed measurements to be made of the fields generated when the poloidal coils were energised at low currents, compatible with personnel being in the machine area. The results showed good internal consistency and allowed a reliable determination of the true coil geometry. However, they also showed that the resulting error field, when translated into the plasma frame of reference, would be substantially less than that thought to be necessary to cause deleterious effects. After careful consideration was made of the relative risks and benefits involved in correcting the small inaccuracies in the coil alignment, a decision was made not to attempt realignment at the present time.

Error field measuerment within the MAST vessel

However, a set of correction coils was designed for the purpose of correcting the error field, at least partially, irrespective of its source. The new correction coils were designed to be compatible with existing power supplies, whilst generating adequate field, and having sufficient strength to withstand the substantial forces involved. Installation was completed with minimum disruption to the MAST programme, using standard engineering components. A fairly conventional design was chosen, comprising 2 pairs of coils arranged in north-south and east-west directions, outside but fairly close to the vessel. The coils themselves are partly made from solid copper bars in glass reinforced plastic housing, and partly from flexible cable. The coils will be commissioned during the first part of the 2004 campaign.

Experimental Programme

The 2004 experimental programme is presently being revised in the light of the delayed start to the campaign. The initial focus will still be on non-solenoid start-up studies and tests of electron Bernstein wave heating. The 2004 campaign is still scheduled to end at the beginning of November to allow work to be carried out on the 400kV supply.

Publications & Presentations

Fraser Lott and James Martin, both of Imperial College London, presented papers on "Thermographic power accounting in MAST" and "Motion and lifetime of dust grains in tokamak plasmas" respectively, at the 16th International Conference on Plasma Surface Interactions (PSI) in Portland, Maine, May 2004.

Vladimir Shevchenko and Francesco Volpe attended the 13th International Workshop on ECRH and ECE in Nizhny Novgorod, Russia, where Vladimir delivered an invited talk on "Prospects of electron Bernstein wave emission diagnostics and electron Bernstein wave heating in spherical tokamaks", as well as a contributed paper on "Electron Bernstein wave current drive start-up scenarios for MAST".

Brian Lloyd presented an invited talk entitled "MAST and the impact of low aspect ratio on tokamak physics" at the 31st EPS Conference in London. The invited talk was supported by a wide range of MAST posters, including one on "Magnetic Reconnections in MAST" for which Gianpaolo Turri, a PhD student from Imperial College London, was awarded an EPS prize.

The following journal publications have recently been issued:
"Spatial & Temporal Structure of Edge-localised Modes",
Kirk A et al 2004, Phys. Rev. Lett. 92 245002.
"The influence of gas fuelling location on H-mode access in MAST",
Field A R et al 2004, Plasma Phys. Control. Fusion 46 981.

Other News

Geof Cunningham and Chippy Thyagaraja visited the TEXTOR team in Julich, Germany to participate in a workshop on turbulence studies in tokamak plasmas.

Franco Alladio, Paolo Micozzi, Alessandro Mancuso and Pietro Costa from ENEA Frascati have been visiting the MAST team to perform simulations of non-solenoid start-up scenarios and prepare for experiments on MAST.

As part of the UKAEA-ENEA collaboration on spherical tokamaks, the pioneering START experiment (operational 1991 - 1998) has been transferred from Culham to Frascati where it will be deployed in the Italian spherical tokamak research programme (see news item).

Removal of the START vacuum vessel from Culham