News from MAST - August 2004

MAST Status

Plasma operation began during the week beginning 19th July. Following an intensive period of plasma conditioning, vessel boronisation was carried out on Friday 6th August, prior to commencement of the main physics programme. The physics campaign is now well under way, and a wide range of discharges has been studied in the new divertor configuration. The new Digital Control System (developed in collaboration with General Atomics in US) is now used routinely. Initial experiments have focussed mainly on:

• studies of start-up without a central solenoid, in collaboration with colleagues from ENEA Frascati,
• exploitation of the new error field correction coils to extend the MAST operational space,
• studies of edge localised modes (ELMs),
• electron Bernstein wave (EBW) heating tests using our high power 60GHz microwave facilities.

Non-solenoid start-up studies have focussed on the so-called ‘Double Null Merging’ (DNM) technique in which plasmas formed at poloidal field nulls above and below the mid-plane are subsequently merged in the mid-plane region. Using this technique, it has been possible to generate hot (Te0 ~ 0.5keV), dense (ne0 ~ 9 x 1019m-3) plasmas and sustain the discharges for greater than 300ms without using the central solenoid. Plasma currents in excess of 300kA have been obtained.

Non-solenoid start-up in MAST

Highlight of the campaign to date, has been the successful demonstration of error field reduction on MAST, using the correction coils installed during the engineering break, to suppress locked modes. This has already allowed a significant extension of the MAST operational space towards low densities.

Use of error field correction coils (EFCC) in MAST to suppress locked modes and extend the operational space to lower densities

Diagnostic News

First results have been obtained from the new bolometer system. The MAST bolometer system was developed in collaboration with the Stellarator Group at IPP Garching, Germany. Gold-foil sensors are used as on other major fusion facilities together with electronics developed at IPP Garching for W7-X. The original 16-channel vertical-viewing camera, commissioned in mid-'02, has routinely provided vertical profile measurements of total radiated power with 1 ms time resolution. Additional tangential viewing bolometer cameras have now been installed, viewing co- and counter- to the NBI with 12- and 4-channels respectively. This system provides profiles of the total radiation at the horizontal mid-plane as well as providing information on the co/counter asymmetry of the fast ion population.

View of the MAST bolometer system from inside the vessel. The new co- and counter-viewing cameras are attached to the side of the original vertical-viewing camera

Collaboration Activities

On 14th July Anthony Field conducted internal transport barrier (ITB) experiments on NSTX by 'remote participation', in conjunction with PPPL physicists. These NSTX-MAST identity experiments on ion ITBs in co-NBI H-mode plasmas were designed to study the response of ITB formation and evolution to scans of the NBI power and momentum.

Anthony Field of the MAST team participating remotely in experiments on NSTX

A number of MAST collaborators, including visitors from the Ioffe Institute St. Petersburg, MIT and PPPL, visited Culham Science Centre for discussions in conjunction with their attendance at the 31st EPS conference in London.

Alexander Saveliev of the A F Ioffe Physico-Technical Institute, St. Petersburg, Russia is visiting Culham Science Centre to carry out theoretical work on electron Bernstein wave heating for MAST.

Publications & Presentations

The following journal publications have recently been issued:

"Beta-dependence of energetic particle-driven instabilities in spherical tokamaks",
M P Gryaznevich and S E Sharapov 2004, Plasma Phys. Contr. Fusion 46 S15.

"Spectroscopic determination of the internal amplitude of frequency sweeping TAE"
S D Pinches et al 2004, Plasma Phys. Contr. Fusion 46 S47.

"Integrated plasma physics modelling for the Culham steady state spherical tokamak fusion power plant"
H R Wilson et al 2004, Nucl. Fusion 44 917.