News from MAST - April 2004

MAST Development

The 9 month MAST Engineering Break (see MAST News Bulletin Dec 2003 for a detailed description) was completed on schedule. Following vessel closure, leak testing was carried out followed by an extensive period (1½ weeks) of high temperature baking, resulting in a base pressure close to 10^-8mbar. Power supply and electrical system commissioning has progressed rapidly and is almost complete. The new central solenoid has been operated at ±45kA ( ~ 80% of maximum capability) and the toroidal field (TF) system has been taken up to 85kA (I_rod ~2MA). The maximum TF power supply current is 92kA

Figure 1. Installation of the new MAST centre column

Commissioning of the 60GHz electron Bernstein wave (EBW) heating system is also well-underway with 5 of the 7 gyrotrons having been operated at high power. EBW experiments are scheduled to take place early in the 2004 (M4) campaign (see below).

Commissioning of the South West (SW) neutral beam injector has also started, power levels > 1MW being rapidly achieved in hydrogen.

Neutral Beam System Upgrade

Until July 2003, the MAST NBI System was operated using two duopigatron neutral beam injectors on loan from Oak Ridge National Laboratory which together gave a maximum total injected deuterium beam power into MAST of 3.3MW for 250ms in plasma pulse 9502. In order to further develop the long pulse high power capability of the NBI system the decision was taken in 2003 to replace the two duopigatron injectors with Positive Ion Neutral Injectors (PINIs), which have been successfully used over many years on JET and other fusion facilities in Europe. The MAST PINI is based on the JET PINI design with a supercusp ion source and tetrode accelerator modified to match the existing geometry of the MAST NBI beamlines. It will be capable of delivering ~ 2.5MW (per injector) into the MAST plasma for a pulse duration in excess of 5 seconds when operated in deuterium at 75kV/65A. The predicted deuterium beam power distribution at the MAST plasma axis is shown in Figure 2.

The first MAST PINI (Figure 3) has been designed, manufactured, fully assembled and installed on the MAST South (S) beamline in April 2004. In addition, many components of the NBI beamlines had to be modified or re-designed: new PINI support and steering structure, new second stage neutraliser, modified bend magnet (reduced pole gap), additional HV area above the injector to accommodate the snubber, etc. A new PLC controlled gas introduction system for the PINI ion source and the neutraliser has been designed, manufactured and installed. New long-pulse calorimeters and Residual Ion Dumps (RIDs), based on hypervapotrons, were installed into both beamlines (see MAST News Bulletin Feb 2004). The calorimeters and RIDs are instrumented with a large number of thermocouples allowing accurate measurement of beam power density distribution, beam position and neutralisation efficiency. Most of the power supplies and corresponding controls are being modified or replaced to allow long-pulse PINI operation. Major changes include fast switching of the arc at high voltage (HV) turn-on (arc notching), reapplication after PINI HV breakdown and HV regulation. This new scheme will also allow pulse width modulation of the beam power. New components required for PINI operation have been designed, procured and are being installed: filament transformer and power supply, gradient (G2) resistor, arc current feedback control, etc. A new HV transmission line, connecting the PINI to the HVPS, has been designed, installed and tested. The new transmission line also incorporates the snubber to protect the PINI from stored energy in the transmission line during HV breakdowns. A new bending magnet control circuit allowing automatic setting of the bending magnet current based on working gas and set beam voltage has been designed and manufactured. The new Beam Interlock System, based on the Fast Beam Interlock System (FBIS) used on JET, has been designed, manufactured and is being installed. This system will protect both beamline and MAST vessel components by stopping the beam extraction in case of the following fault conditions: beamline over-pressure, neutraliser gas introduction valve malfunction, bending magnet current/beam voltage mismatch, excessive shinethrough power on MAST vessel components at low plasma densities (Beam Bremsstrahlung Interlock). Electrical and thermal data acquisition system and timer controls were also upgraded to allow fast collection and storage of an increased number of signals: 16 fast electrical data channels, 96 thermal data channels and 8 timing channels were added to each beamline.

The first MAST PINI will be brought into operation on the S beamline in Summer 2004 whilst continuing to operate the SW beamline with the ORNL duopigatron injector. The second PINI will be assembled in Summer 2004, installed on the SW beamline during the November 2004 MAST Engineering Break and brought into MAST operation at the beginning of 2005.

Experimental Activities

The experimental proposals (>80) for the forthcoming campaign have been reviewed and prioritised by the Campaign Drivers and the MAST Experiments Committee (MEC) has given a provisional indication of the experimental time likely to be allocated to each of the main thrusts. A set of targets, by which the success of the 2004 campaign can be judged, has also been drawn up. Detailed planning of the experimental programme is now underway. After an initial period of plasma conditioning discharges and diagnostic commissioning, the main focus will be on start-up studies and electron Bernstein wave heating tests. Scientists from ENEA Frascati will visit Culham to participate in the start-up studies.

A number of MAST presentations have been given at the recent round of ITPA (International Tokamak Physics Activity) meetings. At the 6th ITPA Pedestal Physics Group Meeting at Culham 1-3 Mar 2004 Andrew Kirk delivered a presentation on the pedestal database whilst Hendrik Meyer gave a presentation on improvements in H-mode access in connected double null configurations in MAST. MAST results were also included in a presentation by Rajesh Maingi (ORNL), of the NSTX team, who compared edge pedestal data in MAST and NSTX. Martin Valovic gave a presentation on the incorporation of new MAST data in the H-mode confinement database at the 6th meeting of the ITPA Confinement Database and Modelling Topical Group at Naka, 8 - 11 March 2004. The incorporation of MAST ITB (Internal Transport Barrier) data in the Profile Database was also discussed by Martin at the Transport Topical Group meeting held in Naka at the same time.

MAST presentations were prominent at the recent APS High Temperature Plasma Diagnostics conference in San Diego, April 2004. Paddy Carolan gave an invited talk on "High Definition Imaging in the MAST Spherical Tokamak from Soft X-rays to Infrared". This was backed up by contributed papers from Mike Walsh of Walsh Scientific ("Incorporation of fast laser beam shunting and a broadband polariser in the MAST Thomson scattering systems"), Ash Patel ("A versatile multi-wavelength imaging diagnostic in the Mega Ampere Spherical Tokamak, MAST") and Michaela Nelson of Queen's University, Belfast, ("THEMIS, a high resolution soft x-ray spectrometer on the MAST tokamak"). Of particular interest is a novel diagnostic that provides narrow spectral bandwidth imaging which accommodates the wide field of view, uniquely available in MAST, of the full plasma diameter. Flexibility was built into the design, first exploited in measuring the Zeff profile from the visible bremsstrahlung and more recently used to measure the neutral beam characteristics as well as availing of the diagnostic opportunities provided by the neutral beams. One configuration of the diagnostic is shown below (Fig. 4) which allowed the beam divergence evolution to be measured (Fig. 5) across the full vessel diameter. The viewing geometry of the beam meant that there was a range of Doppler shifts. A slightly tilted interference filter, used in the convergent region of the optics, coped with this complication by taking advantage of the filter bandpass variation with light incident angle. In another application (Fig 6) the Charge Exchange Recombination, CXR, of two elements were monitored as well as the Balmer D_α light from the beam neutrals. This is important in determining impurity density profiles.

Recent MAST journal publications include:
A. Kirk et al, ELM characteristics in MAST, Plasma Phys. Contr. Fusion 46 (2004) 551
H. Meyer et al, Formation of transport barriers in the MAST spherical tokamak, Plasma Phys. Contr. Fusion 46 (2004) A291
A. Kirk et al, H-mode pedestal characteristics in MAST, Plasma Phys. Contr. Fusion 46 (2004) A187

Other News

A group of senior Polish scientists visited MAST on 30th March 2004 to discuss opportunities for future collaboration on fusion research.

A MAST Data Access Policy has been developed outlining the responsibilities of those granted access to unpublished MAST data.