News from MAST - June 2005

A full experimental programme was carried out on MAST in May/June with a total of 24 operational days. Experiments included neutral beam current drive (NBCD) studies, non-solenoid start up experiments in conjunction with ENEA Frascati, dependence of x-point height on H-mode access, particle transport studies with deuterium pellet injection, impact of target hot spots and ELM pulse duration on interpretation of infra-red measurements of divertor power loading, internal transport barrier (ITB) formation, and effluxes and power loading due to ELMs and sawteeth. In addition, scenario development was carried out in preparation for joint experiments on scrape-off layer (SOL) transport (with MIT) and MAST/NSTX/DIII-D pedestal similarity experiments (with PPPL and GA)

In parallel with the on-going physics programme, improvements to the MAST plant and diagnostics continue to be implemented. Further improvements to the plasma control system include incorporation of plasma density feedback control and improved decoupling of radial position and plasma current control.

In an ideal tokamak, the equilibrium magnetic fields would be axisymmetric. In practice, however, the magnetic fields show small deviations from axisymmetry which are known as error fields. It is well known that error fields with amplitudes B_r/B_T ~ 10^-4, where B_T is the toroidal field at the geometric axis, can induce resistive tearing modes in the plasma and these can grow and lead to non-rotating, or locked, MHD modes in the plasma. These locked modes typically lead to termination of the plasma. To reduce the effect of these error fields, a set of error field correction coils was installed on the MAST tokamak. These are designed to create non-axisymmetric magnetic fields inside the machine to oppose the intrinsic error field. Correction of the intrinsic error field has allowed MAST to operate in previously inaccessible regimes. Experiments have also been carried out by David Howell, Tim Hender and Geof Cunningham to measure the locked mode threshold by applying large error fields using the coils. In this way we have determined how the locked mode threshold scales with density, safety factor and toroidal field (Figure 1). The MAST error field correction system consists of four coils attached to the outside of the vessel. Opposite pairs of coils were wired in series to produce a predominantly n = 1 spectrum. The two pairs of coils are powered by independent power supplies. This enables us to apply an n = 1 field at an arbitrary toroidal phase.

As in conventional tokamaks, the scaling with density is approximately linear and the scalings with toroidal field and safety factor are also in broad agreement with those in conventional tokamaks such as JET and DIII-D. However, direct comparison of MAST and DIII-D data also gives information on the aspect ratio scaling.

Particle transport studies with deuterium pellet injection are being carried out in MAST, led by Martin Valovic. The cryogenic pellet injector, developed and maintained by Ken Axon, was built at Risø , Denmark and donated to UKAEA by FOM, Netherlands . Pellets are launched either vertically through the x-point or radially from the low field side. For vertical launch it is observed that even if the pellet increases the pedestal density by a factor of two the discharge remains in H-mode. The pellet typically triggers an ELM-like event, nevertheless the particle confinement remains good during and after pellet injection. The high spatial resolution (3mm) Thomson scattering system has been synchronized with the pellet allowing detailed investigation of the pellet deposition dynamics. Visible brehmsstrahlung emission allows measurement of the very high density plasma around the pellet (Figure 2).

Approximately 15 MAST-related papers were presented at the 32^nd EPS Plasma Physics Conference in Tarragona , Spain . Many of these papers were presented by authors from collaborating institutions including University College , Cork , Ireland , Oxford University , UK , Imperial College , London , UK , St. Petersburg State Polytechnical University , Russia , Moscow State University , Russia , and the Ioffe Institute, St. Petersburg , Russia .

Geof Cunningham attended the 7^th International Reflectometry Workshop in Garching , Germany , where he presented a paper on reflectometry measurements in MAST using the reflectometer developed by IST, Lisbon , Portugal .

Vladimir Rozhansky and Sergei Voskoboynikov, of St. Petersburg State Polytechnical University, Russia, visited MAST to work with Andrew Kirk and Glenn Counsell of the MAST team on modelling of MAST plasmas using B2SOLPS5.0

Franco Alladio and Paolo Micozzi of ENEA Frascati visited Culham to work on modelling of non-solenoid start-up in MAST and participate in experiments.

Publications

Recent journal publications include:

"Improved H-mode access in connected DND in MAST"
Meyer H, et al 2005, Plasma Physics & Controlled Fusion47 843.

"Comparison of L and H-mode plasma edge fluctuations in MAST"
Dudson B, et al 2005, Plasma Physics & Controlled Fusion47 885.

"Anisotropic fast neutral particle spectra in MAST"
Tournianski M, et al 2005, Plasma Physics & Controlled Fusion47 671.