News from MAST - April 2005

MAST operations continued in March/April 2005. Experiments included non-solenoid start up studies in conjunction with ENEA Frascati, H-mode access investigations, detailed diagnosis of baseline L-mode and H-mode discharges exploiting improved diagnostic capabilities, locked mode threshold scaling, disruption power load measurements and studies of edge turbulence in conjunction with Oxford University (Ben Dudson). Further optimisation of the boronisation procedure on MAST has improved H-mode access.

In parallel with the on-going physics programme, improvements to the MAST plant and diagnostics continue to be implemented. For example, modifications to the PF circuit now allow the current in the P2 coil to be reversed during a discharge. Diagnostic developments include further optimisation of temperature and velocity measurements using a high resolution crystal spectrometer (THEMIS) by Hendrik Meyer and Michaela Nelson. The curved crystal Johann-spectrometer was developed in collaboration with Queens University , Belfast . The instrument relies on Argon impurity puffing for its measurement since the Bragg angle is chosen to observe the spectrum of Helium-like argon.

The design of the spectrometer was adapted from an existing JET design and is extremely compact. It shares the flight line of the Neutral Particle Analyser (NPA) and, therefore, the instrument can be scanned toroidally and poloidally using the scanning mechanism of the NPA. However, the flight line of the NPA has to be open to MAST and, hence, the crystal needs to be in ultra high vacuum (UHV). The separation between the UHV and the low vacuum for the CCD detector is realised by a 75µm thick beryllium foil embedded into a soft-shut vacuum valve. Furthermore, the crystal bending jig needs to be able to maintain the curvature of the crystal accurately after baking of the flight line up to temperatures of 100^ºC, which was tested before implementation using visible light. The toroidal scan includes a radial line of sight, which allows an absolute wavelength calibration of the system.

Two typical spectra during the same MAST discharge are shown in figure 1, and the evolution of the radial profile of the toroidal Ar¹⁶⁺ rotation is shown in figure 2. The velocity and temperature of the He-like argon can be inferred from the line shift and width of the spectral lines respectively. Both quantities are generally strongly coupled to the bulk ions because of the high charge state and low concentration of the argon. In contrast to charge exchange recombination spectroscopy (CXRS), the spectra can be obtained with and without NBI.

Collaboration Activities

Martin Peng (ORNL) visited Culham to discuss NSTX-MAST collaborative activities and issues relating to an ST-based component test facility (volume neutron source).

Mikhail Tournianski of the MAST team visited IPP Greifswald to discuss collaboration on compact NPA (neutral particle analyser) measurements with members of the W7-X team.

Publications

Recent journal publications include:

"Ideal MHD stability of MAST",
Hole M, et al 2005, Plasma Physics & Controlled Fusion47 581

"Theoretical interpretation of frequency sweeping observations in the Mega-Amp spherical tokamak"
Vann R G L et al 2005, Physics of Plasmas12, 032501.

"The poloidal distribution of turbulent fluctuations in the Mega-Ampere spherical tokamak"
Antar G Y et al 2005, Physics of Plasmas12, 032506.

"Generation of toroidal rotation by gas puff. Simulations of MAST experiments with B2SOLPS5.0"
Rozhansky V et al 2005, Journal of Nuclear Materials337-339, 291.

"Thermographic power accounting in MAST"
Lott F et al 2005, Journal of Nuclear Materials337-339, 786.