Investigation of transient phenomena on MAST using high resolution Thomson scattering

The MAST tokamak is equipped with high spatial resolution Ruby laser and high time resolution Nd:YAG laser TS diagnostics. The Nd:YAG lasers are viewed by two separate sets of optics. One of these lens systems views the core region and measures at spatial resolution of 2.5-4cm and the other examines the plasma edge with 1cm resolution. This newly installed edge system has already produced a number of important results. In H-mode and L-mode filaments have been observed using laser time separations of 1-20$\mu $s. The high spatial resolution has allowed determination of the evolution of the outboard pressure pedestal, which plays a critical role in determining plasma stability. The variable time separation between lasers has also been exploited to study pellet deposition and retention in the plasma. A major upgrade to the core Nd:YAG system is now being planned. It is proposed to replace the current four lasers with a combined sampling rate of 200Hz at 1.0J with eight lasers with a combined sampling rate of 240Hz at 1.6J. The increase in laser energy together with new optics will allow the system to sample at high spatial resolution.