[en] The recent upgrade to the MAST YAG Thomson scattering while enhancing the diagnostic capabilities increased the complexity of the system. There are eight YAG lasers now operational, doubling the number from the previous setup. This means alignment between each laser individually and reference points is essential to guarantee data quality and diagnostic reliability. To address this issue an alignment system was recently installed. It mimics the beams alignment in MAST by sampling 1% of the laser beam that is sent into a telescope which demagnifies by a factor of 8. The demagnified beam is viewed with a CCD camera. By scanning the camera the profile and position of the beams in the scattering zone and in a range of several meters inside MAST can be determined. Therefore alignment is checked along the beam path without having to sample it inside the vessel. The experimental apparatus and test procedures are described.

[en] The first measurements of the structure of the edge radial electric field, E_r, in a spherical tokamak (MAST) are presented. Using active Doppler spectroscopy on He⁺ with 120 lines of sight E_r profiles are calculated from the leading terms of the radial momentum balance. A spatial resolution up to Δr ∼ 1.5 mm with a typical time resolution of Δt = 5 ms can be achieved. In L-mode the field is largely determined by the diamagnetic term of the force balance, and fields of only a few kV/m are observed. The measured impurity flow is mostly parallel to B, and is greatly affected by MHD, such as sawteeth or mode locking of tearing modes, or error fields. In H-mode a strong perpendicular flow evolves with poloidal and toroidal velocities up to v^He+_φθ ∼ -20 km/s, and a deep negative electric field well E_r^min ∼> -15 kV/m develops. The profile form is dominated by the diamagnetic term

[en] Neo-classical tokamak plasma theory predicts poloidal rotation driven by the temperature gradient of a few km s^-1. In conventional aspect-ratio tokamak plasmas, e.g. on JET and DIII-D, apparent poloidal velocities considerably in excess of the neo-classical values have been measured, particularly in the presence of internal transport barriers, by means of charge-exchange recombination spectroscopy (CXRS) on the fully ionized C⁶⁺ impurity ions. Comparison between such measurements and theoretical predictions requires careful corrections to be made for apparent 'pseudo' velocities, which can arise from the finite lifetime of the excited atoms in the magnetized plasma and the energy dependence of the charge-exchange excitation process. In present day spherical tokamak plasmas this correction is an order of magnitude smaller than on large conventional tokamaks, which operate at higher temperature and magnetic field, hence reducing any associated systematic uncertainties. On MAST measurements of toroidal and poloidal flows of the C⁶⁺ impurities are available from high-resolution Doppler CXRS measurements, including appropriate corrections for the pseudo-velocities. Comparison of the measured C⁶⁺ velocities with neo-classical theory requires calculation of the impurity flow, which differs from that of the bulk ions due to the respective diamagnetic contributions for each species and inter-species friction forces. Comparisons are made with the predictions of a recent neo-classical theory (Newton 2007 Collisional transport in a low collisionality plasma with strong rotation PhD Thesis University of Bristol, Newton and Helander 2006 Phys. Plasmas 13 102505), which calculates the full neo-classical transport matrix for bulk ions and a single impurity species for a strongly rotating plasma, as well as those of a simpler neo-classical theory (Kim et al 1991 Phys. Fluids B 3 2050-9) for an impure plasma and the NCLASS code (Houlberg et al 1997 Phys. Plasmas 4 3230-42). Initial results for both L- and H-mode plasmas show that, within the measurement uncertainties, the measured poloidal rotation of the core plasma is consistent with the neo-classical predictions.

[en] A Thomson scattering diagnostic designed to measure both edge and core physics has been implemented on MAST. The system uses eight Nd:YAG lasers, each with a repetition rate of 30 Hz. The relative and absolute timing of the lasers may be set arbitrarily to produce fast bursts of measurements to suit the time evolution of the physics being studied. The scattered light is collected at F/6 by a 100 kg six element lens system with an aperture stop of 290 mm. The collected light is then transferred to 130 polychromators by 130 independent fiber bundles. The data acquisition and processing are based on a distributed computer system of dual core processors embedded in 26 chassis. Each chassis is standalone and performs data acquisition and processing for five polychromators. This system allows data to be available quickly after the MAST shot and has potential for real-time operations.

[en] A major upgrade to the ruby Thomson scattering (TS) system has been designed and implemented on the Mega-ampere spherical tokamak (MAST). MAST is equipped with two TS systems, a Nd:YAG laser system and a ruby laser system. Apart from common collection optics each system provides independent measurements of the electron temperature and density profile. This paper focuses on the recent upgrades to the ruby TS system. The upgraded ruby TS system measures 512 points across the major radius of the MAST vessel. The ruby laser can deliver one 10 J 40 ns pulse at 1 Hz or two 5 J pulses separated by 100-800 μs. The Thomson scattered light is collected at F/15 over 1.4 m. This system can resolve small (7 mm) structures at 200 points in both the electron temperature and density channels at high optical contrast; ∼50% modulated transfer function. The system is fully automated for each MAST discharge and requires little adjustment. The estimated measurement error for a 7 mm radial point is <4% of T_e and <3% of n_e in the range of 40 eV to 2 keV, for a density of n_e=2x10¹⁹ m^-3. The photon statistics at lower density can be increased by binning in the radial direction as desired. A new intensified CCD camera design allows the ruby TS system to take two snapshots separated with a minimum time of 230 μs. This is exploited to measure two density and temperature profiles or to measure the plasma background light.

[en] Studies of the pedestal characteristics and quantities determining ELM energy losses in MAST are presented. Progress is reported on the attempts to determine the quantities that affect the pedestal height and understanding ELM losses. High temperature pedestal plasmas have been achieved which have collisionalities one order of magnitude lower than previous results. The pedestal widths obtained in these low collisionality plasmas are in better agreement with banana orbit scalings than previous high collisionality plasmas, suggesting that banana orbits can only play a role in determining the minimum width when the collisionality is sufficiently low. A stability analysis performed on these plasmas shows them to be near the ballooning limit and to have broad mode structures which would predict large ELM energy losses. These ELM energy losses have been observed at the target resulting in peak power densities in excess of ∼20 MWm^-2. The fraction of pedestal energy released by an ELM as a function of collisionality has been compared with data from other devices. A model for ELM energy losses has been proposed and compared to data from MAST and JET. (author)

[en] A new infrared Thomson scattering system has been designed for the MAST tokamak. The system will measure at 120 spatial points with ≅10 mm resolution across the plasma. Eight 30 Hz 1.6 J Nd:YAG lasers will be combined to produce a sampling rate of 240 Hz. The lasers will follow separate parallel beam paths to the MAST vessel. Scattered light will be collected at approximately f/6 over scattering angles ranging from 80 deg. to 120 deg. The laser energy and lens size, relative to an existing 1.2 J f/12 system, greatly increases the number of scattered photons collected per unit length of laser beam. This is the third generation of this polychromator to be built and a number of modifications have been made to facilitate mass production and to improve performance. Detected scattered signals will be digitized at a rate of 1 GS/s by 8 bit analog to digital converters (ADCs.) Data may be read out from the ADCs between laser pulses to allow for real-time analysis.

[en] The newly upgraded MAST Thomson scattering (TS) system provides excellent spatial resolution (∼1 cm) at over 130 radial locations across a full plasma diameter, and utilizes eight individual Nd: :YAG laser systems which can be fired sequentially, providing electron temperature and density profiles approximately every 4 ms throughout a plasma discharge. By operating the system in burst mode, whereby the laser separation can be adjusted to within a few microseconds of each other, it is possible to obtain detailed profiles of transient and periodic phenomena such as sawteeth crashes, massive gas injection for disruption mitigation and the temperature perturbations associated with neoclassical tearing mode (NTM) islands. Following Fitzpatrick et al (1995 Phys. Plasmas 2 825), we consider a simplified model in which finite parallel diffusive heat transport can provide a threshold for NTM island growth and demonstrate that the TS derived electron temperature profiles around an island can be used to obtain both the island width and the critical island width below which temperature gradients are maintained across the island, potentially removing the bootstrap current drive for the NTM. Initial results from high beta, neutral beam injection heated discharges on MAST show that the measured island width inferred from the TS data is in good agreement with magnetic estimates of the island width (considering both a cylindrical approximation and using a full field line tracing estimate). The temporal behaviour of the island width obtained from the magnetic diagnostics indicates that for the scenarios considered to date, finite parallel diffusion is likely to play an important role in NTM threshold physics in MAST.

[en] A new edge Thomson scattering diagnostic has been implemented at MAST to complement an existing high spatial resolution ruby laser system and the high time sampling core Nd:YAG system. The Nd:YAG system comprises of four independently controllable lasers. Scattered light from these lasers is viewed at large scattering angle (153 deg. ) by a special optical arrangement in the new edge system. The Nd:YAG lasers are viewed at 16 contiguous spatial locations separated by ∼1 cm each, located at the plasma outboard pedestal and scrape-off layer region. Here the use of a low f-number lens for the collection of a large solid angle of scattered light is particularly beneficial due to low plasma density (n_e). The spectrum of scattered light is significantly broader at large scattering angles, allowing diagnosis of lower plasma temperatures (T_e) while using the same spectrometer design as the core system. The four Nd:YAG lasers follow two separate slightly offset (<1/3 of a spatial channel) optical paths through the vessel. This is useful when the lasers are used in burst mode for detailed edge studies of fast events such as ELMs. Polychromators have been designed to allow for both Raman and Rayleigh calibration. First results from this diagnostic are presented showing H-mode pedestal behavior. A novel spectral fitting technique has been devised and is applied to edge pedestal fitting

[en] A new Thomson scattering diagnostic has been designed and is currently being installed on the COMPASS tokamak in IPP Prague in the Czech Republic. The requirements for this system are very stringent with approximately 3 mm spatial resolution at the plasma edge. A critical part of this diagnostic is the laser source. To achieve the specified parameters, a multilaser solution is utilized. Two 30 Hz 1.5 J Nd:YAG laser systems, used at the fundamental wavelength of 1064 nm, are located outside the tokamak area at a distance of 20 m from the tokamak. The design of the laser beam transport path is presented. The approach leading to a final choice of optimal focusing optics is given. As well as the beam path to the tokamak, a test path of the same optical length was built. Performance tests of the laser system carried out using the test path are described.