No abstract available

No abstract available

[en] The qualitative assessment of left ventricular wall motion in the RAO view between equilibrium gated radionuclide angiography (MUGA) and single plane left ventricular angiography is compared. (author)

[en] Myocardial imaging was performed at rest and after dipyridamole infusion using ²⁰¹Tl and sup(99m)Tc-gated blood pool studies (MUGA). Of 38 patients studied, 21 had coronary artery disease (CAD) and 17 were normal. With ²⁰¹Tl imaging, 19 of 21 patients with CAD (sensitivity 88%) showed perfusion defects during dipyridamole. Of 17 normals 6 (specificity 65%) showed clear perfusion defects during dipyridamole. With MUGA, 2 of 15 patients with CAD (sensitivity 13%) developed abnormal wall motion during dipyridamole; both patients developed ST depression > 1 mm. Of 10 normals none developed abnormal wall motion. Twenty six of 38 patients developed angina during dipyridamole, which was reversed by aminophylline in 24 of 26. We suggest that dipyridamole infusion is effective for ²⁰¹Tl-perfusion imaging, and easily performed, but is not suitable for MUGA studies where ischaemia is the required endpoint. (orig.)

[en] A multichord motional Stark effect (MSE) system has recently been built on the MAST tokamak. In MAST the π and σ lines of the MSE spectrum overlap due to the low magnetic field typical for present day spherical tokamaks. Also, the field curvature results in a large change in the pitch angle over the observation volume. The measured polarization angle does not relate to one local pitch angle but to an integration over all pitch angles in the observation volume. The velocity distribution of the neutral beam further complicates the measurement. To take into account volume effects and velocity distribution, an ab initio code was written that simulates the MSE spectrum on MAST. The code is modular and can easily be adjusted for other tokamaks. The code returns the intensity, polarized fraction, and polarization angle as a function of wavelength. Results of the code are presented, showing the effect on depolarization and wavelength dependence of the polarization angle. The code is used to optimize the design and calibration of the MSE diagnostic.

[en] Beam emission spectroscopy (BES) of the energetic deuterium (D⁰) heating beams can provide a means of characterizing the density turbulence in tokamak plasmas. First such measurements have been performed on the MAST spherical tokamak using a trial BES system, which shares the collection optics of the charge-exchange recombination spectroscopy system. This system, with eight spatial channels covering the outer part of the plasma cross section, uses avalanche photodiode detectors with custom preamplifiers to provide measurements at 1 MHz bandwidth with a spatial resolution of 4 cm. Simulations of the measurement, including the beam absorption and excitation, line-of-sight integration of the emission spectrum, and the characteristics of the detection system have been benchmarked against the measured absolute intensity of the Doppler shifted D_α fluorescence from the 50 keV beam. This gives confidence in predictions of the performance of a two-dimensional imaging BES system planned for MAST. Correlation techniques have also provided information on the characteristics of the density turbulence at the periphery of L-mode plasmas as well as density perturbations due to coherent magnetohydrodynamic activity at the edge of H-mode plasmas. Precursor oscillations of the density in the pedestal region to edge-localized modes occurring during H-mode plasmas with a single-null diverted magnetic configuration are also observable in the raw signals from the trial BES system.

[en] In the Mega-Amp Spherical Tokamak, MAST, the formation of the edge transport barrier leading to the high-confinement (H-mode) regime is greatly facilitated by operating in a double null diverted (DND) configuration where both X-points are practically on the same flux surface. Ohmic H-modes are presently only obtained in these connected double null diverted (CDND) configurations. The ease of H-mode access is lost if the two flux surfaces passing through the X-points are radially separated by more than one ion Larmor radius (ρ_i ∼ 6 mm) at the low-field-side mid-plane. The change of the magnetic configuration from disconnected to CDND is accompanied by a change in the radial electric field of about ΔE_ψ ∼ -1 kV m^-1 and a reduction of the electron temperature decay length in the high-field-side scrape-off-layer. Other parameters at the plasma edge, in particular those affecting the H-mode access criteria of common L/H transition theories, are not affected by the slight changes to the magnetic configuration. It is believed that the observed change in E_ψ, which may result from differences in ion orbit losses, leads to a higher initial E x B flow shear in CDND configurations which could lead to the easier H-mode access

[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] 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] The tight aspect ratio tokamak, with its high-β capability allows for an increased understanding of MHD stability, including, but not limited to, aspect ratio scaling effects. The MAST tokamak is equipped with neutral beam heating, a range of excellent diagnostics and external error field corrections coils that allow non-axisymmetric (dominantly n=1) fields to be applied. MAST has made considerable progress in studying sawtooth behaviour with co and counter-NBI, in observations of fast ion instabilities and on studies of the threshold for error field locked modes, as reported in this paper. (author)