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Publications (28)38.56 Total impact

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    Nuclear Fusion 10/2013; 53(10):104008. · 2.73 Impact Factor
  • Nuclear Fusion 09/2011; 51(9):094013. · 2.73 Impact Factor
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    ABSTRACT: Major developments on the Mega Amp Spherical Tokamak (MAST) have enabled important advances in support of ITER and the physics basis of a spherical tokamak (ST) based component test facility (CTF), as well as providing new insight into underlying tokamak physics. For example, L H transition studies benefit from high spatial and temporal resolution measurements of pedestal profile evolution (temperature, density and radial electric field) and in support of pedestal stability studies the edge current density profile has been inferred from motional Stark effect measurements. The influence of the q-profile and E B flow shear on transport has been studied in MAST and equilibrium flow shear has been included in gyro-kinetic codes, improving comparisons with the experimental data. H-modes exhibit a weaker q and stronger collisionality dependence of heat diffusivity than implied by IPB98(y,2) scaling, which may have important implications for the design of an ST-based CTF. ELM mitigation, an important issue for ITER, has been demonstrated by applying resonant magnetic perturbations (RMPs) using both internal and external coils, but full stabilization of type-I ELMs has not been observed. Modelling shows the importance of including the plasma response to the RMP fields. MAST plasmas with q > 1 and weak central magnetic shear regularly exhibit a long-lived saturated ideal internal mode. Measured plasma braking in the presence of this mode compares well with neo-classical toroidal viscosity theory. In support of basic physics understanding, high resolution Thomson scattering measurements are providing new insight into sawtooth crash dynamics and neo-classical tearing mode critical island widths. Retarding field analyser measurements show elevated ion temperatures in the scrape-off layer of L-mode plasmas and, in the presence of type-I ELMs, ions with energy greater than 500 eV are detected 20 cm outside the separatrix. Disruption mitigation by massive gas injection has reduced divertor heat loads by up to 70%.
    Nuclear Fusion 01/2011; 51(9). · 2.73 Impact Factor
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    ABSTRACT: 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<sub>e</sub> and <3% of n<sub>e</sub> in the range of 40 eV to 2 keV, for a density of n<sub>e</sub>=2×10<sup>19</sup> m <sup>-3</sup> . 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.
    Review of Scientific Instruments 01/2011; · 1.60 Impact Factor
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    ABSTRACT: 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.
    Plasma Physics and Controlled Fusion 11/2010; 52(12):124041. · 2.37 Impact Factor
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    ABSTRACT: 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.
    The Review of scientific instruments 10/2010; 81(10):10D520. · 1.52 Impact Factor
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    ABSTRACT: 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.
    The Review of scientific instruments 10/2010; 81(10):10D511. · 1.52 Impact Factor
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    ABSTRACT: 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.
    The Review of scientific instruments 10/2010; 81(10):10D521. · 1.52 Impact Factor
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    ABSTRACT: 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.
    Plasma Physics and Controlled Fusion 09/2009; 51(10):105002. · 2.37 Impact Factor
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    ABSTRACT: Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks (STs). Using the increased heating capabilities P-NBI <= 3.8 MW H-mode at I-P = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I-P and more strongly with B-t than in the ITER IPB98(y, 2) scaling. Measurements of the fuel retention of shallow pellets extrapolate to an ITER particle throughput of 70% of its original designed total throughput capacity. The anomalous momentum diffusion, chi(phi), is linked to the ion diffusion, chi(i), with a Prandtl number close to P-phi approximate to chi(phi)/chi(i) approximate to 1, although chi(i) approaches neoclassical values. New high spatial resolution measurements of the edge radial electric field, E-r, show that the position of steepest gradients in electron pressure and E-r (i.e. shearing rate) are coincident, but their magnitudes are not linked. The T-e pedestal width on MAST scales with root beta(ped)(pol) rather than rho(pol). The edge localized mode (ELM) frequency for type-IV ELMs, new in MAST, was almost doubled using n = 2 resonant magnetic perturbations from a set of four external coils (n = 1, 2). A new internal 12 coil set (n <= 3) has been commissioned. The filaments in the inter-ELM and L-mode phase are different from ELM filaments, and the characteristics in L-mode agree well with turbulence calculations. A variety of fast particle driven instabilities were studied from 10 kHz saturated fishbone like activity up to 3.8 MHz compressional Alfven eigenmodes. Fast particle instabilities also affect the off-axis NBI current drive, leading to fast ion diffusion of the order of 0.5 m(2) s(-1) and a reduction in the driven current fraction from 40% to 30%. EBW current drive start-up is demonstrated for the first time in a ST generating plasma currents up to 55 kA. Many of these studies contributed to the physics basis of a planned upgrade to MAST.
    Nuclear Fusion. 01/2009; 49(10):104017.
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    ABSTRACT: 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° to 120° . 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.
    Review of Scientific Instruments 11/2008; · 1.60 Impact Factor
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    ABSTRACT: The first measurements of the structure of the edge radial electric field, Er, in a spherical tokamak (MAST) are presented. Using active Doppler spectroscopy on He+ with 120 lines of sight Er 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 vHe+θ ≈ -20 km/s, and a deep negative electric field well Ermin -15 kV/m develops. The profile form is dominated by the diamagnetic term.
    Journal of Physics Conference Series 08/2008; 123(1):012005.
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    ABSTRACT: Studies of the pedestal characteristics and quantities determining edge-localized mode (ELM) energy losses in MAST are presented. High temperature pedestal plasmas have been achieved which have collisionalities one order of magnitude lower than previous results . A stability analysis performed on these plasmas shows them to be near the ballooning limit. The fraction of pedestal energy released by an ELM as a function of collisionality on MAST is consistent with data from other devices. The evolution of the filamentary structures observed during ELMs has been studied and has shown that they exist near to the last closed flux surface for the time over which the majority of particles and energy are being released from the pedestal region into the scrape off layer. A simple model has been developed, which is in reasonable agreement with the observed ELM energy losses and target profiles.
    Plasma Physics and Controlled Fusion 07/2007; 49(8):1259. · 2.37 Impact Factor
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    ABSTRACT: Substantial advances have been made on the Mega Ampère Spherical Tokamak (MAST). The parameter range of the MAST confinement database has been extended and it now also includes pellet-fuelled discharges. Good pellet retention has been observed in H-mode discharges without triggering an ELM or an H/L transition during peripheral ablation of low speed pellets. Co-ordinated studies on MAST and DIII-D demonstrate a strong link between the aspect ratio and the beta scaling of H-mode energy confinement, consistent with that obtained when MAST data were merged with a subset of the ITPA database. Electron and ion ITBs are readily formed and their evolution has been investigated. Electron and ion thermal diffusivities have been reduced to values close to the ion neoclassical level. Error field correction coils have been used to determine the locked mode threshold scaling which is comparable to that in conventional aspect ratio tokamaks. The impact of plasma rotation on sawteeth has been investigated and the results have been well-modelled using the MISHKA-F code. Alfvén cascades have been observed in discharges with reversed magnetic shear. Measurements during off-axis NBCD and heating are consistent with classical fast ion modelling and indicate efficient heating and significant driven current. Central electron Bernstein wave heating has been observed via the O-X-B mode conversion process in special magnetically compressed plasmas. Plasmas with low pedestal collisionality have been established and further insight has been gained into the characteristics of filamentary structures at the plasma edge. Complex behaviour of the divertor power loading during plasma disruptions has been revealed by high resolution infra-red measurements.
    Nuclear Fusion. 01/2007; 47(10):S658-S667.
  • R. Scannell, M. J. Walsh, M. Dunstan
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    ABSTRACT: 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 1-20mus. 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.
    01/2007;
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    ABSTRACT: Important advances have been made on MAST, aided by substantial developments to plasma control, diagnostics and heating systems. The parameter range of the MAST confinement database has been extended and it now also includes pellet-fuelled discharges. Co-ordinated studies on MAST and DIII-D provide a strong link between the aspect ratio and beta scaling of H-mode energy confinement, consistent with that obtained when MAST data were merged with a subset of the ITPA database. Efficient pellet fuelling has been observed in H-mode discharges and post-pellet losses are low. Electron and ion ITBs are readily formed and their evolution has been investigated. Electron and ion thermal diffusivities have been reduced to values close to the ion neoclassical level. Non-linear GS2 calculations predict transport from the ETG mode at mid-radius in MAST H-mode comparable with experimental values. Error field correction coils have been used to determine the locked mode threshold scaling which is comparable with that in conventional tokamaks. The impact of plasma rotation on sawteeth has been investigated with co- and counter-NBI and the results have been well-modelled using the MISHKA-F code. The supra-Alfvénic ion population in MAST leads to a rich variety of fast particle driven instabilities. Their characteristics, beta dependence and impact on the fast ion population have been investigated. Off-axis NBCD and heating has been studied. Measurements are consistent with classical fast ion modelling and indicate efficient heating and significant driven current. Electron Bernstein wave heating has been observed via the O-X-B mode conversion process. Further advances in non-solenoid start-up techniques have been made. High pedestal temperature plasmas have been produced with collisionalities one order of magnitude lower than in previous MAST experiments. Pedestal widths in these plasmas agree better with banana orbit scalings and ELM losses are increased, consistent with the broad mode structures predicted by stability analyses. New measurements clearly show that ELM filaments persist for ∼200
    21st IAEA Fusion Energy Conference; 01/2006
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    ABSTRACT: 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°) 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 (ne). The spectrum of scattered light is significantly broader at large scattering angles, allowing diagnosis of lower plasma temperatures (Te) while using the same spectrometer design as the core system. The four Nd:YAG lasers follow two separate slightly offset (
    Review of Scientific Instruments 01/2006; 77(10). · 1.60 Impact Factor
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    ABSTRACT: Significant progress has been made on the Mega Ampere Spherical Tokamak (MAST) towards a fundamental understanding of transport, stability and edge physics and addressing technological issues for future large devices. Collaborative studies of the L–H transition with NSTX and ASDEX Upgrade confirm that operation in a connected double-null configuration significantly reduces the threshold power, Pthr. The MAST data provide support for a theory for the transition based on finite β drift wave turbulence suppression by self-generated zonal flows. Analysis of low and high field side density gradients in the H-mode pedestal provides support for an analytical model of the density pedestal width dependent on the neutral penetration depth. Adding MAST data to international confinement databases has enhanced confidence in scalings for ITER by significantly expanding the range of β and ε explored and indicates a slightly stronger ε dependence than in current scalings. Studies of core transport have been conducted for well-diagnosed L-mode, H-mode and internal transport barrier (ITB) discharges using TRANSP, and microstability and turbulence studies have been carried out using GS2. Linear micro-stability analysis indicates that ITG modes are typically unstable on all flux surfaces with growth rates that are comparable to the equilibrium E × B flow shearing rate. Mixing length estimates of transport coefficients from ITG (neglecting flow shear) give diffusion coefficients that are broadly comparable with observed thermal diffusivities. Non-linear, collisionless ETG calculations have been performed and suggest radially extended electrostatic streamers up to 100ρe across in radius. Transport from ITG could easily be suppressed in regions where the E × B shear flow rate, ωSE, exceeds the ITG growth rate, possibly contributing to ITBs. Toroidal rotation, driven by neutral beam torque, is the dominant contribution to ωSE via the vBθ term in the radial electric field. Early edge localized mode activity on MAST is associated with the formation of narrow filamentary structures following field lines in the edge. These filaments rotate toroidally with the edge plasma and, away from the X-points, accelerate radially outwards from the edge up to 20 cm. Studies of disruptions on MAST demonstrate a complex evolution of core energy loss and resultant divertor power loads, including phases where the target heat flux width is broadened by a factor of 8. Observations of energetic particle modes driven by super-Alfvénic beam ions provide support for a model for the non-linear evolution of toroidal Alfvén eigenmodes (AEs) forming Bernstein–Green–Krushal waves. The AE activity reduces to low levels with increasing β. Plasma start-up without a central solenoid and in a manner compatible with future large spherical tokamak (ST) devices has been demonstrated using breakdown at a quadrupole magnetic null. Closed flux surface plasmas with peak plasma currents up to 370 kA have been generated and sustained for 0.3 s. New error field correction coils have extended the operational space for low density plasmas and enabled scaling studies of error field induced locked mode formation in the ST.
    Nuclear Fusion 10/2005; 45(10):S157. · 2.73 Impact Factor
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    ABSTRACT: Low aspect ratio plasmas in devices such as the mega ampere spherical tokamak (MAST) are characterized by strong toroidicity, strong shaping and self fields, low magnetic field, high beta, large plasma flow and high intrinsic E × B flow shear. These characteristics have important effects on plasma behaviour, provide a stringent test of theories and scaling laws and offer new insight into underlying physical processes, often through the amplification of effects present in conventional tokamaks (e.g. impact of fuelling source and magnetic geometry on H-mode access). The enhancement of neoclassical effects makes MAST ideal for the study of particle pinch processes and neoclassical resistivity corrections, which can be assessed with unique accuracy. MAST data have an important influence on scaling laws for confinement and H-mode threshold power, exerting strong leverage on the form of these scaling laws (e.g. scaling with aspect ratio, beta, magnetic field, etc). The high intrinsic flow shear is conducive to transport barrier formation by turbulence suppression. Internal transport barriers are readily formed in MAST with both co- and counter-NBI, and electron and ion thermal diffusivities have been reduced to the ion neoclassical level. The strong variation in toroidal field (~ × 5 in MAST) between the inboard and outboard plasma edges, provides a useful test of edge models prompting, for example, a comparison of inboard and outboard scrape-off-layer transport to highlight magnetic field effects. Low aspect ratio plasmas are also an ideal testing ground for plasma instabilities, such as neoclassical tearing modes, edge localized modes (ELMs) and Alfvén eigenmodes, which are readily generated due to the supra-Alfvénic ion population. Examples of how MAST is providing new insights into such instabilities (e.g. ELM structure) are described.
    Plasma Physics and Controlled Fusion 11/2004; 46(12B):B477. · 2.37 Impact Factor
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    ABSTRACT: Both ruby laser (300 points) and NdYAG laser (19 points) Thomson scattering systems are used on MAST. Fast nonlinear optical switching shunts laser beams from the four 50 Hz NdYAG lasers to obtain coaxial plasma illumination. The technique allows for future expandability to many laser systems. The ruby laser system is used in parallel. A broadband thin-plate polariser, based on nanotechnology, has been incorporated in the collection optics. It has a wide field of view and it almost halves the detected laser stray light and plasma background for both laser spectral regions. This allows much of the collection optics to function effectively in both spectral regions simultaneously.
    Review of Scientific Instruments 10/2004; 75(10):3909-3911. · 1.60 Impact Factor