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A. Kirk,
E. Nardon,
R. Akers,
M. Bécoulet,
G. De Temmerman,
B. Dudson,
B. Hnat,
Y.Q. Liu, R. Martin,
P. Tamain,
D. Taylor,
the MAST team
[show abstract]
[hide abstract]
ABSTRACT: Experiments have been performed on MAST using both external (n = 1, 2) and internal (n = 3) resonant magnetic perturbation (RMP) coils. ELM suppression has not been achieved even though vacuum modelling shows that either set of coils can produce a region (ΔΨpol > 0.17), for which the Chirikov parameter is greater than 1, wider than that correlated with ELM suppression in DIII-D. Although complete ELM suppression has not been achieved, application of RMPs has triggered ELMs in ELM free H-mode periods (n = 3) and increased the ELM frequency in regularly ELM-ing discharges (n = 2, 3). In addition, the application of RMPs in an n = 3 configuration has produced large changes to the edge turbulence in L-mode discharges.
Nuclear Fusion 02/2010; 50(3):034008. · 4.09 Impact Factor
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H. Meyer,
R. J. Akers,
F. Alladio,
L. C. Appel,
K. B. Axon,
N. Ben Ayed,
P. Boerner,
R. J. Buttery,
P. G. Carolan,
D. Ciric, [......],
G. Voss,
M. J. Walsh,
S. E. V. Warder,
R. Watkins,
H. R. Wilson,
M. Windridge,
M. Wisse,
A. Zabolotski,
S. Zoletnik,
O. Zolotukhin
[show abstract]
[hide abstract]
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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M.P. Gryaznevich,
S.E. Sharapov,
M. Lilley,
S.D. Pinches,
A.R. Field,
D. Howell,
D. Keeling, R. Martin,
H. Meyer,
H. Smith,
R. Vann,
P. Denner,
E. Verwichte,
the MAST Team
[show abstract]
[hide abstract]
ABSTRACT: The developments of advanced tokamak scenarios as well as the employment of a new neutral beam injection (NBI) source with higher power and beam energy up to ≈65 keV have significantly broadened the frequency range and the variety of Alfvén eigenmodes (AEs) excited by the super-Alfvénic NBI on the spherical tokamak MAST. During recent experiments on MAST, several distinct classes of beam-driven AEs have been identified, with different modes being most unstable in different MAST scenarios. In MAST discharges with elevated monotonic q(r)-profiles and NBI power ≥3 MW, chirping modes starting in the frequency range ≤150 kHz decreased in frequency down to ≈20 kHz as q(0) decreased and then smoothly transformed to long-living modes with a weakly-varying frequency and a n = 1 kink-mode structure. The bolometer data suggest that the long-living modes can be responsible for fast ion losses on MAST, while the charge-exchange data show that a coupling between these modes and other low-frequency modes can cause a collapse of toroidal plasma rotation with a subsequent disruption. In MAST discharges with reversed magnetic shear, Alfvén cascade eigenmodes in the frequency range 40–180 kHz were observed at a moderate NBI power ≤2 MW allowing an additional assessment of q(r)-profile evolution in time. A robust reproducible scenario was found on MAST, in which the instability of high-frequency modes in the range 0.4–3.8 MHz and typically with negative toroidal mode numbers was dominating the spectrum of beam-driven AEs. Since the highest frequency of such modes is close to the on-axis ion cyclotron frequency and the polarization study of these modes show a significant parallel perturbed magnetic field, these modes are identified as compressional Alfvén eigenmodes. For investigating the AE spectrum in plasmas with high β, an active AE antenna has been installed on MAST. First measurements of stable AE modes in MAST have been performed successfully and are described here.
Nuclear Fusion 06/2008; 48(8):084003. · 4.09 Impact Factor
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B. Lloyd,
R. J. Akers,
F. Alladio,
Y. Andrew,
L. C. Appel,
D. Applegate,
K. B. Axon,
N. Ben Ayed,
C. Bunting,
R. J. Buttery, [......],
J. Urban,
M. Valovic,
R. G. L. Vann,
F. Volpe,
G. Voss,
M. J. Walsh,
S. E. V. Warder,
R. Watkins,
H. R. Wilson,
M. Wisse
[show abstract]
[hide abstract]
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.
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A Kirk,
N Ben Ayed,
G Counsell,
B Dudson,
T Eich,
A Herrmann,
B Koch, R Martin,
A Meakins,
S Saarelma,
R Scannell,
S Tallents,
M Walsh,
H R Wilson,
the MAST team
[show abstract]
[hide abstract]
ABSTRACT: The boundary of the tokamak core plasma, or scrape-off layer, is normally characterized in terms of average parameters such as density, temperature and e-folding lengths suggesting diffusive losses. However, as is shown in this paper, localized filamentary structures play an important role in determining the radial efflux in both L mode and during edge localized modes (ELMs) on MAST. Understanding the size, poloidal and toroidal localization and the outward radial extent of these filaments is crucial in order to calculate their effect on power loading both on the first wall and the divertor target plates in future devices. The spatial and temporal evolution of filaments observed on MAST in L-mode and ELMs have been compared and contrasted in order to confront the predictions of various models that have been proposed to predict filament propagation and in particular ELM energy losses.
Plasma Physics and Controlled Fusion 11/2006; 48(12B):B433. · 2.42 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Edge-localized modes (ELMs) are repetitive instabilities that occur in the outer region of tokamak plasmas. This Letter provides new information on and the implications of the evolution of the filament structures observed during ELMs in the MAST tokamak. The filaments exist for the time over which particles are being released into the scrape off layer. They start off at the plasma edge rotating at the velocity of the pedestal, and then decelerate toroidally and accelerate radially outwards. As the filaments propagate radially they remain aligned with the local magnetic field line.
Physical Review Letters 06/2006; 96(18):185001. · 7.37 Impact Factor
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B. Lloyd,
R. J. Akers,
F. Alladio,
Y. Andrew,
L. C. Appel,
D. Applegate,
K. B. Axon,
N. Ben Ayed,
C. Bunting,
R. J. Buttery, [......],
J. Urban,
M. Valovic,
R. G. L. Vann,
F. Volpe,
G. Voss,
M. J. Walsh,
S. E. V. Warder,
R. Watkins,
H. R. Wilson,
M. Wisse
[show abstract]
[hide abstract]
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, Chengdu, China; 01/2006
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G.F. Counsell,
R.J. Akers,
L.C. Appel,
D. Applegate,
K.B. Axon,
Y. Baranov,
C. Brickley,
C. Bunting,
R.J. Buttery,
P.G. Carolan, [......],
M. Valovic,
F. Volpe,
G. Voss,
M.J. Walsh,
J.R. Watkins,
H.R. Wilson,
M. Wisse,
the MAST,
NBI,
ECRH Teams
[show abstract]
[hide abstract]
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. · 4.09 Impact Factor
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B Lloyd,
J-W Ahn,
R J Akers,
L C Appel,
D Applegate,
K B Axon,
Y Baranov,
C Brickley,
C Bunting,
R J Buttery, [......],
M Valovic,
F Volpe,
G Voss,
M J Walsh,
J R Watkins,
H R Wilson,
M Wisse,
the MAST,
NBI,
ECRH Teams
[show abstract]
[hide abstract]
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. · 2.42 Impact Factor
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A Kirk,
G F Counsell,
H R Wilson,
J-W Ahn,
R Akers,
E R Arends,
J Dowling, R Martin,
H Meyer,
M Hole,
M Price,
P B Snyder,
D Taylor,
M J Walsh,
Y Yang,
the MAST team
[show abstract]
[hide abstract]
ABSTRACT: Edge localized mode (ELM) characteristics in a large spherical tokamak (ST) with significant auxiliary heating are explored. High confinement is achieved in mega ampere spherical tokamak (MAST) at low ELM frequencies even though the ELMs exhibit many type III characteristics. These ELMs are associated with a reduction in the pedestal density but no significant change in the pedestal temperature or temperature profile, indicating that energy is convected from the pedestal region into the scrape-off layer. Power to the targets during an ELM arrives predominantly at the low field outboard side. ELM effluxes are observed up to 20 cm from the plasma edge at the outboard mid-plane and are associated with the radial motion of a feature at an average velocity of 0.75 km s−1. The target balance observed in MAST is potentially rather favourable for the ST since H-mode access is facilitated in a regime where ELM losses flow mostly to the large wetted area, outboard targets and, in addition, the target heat loads are reduced by an even distribution of power between the upper and lower targets.
Plasma Physics and Controlled Fusion 02/2004; 46(3):551. · 2.42 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Results from MAST provide a first test of neoclassical tearing mode physics in the spherical tokamak (ST). The mode accounts for the main performance limit in conventional tokamaks. Its behavior in the ST is remarkably well described by existing theoretical models, although it is more readily seeded by sawtooth events in these scenarios. Modeling confirms the significance of stabilizing field-curvature effects. This provides good grounds for optimism that with suitable control of profiles, it may be possible to avoid these modes in the ST.
Physical Review Letters 04/2002; 88(12):125005. · 7.37 Impact Factor
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R J Akers,
G F Counsell,
A Sykes,
L C Appel,
E R Arends,
C Byrom,
P G Carolan,
N J Conway,
G Cunningham,
A Dnestrovskij, [......],
A Kirk,
S Korsholm, R Martin,
H Meyer,
M P S Nightingale,
C M Roach,
V Shevchenko,
M Tournianski,
M J Walsh,
C D Warrick
[show abstract]
[hide abstract]
ABSTRACT: H-mode plasmas have been achieved on the MAST spherical tokamak at input power considerably higher than predicted by conventional threshold scalings. Following L- H transition, a clear improvement in energy confinement is obtained, exceeding recent international scalings even at densities approaching the Greenwald density limit. Transition is accompanied by an order-of-magnitude increase in edge-density gradient, a marked decrease in turbulence, the efficient conversion of internal electron Bernstein waves into free space waves, and the onset and saturation of edge poloidal rotation.
Physical Review Letters 02/2002; 88(3):035002. · 7.37 Impact Factor
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A. Sykes,
J.-W. Ahn,
R.J. Akers,
E. Arends,
K. Axon,
R.J. Buttery,
C. Byrom,
P.G. Carolan,
D. Ciric,
N.J. Conway, [......],
V. Shevchenko,
K. Stammers,
A. Tabasso,
D. Taylor,
M. Tournianski,
M. Valovic,
G.M. Voss,
M.J. Walsh,
S.E.V. Warder,
H.R. Wilson
[show abstract]
[hide abstract]
ABSTRACT: The MAST (Mega-Amp Spherical Tokamak) experiment has been operational since Jan 2000. Results from MAST are important both in evaluating the potential of future ST fusion devices, and in developing understanding of processes relevant to conventional aspect ratio tokamaks. In this paper methods of initiating, ramping up, and sustaining the high plasma currents associated with ST fusion devices are discussed. New physics results, including the effectiveness of inboard gas puffing, the threshold power required for the L-H transition, and the 'natural' divertor configuration are given. Plans for improvements during 2002 are outlined.
Fusion Engineering, 2002. 19th Symposium on; 02/2002
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[show abstract]
[hide abstract]
ABSTRACT: The mega-ampere spherical tokamak (MAST) experiment is a new, large, low aspect ratio device (R=0.7–0.8 m, a=0.5–0.65 m, maximum BT ∼ 0.63 T at R=0.7 m) operating its first experimental physics campaign. Designed to study a wide variety of plasma shapes with up to 2 MA of plasma current with an aspect ratio down to 1.3, the poloidal field (PF) coils used for plasma formation, equilibrium and shaping are inside the main vacuum vessel. For plasma control and to investigate a wide range of plasma phenomena, an extensive set of magnetic diagnostics have been installed inside the vacuum vessel. More than 600 vacuum compatible, bakeable diagnostic coils are configured in a number of discrete arrays close to the plasma edge with about half the coils installed behind the graphite armour tiles covering the center column. The coil arrays measure the toroidal and poloidal variation in the equilibrium field and its high frequency fluctuating components. Internal coils also measure currents in the PF coils, plasma current, stored energy and induced currents in the mechanical support structures of the coils and graphite armour tiles. The latter measurements are particularly important when halo currents are induced following a plasma termination, for example, when the plasma becomes vertically unstable. The article describes the MAST magnetic diagnostic coil set and their calibration. The way in which coil signals are used to control the plasma equilibrium is described and data from the first MAST experimental campaign presented. These coil data are used as input to the code EFIT [L. Lao et al., Nucl. Fusion 25, 1611 (1985)], for measurement of halo currents in the vacuum vessel structure and for measurements of the structure of magnetic field fluctuations near the plasma edge. © 2001 American Institute of Physics.
Review of Scientific Instruments 12/2000; 72(1):421-425. · 1.37 Impact Factor
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C Ribeiro,
R Akers,
F Alladio,
K Axon,
L Baylor,
G.F Counsell,
J Dowling,
S Fielding,
L Garzotti,
M Gryaznevich, [......], R Martin,
P Micozzi,
B Sass,
A Sykes,
D Terranova,
T.N Todd,
P Twynam,
M Wakatani,
M.J Walsh,
S You
[show abstract]
[hide abstract]
ABSTRACT: Cryogenic pellet injection has been used in START and it is under commissioning for MAST for fuelling and diagnostics. In START NBI, heated plasmas fuelled with deuterium pellets, high normalized density (Greenwald number NG≤1.1) with good energy confinement time (τE/τEelmfITER97≤1.3) both at high beta (βT≤25%, βN≤4.9) have been obtained for time scales close to the energy confinement time (∼τE). In addition, high fuelling efficiency (∼100%) have also been simultaneously observed. A complete pellet injection programme is currently under development in MAST, including the commissioning of a eight pellet injector, technical aspects of launching pellets through guide tubes, dedicated diagnostics, and modelling.
Fusion Engineering and Design.
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[show abstract]
[hide abstract]
ABSTRACT: This is a preprint of a paper intended for presentation at a scientific meeting. Because of the provisional nature of its content and since changes of substance or detail may have to be made before publication, the preprint is made available on the understanding that it will not be cited in the literature or in any way be reproduced in its present form. The views expressed and the statements made remain the responsibility of the named author(s); the views do not necessarily reflect those of the government of the designating Member State(s) or of the designating organization(s). In particular, neither the IAEA nor any other organization or body sponsoring this meeting can be held responsible for any material reproduced in this preprint.
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B. Lloyd,
J. W. Ahn,
R. J. Akers,
L. C. Appel,
E. R. Arends,
K. B. Axon,
R. J. Buttery,
C. Byrom,
P G Carolan,
C. Challis, [......],
M. R. Tournianski,
G. Turri,
M. Valovic,
G. Voss,
M J Walsh,
S. Warder,
J. R. Watkins,
H. R. Wilson,
Y Yang,
S. You
[show abstract]
[hide abstract]
ABSTRACT: The low aspect ratio of the mega amp spherical tokamak (MAST) allows differentiation between different forms of the H-mode threshold scaling. With optimized fuelling using inboard puffing, and a connected double null divertor (DND) magnetic configuration, the H-mode power threshold data lie about 1.7 times higher than recent scaling laws. Slight magnetic configuration changes, of the order of the ion Larmor radius, around a connected DND significantly influence H-mode access. H-mode confinement in discharges with low frequency edge localized modes (ELMs) is generally consistent with international scaling laws, e.g. IPB98(y,2). Strong indications of both particle and energy internal transport barriers have been seen. Normalized beta values beta(N) > 5 have been obtained, approaching the ideal n = I no wall external kink stability limit. Sawtooth triggered neo-classical tearing modes have been observed; numerical modelling of the island evolution reproduces mode behaviour well and confirms the significance of stabilizing field curvature effects. Divertor power loading studies, including transient effects due to ELMs, show a strong bias of power efflux to the outboard targets, where it is more easily handled. ELM energy losses, DeltaW(ELM), are less than 4% of the stored energy in all regimes explored so far, but ELM effluxes extending 30 cm outside the outboard separatrix have been measured. Toroidally asymmetric divertor biasing resulted in significant broadening of the D-alpha profile on the biased components and a reduction in the total power to the unbiased components. Halo current magnitudes and asymmetries are generally small compared with conventional tokamaks; recent measurements show that the plasma behaves more as a voltage source than a current source. Initial neutral beam current drive experiments indicate non-inductively driven current values (I-NBI similar to 0.3I(p)) comparable with code predictions.
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R. J. Buttery,
R. Akers,
E. Arends,
N. J. Conway,
G. F. Counsell,
G. Cunningham,
C. G. Gimblett,
M. Gryaznevich,
R. J. Hastie,
M. J. Hole,
I. Lehane, R Martin,
A Patel,
T. Pinfold,
O. Sauter,
D. Taylor,
G. Turri,
M. Valovic,
M J Walsh,
H. R. Wilson
[show abstract]
[hide abstract]
ABSTRACT: The development of reliable H-modes on MAST, together with advances in heating power and a range of high spatial resolution diagnostics, has provided a platform to enable MAST to address some of the most important issues of tokamak stability. In particular the high beta potential of the spherical tokamak is highlighted with stable operation at beta(N) similar to 5-6, beta(T) similar to 16% and beta(p) up to similar to2. Magnetic diagnostic evaluation of the global beta parameters is independently confirmed by kinetic profile data. Calculations indicate that the beta(N) values are in the vicinity of no-wall stability limits. Studies of neoclassical tearing modes (NTMs) have been extended to explore their effects and develop avoidance strategies. Experiments have demonstrated that sawteeth play a strong role in triggering NTMs-by avoiding large sawteeth a much higher beta(N) value has been reached. The significance of NTMs is confirmed, with large islands observed using the 300 point Thomson scattering diagnostic, and locking of large n = 1 modes frequently leading to disruptions, which become more rapid at low q(95). The role of error fields has been explored. H-mode plasmas are also limited by edge localized modes (ELMs), with confinement degraded as the ELM frequency rises. However, in contrast to the conventional tokamak, the ELMs in high performing regimes on MAST (H-IPB98Y2 similar to 1) appear to be type III in nature. Modelling using the ELITE code, which incorporates finite n corrections, identifies instability to peeling modes, consistent with a type III interpretation. It also shows considerable scope to raise pressure gradients before ballooning type modes (perhaps associated with type I ELMs) occur. The calculations show that narrow pedestals can support much stronger pressure gradients than might be expected from simple n = infinity ballooning calculations. Finally sawteeth are shown to degrade confinement by similar to10-15% in particular cases examined. They are observed not to remove the q = 1 surface in the cases where snakes are present-various physics models of the sawteeth are now being explored. Thus research on MAST is not only demonstrating stable operation at high performance levels and developing methods to control instabilities; it is also providing detailed tests of the stability physics and models applicable to conventional tokamaks, such as ITER.
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S. E. Sharapov,
B. Alper,
F Andersson,
Y. F. Baranov,
H. L. Berk,
L. Bertalot,
D. Borba,
C. Boswell,
B. N. Breizman,
R. Buttery, [......],
T. Pinfold,
S. V. Popovichev,
P. Sandquist,
D Stork,
D. Testa,
A. Tuccillo,
I. Voitsekhovich,
V. Yavorskij,
N. P. Young,
F. Zonca
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ABSTRACT: In preparation for next step burning plasma devices such as ITER, experimental studies of instabilities and confinement of energetic ions were performed on Joint European Torus (JET) and on Mega-Amper Spherical Tokamak (MAST) with innovative diagnostic techniques, in conventional and shear-reversed plasmas, exploring a wide range of effects for energetic ions. A compendium of recent results testing capabilities of the present-day facilities for burning plasma relevant study is presented in this paper. 'Alpha tail' production using 3rd harmonic ion-cyclotron resonance heating (ICRH) of He-4 beam ions has been employed on JET for studying He-4 of the megaelectronvolt energy range in a 'neutron-free' environment. The evolution of ICRH-accelerated ions of He-4 with E >= 1.7 MeV and D with E >= 500 keV was assessed from nuclear gamma-ray emission born by the fast ions colliding with Be and C impurities. A simultaneous measurement of spatial profiles of fast He-4 and fast D ions relevant to ITER was performed for the first time in positive and strongly reversed magnetic shear discharges. Time-resolved gamma-ray diagnostics for ICRH-accelerated He-3 and H minority ions allowed changes in the fast ion distribution function to be assessed in the presence of unstable toroidal Alfven eigenmodes (TAEs) and sawteeth. A significant decrease of gamma-ray intensity from protons with E >= 5 MeV was detected during the 'tornado' modes. This was interpreted as 'tomado'-induced loss of fast ions with the drift orbit width, Delta(f), comparable to the minor radius of tokamak a. Experiments performed in the opposite case, Delta f/a << 1, for ICRH-accelerated He-3 ions with E >= 500 keV, have shown excitation of numerous Alfven eigenmodes without a significant degradation of the fast ion confinement. The stabilizing effect of fast particles on 'monster' sawteeth was experimentally found to fail in low-density plasmas with high power ion cyclotron resonance frequency (ICRF)-heating. The transition from the 'monster' to short-period 'grassy' sawteeth was investigated with different ICRF phasing, which controls the pinch-effect and radial distribution of ICRF-accelerated ions. Instabilities excited by super-Alfvenic beam ions were investigated on the spherical tokamak MAST. Due to higher values of beta and a higher proportion of fast ions on MAST than on JET, a wider variety of modes and nonlinear regimes for the Alfven instabilities were observed, including the explosive TAE-regimes leading to the formation of hole-clump pairs on the fast ion distribution function. The MAST and START data showed that TAE and chirping modes decrease both in their mode amplitudes and in the number of unstable modes with increasing beta.
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A. Sykes,
G.W. Crawford,
G. Cunningham,
N.A. Fawlk,
D.H.J. Goodall,
M. Gryaznevich,
J. Hugill,
I. Jenkins, R. Martin,
C. Ribeiro,
D.C. Robinson,
R.T.C. Smith,
T.N. Todd,
M.J. Walsh,
B.J. Ward
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ABSTRACT: START continues to provide key results on the `spherical tokamak'
concept. This paper outlines the engineering solutions adopted to
overcome problems of space restrictions in a very low aspect ratio
tokamak, whilst providing high performance at low cost
Fusion Engineering, 1995. SOFE '95. 'Seeking a New Energy Era'., 16th IEEE/NPSS Symposium;
