Publications (8)6.52 Total impact
-
Article: Overview of physics results from MAST
[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. -
Article: Overview of physics results from MAST
[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. -
Conference Proceeding: Overview of physics results from MAST
[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 ∼20021st IAEA Fusion Energy Conference, Chengdu, China; 01/2006 -
Article: Overview of MAST results
[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 -
Conference Proceeding: Plasma current formation in MAST without use of central solenoid
[show abstract] [hide abstract]
ABSTRACT: Designs of a Spherical Tokamak Component Test Facility (CTF) or ST Power Plant feature high plasma currents but a central solenoid is not possible due to the high neutron flux (there being no space for an effective shield). However the design and the equilibrium properties of an ST lend themselves to novel techniques both for obtaining an initial current, and for ramping it up to the operational value. Two methods of initial start-up which do not require use of the central solenoid have been tested on MAST, and are described in this paper. The merging-compression (M-C) scheme, pioneered on START, is routinely used on MAST and can generate ST plasmas of 0.4MA (Section 1). The novel ‘Double Null Merging’ method (DNM) also appears very promising and plasma current of 340kA has been generated on MAST (Section 2).32nd EPS Conference on Plasma Physics, Tarragona; 06/2005 -
Article: MAST and the impact of low aspect ratio on tokamak physics
[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 -
Article: Electron Bernstein Wave Heating Experiments on MAST
[show abstract] [hide abstract]
ABSTRACT: Burning plasma spherical tokamak (ST) designs rely on off-axis current drive and non-solenoid start-up techniques. Electron Bernstein waves (EBW) may provide efficient off-axis heating and current drive (CD) in high density ST plasmas. EBW may also be used in the plasma start-up phase due to the fact that EBW absorption and CD efficiency remain high even in relatively cold plasmas. EBW studies on the Mega Amp Spherical Tokamak (MAST) can be subdivided into four separate subjects: thermal EC emission observations from overdense plasmas; EBW modeling; proof-of-principle EBW heating experiments with the existing 60 GHz gyrotrons; EBW assisted plasma start-up at 28 GHz. These studies are also aimed at determining the potential for a high power EBW system for heating and CD on MAST; the optimum choices for the frequency and launch configuration of such as system are key issues. A major advance in the integrated modelling of EBW, involving a suite of codes, has been achieved. The EBW excitation in the plasma is first considered as a full wave 1D mode-coupling problem in slab geometry. Then propagation of the EBW is computed using an EBW ray-tracing code, which implements the fully electromagnetic, hot plasma dispersion function. The ray tracing data are then used in the BANDIT code in a self-consistent, relativistic 3D Fokker-Planck treatment to calculate the heating and driven current profiles. The modelling shows that the operating frequency for efficient EBW heating and CD on MAST should be in the range of the fundamental EC resonance or its lower harmonics, with the best frequency choice for the high power RF source in the range 16–30 GHz. Proof-of-principle EBW heating experiments have been conducted on MAST using the existing 60 GHz, 1 MW complex. EBW heating effects have clearly been observed for the first time in an ST. A 28 GHz EBW start-up system (200 kW, 40 ms) is being commissioned, and according to our modelling should generate plasma current up to ~100 kA during the plasma start-up phase giving the prospect of a fully non-inductive plasma start-up scenario. -
Article: Parametric decay instability accompanying electron Bernstein wave heating in MAST
Top Journals
Institutions
-
2004–2005
-
Culham Centre for Fusion Energy
Abingdon, ENG, United Kingdom
-
