Amp Spherical Tokamak Research Articles

MAST and the impact of low aspect ratio on tokamak physics

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.

Z eff profile measurements from bremsstrahlung imaging in the MAST spherical tokamak

A bremsstrahlung diagnostic has been deployed on the Mega Ampere Spherical Tokamak (MAST), with matrix viewing of the full plasma cross section in a narrow region of the continuum free of spectral lines. An uncooled CCD detector is used to provide 100 Hz 128×128 matrix viewing of the plasma emissivity which is spectrally selected using an interference filter of only ∼0.5 nm bandwidth. The apparently conflicting demands of a large field of view for the plasma and the small solid angle for narrow spectral bandwidth is accommodated in a compact lens, filter, and detector arrangement. Using measured electron temperature and density profiles the Zeff profile evolution is determined.

High definition imaging in the Mega Amp Spherical Torus spherical tokamak from soft x rays to infrared (invited)

The Mega Amp Spherical Torus (MAST) diagnostic needs are strongly influenced by physics goals that often require diagnostic integration and cross-mapping, especially in fine-scale investigations, such as transport barriers. Conversely, the unrivalled viewing access to the edge, scrape-off layer (SOL) and divertor regions, provided by the MAST open geometry, impacts on the physics program priorities. A supporting suite diagnostics, such as the high definition Thomson scattering systems, provide considerable added value in detailed data interpretation (e.g., bremsstrahlung emissivity in terms of Zeff). Thus, to exploit these advantages, an extensive set of high-resolution imaging diagnostics have been installed, encompassing soft x-rays, visible bremsstrahlung, charge exchange recognisation radiation, Dα from NBI, and edge plasma neutrals, and infrared (IR) from the divertor and wall regions. Plasma light collection optics provide near parallel illumination of narrow bandpass interference filters to give monochromatic images. One adaptation provides multiwavelength images; another accommodates smooth variation of wavelength across an image (e.g., for a range of Doppler shifts beam fast neutrals). Diagnostic synergy is enhanced by combining such diagnostics to common viewing optics which allow exact-mapping. Soft x-ray tangential imaging has been achieved by using a two dimensional charge coupled device detector in a pinhole camera. Finally, a fast IR camera monitors the power deposition on the first wall and divertor plates, important in quantifying power losses (e.g., ELMs, disruptions), and complemented by visible viewing of the SOL, and linear Dα cameras.

Enhanced microchannel plate performance at high particle fluxes by pulsed exposure mode of operation

Operating a microchannel plate (MCP) in saturated mode provides a simple photon and particle counting detector. However, there is a finite recovery time after individual events during which individual microchannels no longer respond, reducing the overall sensitivity. At continuous high flux levels, the corrections from measured to true flux become increasingly large as the fraction of live microchannels rapidly decays to low values. Gating the flux arriving at the MCP greatly increases the proportion of live microchannels allowing periodic measurements to be made that accommodate high fluxes and associated low errors. Such improvements have been observed in a neutral particle analyzer on the Mega Ampere Spherical Tokamak. A simple analytical treatment accounts for the measured improvements.

ELM characteristics in MAST

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.

Overview of recent experimental results on MAST

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 βN > 5 have been obtained, approaching the ideal n = 1 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, Δ WELM, 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 Dα 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 (INBI ∼ 0.3Ip) comparable with code predictions.

Analysis of the progress to detachment in the divertor of the MAST tokamak

Power handling and erosion of the divertor target plates are critical issues in the design of a next step fusion experimental reactor. Several steps can be taken to mitigate the effect of the plasma interaction with the divertor target, one of the most effective is plasma detachment. In MAST (Mega Ampere Spherical Tokamak) a series of experiments were conducted which aimed to achieve detachment for the first time on a spherical tokamak with rather open divertor geometry. This paper presents the results of these experiments, concentrating on a series of L-mode discharges, additionally heated with 750 kW of NB injection, for which the degree of detachment [Nucl. Fusion 38 (1998) 331] is calculated and discussed. Evidence for deuterium volume recombination in the lower outer divertor is shown and used to estimate with greater accuracy T e (overestimated by the Langmuir probes for detached plasmas) in the divertor region.

Combined visible and infrared Thomson scattering on the MAST experiment

A dual laser Thomson scattering system is implemented in the Mega Amp Spherical Tokamak (MAST). The complementary features of each approach are exploited. One system is ideally suited to measuring detailed profile behavior across the plasma while the other is ideally suited though not exclusively to studying fast changing events and general characterization of the plasma. Both systems are designed to be flexible and allow future expansion to suit the needs of a changing physics program.

A review of plasma boundary phenomena in the mega ampere spherical tokamak

Enhanced diagnostics and an expanded operational space in Mega Ampère Spherical Tokamak (MAST) has lead to a wealth of data on the plasma boundary becoming available over the last year of operations and has significantly advanced the understanding of this region in the spherical tokamak and, more generally, is contributing to an improved understanding of the underlying plasma phenomena for this important region of the tokamak. Amongst the observations reported in this paper are that MAST is able to achieve simultaneous high confinement and high density with Type III ELMs for connected double-null discharges in which most (>93%) of the power efflux from the core observed at the targets (including during ELMs) arrives at the large wetted area outboard targets and is evenly distributed between the upper and lower divertor regions. Significant particle fluxes are reported well beyond the outboard separatrix during ELMs (up to 20 cm distant from the plasma) and appear to be associated with the radial expansion of a localised structure at around 1 km s −1. The result of preliminary experiments to broaden the SOL by asymmetric divertor biasing are also presented which show an encouraging, qualitative agreement with theory and demonstrate clear evidence for both SOL broadening and target power reduction.

Simulations of symmetric halo currents

It is proposed that halo current magnitude during vertical disruptions can be significantly affected by a number of strongly interacting processes, involving the generation of neutrals, their ionization and recombination, open-surface parallel losses, Ohmic heating due to the large back-emfs, the generation of impurities, and mixed-circuit effects. To investigate the balance of such processes we present and interpret results from one-dimensional multi-fluid transport simulations. The simulations show that, as expected, core plasma current is mainly lost due to the reduction of the core cross-sectional area rather than the thermal quench, and that large values of the safety factor tend to produce low poloidal halo current. In addition, it is seen that increasing the resistance of the external halo path suppresses halo current, as do impurities, naturally released from the divertor during the termination event. High values of the safety factor insulate the halo plasma from parallel transport effects, increasing electrical conductance and toroidal halo current. For mega ampere spherical tokamak device parameters, values of the poloidal halo current are of the order of 15% of the original plasma current.

Indirectly Heated Cathode Development for Long-Pulse Neutral Beam Injectors

Record beta and density values have been obtained at the Small Tight Aspect Ratio Tokamak in the United Kingdom Atomic Energy Agency (UKAEA) Fusion Culham Science Centre by using Oak Ridge National Laboratory's (ORNL's) neutral beam injector for plasma heating. This result has improved the prospects for a future spherical tokamak (ST) fusion core device. To address the physics issues of ST plasmas and the technology of neutral beam heating, ORNL neutral beam injectors have been installed on the Mega Amp Spherical Tokamak (MAST) at UKAEA Culham. The goal of the injectors is to provide a neutral beam heating power of 5 MW for 0.5 s, or up to 4 MW for 5 s. To achieve 5-s operation at the required power level of 4 MW, the existing oxide-filament cathode must be replaced with a cathode having long-pulse capability.In 1983 ORNL developed an advanced positive ion source having long-pulse capability for 50-A and 80-keV hydrogen ion beams. The indirectly heated cathode technology developed for the advanced positive ion source will be utilized to fulfill requirements of long-pulse neutral beam heating on MAST plasmas. The cathode utilizes an electron emitter made of lanthanum oxide (La2O3) doped molybdenum. The cathode is heated by a graphite heater and insulated by a heat shield. The heat shield is made of multiple layers of tantalum sheet. Details of design and performance of such long-pulse cathodes are reported and discussed.

The prospects for electron Bernstein wave heating of spherical tokamaks

Electron Bernstein waves are analyzed as possible candidates for heating spherical tokamaks. An inhomogeneous plane slab model of the plasma with a sheared magnetic field is used to calculate the linear conversion of the ordinary mode (O-mode) to the extraordinary mode (X-mode). A formula for the fraction of the incident O-mode energy which is converted to the X-mode at the O-mode cutoff is derived. This fraction is then able to propagate to the upper hybrid resonance where it is converted to the electron Bernstein mode. The damping of electron Bernstein waves at the fourth harmonic resonance, corresponding to a 60 GHz source on the Mega Amp Spherical Tokamak MAST [A. C. Darke et al., Proceedings of the 16th Symposium on Fusion Energy, Champaign-Urbana, Illinois (IEEE, Piscataway, NJ, 1995, Vol. 2, p. 1456)], is computed. For comparison, results are also presented for a lower frequency source, close to the fundamental electron cyclotron resonance. Both the fundamental and the fourth harmonic are shown to be so strongly absorbing that the electron Bernstein wave would be totally absorbed in the outer regions of the resonance. This feature implies that electron Bernstein wave current drive (on- or off-axis) could be very efficient.

The physics of spherical torus plasmas

Broad and important progress in plasma tests, theory, new experiments, and future visions of the spherical torus (ST, or very low aspect ratio tokamaks) have recently emerged. These have substantially improved our understanding of the potential properties of the ST plasmas, since the preliminary calculation of the ST magnetohydrodynamic equilibria more than a decade ago. Exciting data have been obtained from concept exploration level ST experiments of modest capabilities (with major radii up to 35 cm), making important scientific contributions to toroidal confinement in general. The results have helped approval and construction of new and/or more powerful ST experiments, and stimulated an increasing number of theoretical calculations of interest to magnetic fusion energy. Utilizing the broad knowledge base from the successful tokamak and advanced tokamak research, a wide range of new ST physics features has been suggested. These properties of the ST plasma will be tested at the 1 MA level with major radius up to ∼80 cm in the new proof of principle devices National Spherical Torus Experiment (NSTX, U.S.) [M. Peng et al., European Conf. Abst. 22C, 451 (1998); S. M. Kaye et al., Fusion Technol. 36, 16 (1999); M. Ono et al., “Exploration of Spherical Torus Physics in the NSTX Device,” 17th IAEA Fusion Energy Conf., paper IAEA-CN-69/ICP/01 (R), Yokohama, Japan (1998)], Mega Ampere Spherical Tokamak (MAST, U.K.) [A. C. Darke et al., Fusion Technol. 1, 799 (1995); Q. W. Morris et al., Proc. Int. Workshop on ST (Ioffe Inst., St. Petersburg, 1997), Vol. 1, p. 290], and Globus-M (R.F.) [V. K. Gusev et al., European Conf. Abst. 22C, 576 (1998)], which have just started full experimental operation. New concept exploration experiments, such as Pegasus (University of Wisconsin) [R. Fonck and the PEGASUS Team, Bull. Am. Phys. Soc. 44, 267 (1999)], Helicity Injected Tokamak-II (HIT-II, University of Washington) [T. R. Jarboe et al., Phys. Plasmas 5, 1807 (1998)], and Current Drive Experiment-Upgrade (CDX-U, Princeton Plasma Physics Laboratory) [M. Ono et al., Proceedings of the 16th International Conference on Fusion Energy (IAEA, Vienna, 1997), Vol. 2, p. 71] and other experiments in Japan and Brazil, etc., present additional opportunities for important progress. This tutorial paper summarizes our understanding and projections of the physics of the ST plasmas, the investigation of which will hopefully bring new enthusiasm and advancements for fusion energy science research in the U.S. and the world.

The Mega Amp Spherical Tokamak

Results from START at Culham have been very impressive and have triggered the construction of several new Spherical Tokamaks ( R/ a≤1.5), including the Mega Amp Spherical Tokamak (MAST), which was given EURATOM approval in April 1996. MAST ( R=0.7 m, a=0.5 m, k∼2, I p≤2 MA) is twice the size of START and will allow plasmas with cross-section similar to medium-sized conventional tokamaks (e.g. ASDEX-U, D-IIID) to be studied. A summary is presented of the overall design of MAST, which as far as possible has been based on the successful approach used on START and has been arranged to maximise the use of existing COMPASS power supplies. Details of some of the challenging design issues associated with the components in the central regions of the machine, where space is restricted, are described.

Effect of the sample volume splitting in heavy ion-beam probing (HIBP) for spherical tokamaks (by the example of HIBP for mega ampere spherical tokamak)

Here, we propose the heavy ion-beam probing (HIBP) project for the mega ampere spherical tokamak (MAST) to establish systematic research of the radial electric field as an idea to investigate both the reactor-related physics (H mode and advanced tokamak mode with reversed shear) and basic plasma physics (the nature of anomalous transport). The calculations of the probing beam trajectories made for typical MAST parameters: B0=0.6 T, Ipl=1 MA, and Cs+ probing ions show the applicability of HIBP for the spherical torus MAST with energy of about a few hundred keV. The detector line connecting the center and the edge of the plasma allows us to obtain a radial profile by variation of the energy shot by shot. A detector line of equal energy E=110 keV allows us to get a series of profiles in the internal part of the plasma 0<ρ<0.4 during a single shot by a fast scan of the injection angle. The effect of splitting of the sample volume due to the intersection of the secondary fan in a rapidly decreasing toroidal magnetic field of the spherical tokamak was observed: two sample volumes can appear instead of the conventional one. It changes the principles of HIBP: the notion of a detector grid became more general and a new problem appears: how to separate the signals coming from different sample volumes. Using this effect we can spread the detector grid and observe the outer part of the plasma, 0.4<ρ<1. It was shown that these two parts can be united and the total plasma profile can be observed in one shot.