Search

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 Alfven eigenmodes, which are readily generated due to the supra-Alfvenic ion population. Examples of how MAST is providing new insights into such instabilities (e.g. ELM structure) are described.

Note: Proc. 19th IAEA Fusion Energy Conference, Lyon, France, October 2002, IAEA-CN-94/EX/OV2-3 Reference CRPP-CONF-2002-068 Record created on 2008-05-13, modified on 2017-05-12

A combination of recently installed state-of-the-art imaging and profile diagnostics, together with established plasma simulation codes, are providing for the first time on Mega Ampere Spherical Tokamak (MAST) the tools required for studying confinement and transport, from the core through to the plasma edge and scrape-off-layer (SOL). The H-mode edge transport barrier is now routinely turned on and off using a combination of poloidally localized fuelling and fine balancing of the X-points. Theory, supported by experiment, indicates that the edge radial electric field and toroidal flow velocity (thought to play an important role in H-mode access) are largest if gas fuelling is concentrated at the inboard side. H-mode plasmas show predominantly type III ELM characteristics, with confinement HH factor (w.r.t. scaling law IPB98[y, 2]) around ~1.0. Combining MAST H-mode data with the International Tokamak Physics Activities (ITPA) analyses, results in an L–H power threshold scaling proportional to plasma surface area (rather than PLH ~ R2). In addition, MAST favours an inverse aspect ratio scaling PLH ~ e0.5. Similarly, the introduction of type III ELMing H-mode data to the pedestal energy regression analysis introduces a scaling Wped ~ e−2.13 and modifies the exponents on R, BT and κ. Preliminary TRANSP simulations indicate that ion and electron thermal diffusivities in ELMing H-mode approach the ion-neoclassical level in the half-radius region of the plasma with momentum diffusivity a few times lower. Linear flux-tube ITG and ETG microstability calculations using GS2 offer explanations for the near-neoclassical ion diffusivity and significantly anomalous electron diffusivity seen on MAST. To complement the baseline quasi-steady-state H-mode, newly developed advanced regimes are being explored. In particular, 'broad' internal transport barriers (ITBs) have been formed using techniques developed at conventional aspect ratio. Electron and ion energy diffusivities are reduced towards the ion-neoclassical level in the ITB region of both co- and counter-injection NBI heated plasmas, with momentum diffusivity up to 10 times lower. Moving out to the edge and SOL, OSM2/EIRENE modelling is being used to extract edge perpendicular particle and heat diffusivities, results being consistent with the ballooning nature of power-flow seen in L-mode and reduction in outboard turbulence seen in ELM-free and inter-ELM H-mode. Modelling of parallel SOL transport requires the inclusion of the mirror force (~10 times higher in MAST than at the conventional aspect ratio) and B2SOLPS5.0 simulations show the edge electric field to be well modelled by neoclassical theory. Transient edge transport phenomena are being studied in detail using a variety of techniques (e.g. probability density function (PDF) and power spectrum analysis, differencing and rescaling methods). Intermittent transport is associated with a radial efflux at up to a tenth of the sound speed and up to 30 cm from the separatrix. Arguably, the most dramatic edge events seen in the plasma periphery are the ELMs. Recent results using fast, high-resolution visible imaging confirm the hypothesis that ELMs have both poloidal and toroidal structures (n ~ 10 at q = 4), consistent with recent theories of the non-linear evolution of ballooning modes.

A control and data acquisition system has been developed for the Mega-Amp Spherical Tokamak (MAST) multi-barrel pellet injector to allow reliable operation with multiple pellet sizes. The system operates with one PC running Windows NT with four embedded PCI cards. All measurements and control functions are implemented on a PC. The system software consists of two programs. The first program provides pellet firing and registration of up to four fast signals associated with each pellet–pellet speed, mass, and other signals. The second program controls pellet preparation and all the pellet injector equipment. This program is realised as state machine—allowing a sequence of any number of states, it includes independent digital PID control of the temperature of each barrel. Both programs run conjointly using message exchange. The system is integrated and synchronised with the MAST control and data acquisition systems.

Spontaneous transitions from the low “L-mode” to high “H-mode” of tokamak plasma confinement, first observed during neutral beam heating experiments on ASDEX, are now routinely achieved in many tokamak experiments. The H-mode regime is attractive as it offers the possibility of enhanced confinement, and thus a route towards a more “compact” and cost-efficient fusion power-plant. Transition to H-mode is now routinely achievable in the Mega-Amp Spherical Tokamak (MAST) [A. C. Darke et al., Fusion Technology 1994 (Elsevier, Amsterdam, 1995), Vol. 1, p. 799] for both Ohmically and neutral beam injection (NBI) heated plasmas (PNBI∼0.5–1.7 MW). H-mode plasmas can be either center stack limited or X-point diverted, exhibiting regular Type III edge localized modes (ELMs). Global confinement in H-mode with low frequency ELMs is consistent with the international IPB(y,2) scaling and exceeds the scaling by a factor ∼1.5–2.0 for high performance discharges. Confinement degrades with increasing ELM frequency (which in...

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.

On COMPASS-D, distortions in the density profile at the divertor target are observed in all discharge phases. These distortions are strongly affected by the toroidal field direction, and for B T > 0 appear as a large asymmetry between the inner and outer divertor regions. In this paper we investigate the effect of both drifts and scrape-off layer (SOL) currents on divertor plasma parameters. Large currents are observed in the divertor region, which change the plasma potential creating large electric fields in the divertor target. The resulting E x B drift is suggested as a possible mechanism causing the observed distortions. A combination of thermoelectrical and Pfirsch-Schliiter terms are proposed as driving mechanisms for the SOL currents.

In this paper we report the effect of both edge localized modes (ELMs) and drifts on divertor plasma parameters in COMPASS-D. Profile changes during ELMs are measured with high spatial resolution, and power and particle losses per ELM are estimated. The time evolution of the divertor parameters is shown to be consistent with the ELM being associated with a fast collapse of the transport barrier. Asymmetries between inboard and outboard divertor targets in L-mode plasmas are shown to depend on the plasma parameters, in particular the direction of the toroidal magnetic field, the average plasma density and the input power. In general, the outer divertor region receives more power than the inner one, the asymmetry increasing for ion drift away from the X-point, contrary to observation in other devices. The drift together with scrape-off layer (SOL) currents are suggested as possible mechanisms causing the asymmetry.

In this paper, the array of Langmuir probes at the COMPASS-D divertor and the analysis and interpretation of the probe data are described. The divertor probe system consists of 39 graphite single Langmuir probes embedded in the divertor target. The probe array is being used to investigate the properties of the divertor plasma on COMPASS-D single-null X-point discharges; its high spatial and temporal resolutions allow a detailed study of a variety of fast phenomena such as fluctuations and ELMs. Results on these topics are also presented.

ELM experimental analysis on COMPASS-D, which has previously catalogued magnetic precursor behaviour, has been extended to consider effects in the SOL. ELM precursors are observed at the divertor. Profile changes are measured with high resolution and power and particle losses in ELMs are estimated. High speed videos of an ELM show a turbulent edge extending to the outboard limiter, in broad agreement with reciprocating probe observations. A model which describes well the magnetic features of the ELM, is being extended to include predictions of power deposition structure at the divertor target, for comparison with experimental data.

The effect of externally applied resonant magnetic perturbations on the power load to the divertor target is studied in the COMPASS-D tokamak. A theoretical model is constructed, based on detailed field-line tracing, allowing for cross-field transport in the scrape-off layer (SOL). In this model, the perturbations give rise to an `ergodic' layer that modifies the transport properties in the SOL, causing a net radial spread in the power loading. Spiral magnetic structures form on the divertor target, giving rise to `hotspots' near their toroidal turning points and a double peak structure to the inboard and outboard divertor strike zones. For the first time, detailed comparisons are made between model and experiment, obtaining good agreement when the amplification of the applied field, due to the requirements of pressure balance in the plasma, is included

Single null X-point discharges on COMPASS-D show a gradual L to H transition. Frequent boronization gives long ELM-free H-mode periods, duration > 100 ms, with continuously rising density to > 1.5 × 1010 m−3. The LH transition threshold behaviour is more complex than that predicted by present simple scaling laws, with a well-defined low-density limit being observed and sensitivity to small perturbations at the plasma boundary. Density and recycling behaviour is analysed. In discharges refuelled by recycling alone, the coefficient of recycling as well as the particle confinement time is higher in H-mode periods.

The COMPASS-D tokamak has recently obtained H-mode plasmas in a single-null X-point configuration with no additional heating. Clear H-mode signatures have been observed including reduction in Dalpha light and the unmistakable appearance of ELMs and ELM-free periods. Recycling and gas puff can play crucial roles in achieving and terminating the H-modes. The emergence of distinct ELMs, from the apparently-random fluctuations, of the background Dalpha light, is usually one of progression, taking some 10s of ms rather than a sudden discontinuity. The relatively slow evolution allows for detail examination of important phenomena, such as improvements to confinement and fluid rotation. The H-mode quality improves with density, unless impurity accumulation becomes important. Improvements to the energy and particle confinement are observed and are accompanied by positive increases in the radial electric field at approximately=4 cm inside the last closed flux surface (LCFS). The ohmic powers in discharges with clearly defined H-modes have not been less, so far, than the power obtained from power threshold scalings from larger machines.

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.

Fast visible imaging of Mega-Ampere Spherical Tokamak plasmas during injection of refueling pellets of frozen deuterium can sometimes vividly reveal their internal magnetic structure. Concentrated deposition of particles around the flux surface with unit value of field-line helicity results in locally increased bremsstrahlung emission which illuminates this whole surface or, occasionally, a single flux tube within it. Analysis of such images could be exploited to check equilibrium reconstruction models.

Pellet material deposited in a tokamak plasma experiences a drift towards the low field side of the torus induced by the magnetic field gradient. Plasma fuelling in ITER relies on the beneficial effect of this drift to increase the pellet deposition depth and fuelling efficiency. It is therefore important to analyse this phenomenon in present machines to improve the understanding of the ∇B induced drift and the accuracy of the predictions for ITER. This paper presents a detailed analysis of pellet material drift in MAST pellet injection experiments based on the unique diagnostic capabilities available on this machine and compares the observations with predictions of state-of-the-art ablation and deposition codes.

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, χϕ, is linked to the ion diffusion, χi, with a Prandtl number close to P ϕ ≈ χϕ/χi ≈ 1, although χi 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 rather than ρpol. 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 Alfvén eigenmodes. Fast particle instabilities also affect the off-axis NBI current drive, leading to fast ion diffusion of the order of 0.5 m2 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.

Results from the DITE tokamak experiment with the Mk II bundle divertor and neutral beam heating are described. Particle and energy balances have been investigated and both show substantial fractions of the inputs flowing to the divertor. Observations of asymmetries in power deposition in the torus are presented. Measurements of the plasma parameters in the divertor scrape-off layer and in the diverted plasma are reported. Flow into the divertor is adequately described by convective processes.

The soft X-ray emission from the DITE tokamak for a wide range of different conditions has been measured. The coronal equilibrium model has been used to interpret the results and to show that the plasma observed ranges from a nearly pure hydrogen plasma to one completely dominated by impurities. The effects on the impurities of the DITE bundle divertor and of gettering the torus walls have been clearly established.

The DITE bundle divertor MkII has been operated with high power neutral beam injection, up to 2.1 MW, and with gaseous fuelling in the divertor. Discharges with good confinement have been used to study the properties of the plasma exhaust channel in the presence of a high density of neutral molecules in the divertor, up to ~ 1 ×1021 m−3. An inner layer of the channel carries a large part of the heat exhaust by collisionless convection at a power density ~ 100 MW m−2 and particle flux ~ 1024 m−2 s−1. Ionisation in the divertor has little effect on this layer. A collisional, low power density outer layer is strongly affected by the neutral particles in the divertor, with plasma flow out of the divertor at high neutral pressure, and ionisation losses supplied by electron heat transport from the torus. This layer has not previously been demonstrated in the plasma exhaust channel of a fusion research device. Calculations using the DEGAS code predict blocking of the divertor ducts by the exhaust plasma: retention of neutral particles in the divertor is seen experimentally. The amount of ionisation in the divertor is however lower than predicted, and it is shown that collisional interaction between neutral particles from the diverted plasma and the molecular background could be responsible for this reduction.

Detached discharges are described in which the radiated power loss is from within a maximum radius up to 50 mm less than the limiter radius. Outside this radiating zone the plasma density and temperature profiles are flat with values of approximately 5 × 1018 m−3 and 7 eV respectively. Typical conditions for detached discharges in DITE are a plasma current of 100 kA ,a safety factor of ≈ 7 (toroidal field 2.5 T), and a line average density of 3 × 1019 m−3 in deuterium. Detachment can also be observed in similar discharges in helium at a density of 6 × 1019 m−3. These values are 70–100% of the critical density for disruption.

The latest analysis of results from the Bundle Divertor and prospects for application to a tokamak reactor are presented. The upgraded version of the machine, DITE IB is described briefly. Initial results of a programme to study the causes of disruption at high density in ohmic and beam-heated discharges are also described.

Characteristics of the operation of the DITE tokamak with the MkII bundle divertor and neutral beam heating are described. In these experiments the divertor was pumped by a liquid helium cooled cryopump, and fuelling was by gas input into the tokamak vacuum vessel. Generally a fuelling rate of 1–2 × 1021 atoms/s is required to sustain a constant density with the divertor operating, and about 50% of the gas input flows to the divertor. Global power balances have been carried out using instrumentation distributed around the tokamak and in the divertor. It is found that the wall loading exhibits substantial toroidal and poloidal asymmetries, with a high loading near the gas feed when the divertor is operating. The percentage power exhaust to the divertor is highest, up to 70%, in ohmic discharges with the gas feed turned off and the density falling. With 1.4 MW of neutral beam heating, about 24% of the power is seen at the divertor plates. Typically, between 70% and 80% of the input power can be counted for. Measurements in the scrape-off plasma show that the density is about 1× 1019 m−3 at 30 to 40 mm outside the separatrix, when the line average density in the tokamak is 2−2.5 × 1019 m−3. However there is less than 10% reionisation of recycled neutrals in the divertor, as determined from Hα measurements. In addition, the strong coupling observed between the particle and power flow to the divertor suggests that the exhaust is convective, and this may be a consequence of the strong mirror field characteristic of the bundle divertor magnetic geometry.