Mega Amp Spherical Tokamak Research Articles

Two-fluid and magnetohydrodynamic modelling of magnetic reconnection in the MAST spherical tokamak and the solar corona

Twisted magnetic flux ropes are ubiquitous in laboratory and astrophysical plasmas, and the merging of such flux ropes through magnetic reconnection is an important mechanism for restructuring magnetic fields and releasing free magnetic energy. The merging-compression scenario is one possible start-up scheme for spherical tokamaks, which has been used on the Mega Amp Spherical Tokamak (MAST). Two current-carrying plasma rings or flux ropes approach each due to mutual attraction, forming a current sheet and subsequently merge through magnetic reconnection into a single plasma torus, with substantial plasma heating. Two-dimensional resistive and Hall-magnetohydrodynamic simulations of this process are reported, including a strong guide field. A model of the merging based on helicity-conserving relaxation to a minimum energy state is also presented, extending previous work to tight-aspect-ratio toroidal geometry. This model leads to a prediction of the final state of the merging, in good agreement with simulations and experiment, as well as the average temperature rise. A relaxation model of reconnection between two or more flux ropes in the solar corona is also described, allowing for different senses of twist, and the implications for heating of the solar corona are discussed.

Intermediate-k density and magnetic field fluctuations during inter-ELM pedestal evolution in MAST

Measurements of local density and magnetic field fluctuations near the pedestal top, conditionally averaged over the edge localized mode (ELM) cycle, have been made in Mega Amp Spherical Tokamak (MAST). A Doppler backscattering (DBS) system installed at MAST was used to measure intermediate-k density fluctuations at the top of the pedestal. A novel diagnostic technique combining DBS with cross-polarization scattering (CP-DBS) enabled magnetic field fluctuations to also be locally measured at similar wave numbers. Polarization isolation and other effects for CP-DBS are discussed. Both measurements were used in a series of high-β –4.5) MAST plasmas with large type-I ELMs with an period where microtearing modes (MTMs) had been predicted to be unstable in similar conditions (Dickinson 2012 Phys. Rev. Lett. 108 135002). The measured density fluctuation level increased by a factor of about 4 between 2 and 4 ms after the ELM, which was correlated with the recovery of the density profile while the temperature pedestal height continued to increase slowly. Magnetic field fluctuations showed different temporal behaviors, slowly increasing throughout the ELM cycle as the local β increased. Linear GS2 calculations show both MTM and electron temperature gradient (ETG) modes unstable at similar wave numbers as the measurements (although with more overlap between ETG wave numbers and diagnostic spectral resolution) at the top of the pedestal, along with kinetic ballooning modes are unstable lower in the pedestal (at larger wavelengths). The inferred ratio of fluctuation levels from experiment was . The comparable ratios from GS2 were for the MTM and for the ETG. Both the experimental wave number range and the fluctuation ratio are more similar to the linear characteristics of the ETG than the MTM. These results imply that intermediate-k fluctuations due to the ETG play a role in inter-ELM pedestal evolution.

Numerical investigation of isolated filament motion in a realistic tokamak geometry

This paper presents a numerical investigation of isolated filament dynamics in a simulation geometry representative of the scrape-off layer (SOL) of the Mega Amp Spherical Tokamak (MAST) previously studied in Walkden et al 2013 (Plasma Phys. Control. Fusion 55 105005). This paper focuses on the evolution of filament cross-sections at the outboard midplane and investigates the scaling of the centre of mass velocity of the filament cross-section with filament width and electron temperature.By decoupling the vorticity equation into even and odd parity components about the centre of the filament in the bi-normal direction parallel density gradients are shown to drive large velocities in the bi-normal (approximately poloidal) direction which scale linearly with electron temperature. In this respect increasing the electron temperature causes a departure of the filament dynamics from two-dimensional (2D) behaviours.Despite the strong impact of three-dimensional effects the radial motion of the filament is shown to be relatively well predicted by 2D scalings. The radial velocity is found to scale positively with both electron temperature and cross-sectional width, suggesting an inertially limited nature. Comparison with the two-region model (Myra et al 2006 Phys. Plasmas 13 112502) achieves reasonable agreement when using a corrected parallel connection length due to the neglect of diamagnetic currents driven in the divertor region of the filament.Analysis of the transport of particles due to the motion of the filament shows that the background temperature has a weak overall effect on the radial particle flux whilst the filament width has a strong effect.

Charge dependence of neoclassical and turbulent transport of light impurities on MAST

Carbon and nitrogen impurity transport coefficients are determined from gas puff experiments carried out during repeat L-mode discharges on the Mega-Amp Spherical Tokamak (MAST) and compared against a previous analysis of helium impurity transport on MAST. The impurity density profiles are measured on the low-field side of the plasma, therefore this paper focuses on light impurities where the impact of poloidal asymmetries on impurity transport is predicted to be negligible. A weak screening of carbon and nitrogen is found in the plasma core, whereas the helium density profile is peaked over the entire plasma radius. Both carbon and nitrogen experience a diffusivity of the order of 10 m2s−1 and a strong inward convective velocity of ∼40 m s−1 near the plasma edge, and a region of outward convective velocity at mid-radius. The measured impurity transport coefficients are consistent with neoclassical Banana-Plateau predictions within . Quasi-linear gyrokinetic predictions of the carbon and helium particle flux at two flux surfaces, and , suggest that trapped electron modes are responsible for the anomalous impurity transport observed in the outer regions of the plasma. The model, combining neoclassical transport with quasi-linear turbulence, is shown to provide reasonable estimates of the impurity transport coefficients and the impurity charge dependence.

The effects of resonant magnetic perturbations on fast ion confinement in the Mega Amp Spherical Tokamak

The effects of resonant magnetic perturbations (RMPs) on the confinement of energetic (neutral beam) ions in the Mega Amp Spherical Tokamak (MAST) are assessed experimentally using measurements of neutrons, fusion protons and fast ion Dα (FIDA) light emission. In single null-diverted (SND) MAST pulses with relatively low plasma current (400 kA), the total neutron emission dropped by approximately a factor of two when RMPs with toroidal mode number n = 3 were applied. The measured neutron rate during RMPs was much lower than that calculated using the TRANSP plasma simulation code, even when non-classical (but axisymmetric) ad hoc fast ion transport was taken into account in the latter. Sharp drops in spatially-resolved neutron rates, fusion proton rates and FIDA emission were also observed. First principles-based simulations of RMP-induced fast ion transport in MAST, using the F3D-OFMC code, show similar losses for two alternative representations of the MAST first wall, with and without full orbit effects taken into account; for n = 6 RMPs in a 600 kA plasma, the additional loss of beam power due to the RMPs was found in the simulations to be approximately 11%.

Hybrid guiding-centre/full-orbit simulations in non-axisymmetric magnetic geometry exploiting general criterion for guiding-centre accuracy

To identify under what conditions guiding-centre or full-orbit tracing should be used, an estimation of the spatial variation of the magnetic field is proposed, not only taking into account gradient and curvature terms but also parallel currents and the local shearing of field-lines. The criterion is derived for general three-dimensional magnetic equilibria including stellarator plasmas. Details are provided on how to implement it in cylindrical coordinates and in flux coordinates that rely on the geometric toroidal angle. A means of switching between guiding-centre and full-orbit equations at first order in Larmor radius with minimal discrepancy is shown. Techniques are applied to a MAST (mega amp spherical tokamak) helical core equilibrium in which the inner kinked flux-surfaces are tightly compressed against the outer axisymmetric mantle and where the parallel current peaks at the nearly rational surface. This is put in relation with the simpler situation B(x, y, z) = B0[sin(kx)ey + cos(kx)ez], for which full orbits and lowest order drifts are obtained analytically. In the kinked equilibrium, the full orbits of NBI fast ions are solved numerically and shown to follow helical drift surfaces. This result partially explains the off-axis redistribution of neutral beam injection fast particles in the presence of MAST long-lived modes (LLM).

Publisher's Note: “Bi-directional Alfvén cyclotron instabilities in the mega-amp spherical tokamak” [Phys. Plasmas 21, 082501 (2014)

First Page

MAST Upgrade – Construction Status

The Mega Amp Spherical Tokamak (MAST) is the centre piece of the UK fusion research programme. In 2010, a MAST Upgrade programme was initiated with three primary objectives, to contribute to: (1) testing reactor concepts (in particular exhaust solutions via a flexible divertor allowing Super-X and other extended leg configurations); (2) adding to the knowledge base for ITER (by addressing important plasma physics questions and developing predictive models to help optimise ITER performance of ITER) and (3) exploring the feasibility of using a spherical tokamak as the basis for a fusion Component Test Facility.With the project mid-way through its construction phase, progress will be reported on a number of the critical subsystems. This will include manufacture and assembly of the coils, armour and support structures that make up the new divertors, construction of the new set coils that make up the centre column, installation of the new power supplies for powering the divertor coils and enhanced TF coil set, progress in delivering the upgraded diagnostic capability, the modification and upgrading of the NBI heating systems and the complete overhaul of the machine control infrastructure, including a new control room with full remote participation facilities.

Electron kinetics inferred from observations of microwave bursts during edge localized modes in the mega-amp spherical tokamak.

Recent measurements of microwave and x-ray emission during edge localized mode (ELM) activity in tokamak plasmas provide a fresh perspective on ELM physics. It is evident that electron kinetics, which are not incorporated in standard (fluid) models for the instability that drives ELMs, play a key role in the new observations. These effects should be included in future models for ELMs and the ELM cycle. The observed radiative effects paradoxically imply acceleration of electrons parallel to the magnetic field combined with rapid acquisition of perpendicular momentum. It is shown that this paradox can be resolved by the action of the anomalous Doppler instability which enables fast collective radiative relaxation, in the perpendicular direction, of electrons accelerated in the parallel direction by inductive electric fields generated by the initial ELM instability.

Divertor impurity injection using high voltage arcs for impurity transport studies on the Mega Amp Spherical Tokamak.

The operation of next-generation fusion reactors will be significantly affected by impurity transport in the scrape-off layer (SOL). Current modelling efforts are restricted by a lack of detailed data on impurity transport in the SOL. In order to address this, a carbon injector has been designed and installed on the Mega Amp Spherical Tokamak (MAST). The injector creates short lived carbon plumes originating at the MAST divertor lasting less than 50 μs. High voltage capacitor banks are used to create a discharge across concentric carbon electrodes located in a probe mounted on the Divertor Science Facility in the MAST lower divertor. This results in a very short plume duration allowing observation of the evolution of the plume and precise localisation of the plume relative to the X-point on MAST. The emission from the carbon plume was imaged using fast visible cameras filtered in order to isolate the carbon II and carbon III emission lines centered around 514 nm and 465 nm.

Isothermal velocity measurements in two HyperVapotron geometries using Particle Image Velocimetry (PIV)

HyperVapotron beam stopping elements are high heat flux devices able to transfer large amounts of heat (of the order of 10–20MW/m2) efficiently and reliably making them strong candidates as plasma facing components for future nuclear fusion reactors or other applications where high heat flux transfer is required. They employ the Vapotron effect, a two phase complex heat transfer mechanism. The physics of operation of the device are not well understood and are believed to be strongly linked to the evolution of the flow fields of coolant flowing inside the grooves that form part of the design. An experimental study of the spatial and temporal behaviour of the flow field under isothermal conditions has been carried out on two replicas of HyperVapotron geometries taken from the Mega Amp Spherical Tokamak (MAST) and the Joint European Torus (JET) experiments. The models were tested under three isothermal operating conditions to collect coolant flow data and assess how the design and operational conditions might affect the thermal performance of the devices for single phase heat transfer. It was discovered that the in-groove speeds of MAST are lower and the flow structures less stable but less sensitive to free stream speed perturbations compared to the JET geometry. The MAST geometry was found to suffer from hydrodynamic end effects. A wake formation was discovered at the top of the groove entrance for the JET geometry, while this is absent from the MAST geometry. The wake does not affect significantly the mean operation of the device but it may affect the coolant pumping load of the device. For the JET variant, there is evidence that the typical operation with free stream flow speed of 6m/s is advantageous.

The effect of tangled magnetic fields on instabilities in tokamak plasmas

The high pressure gradients in the edge of a tokamak plasma can lead to the formation of explosive plasma instabilities known as edge localised modes (ELMs). The control of ELMs is an important requirement for the next generation of fusion devices such as ITER. Experiments performed on the Mega Amp Spherical Tokamak (MAST) at Culham have shown that the application of non-axisymetric resonant magnetic perturbations (RMPs) can be used to mitigate ELMs. During the application of the RMPs, clear structures are observed in visible- light imaging of the X-point region. These lobes, or tangles, have been observed for the first time and their appearance is correlated with the mitigation of ELMs. Tangle formation is seen to be associated with the RMPs penetrating the plasma and may be important in explaining why the ELM frequency increases during ELM mitigation. Whilst the number and location of the tangles can be explained by vacuum magnetic field modelling, obtaining the correct radial extent of the tangles requires the plasma response to be taken into account.

Recent developments in Bayesian inference of tokamak plasma equilibria and high-dimensional stochastic quadratures

We present recent results and technical breakthroughs for the Bayesian inference of tokamak equilibria using force-balance as a prior constraint. Issues surrounding model parameter representation and posterior analysis are discussed and addressed. These points motivate the recent advancements embodied in the Bayesian Equilibrium Analysis and Simulation Tool (BEAST) software being presently utilized to study equilibria on the Mega-Ampere Spherical Tokamak (MAST) experiment in the UK (von Nessi et al 2012 J. Phys. A 46 185501). State-of-the-art results of using BEAST to study MAST equilibria are reviewed, with recent code advancements being systematically presented though out the manuscript.

Visible Imaging of Global MHD on MAST

Eulerian video magnification is a video processing technique for amplifying subtle temporal oscillations in video footage. Applying this technique to footage of an H-mode plasma shot on the mega-ampere spherical tokamak, a global magnetohydrodynamic mode is amplified, and its spatial structure is made readily visible. This technique has broad applicability across plasma physics.

Conceptual design of a neutron camera for MAST Upgrade.

This paper presents two different conceptual designs of neutron cameras for Mega Ampere Spherical Tokamak (MAST) Upgrade. The first one consists of two horizontal cameras, one equatorial and one vertically down-shifted by 65 cm. The second design, viewing the plasma in a poloidal section, also consists of two cameras, one radial and the other one with a diagonal view. Design parameters for the different cameras were selected on the basis of neutron transport calculations and on a set of target measurement requirements taking into account the predicted neutron emissivities in the different MAST Upgrade operating scenarios. Based on a comparison of the cameras' profile resolving power, the horizontal cameras are suggested as the best option.

Bi-directional Alfvén cyclotron instabilities in the mega-amp spherical tokamak

Alfvén cyclotron instabilities excited by velocity gradients of energetic beam ions were investigated in MAST experiments with super-Alfvénic neutral beam injection over a wide range of toroidal magnetic fields from ∼0.34 T to ∼0.585 T. In MAST discharges with high magnetic field, a discrete spectrum of modes in the sub-cyclotron frequency range is excited toroidally propagating counter to the beam and plasma current (toroidal mode numbers n < 0). At lower magnetic field ≤0.45 T, a discrete spectrum of Compressional Alfvén Eigenmodes (CAEs) with n > 0 arises, in addition to the modes with n < 0. At lowest magnetic fields, the CAEs with n > 0 become dominant, they are observed in frequency range from ∼250 kHz for n=1 to ∼3.5 MHz for n=15, well above the on-axis ion cyclotron frequency (∼2.5 MHz). The data is interpreted in terms of normal and anomalous Doppler resonances modified by magnetic drift terms due to inhomogeneity and curvature of the magnetic field. A Hall MHD model is applied for computing the eigenfrequencies and the spatial mode structure of CAEs and a good agreement with the experimental frequencies is found.

Investigating fusion plasma instabilities in the Mega Amp Spherical Tokamak using mega electron volt proton emissions (invited).

The proton detector (PD) measures 3 MeV proton yield distributions from deuterium-deuterium fusion reactions within the Mega Amp Spherical Tokamak (MAST). The PD's compact four-channel system of collimated and individually oriented silicon detectors probes different regions of the plasma, detecting protons (with gyro radii large enough to be unconfined) leaving the plasma on curved trajectories during neutral beam injection. From first PD data obtained during plasma operation in 2013, proton production rates (up to several hundred kHz and 1 ms time resolution) during sawtooth events were compared to the corresponding MAST neutron camera data. Fitted proton emission profiles in the poloidal plane demonstrate the capabilities of this new system.

The effect of resonant magnetic perturbations on the divertor heat and particle fluxes in MAST

Edge localized modes (ELMs) are a concern for future devices, such as ITER, due to the large transient heat loads they generate on the divertor surfaces which could limit the operational lifetime of the device. This paper discusses the application of resonant magnetic perturbations (RMPs) as a mechanism for ELM control on Mega Amp Spherical Tokamak (MAST). Experiments have been performed using an n = 3 toroidal mode number perturbation and measurements of the strike point splitting performed. The measurements have been made using both infrared and visible imaging to measure the heat and particle flux to the divertor. The measured profiles have shown clear splitting in L-mode which compares well with the predication of the splitting location from modelling including the effect of screening. The splitting of the strike point has also been studied as a function of time during the ELM. The splitting varies during the ELM, being the strongest at the time of the peak heat flux and becoming more filamentary at the end of the ELM (200 µs after the peak midplane Dα emission). Variation in the splitting profiles has also been seen, with some ELMs showing clear splitting and others no splitting. A possible explanation of this effect is proposed, and supported by modelling, which concerns the relative phase between the RMP field and the ELM filament location.

Kinetics of dust particles around the scrape off layer in fusion devices

A kinetic model based on the balance of charge and energy over the dust particle surface around the scrape off layer (SOL) region in fusion devices has been developed; for describing the dust mass diminution, its temperature evolution and phase change process have been taken into account. The formulation has been utilized to determine the lifetime of cylindrical and spherical dust particles. A realistic situation in fusion devices, when the plasma exhibits meso-thermal flow, has been taken into account; for this purpose a rigorous approach, pioneered by Mott-Smith and Langmuir (1926 Phys. Rev. 28 727), has been adopted to derive the general expressions for the electron (ion) current on cylindrical dust surfaces and the corresponding mean energy of accreting electrons/ions in a flowing plasma. On the basis of analytical modelling the numerical results for the dust electric potential energy and the lifetime of the dust particles corresponding to a typical plasma environment near the SOL region of Mega Ampere Spherical tokamak (MAST)/Joint European Torus (JET) fusion devices have been evaluated for graphite and tungsten dust particles. The results are graphically illustrated as functions of particle size, electron/ion temperature and plasma ionization. It is seen that a large dust particle immersed in low temperature plasma can survive for long time; as an important outcome it is also noticed that the cylindrical particles of tungsten last longer than spherical particles. The findings are of relevance in characterizing and simulating the effects of a variety of dusts for experimental campaigns in large scale (ITER/Demo-like) fusion devices.

The 2.5 MeV neutron flux monitor for MAST

A proof-of-principle collimated Neutron flux Camera (NC) monitor for the measurement of the 2.45MeV neutron emission from the deuterium–deuterium fusion reactions has been developed, installed and put into use at the Mega Ampere Spherical Tokamak (MAST). The NC measures the spatial and time resolved volume integrated neutron emissivity in deuterium fusion plasmas in the presence of auxiliary plasma heating along two equatorial and two diagonal lines of sight whose tangency radius can be varied between plasma discharges. This paper describes the NC design principles, their technical realization and its performances illustrated with experimental observations of different plasma scenarios. Neutron count rates in the range 0.1–1.5MHz are routinely observed allowing time resolutions as high as 1ms with a statistical uncertainty less than 10% and an energy threshold of 0.5MeV. Examples of the effect of plasma instabilities on the neutron emission are presented. The good results obtained will be used for the design of the neutron flux camera monitor for MAST Upgrade.