Mega Ampere Spherical Tokamak Upgrade Research Articles

Validating the simulation of beam-ion charge exchange in MAST Upgrade

Simulation of the impact of charge-exchange (CX) reactions on beam ions in the Mega Amp Spherical Tokamak (MAST) Upgrade was compared to measurements carried out with a fission chamber (neutron fluxes) and a fast ion deuterium-alpha (FIDA) diagnostic. A simple model was developed to reconstruct the outer-midplane neutral density based on measurements of deuterium-alpha emission from edge neutrals, and on Thomson scattering measurements of electron density and temperature. The main computational tools used were the ASCOT orbit-following code and the FIDASIM code for producing synthetic FIDA signals. The neutral density reconstruction agrees qualitatively with SOLPS-ITER modelling and yields a synthetic passive FIDA signal that is consistent with measurement. When CX losses of beam ions are accounted for, predicted neutron emission rates are quantitatively more consistent with measurement. It was necessary to account for CX losses of beam ions in simulations to reproduce the measured passive FIDA signal quantitatively and qualitatively. The results suggest that the neutral density reconstruction is a good approximation, that CX with edge neutrals causes significant beam-ion losses in MAST Upgrade, typically 20% of beam power, and that the ASCOT fast-ion CX model can be used to accurately predict the redistribution and loss of beam ions due to CX.

A radiometer to diagnose parametric instabilities during linear excitation of electron Bernstein waves in the Mega Amp Spherical Tokamak (MAST) Upgrade.

Highly overdense magnetically confined fusion plasmas, such as the Mega Amp Spherical Tokamak (MAST) Upgrade, cannot easily be heated using conventional electron cyclotron resonance heating because high density cutoffs block microwave access to the plasma core. Instead, electromagnetic waves can be coupled to electron Bernstein waves (EBWs) through the O-X-B mode coupling scheme, and the EBWs can then be absorbed at higher densities. The excitation of EBWs occurs at the upper hybrid (UH) layer where nonlinear wave interactions, called parametric decay instabilities (PDIs), are known to occur at reduced power thresholds. We present a design for a radiometer to detect PDIs during O-X-B in MAST Upgrade. The radiometer will aid in determining at what power levels PDIs become important as well as inferring various parameters about both electrons and ions near the UH layer. We estimate a gyrotron power density threshold for PDI and expected frequency shifts to be produced in them. The design allows for shifts from several decays involving lower hybrid (LH) waves to be observed.

Kinetic equilibrium reconstructions of plasmas in the MAST database and preparation for reconstruction of the first plasmas in MAST upgrade

Reconstructions of plasma equilibria using magnetic sensors were routine during operation of the Mega Ampere Spherical Tokamak (MAST) device, but reconstructions using kinetic profiles were not. These are necessary for stability and disruption analysis of the MAST database, as well as for operation in the upgrade to the device, MAST-U. The three-dimensional (3D) code VALEN is used to determine eddy currents in the 3D vessel structures for vacuum coil test shots, which are then mapped to effective resistances in the two-dimensional vessel groupings in the EFIT equilibrium reconstruction code to be used in conjunction with nearby loop voltage measurements for estimated currents in the structures during reconstruction. Kinetic equilibrium reconstructions with EFIT, using all available magnetic sensors as well as Thomson scattering measurements of electron temperature and density, charge exchange recombination spectroscopy measurements of ion temperature, and internal magnetic field pitch angle measurements from a motional Stark effect (MSE) diagnostic are performed for a large database of MAST discharges. Excellent convergence errors are obtained for the portions of the discharges where the stored energy was not too low, and it is found that reconstructions performed with temperature and density measurements but without MSE data usually already match the pitch angle measurements well. A database of 275 kinetic equilibria is used to test the ideal MHD stability calculation capability for MAST. Finally, the necessary changes to conducting structure in VALEN, and diagnostic setup in EFIT have been completed for the upgrade from MAST to MAST-U, enabling kinetic reconstructions to commence from the first plasma discharges of the upgraded device.

Overview of new MAST physics in anticipation of first results from MAST Upgrade

The mega amp spherical tokamak (MAST) was a low aspect ratio device (R/a = 0.85/0.65 ~ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics issues for the operation of ITER, design of DEMO and future spherical tokamaks by utilising high resolution diagnostic measurements closely coupled with theory and modelling to significantly advance our understanding. An empirical scaling of the energy confinement time that favours higher power, lower collisionality devices is consistent with gyrokinetic modelling of electron scale turbulence. Measurements of ion scale turbulence with beam emission spectroscopy and gyrokinetic modelling in up-down symmetric plasmas find that the symmetry of the turbulence is broken by flow shear. Near the non-linear stability threshold, flow shear tilts the density fluctuation correlation function and skews the fluctuation amplitude distribution. Results from fast particle physics studies include the observation that sawteeth are found to redistribute passing and trapped fast particles injected from neutral beam injectors in equal measure, suggesting that resonances between the m = 1 perturbation and the fast ion orbits may be playing a dominant role in the fast ion transport. Measured D–D fusion products from a neutron camera and a charged fusion product detector are 40% lower than predictions from TRANSP/NUBEAM, highlighting possible deficiencies in the guiding centre approximation. Modelling of fast ion losses in the presence of resonant magnetic perturbations (RMPs) can reproduce trends observed in experiments when the plasma response and charge-exchange losses are accounted for. Measurements with a neutral particle analyser during merging-compression start-up indicate the acceleration of ions and electrons. Transport at the plasma edge has been improved through reciprocating probe measurements that have characterised a geodesic acoustic mode at the edge of an ohmic L-mode plasma and particle-in-cell modelling has improved the interpretation of plasma potential estimates from ball-pen probes. The application of RMPs leads to a reduction in particle confinement in L-mode and H-mode and an increase in the core ionization source. The ejection of secondary filaments following type-I ELMs correlates with interactions with surfaces near the X-point. Simulations of the interaction between pairs of filaments in the scrape-off layer suggest this results in modest changes to their velocity, and in most cases can be treated as moving independently. A stochastic model of scrape-off layer profile formation based on the superposition of non-interacting filaments is in good agreement with measured time-average profiles. Transport in the divertor has been improved through fast camera imaging, indicating the presence of a quiescent region devoid of filament near the X-point, extending from the separatrix to ψn ~ 1.02. Simulations of turbulent transport in the divertor show that the angle between the divertor leg on the curvature vector strongly influences transport into the private flux region via the interchange mechanism. Coherence imaging measurements show counter-streaming flows of impurities due to gas puffing increasing the pressure on field lines where the gas is ionised. MAST Upgrade is based on the original MAST device, with substantially improved capabilities to operate with a Super-X divertor to test extended divertor leg concepts. SOLPS-ITER modelling predicts the detachment threshold will be reduced by more than a factor of 2, in terms of upstream density, in the Super-X compared with a conventional configuration and that the radiation front movement is passively stabilised before it reaches the X-point. 1D fluid modelling reveals the key role of momentum and power loss mechanisms in governing detachment onset and evolution. Analytic modelling indicates that long legs placed at large major radius, or equivalently low at the target compared with the X-point are more amenable to external control. With MAST Upgrade experiments expected in 2019, a thorough characterisation of the sources of the intrinsic error field has been carried out and a mitigation strategy developed.

A rotary and reciprocating scintillator based fast-ion loss detector for the MAST-U tokamak.

The design and unique feature of the first fast-ion loss detector (FILD) for the Mega Amp Spherical Tokamak - Upgrade (MAST-U) is presented here. The MAST-U FILD head is mounted on an axially and angularly actuated mechanism that makes it possible to independently adapt the orientation [0°, 90°] and radial position [1.40 m, 1.60 m] of the FILD head, i.e., its collimator, thus maximizing the detector velocity-space coverage in a broad range of plasma scenarios with different q95. The 3D geometry of the detector has been optimized to detect fast-ion losses from the neutral beam injectors. Orbit simulations are used to calculate the strike map and predict the expected signals. The results show a velocity-space range of [4 cm, 13 cm] in gyroradius and [30°, 85°] in pitch angle, covering the entire neutral beam ion energy range. The optical system will provide direct sight of the scintillator and simultaneous detection with two cameras, giving high spatial and temporal resolution. The MAST-U FILD will shed light on the dominant fast-ion transport mechanisms in one of the world's two largest spherical tokamaks through absolute measurements of fast-ion losses.

The neutron camera upgrade for MAST Upgrade

The Neutron Camera Upgrade (NCU) is a neutron flux monitor consisting of six lines of sight (LoSs) under installation on Mega Ampere Spherical Tokamak (MAST) Upgrade. The NCU is expected to contribute to the study of the confinement of fast ions and on the efficiency of non-inductive current drive in the presence of on-axis and off-axis neutral beam injection by measuring the neutron emissivity profile along the equatorial plane. This paper discusses the NCU main design criteria, the engineering and interfacing issues, and the solutions adopted. In addition, the results from the characterization and performance studies of the neutron detectors using standard γ-rays sources and a 252Cf source are discussed. The proposed design has a time resolution of 1 ms with a statistical uncertainty of less than 10% for all MAST Upgrade scenarios with a spatial resolution of 10 cm: higher spatial resolution is possible by moving the LoSs in-between plasma discharges. The energy resolution of the neutron detector is better than 10% for a light output of 0.8 MeVee, and the measured pulse shape discrimination is satisfactory.

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.

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.

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.

MAST Accomplishments and Upgrade for Fusion Next-Steps

The Mega Amp Spherical Tokamak (MAST) program and a major upgrade are aimed at using MASTs capabilities and parameter ranges to address specific issues for ITER physics, help find solutions for DEMO (in particular plasma exhaust), and develop a credible physics basis for possible ST-based Component Test Facilities/Fusion Nuclear Science Facilities. MAST also provides an attractive access point for scientists and engineers joining fusion. The general approach is to combine experiments, theory, and modeling intimately to help create usable predictive models for future devices. Physics results include edge-localized mode (ELM) mitigation with the flexible resonant magnetic perturbation coils (up to n=6), where smaller, more frequent ELMs are generated, challenging conventional 2-D stability theory. Gyrokinetic theory and models are combined with experiments in the areas of pedestal stability, pellet fueling optimization, and impurity transport. Fast particle stability and confinement are used to optimize scenarios and neutral beam geometry. Exhaust modeling includes turbulence processes that enhance cross-field transport to help ease the power handling. The first major phase of the upgrade program is well under way, to increase the toroidal field and the transformer flux swing and especially implement the flexible exhaust physics platform by means of 17 new poloidal field coils and a closed pumpable divertor. Changes in the neutral beam systems (off-axis as well as on-axis) and new diagnostics especially of the divertor complete the present upgrade scope.

New magnetic real time shape control for MAST

The Mega Ampere Spherical Tokamak (MAST) real time plasma position controller is based on an optical linear camera placed on the mid plane of the vessel. This solution has the advantage of being a direct observation of the Dα emissions coming from the interaction between the boundary of the plasma and neutral gas, but, on the other hand, it restricts the control to the outer radius of the plasma only.A complete chain of tools has been set up to implement, test and simulate a new real time magnetic plasma shape controller based on the rtEFIT code. The complete working path consists of three elements: a linear static relationship between control parameters and current demands, a linear state space model needed to represent the plasma dynamic response in closed loop simulations, and the possibility to run simulations inside the Plasma Control System (PCS). The linear relationship has been calculated using the FIESTA code, which is developed using Matlab at CCFE. The linear state space model was generated using the CREATE-L code developed by the CREATE Consortium. It has already been successfully used to model JET, FTU and TCV tokamaks. Using this working path many simulations have been carried out allowing fine tuning of the control gains before the real experiment. The simulation testing includes the plasma shape control law as implemented in PCS itself, so intensive debugging has been possible prior to operation. Successful control using rtEFIT was established in the second dedicated experiment during the MAST 2011–12 campaign. This work is a stepping stone towards divertor control which is ultimately intended for application to the super-X divertor in the MAST Upgrade experiment.

MAST Upgrade centre column design and analysis

The MAST-U (Mega-Amp Spherical Tokamak Upgrade) experiment is now being designed and will involve significant advances over the existing MAST machine, including increasing the toroidal field from 0.52 to 0.78 Tesla, the solenoid flux swing from 0.9 to 1.7 volt-seconds and the pulse length from 0.5 to 5 seconds (at 0.54 Tesla toroidal field). To meet these and other enhancements, the existing MAST centre column sub-assembly will be completely replaced in MAST-U. This sub-assembly comprises (a) the centre rod which forms the inner legs of the TF circuit, (b) the solenoid which fits closely over the centre rod, (c) the “air side” coils which are for plasma shaping – one within the bore of the centre rod (Pc) and one outside the solenoid (Px), (d) the two stainless steel crowns which are keyed and bonded to the ends of the centre rod for torsional support and (e) axial supports for the solenoid and airside coils. This paper describes the detailed design of the centre rod and the solenoid magnets and the means to manage the increased twisting torque on the centre rod (138 kNm in MAST-U compared with 58 kNm on MAST) due to the interaction of these two magnets.

MAST upgrade closed pumped divertor design and analysis

The design of the Mega Amp Spherical Tokamak (MAST) Upgrade project provides an excellent opportunity for advancements in tokamak physics in support of ITER and DEMO as well as CTF. One of the main areas of contribution is the divertor. MAST Upgrade will be capable of operating with the outer leg of the divertor being either conventional or with the novel Super-X Divertor (SXD) configuration, in which additional shaping of the magnetic field takes the divertor leg out to large radius. A region of low poloidal field in the chamber of the divertor allows the outer leg to traverse a considerable distance toroidally, reaching a large major radius. Cryopumps outboard the divertor targets will provide density control. This paper describes the current design and layout of the closed pumped divertor geometry to meet the configurations envisaged. The assembly supports symmetrical plasmas with both the upper and lower divertor assemblies able to accommodate either the conventional or SXD.

Engineering design of the double neutral beam injection system for MAST Upgrade

The objectives of the upcoming upgrade to the Mega-Ampere Spherical Tokamak (MAST) require the construction of a new neutral beam injection system. Although the principle of operation is similar to other beamlines including those on JET, the design process is complicated by strict requirements such as the need for intensive ion source commissioning. A double neutral beamline has been designed which satisfies the requirements. This paper reports the engineering design of the MAST Upgrade double beamline, describing the key challenges and unusual solutions adopted.