K. J. Gibson

Design of a new Nd:YAG Thomson scattering system for MAST

A new infrared Thomson scattering system has been designed for the MAST tokamak. The system will measure at 120 spatial points with approximately 10 mm resolution across the plasma. Eight 30 Hz 1.6 J Nd:YAG lasers will be combined to produce a sampling rate of 240 Hz. The lasers will follow separate parallel beam paths to the MAST vessel. Scattered light will be collected at approximately f/6 over scattering angles ranging from 80 degrees to 120 degrees. The laser energy and lens size, relative to an existing 1.2 J f/12 system, greatly increases the number of scattered photons collected per unit length of laser beam. This is the third generation of this polychromator to be built and a number of modifications have been made to facilitate mass production and to improve performance. Detected scattered signals will be digitized at a rate of 1 GS/s by 8 bit analog to digital converters (ADCs.) Data may be read out from the ADCs between laser pulses to allow for real-time analysis.

A 130 point Nd:YAG Thomson scattering diagnostic on MAST.

A Thomson scattering diagnostic designed to measure both edge and core physics has been implemented on MAST. The system uses eight Nd:YAG lasers, each with a repetition rate of 30 Hz. The relative and absolute timing of the lasers may be set arbitrarily to produce fast bursts of measurements to suit the time evolution of the physics being studied. The scattered light is collected at F/6 by a 100 kg six element lens system with an aperture stop of 290 mm. The collected light is then transferred to 130 polychromators by 130 independent fiber bundles. The data acquisition and processing are based on a distributed computer system of dual core processors embedded in 26 chassis. Each chassis is standalone and performs data acquisition and processing for five polychromators. This system allows data to be available quickly after the MAST shot and has potential for real-time operations.

L–H transition and pedestal studies on MAST

On MAST studies of the profile evolution of the electron temperature (Te), electron density (ne), radial electric field (Er) as well as novel measurements of the ion temperature (Ti) and toroidal current density (j) in the pedestal region allow further insight into the processes forming and defining the pedestal such as the H-mode access conditions and MHD stability. This includes studies of fast evolution of Te, ne and Er with Δt = 0.2 ms time resolution and the evolution of pe and j through an edge-localized mode (ELM) cycle. Measurements of the H-mode power threshold, PL−H revealed that about 40% more power is required to access H-mode in 4He than in D and that a change in the Z-position of the X-point can change PL−H significantly in single and double null configurations. The profile measurements in the L-mode phase prior to H-mode suggest that neither the gradient nor the value of the mean Te or Er at the plasma edge play a major role in triggering the L–H transition. After the transitions, first the fluctuations are suppressed, then the Er shear layer and the ne pedestal develops followed by the Te pedestal. In the banana regime at low collisionality (ν) ∇Ti ≈ 0 leading to Ti > Te in the pedestal region with Ti ~ 0.3 keV close to the separatrix. A clear correlation of ∇Ti with ν is observed. The measured j (using the motional Stark effect) Te and ne are in broad agreement with the common peeling–ballooning stability picture for ELMs and neoclassical calculations of the bootstrap current. The j and ∇pe evolution Δt ≈ 2 ms as well as profiles in discharges with counter current neutral beam injection raise questions with respect to this edge stability picture.

The influence of finite radial transport on the structure and evolution of m/n?=?2/1 neoclassical tearing modes on MAST

Finite radial transport around magnetic islands is believed to play an important role in the threshold, spatial structure and temporal evolution of neoclassical tearing modes (NTMs). We report on novel measurements of NTMs with mode structure m/n?=?2/1 on the MAST spherical tokamak (ST), which have allowed a direct evaluation of the effect of transport on island behaviour for the first time on an ST. Temperature profiles obtained with the upgraded Thomson scattering (TS) system on MAST have been used to constrain the solutions of a heat transport equation for the NTM magnetic island (Fitzpatrick 1995 Phys. Plasmas 2 825), allowing the determination of the critical width for temperature flattening across an island wc, an important parameter in the modified Rutherford equation (MRE) for NTM evolution. The measured value of wc?=?0.7???0.2?cm obtained for an ensemble of high ?N MAST discharges has been used in an analysis of the MRE for 2/1 NTM growth and saturation on MAST. Using a probabilistic method for parameter and error estimation, which takes account of the experimental uncertainty on measured equilibrium parameters, it is found that the temporal evolution of island size is well described by marginally, classically unstable NTMs with strongly destabilizing bootstrap current and stabilizing curvature terms. Finally, further analysis of a ? ramp-down discharge is presented, in which the measured wc value explains the observed threshold width well.

Hybrid simulations of mini-magnetospheres in the laboratory

Solar energetic ions are a known hazard to both spacecraft electronics and to manned space flights in interplanetary space missions that extend over a long period of time. A dipole-like magnetic field and a plasma source, forming a mini-magnetosphere, are being tested in the laboratory as means of protection against such hazards. We investigate, via particle-in-cell hybrid simulations, using kinetic ions and fluid electrons, the characteristics of the mini-magnetospheres. Our results, for parameters identical to the experimental conditions, reveal the formation of a mini-magnetosphere, whose features are scanned with respect to the plasma density, the plasma flow velocity and the intensity of the dipole field. Comparisons with a simplified theoretical model reveal a good qualitative agreement and excellent quantitative agreement for higher plasma dynamic pressures and lower B-fields.

New physics capabilities from the upgraded Thomson scattering diagnostic on MAST

The newly upgraded MAST Thomson scattering (TS) system provides excellent spatial resolution (~1?cm) at over 130 radial locations across a full plasma diameter, and utilizes eight individual Nd:?:YAG laser systems which can be fired sequentially, providing electron temperature and density profiles approximately every 4?ms throughout a plasma discharge. By operating the system in burst mode, whereby the laser separation can be adjusted to within a few microseconds of each other, it is possible to obtain detailed profiles of transient and periodic phenomena such as sawteeth crashes, massive gas injection for disruption mitigation and the temperature perturbations associated with neoclassical tearing mode (NTM) islands. Following Fitzpatrick et al (1995 Phys. Plasmas 2 825), we consider a simplified model in which finite parallel diffusive heat transport can provide a threshold for NTM island growth and demonstrate that the TS derived electron temperature profiles around an island can be used to obtain both the island width and the critical island width below which temperature gradients are maintained across the island, potentially removing the bootstrap current drive for the NTM. Initial results from high beta, neutral beam injection heated discharges on MAST show that the measured island width inferred from the TS data is in good agreement with magnetic estimates of the island width (considering both a cylindrical approximation and using a full field line tracing estimate). The temporal behaviour of the island width obtained from the magnetic diagnostics indicates that for the scenarios considered to date, finite parallel diffusion is likely to play an important role in NTM threshold physics in MAST.

The H-mode pedestal structure and its role on confinement in JET with a carbon and metal wall

We present the pedestal structure, as determined from the high-resolution Thomson scattering measurements, for a database of low and high triangularity (????0.22?0.39) 2.5?MA, type I ELMy H-mode JET plasmas after the installation of the new ITER-like wall (JET-ILW). The database explores the effect of increasing deuterium fuelling and nitrogen seeding with a view to explain the observed changes in performance (edge and global). The low triangularity JET-ILW plasmas show no significant change in performance and pedestal structure with increasing gas dosing. These results are in good agreement with EPED1 predictions. At high triangularity, for pure deuterium fuelled JET-ILW plasmas, there is a 20?30% reduction in global performance and pressure pedestal height in comparison to JET-C plasmas. This reduction in performance is primarily due to a degradation of the temperature pedestal height. The global performance and pressure pedestal height of JET-ILW plasmas can be partially recovered to that of JET-C plasmas with additional nitrogen seeding (Giroud et al 2013 Nucl. Fusion 53 113025). This observed improvement in performance is predominately due to a significant increase in density pedestal height as well as a small increase in the temperature pedestal height. A key result with increasing deuterium fuelling for JET-ILW plasmas is there is no improvement in pressure pedestal height however the pedestal still widens which is inconsistent with the scaling. Furthermore, a key result with increasing nitrogen seeding is the pressure pedestal widening is due to an increase in the temperature pedestal width whilst the density pedestal shows no clear trend. The comparison of EPED1 predictions with the measurements at high triangularity is complex as, for example, for pure deuterium fuelled plasmas there is very good agreement for the pedestal height but not the width. In addition, current EPED1 runs under-predict the pedestal height and width at high nitrogen seeding for JET-ILW plasmas however further work is required to determine the significance of these deviations. Understanding these deviations is essential as provides an insight to the physical mechanisms governing the pedestal structure and edge performance.

Plasma profile evolution during disruption mitigation via massive gas injection on MAST

Massive gas injection (MGI) is one means of ameliorating disruptions in future devices such as ITER, where the stored energy in the plasma is an order of magnitude larger than in present-day devices. The penetration of the injected impurities during MGI in MAST is diagnosed using a combination of high-speed (20?kHz) visible imaging and high spatial (1?cm) and temporal (0.1?ms) resolution Thomson scattering (TS) measurements of the plasma temperature and density. It is seen that the rational surfaces, in particular q?=?2, are the critical surfaces for disruption mitigation. The TS data shows the build-up of density on rational surfaces in the edge cooling period of the mitigation, leading to the collapse of the plasma in a thermal quench. The TS data are confirmed by the visible imaging, which shows filamentary structures present at the start of the thermal quench. The filamentary structures have a topology which matches that of a q?=?2 field line in MAST, suggesting that they are located on the q?=?2 surface. Linearized magnetohydrodynamic stability analysis using the TS profiles suggests that the large density build-up on the rational surfaces drives modes within the plasma which lead to the thermal quench. The presence of such modes is seen experimentally in the form of magnetic fluctuations on Mirnov coils and the growth of an n?=?1 toroidal mode in the period prior to the thermal quench. These results support the observations of other machines that the 2/1 mode is the likely trigger for the thermal quench in a mitigated disruption and suggests that the mitigation process in spherical tokamaks is similar to that in conventional aspect ratio devices.

Pedestal study across a deuterium fuelling scan for high δ ELMy H-mode plasmas on JET with the carbon wall

We present the results from a new fuelling scan database consisting of 14 high triangularity (????0.41), type I ELMy H-mode JET plasmas. As the fuelling level is increased from low, (?D???0.2???1022?el?s?1, ne,ped/nGW?=?0.7), to high dosing (?D???2.6???1022?el?s?1, ne,ped/nGW?=?1.0) the variation in ELM behaviour is consistent with a transition from ?pure type I? to ?mixed type I/II? ELMs (Saibene et al 2002 Plasma Phys. Control. Fusion 44 1769). However, the pulses in this new database are better diagnosed in comparison to previous studies and most notable have pedestal measurements provided by the JET high resolution Thomson scattering (HRTS) system. We continue by presenting, for the first time, the role of pedestal structure, as quantified by a least squares mtanh fit to the HRTS profiles, on the performance across the fuelling scan. A key result is that the pedestal width narrows and peak pressure gradient increases during the ELM cycle for low fuelling plasmas, whereas at high fuelling the pedestal width and peak pressure gradient saturates towards the latter half of the ELM cycle. An ideal MHD stability analysis shows that both low and high fuelling plasmas move from stable to unstable approaching the ideal ballooning limit of the finite peeling?ballooning stability boundary. Comparison to EPED predictions show on average good agreement with experimental measurements for both pedestal height and width however when presented as a function of pedestal density, experiment and model show opposing trends. The measured pre-ELM pressure pedestal height increases by ?20% whereas EPED predicts a decrease of 25% from low to high fuelling. Similarly the measured pressure pedestal width widens by ?55%, in poloidal flux space, whereas EPED predicts a decrease of 20% from low to high fuelling. We give two possible explanations for the disagreement. First, it may be that EPED under predicts the critical density, which marks the transition from kink-peeling to ballooning-limited plasmas. Second, the stronger broadening of the experimental pedestal width than predicted by EPED is an indication that other transport related processes contribute to defining the pedestal width such as enhanced inter-ELM transport as observed at high fuelling, for mixed type I/II ELMy pulses.

Coherence imaging of scrape-off-layer and divertor impurity flows in the Mega Amp Spherical Tokamak (invited)a)

A new coherence imaging Doppler spectroscopy diagnostic has been deployed on the UKs Mega Amp Spherical Tokamak for scrape-off-layer and divertor impurity flow measurements. The system has successfully obtained 2D images of C III, C II, and He II line-of-sight flows, in both the lower divertor and main scrape-off-layer. Flow imaging has been obtained at frame rates up to 1 kHz, with flow resolution of around 1 km/s and spatial resolution better than 1 cm, over a 40° field of view. C III data have been tomographically inverted to obtain poloidal profiles of the parallel impurity flow in the divertor under various conditions. In this paper we present the details of the instrument design, operation, calibration, and data analysis as well as a selection of flow imaging results which demonstrate the diagnostics capabilities.