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

Scannell, R.; Walsh, M. J.; Carolan, P. G.; Darke, A.; Dunstan, M. R.; Huxford, R. B.; McArdle, G.; Морган, Д.; Naylor, G.; O’Gorman, T.; Shibaev, S.; Barratt, N C; Gibson, K. J.; Tallents, G. J.; Wilson, H. R.

doi:10.1063/1.2971971

Cited by 68 publications

(62 citation statements)

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“…Recent advances in imaging electron temperature fluctuations with high temporal and spatial resolution [18] suggest that the global stochasticity of the magnetic field [13] is not the dominant crash mechanism since the heat transport exhibits well organised, collective behaviour. Whilst previous experimental data has given great insight into the phenomenology of the crash [7,18,19], it had insufficient radial resolution to provide either validation or vitiation of the concept of triggering of secondary instabilities [8,15,16], and theoretical or numerical comparison has been stifled by this.The recently upgraded infrared Thomson Scattering (TS) system on MAST [20], with radial resolution < 10mm and the possibility of temporal resolution of 1µs, has allowed detailed analysis of the electron density and temperature profiles during a sawtooth crash. The system is designed to measure at high spatial resolution and achieve low systematic and random errors, allowing observation of changes in the gradients over narrow regions associated with magnetic islands.…”

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confidence: 99%

“…The recently upgraded infrared Thomson Scattering (TS) system on MAST [20], with radial resolution < 10mm and the possibility of temporal resolution of 1µs, has allowed detailed analysis of the electron density and temperature profiles during a sawtooth crash. The system is designed to measure at high spatial resolution and achieve low systematic and random errors, allowing observation of changes in the gradients over narrow regions associated with magnetic islands.…”

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confidence: 99%

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Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma

et al. 2010

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High resolution Thomson scattering measurements with sub-centimetre radial resolution have been made during a sawtooth crash in a fusion plasma in MAST. As magnetic reconnection occurs, a growing magnetic island induces an increase in the temperature gradient at the island boundary layer, before a very rapid collapse of the core temperature. The increase in the local temperature gradient is sufficient to make the plasma core unstable to ideal magnetohydrodynamic instabilities, thought to be responsible for the rapidity of the collapse.PACS numbers: 52.55Fa, 52.35PyMagnetic reconnection is the phenomenon of the breaking and rejoining of magnetic field lines in a plasma. Examples of this process are solar flares in astrophysical plasmas [1,2] and the sawtooth instability in tokamak plasmas [3,4]. Whilst the sawtooth instability was first observed in 1974 [5], the process by which this periodic collapse of the core plasma temperature occurs is still only partially understood. Detailed diagnosis of the sawtooth crash has shown that the temperature profile is initially essentially axisymmetric, but is then deformed by a helical instability before a very rapid temperature collapse re-establishes an axisymmetric profile with a lower value at the magnetic axis [6,7].Tokamak plasmas are susceptible to sawtooth oscillations when the safety factor, q = rB φ /RB θ is less than unity [8], where r, R are the minor and major radii and B φ , B θ are the toroidal and poloidal magnetic fields. The helical perturbation which arises during the crash has an m = n = 1 structure, where m, n are the poloidal and toroidal periodicity of the wave. The first explanation of the periodic temperature collapses was proposed by Kadomtsev [3], who showed that in the nonlinear regime of the m = 1 mode, reconnection occurs at the separatrix on the characteristic Sweet-Parker timescale [1,2] 2 /ηc 2 is the resistive diffusion time, ρ is the mass density, η is the plasma resistivity and S = τ R /τ A is the Lundquist number. This timescale is up to two orders of magnitude too large to explain crash times in large modern-day tokamaks, where τ crash ∼ 20 − 100µs, whereas τ K = 2 − 10ms.The three principal observations which any theory must explain are (i) the rapidity of the temperature collapse, (ii) the sudden onset of the collapse and (iii) the incomplete relaxation of the current profile whereby q remains below unity whilst the temperature profile relaxes completely. The onset of the crash represents a theoretical challenge, since the incremental change in the safety factor which governs the stability of the m = n = 1 mode is unacceptably small to explain the rapid onset. Many alternative crash models have since been proposed, including resistive two-fluid MHD [9], collisionless kinetic effects [10,11], accelerated complete reconnection due to nonlinear collisionless effects [12], magnetic stochastization leading to enhanced perpendicular transport [13] and triggering of secondary instabilities [14][15][16]. Each of these models has had proponents...

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Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma

et al. 2010

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“…On MAST, a good discriminator of ELM type is the electron temperature at the pedestal top, whereby type I ELMs are seen to have pedestal top temperatures above 100 eV [15]. The type I ELMs in the database are selected based on this threshold on the pedestal top temperature which is found using Thomson scattering [24] profiles of the plasma and performing a tanh fit to determine the pedestal parameters [25]. The parameters for a given ELM are taken in the last 20% of the ELM cycle.…”

Section: Elm Database Used For Analysis Of Target Datamentioning

confidence: 99%

The role of ELM filaments in setting the ELM wetted area in MAST and the implications for future devices

Thornton

Allan

Dudson

et al. 2016

Plasma Phys. Control. Fusion

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Abstract. The ELM wetted area is a key factor in the peak power load during an ELM, as it sets the region over which the ELM energy is deposited. The deposited heat flux at the target is seen to have striations in the profiles that are generated by the arrival of filaments ejected from the confined plasma. The effect of the filaments arriving at the target on the ELM wetted area, and the relation to the midplane mode number is investigated in this paper using infrared (IR) thermography and high speed visible imaging (>10kHz). Type I ELMs are analysed, as these have the largest heat fluxes and are observed to have toroidal mode numbers of between 5 and 15. The IR profiles during the ELMs show clear filamentary structures that evolve during the ELM cycle. An increasing number of striations at the target is seen to correspond to an increase in the wetted area. Analysis shows that the ratio of the ELM wetted area to the inter-ELM wetted area, a key parameter for ITER, for the type I ELMs is between 3 and 6 for lower single null plasmas and varies with the ELM midplane mode number, as determined by visible measurements. Monte-Carlo modelling of the ELMs is used to understand the variation seen in the wetted area and the effect of an increased mode number; the modelling replicates the trends seen in the experimental data and supports the observation of increased toroidal mode number generating larger target ELM wetted areas. ITER is thought to be peeling unstable which would imply a lower ELM mode number compared to MAST which is peeling-ballooning unstable. The results of this analysis suggest that the lower n peeling unstable ELMs expected for ITER will have smaller wetted areas than peeling-ballooning unstable ELMs. A smaller wetted area will increase the level of ELM control required, therefore a key prediction required for ITER is the expected ELM mode number.* For full list of contributors, see http://www.euro-fusionscipub.org/mst1

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“…One of the principal diagnostics on MAST is the Thomson Scattering system [56]. The diagnostic is based on the scattering of light emitted from 8 30Hz 1.6 Joule lasers directed into the plasma.…”

Section: Thomson Scatteringmentioning

confidence: 99%

“…Thomson scattering diagnostic (figure 5.8) [56], that measures such information at approximately 1cm resolution, with a nominal temporal resolution of nearly 4ms. As the diagnostic information collected is input into a time-independent plasma transport Figure 5.7: Summary of three discharges carried out during this study.…”

Section: Thomson Scatteringmentioning

confidence: 99%

Characterisation of detached plasmas on the MAST tokamak

Harrison

2011

Journal of Nuclear Materials

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The mitigation of the steady-state heat loading to plasma-facing components in the divertor structures is an essential prerequisite for the operation of next-generation conventional and tight aspect-ratio Tokamaks. Currently, the most attractive means of reducing this heat loading is the so-called "detached" regime, which is the primary focus of this study.This thesis describes work that was carried out on the MAST tokamak to accurately characterise the plasma conditions within divertor plasmas, and to test the validity of those measurements. To assist in this study, an interpretive code, OSM [1], has been upgraded with additional numerical schemes to ascertain the dominant mechanisms governing parallel scrape-off layer transport including flux expansion, dynamic viscosity and cross-field drift motion in attached conditions. It has been found that the invocation of flux expansion, parallel viscosity and diamagnetic drift motion assist the OSM numerical scheme to converge in the presence of (static+dynamic) parallel pressure gradients often observed between the outboard midplane and lower outer divertor leg.It has also been found that the ion pressure, which is currently unknown, could resolve the observed pressure discrepancy.Line-of-sight spectroscopy of high-n Balmer emission lines has been used to ascertain line averaged T e and line integrated n e during the detached phase of MAST discharges with high spatial resolution (≈ 7mm) to determine electron static pressure profiles along flux contours. Furthermore, narrow-bandwidth imaging spectroscopy of D α , D γ , CII and CIII emission during the onset and sustainment of detachment has been carried out with high temporal (5kHz) and spatial (≈ 3mm) resolution. These data have been combined with ion flux measurements to the divertor target plates using embedded Langmuir probes (6-9mm radial resolution) and upstream Thomson scattering measurements of n e , T e . The data was input into the OSM code, coupled with the kinetic neutral transport code EIRENE [2], to check the experimental data set for internal consistency. The experimental data collected has been used to reconstruct the plasma conditions within the detached divertor leg, heavily constrained by experimental data. EIRENE calculations with re-constructed plasma conditions are able to reproduce experimentally observed D γ /D α line emission ratios to within a factor iii iv of 2 within the recombining region. AcknowledgmentsThe work presented in this thesis is the culmination of the work of many people, who I gratefully acknowledge here. Foremost, I thank Dr. Steven Lisgo for his guidance, enthusiasm and assistance over the last 4 years. It is only on reflection that I appreciate how he kept this work on track, whilst encouraging me to explore my own ideas and how doing so has enhanced the quality of the work presented here. Drs. Andrew Kirk and Geoff Fishpool also deserve special thanks for their guidance, and above all, their understanding in the final months of this work. I also owe great thanks to...

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

Cited by 68 publications

References 6 publications

Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma

Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma

The role of ELM filaments in setting the ELM wetted area in MAST and the implications for future devices

Characterisation of detached plasmas on the MAST tokamak

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