Overview of Physics Results from MAST

Transcription

1 23 rd IAEA Fusion Energy Conference, Daejeon, South Korea, October 2010 Overview of Physics Results from MAST Brian Lloyd for the MAST Team & Collaborators EURATOM / CCFE Fusion Association CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority Jointly funded by EURATOM & RCUK 1Energy Programme

2 Introduction MAST is equipped with a wide range of tools e.g. long pulse, high power NBI digital plasma control error field compensation coils ELM control coils Adaptable fuelling incl. pellet injection divertor science facility (manipulator) disruption mitigation system ELM coils.etc and powerful diagnostics TS collection lens e.g. (very) high resolution Thomson scattering MSE (35 chords, ~1ms resolution) CXRS (toroidal, poloidal) edge Doppler spectroscopy high speed imaging (visible, IR) high frequency magnetics extensive edge measurements etc 2

3 MAST Mission to explore the long term potential of the spherical tokamak as a fusion component test facility (CTF) and/or ST power plant (STPP) to advance key tokamak physics for optimal exploitation of ITER and DEMO design optimisation to provide unique insight into underlying tokamak physics 3

4 Introduction Outline Stability - macroscopic stability - fast particle instabilities Confinement & transport - confinement scaling (ν*, q) - transport barrier physics Pedestal Physics ELM control application of RMPs Exhaust Physics - SOL ion energy measurements - disruption mitigation Summary & Future Plans 4

5 Stability internal kink mode High resolution MSE allows improved understanding of performance limiting instabilities e.g. in AT regimes with weak/reversed shear Fishbones 1/1 long-lived mode MSE Long-lived mode found unstable as q 1. Identified as internal n = 1 kink mode - damps rotation, degrades confinement, expels fast ions (reducing fishbone drive) - measured braking compares well with NTV theory (M-D Hua et al PPCF 2010) n = 3 n = 2 I.T. Chapman et al, Nuc. Fus n = 1 Higher-n modes progressively more unstable as q decreases - q (= q min - 1) threshold larger in STs EXS/P5-04 5

6 NTM critical island widths High resolution Thomson scattering system used to probe NTM critical island width - lasers in burst mode (20µs) Each measurement samples different phase of island; mapped to φ using Mirnov data K. Gibson et al I.T. Chapman et al EXS/P5-04 6

7 NTM critical island widths High resolution Thomson scattering system used to probe NTM critical island width - lasers in burst mode (20µs) Finite parallel transport model, constrained by TS measurements, used to calculate critical island width w(cm) TS Calculated critical width below observed threshold Future work to develop free streaming parallel transport model and full kinetic model + include other threshold effects K. Gibson, T O Gorman et al, I.T. Chapman et al EXS/P5-04 7

8 Sawtooth crash mechanism Growing island structure causes increase in T e at island boundary island observed for ~ 80µs Radial location of strongest T e moves to region of lower magnetic shear triggering secondary instability which explains rapid crash (<20 µs) t = 20µs Core becomes ideally MHD unstable when destabilising pressure gradient overcomes stabilising shear (ballooning or Mercier unstable depending on shaping) 3D modelling shows helical core is more unstable to ballooning modes than axisymmetric modelling I.T. Chapman et al 8

9 Sawtooth crash mechanism Growing island structure causes increase in T e at island boundary island observed for ~ 80µs Radial location of strongest T e moves to region of lower magnetic shear triggering secondary instability which explains rapid crash (<20 µs) t = 20µs Core becomes ideally MHD unstable when destabilising pressure gradient overcomes stabilising shear (ballooning or Mercier unstable depending on shaping) 3D modelling shows helical core is more unstable to ballooning modes than axisymmetric modelling I.T. Chapman et al 9

10 Stability Fast ion physics Conf/transport Pedestal ELM control Exhaust frequency Fast particle driven modes in MAST cover a broad frequency range - Alfvén Cascades (RSAE) TAE (ω ~ va/2qr) CAE (ω ~ ωci) Dynamical friction (drag) shown to be important for describing non-linear wave evolution with a super-alfvénic fast ion source - in MAST vb >> va and a good proxy for α-particles in ITER & DEMO time Frequency sweeping TAE in MAST # D bump-on-tail model M Lilley et al (Chalmers U.) EXW/P7-14 Realistic tokamak simulation of α-driven n = 3 core localized TAE using HAGIS (non-linear driftkinetic δf code) B Breizman et al (IFS, Texas) THW/P

11 Confinement scaling (ν*, q) Dedicated scans: cf. IPB98y2 Consistent with reported dependence on engineering parameters GYRO: micro-tearing modes might be a candidate to explain ν* e dependence B T dependence of neutron emission consistent with ν* scaling S B, B τ 4 xν+ 6 x DD T E, th * ν* e scaling is important because ν* e represents largest extrapolation gap towards an ST - based Component Test Facility M. Valovic et al ν ν EXC/P

12 ν* scan transport analysis TRANSP q nc q t q i Ion transport close to neo-classical, local heat transport dominated by electrons consistent with global scaling Fast ion diffusion rate D fast = 2 3 m 2 /s in TRANSP is necessary to match the observed neutron rates M. Valovic et al EXC/P

13 Transport studies MSE/CXRS enable influence of q(r) and flow shear to be studied in MAST Poloidal rotation is small profiles consistent with neoclassical predictions (A. Field, PPCF 2009) - strong driven toroidal rotation dominates ExB flow shear co-nbi Magnetic shear H-mode: χ i ~ 1-3 x χ i neo over most of radius L-mode: χ i >> χ i neo at large r/a, but ion transport strongly suppressed by flow shear at mid-radius ITB formation favoured by early NBI negative magnetic shear in the core T i gradient toroidal rotation gradient With co-nbi, ITBs form in ion and momentum channels just inside q min and χ i χ neo i Some correlation between magnitude of normalized toroidal rotation gradient and the passing of q min through rational values A.R. Field et al EXC/P

14 Microstability Linear, GS2 calculations with kinetic electrons & with/without flow shear (co-nbi) (C. Roach et al) ρ= 0.3 (core): linearly stable to all modes with/without flow shear due to weak negative magnetic shear. Results unchanged by inclusion of e.m. effects 10 #22807 γ (v ti /a) = stable k ρ i ρ = 0.52: Adiabatic electrons ITG modes fully stabilised by flow shear Kinetic electrons appreciable TEM drive, only partially stabilised by flow shear ρ = 0.7: Weak flow shear insufficient to stabilise strongly growing ITG modes. TEM modes stable due to reduced drive at higher collisionality A.R. Field et al EXC/P8-04 M. Barnes et al THC/P4-01 (Non-linear simulations) 14

15 Pedestal edge T i (r) measurements L-H transition (see H. Meyer et al EXC/2-3Ra) Threshold scaling studies (variation with X-pt height, kappa, He vs. D, effect of RMPs (A Kirk et al EXD/8-2 etc)) High resolution pedestal profile measurements (E r, T e, n e ) Edge T i measurements Novel high resolution edge T i measurements from CX emission (C 6+ ) using cold deuterium gaspuff to localize measurement Collisionality dependence consistent with analysis of Kagan & Catto (PPCF 50, (2008)) which showed that in the banana regime L Ti >> ρ i pol T Morgan (U. York) H. Meyer et al EXC/2-3Ra 15

16 Pedestal edge j(r) measurements Large magnetic field line tilt in the ST enables MSE measurements of edge j φ (r) evolution with 2ms resolution. Effect of measured radial electric field included (small) Large increase in edge current density observed at L-H transition (x5) ELITE calculations show pedestal in vicinity of ballooning stability boundary at time of type I ELMs but time-dependence of j φ, p ( MSE & high resolution TS) exhibit unexplained features Furthermore, EBW emission shows evidence of a more complex edge current structure 2D EBW imaging system being developed to investigate further. M. De Bock et al H. Meyer et al EXC/2-3Ra 16

17 ELM control internal array: 1.4kA, 4-turn coils (n = 3) - similar to DIII-D I-coils Even parity (Same sign current in upper and lower coils at same toroidal location) I coil up I coil down Odd parity (Opposite sign current in upper and lower coils at same toroidal location) I coil up I coil down Resonant effects observed in L-mode with similar I coil threshold (~1kA) - density pump-out - enhanced fluctuations (inside LCFS) - increased (more positive) E r ped ELMs can be triggered in ELM free discharges or the ELM frequency increased in type III ELM-ing discharges Initially no effect on type I ELMs could be observed despite a wide region of island overlap (Chirikov parameter σ chir > 1) type I ELM mitigation subsequently observed if q 95 carefully optimised A Kirk et al EXD/8-2 17

18 Mitigation of type I ELMs q scan to find optimum alignment with applied perturbation Chirikov parameter little changed but resonant field changes q 95 = 4.9 DN T e (ev) n e (x10 19 m -3 ) Clear effect on density Little effect on T e f ELM increases by 5 W ELM reduces from 5 kj to ~ 1kJ (f ELM. W ELM ~ constant) W MHD reduces by ~ 8% Pedestal parameters suggest a transition from type I to type IV (low collisionality branch of type III) ELMs ELM mitigation also observed in SN discharges 0.2 R(m) 1.6 A Kirk et al EXD/8-2 18

19 Effect of plasma response (L-mode) Pump-out different for even and odd parity in discharge for which alignment and Chirikov parameter are similar Chirikov not the only important parameter resonant component of applied field different Effect of plasma response MARS-F - single fluid linear MHD code, which solves the full resistive MHD equations in toroidal geometry - takes into account plasma response and screening due to toroidal rotation Large reduction in size of resonant components and reduction in Chirikov parameter Y. Liu THS/P5-10 A Kirk et al EXD/8-2 Enhances relative difference in b r res between even and odd parity 19

20 SOL ion energy measurements Motivation: interpretation of probe data (n e, P div ) determines physical sputtering rates from plasma facing materials ELM ion energies in the far SOL unknown Retarding Field Analyzer (CEA) First measurements show T i ~ (1 2.6) x T e at outboard mid-plane (much higher energies observed in fluctuations) I p = 0.63MA L-mode Ion energy in ELM filaments - large signals observed as far as 20cm from the LCFS and up to 500V of biasing P. Tamain, S. Allan, S. Elmore (Liverpool U.) RFA supplied by CEA Cadarache 20

21 Disruption mitigation particles (10% Argon 90% Helium) Fast gas valve supplied by FZ Jülich Impurity ions penetrate to q = 2 surface prior to thermal quench (high speed imaging) Local density build-up and initiation of thermal quench when cooling front reaches q = 2 surface 60 70% reduction in peak divertor power loads A. Thornton et al 21

22 Summary & Future Plans MAST is addressing a wide range of important physics issues for ITER and future STs, exploiting powerful control tools (e.g. ELM control coils, disruption mitigation system etc), increasingly sophisticated diagnostics and supported by extensive theory and numerical modelling capability. The MAST programme is underpinned by strong and wide-ranging collaborator contributions New capabilities in 2011 Additional ELM control coils 2D BES system (with RMKI Hungary) collimated neutron detector (with Uppsala U.) fast ion D α (FIDA) diagnostic fast edge Doppler spectroscopy ( 50kHz) electron Bernstein wave imaging (with University of York) 22

23 MAST Upgrade Project kick-off July 2010 Construction Goals Demonstrate physics viability of a ST based Component Test Facility Contribute to the ITER/DEMO physics base Demonstrate effectiveness of a flexible Super-X divertor 23 D. Stork, H. Meyer et al ICC/P5-06