MAST and the Impact of Low Aspect Ratio on Tokamak Physics

1. MAST and the Impact of Low Aspect Ratio on Tokamak Physics Brian Lloyd for the MAST Team Euratom/UKAEA Fusion Association This work was jointly funded by the UK Engineering & Physical Sciences Research Council and Euratom

2. B. Lloyd 1), J-W. Ahn 2), R.J. Akers 1), L.C. Appel 1), D. Applegate 1), K.B. Axon 1), Y. Baranov 1), C. Brickley 1), • C. Bunting 1), R.J. Buttery 1), P.G. Carolan 1), C. Challis 1), D. Ciric 1), N.J. Conway 1), M. Cox 1), G.F. Counsell 1), • G. Cunningham 1), A. Darke 1), A. Dnestrovskij 3), J. Dowling 1), B. Dudson 4), M.R. Dunstan 1), A.R. Field 1), S. Gee 1), • M.P. Gryaznevich 1), P. Helander 1), T.C. Hender 1), M. Hole 1), N. Joiner 2), D. Keeling 1), A. Kirk 1), I.P. Lehane 5), • F. Lott 2), G.P. Maddison 1), S.J. Manhood 1), R. Martin 1), G.J. McArdle 1), K.G. McClements 1), H. Meyer 1), • A.W. Morris 1), M. Nelson 6), M. R. O'Brien 1), A. Patel 1), T. Pinfold 1), J Preinhaelter 7), M.N. Price 1), C.M. Roach 1), • V. Rozhansky 8), S. Saarelma 1), A. Saveliev 9), R. Scannell 5), S. Sharapov 1), V. Shevchenko 1), S. Shibaev 1), • K. Stammers 1), J. Storrs 1), A. Sykes 1), A. Tabasso 1), D. Taylor 1), M.R. Tournianski 1), A. Turner 1), G. Turri 2), • M. Valovic 1), F. Volpe 1), G. Voss 1), M.J. Walsh 10), J.R. Watkins 1), H.R. Wilson 1), M. Wisse 5) • and the MAST, NBI and ECRH Teams. • 1)EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, UK • 2)Imperial College, Prince Consort Road, London SW7 2BZ, UK • 3)Kurchatov Institute, Moscow, Russia • 4)Oxford University, UK • 5)University College, Cork, Ireland • 6)Queens University, Belfast, UK • 7)EURATOM/IPP.CR Fusion Association, Institute of Plasma Physics, Prague, Czech Republic • 8)St. Petersburg State Politechnical University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia • 9)A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia • 10)Walsh Scientific Ltd, Culham Science Centre, Abingdon, OX14 3EB, UK

3. Outline Properties of Low Aspect Ratio Plasmas MAST - Mega Ampere Spherical Tokamak Impact of Low A on Tokamak Physics: - confinement & transport - stability - exhaust Summary

4. Bp(R+a) ~ Bt  large field line tilt & low parallel power density in the outboard SOL Bt(R-a) / Bt(R+a) ~ 5  enhanced trapping Strong paramagnetism Impact on transport, resistivity Low moment of inertia  high flow velocity (V ~ Vith) Large inherent ExB flow shear  suppression of micro-instabilities (ITG) Properties of Low A Plasmas Improved stability + good confinement  high beta ( ~ 40% in START, NSTX) Increased decoupling of j(r) & q(r) High shaping (, )  high Ip capability High performance at low B  super-Alfvenic ions Fast particle driven instabilities

5. pelletablation field-line angle cloud (from EFIT) 1 2 3 4 50 40 30 angle (degrees) 20 separatrix 10 0 0.2 0.4 0.6 0.8 1 1.2 -10 R (m) -20 -30 #4917 -40 5 6 7 8 Large field line tilt - emphatically illustrated during pellet injection

6. #2948 L-mode (t= 170ms) H-mode (t = 290ms) MAST Plasma cross-section and current comparable to ASDEX-U and DIII-D. R/a = 0.85/0.65m,   2 Ip  2MA, B = 0.52T@R Open divertor, up-down symmetric - upgraded 2004 Graphite protection on all plasma contacting surfaces Adaptable fuelling systems - inboard & outboard gas puffing/multi-pellet injection Digital plasma control (PCS supplied by GA)

7. Reconstruction of Zeff from visible bremsstrahlung: ZEBRA 2D image Abel inverted emission Z=1 prediction from TS 2D Zeff CCD detector (ZEBRA) 128x128 @200Hz, 256x256 @100Hz

8. Resolution of original CXRS system (19 chords) marginal for steep ITBs Upgraded CXRS facilitated by adaptable low A configuration  200+ chord spectrometer spatial resolution ~ i poloidal and toroidal chords separate views of two NBI beams Charge-exchange recombination spectroscopy: Ion temperature & flow velocity measurements co-NBI cntr-NBI

9. q=1 surface appears (from SXR data) Strong toroidicity in the ST  significant neoclassical enhancement of plasma resistivity: neoclassical Spitzer Ultra-high resolution TS and visible bremsstrahlung measurements of Te and Zeff in MAST allow neoclassical resistivity to be assessed with unique accuracy.

10. Confinement & Transport MAST data significantly extend confinement databases e.g. should give greater confidence in  and  dependencies Dataset improved e.g. spread in  mainly determined by plasmas with conventional D-shaped cross-section  EMAST ~ EIPB98y2 but MAST data support somewhat stronger  dependence (E  0.8) than IPB98y2 scaling [Valovic IAEA 2004] MAST data also exert strong leverage on two-term models of confinement: Wped  –2.13±0.28 [Cordey et al NF 2003]

11. Pth(MW) Pscal(MW) H-mode Power Threshold Low A data: - clearly favour Pth ~ S rather than Pth ~ R2 - favour dependence on |Bout| rather than Bt(0) |Bout|2 = Bt2 + Bp2 The (non-linear) aspect ratio dependence is not yet well-determined - postulated by Takizuka et al that it may take a form related to fraction of untrapped particles Pscal |Bout|0.7ne0.7S0.9F(A) [Takizuka et al PPCF 2004]

12. Inboard Gas Puff Linked to effect on edge rotation H-mode Access H-mode access improved with inboard fuelling - also seen in NSTX and in COMPASS-D (effects less pronounced). Two effects may contribute (both enhanced at low A) - neutral viscosity contribution to angular momentum transport (Helander et al) - net torque arising from B drift effects (Rozhansky et al) H-mode access also optimised for a connected DND configuration with |rsep| < i , i (i  6 mm  q ) - indications of a similar (but weaker) effect in AUG Dependence on fuelling location & magnetic geometry appears to be amplified at low A - helps to provide improved insight into factors influencing H-mode transition

13. Microstability in STs Re Im In STs ITG turbulence might be suppressed by intrinsic pressure driven flow shear since SE/m ~ i* [Kotschenreuther et al 2000] GS2  ITG growth rate  ExB shearing rate in typical MAST H-mode discharge [Applegate et al EPS 2004] ETG growth rate much larger - ‘mixing length’ estimates too low to account for observed transport but non-linear effects (e.g. ‘streamers’) may play a role. Electromagnetic effects  stabilising effect on ETGs but in the plasma core can give rise to dominantly unstable tearing parity modes in the ITG wavelength regime. At high , tearing parity modes may be important at both ETG and ITG r length scales - expected to enhance electron transport [Joiner et al EPS 2004] A|| radians

14. ne x1019m-3 Te Ti H-mode transport coefficients are close to ion neoclassical e ~ i around mid-radius & close to iZ-CH [Chang & Hinton] [ exact values very sensitive to relative values of Te, Ti ] TRANSP simulations

15. [keV] High performance in sawtooth-free L-mode HHIPB98y2 ~ 0.8 HHITER96L ~ 1.6 e ~ i >> iZ-CH [m2/s] TRANSP simulations

16. H-mode/L-mode comparison discharges Strong dependence of H-mode access on magnetic configuration facilitates sensitive H-mode access control mechanism  e.g. allows H-mode/L-mode comparison at same engineering parameters WL-mode/WH-mode ~ 2/3

17. High confinement in counter-NBI Measured neutron rates consistent with LOCUST modelling - only ~ 1/3 fast ion energy absorbed But stored energy comparable to co-NBI  HHIPB98y2 ~ 2 ne(r) more peaked, Te(r) broader than co-NBI High rotation (V0 ~ 340km/s) due to rapid loss of co-moving ions  in-out Zeff asymmetry x2 (dominated by C6+)

18. (ms) (ms) cntr-NBI co-NBI Sawtooth-free discharges HH 1.5 with co-NBI; HH 2 with cntr-NBI HH=2 HH=1

19. cntr-NBI co-NBI Density profile peaking strongly correlated with Ware pinch Neoclassical Ware pinch stronger at low A (  1/2) and augmented by beam-driven pinch for cntr-NBI (Ware pinch dominant) - further increased due to higher Zeff of cntr-NBI discharges [Akers et al EPS 2004]

20. Ion & Electron ITBs ITB existence criteria Criteria based on critical values of R/LT or s/LT fail - readily satisfied even when no ITB In these discharges the driven toroidal flow is the dominant contribution to the ExB flow shear In this case ITB formation may be linked to a critical Mach number [Field et al EPS 2004] x1019m-3 Co-NBI Strong ion ITB i ~ iZ-CH Weaker eITB e ~ 2-3 x iZ-CH Cntr-NBI Strong eITB at large radius [keV] TRANSP simulations

21. By avoidance of NTMs N > 5, (N > 5li) has been achieved in MAST  approaching ideal no-wall beta limit. unstable stable Taking the Sauter NTM model, benchmarked against MAST it appears that the STPP may be stable to NTMs KINX calculations b) a) Menard calculation: - q*/q0=3 - q*/q0=1.5 Stability fBS ~ 40%, Wfast ~ 15 - 20% Sawtooth triggered NTMs have been observed in MAST - island evolution confirms strong role of field curvature stabilisation (Glasser) term at low A

22. High Bootstrap High elongation at low A helps Low internal inductance High Elongation fBS ~ Nh()Irod/Ip {h()   approx.} Bootstrap Current Steady-state regimes rely on high bootstrap fraction fBS ~ 40% in a Component Test Facility (CTF) ~ 90% in an ST Power Plant (STPP) Primary auxiliary current drive scheme - neutral beam current drive (NBCD) - implementation of an off-axis NBCD capability in MAST under investigation

23. Discrete TAE & EAE activity in MAST - relatively benign Chirping modes also observed incl. simultaneous up-down chirping as predicted by hole-clump model (Berk et al, Pinches et al, Vann et al EPS 2004) Fast Particle Driven Instabilities Low Alfven velocity (low B) + wide gaps in Alfven continuum (high toroidicity & ellipticity)  TAEs & EAEs can be driven unstable by energetic ions. f (x100kHz) f vA/2qR (EAE) f vA/4qR (TAE) t(s) Fishbone modes are seen at higher  than TAEs or chirping modes - can trigger longer lasting modes

24. MISHKA TAE eigenfunctions at different values of thermal beta  theory predicts TAE modes stabilised in MAST at high beta (as observed experimentally) TAEs stabilised at high  For TAEs and chirping modes both the amplitudes and number of unstable modes decrease with increasing , in accord with theory, due to plasma pressure and thermal ion Landau damping [Gryaznevich & Sharapov PPCF 2004] x10-1 Increasing  - TAE mode stabilised CAEs having k >> k||, can be expected to be of increasing importance at high beta due to relatively weak Landau damping. Preliminary evidence for CAE mode activity in MAST [Appel et al EPS 2004]

25. Ballooning nature of ELMs RP data + edge plasma toroidal velocity measurements consistent with n ~ 10 filamentary structure ELMs ELMs associated with large radial effluxes at outboard side (<vr> ~ 0.75 kms-1) RP observes large jsat out to ~15 cm Thomson scattering Linear D Kirk et al Phys. Rev. Lett. 92 (2004) 245002

26. ELMs Structure sometimes seen on TS profiles outboard of separatrix. Also observed on the mid-plane linear D array but not on divertor target power footprint Edge profile broadening or other structure only observed in 20 - 25% of cases in which TS fires during ELM D rise.

27. Features are consistent with non-linear ballooning mode theory [Cowley/Wilson]  ELM composed of several narrow filaments, each extended along a field line, projecting into SOL at outboard mid-plane and connecting back into the core plasma far along the field line ELM spatial structure (theory+experiment) Image simulation of an extended structure @q=4, n=10, #8814

28. The spatial and temporal evolution of an ELM Filament eventually detaches at outboard mid-plane Wfil < 0.03WELM Filament remains attached to core plasma & acts as conduit for enhanced transport into SOL Wfil << WELM

29. Increases practical feasibility of advanced divertor schemes such as the cascading pebble divertor Exhaust STs - small area of inboard divertor targets may lead to high power densities But favourable divertor target power distribution: - large ratio of outboard to inboard separatrix area ( ~ x4) in low A plasmas - equal up-down power distribution in DND High Bp/Bt in outboard SOL leads to low parallel power densities:  local target protrusions intercept a small fraction of power efflux so ST is less sensitive to tile mis-alignment for example Pout >> Pin

30. The ST SOL Modelling of the MAST SOL (OSM2/EIRENE) has shown the importance of including the mirror force ||B/B for the ST [Kirk et al PPCF 2003] - x 10 larger in MAST than in JET In MAST the magnetic field is x5 greater in the i/b SOL than the o/b SOL, but the connection length is similar  experimental measurements indicate that outboard transport coefficients exceed inboard coefficients by a factor ~ 2 - 10

31. Summary Properties of low A plasmas (high beta, low field, strong shaping, high rotation, high ExB flow shear, large mirror ratio, large field line pitch angle etc) have important effects on plasma behaviour. Strong synergy between STs and conventional aspect ratio tokamaks: - rapid ST development due to tokamak knowledge base - STs are unique testing ground for tokamak physics STs are providing valuable input to international databases and helping to provide improved insight into tokamak behaviour (eg. ELM structure, H-mode access…) as well as providing stringent tests of theory (neoclassical resistivity, Ware pinch, etc.) The new generation of STs are confirming the excellent confinement and stability properties of low A plasmas (i ~ e ~ ineo, N ~ ideal no-wall limit) Low A plasmas exhibit several favourable properties (e.g. large outboard/inboard power efflux ratio, low parallel power density in the outboard SOL etc) to aid development of viable divertor solutions for future ST devices.