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EU Spherical Tokamak Approach to High Beta Steady State Operation

EU Spherical Tokamak Approach to High Beta Steady State Operation. Brian Lloyd Euratom/UKAEA Fusion Association. This work was jointly funded by the UK Engineering & Physical Sciences Research Council and Euratom. Outline. Introduction. - EU ST programme

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EU Spherical Tokamak Approach to High Beta Steady State Operation

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  1. EU Spherical Tokamak Approach to High Beta Steady State Operation Brian Lloyd Euratom/UKAEA Fusion Association This work was jointly funded by the UK Engineering & Physical Sciences Research Council and Euratom

  2. Outline Introduction - EU ST programme - ST Power Plant & CTF Requirements - Key issues for high beta steady state operation Progress towards high beta steady-state operation - stability - confinement - current drive - exhaust MAST developments Summary & Conclusions

  3. Introduction EU ST programme is centred at UKAEA Culham and focussed on the Mega Ampere Spherical Tokamak (MAST) Other EU Associations (e.g. IPP, ENEA, CRPP, IST, FOM) provide important contributions through collaborations and there are strong international links with the U.S., Russia, Japan and Brazil. ENEA is studying the SPHERA concept. MAST is the successor (first physics operation 2000) to the pioneering START experiment The goals of MAST are twofold: - to advance key tokamak physics issues for optimal exploitation of ITER - to explore the long-term potential of the spherical tokamak (ST).

  4. MAST Parameters Plasma cross-section and current comparable to ASDEX-U and DIII-D. Open divertor, up-down symmetric Graphite protection on all plasma contacting surfaces Adaptable fuelling systems - inboard & outboard gas puffing plus multi-pellet injector Digital plasma control implemented June 2003 (PCS supplied by GA)

  5. 1GW(e) ST Power Plant: physics design parameters Howard Wilson, Garry Voss et al E

  6. High Bootstrap Low internal inductance High Elongation STPP parameter drivers • Neutron wall loading (3.5MWm-2) drives the size: R=3.4m • Cost of electricity limits toroidal field, Irod ~Ip • MHD limits N=8.2 • High elongation required for ~90% pressure-driven current; vertical instability  =3.2 (fs = 3.0) • Required fusion power (~3GW)  Irod=30.2MA (Ip=31MA) • Non-inductive current drive requires low density ~1.1x1020m-3 (~60% Greenwald) • Confinement, E=1.6 IPB98(y,2) or 1.4 IPB98(y,1) fBS ~ Nh()Irod/Ip Howard Wilson et al

  7. STPP Stability • Ballooning modes - a 2nd stable solution exists with 90% pressure driven current • High central safety factor • ~uniform magnetic shear across the plasma  hollow current profile • BUT in an ST q() can remain monotonic Close fitting wall and high q(0) ensures n=1, 2 and 3 kink mode stable NTMs? Considerable uncertainty but stabilising Glasser term very strong. High q(0) avoids low order modes. Howard Wilson et al

  8. Current drive in the STPP Required aux CD profile 0.14MA 2.2MA 0.5MeV -ve D beam 80keV +ve D beam Rob Akers, Howard Wilson et al

  9. Confinement • Pressure profiles chosen to be consistent with ideal MHD stability • The thermal diffusivities required have been calculated: • - A broad minimum in c is required towards the edge • - Control of c may be necessary, eg through ITBs Howard Wilson Alexei Dnestrovskij et al

  10. Non-monotonic B(R) profile helps confine a’s The full orbit code CUEBIT has been used to calculate the lost a-fraction Despite large orbits in the core, the increasing B towards the edge “pinches” the orbits, improving confinement: • Taking account of the ~1% TF ripple, losses are low, at the level <~1% R(m)

  11. Component Test Facility: Physics design parameters Example: T = 11keV, ne = 1.8x1020m-3 Component Test Facility (CTF) Garry Voss, Howard Wilson et al

  12. Key issues for high beta steady state operation: Stability - the STPP is confinement & stability limited. A stable route to N ~ 8 has to be demonstrated. Control/avoidance of NTMs. Bootstrap predictions need to be validated experimentally. Consistency of implied  profiles to be determined. Confinement - the CTF is primarily ‘confinement limited’; confinement scaling with P, Ip important. Confinement scaling with beta important for the STPP. Current drive - experimental demonstration of suitable schemes for off-axis current drive, e.g. NBCD, electron Bernstein wave current drive Exhaust - SOL characteristics and extrapolation to the STPP and CTF; testing of novel divertor concepts e.g. ‘biased’ divertor, cascading pebble divertor etc.

  13. The inherent high beta capabilities of the ST were first confirmed in START Alan Sykes, Mikhail Gryaznevich et al

  14. tmax = 16% (%) MAST Beta Operating Space 2002-03 N > 5, approaching ideal n=1 no-wall external kink limit Avoidance of neo-classical tearing modes (NTMs) by operating in regimes where sawteeth are small or absent altogether. Mikhail Gryaznevich, Richard Buttery et al

  15. - #7020 - #8789 Ip PNBI N W D t(s) Typical high N discharges in MAST Mikhail Gryaznevich et al

  16. unstable stable b) a) Menard calculation: - q*/q0=3 - q*/q0=1.5 Ideal no-wall beta limit approached By avoidance of NTMs N > 5, (N > 5li) has been achieved approaching the ideal no-wall beta limit - no obvious MHD limit to performance. Main limit so far due to initial NBI system capabilities High N confirmed by kinetic measurements fBS ~ 40 - 50% Wfast ~ 15 - 20% PNBI = 2.8MW Matthew Hole, Richard Buttery et al KINX calculations

  17. 0.6 0.5 0.4 Neoclassical tearing modes (NTMs) Sawtooth triggered NTMs (m/n = 3/2, m/n = 2/1) have been observed in MAST 3/2 NTM is excited close to its saturated size and at p close to pcrit  strong seeding process 3/2 NTM reduces confinement by typically ~ 10%; approximate agreement with Chang & Callen belt model 2/1 NTM can trigger HL transition followed by mode locking and disruption Richard Buttery et al

  18. Neo-classical island evolution Island evolution confirms strong role of field curvature stabilisation term (Glasser term) (which cancels ~60% of the drive provided by the bootstrap current in MAST) Small island stabilisation effects (finite island transport and/or polarisation current effects) also necessary to explain observed island evolution. Typical island widths: ~ 4cm for m/n = 3/2 ~ 10cm for m/n = 2/1 Magnetic estimates confirmed by Thomson scattering Richard Buttery, Olivier Sauter et al

  19. 0 1  Energetic particle driven modes STs support a rich variety of EPD modes - but TAEs and chirping modes found to be stabilised for  > 5% and  > 15% respectively. MISHKA TAE eigenfunctions at different values of thermal beta  theory predicts TAE modes stabilised in MAST at high beta as observed experimentally  = 4.3%  = 5.7% Chirping modes - experimental measurements show mode amplitude reduces as beta increases.  = 6.1% Sergei Sharapov, Mikhail Gryaznevich et al

  20. Highest t discharge subject to n=1, n=2 tearing modes Mikhail Gryaznevich, Richard Buttery et al

  21. CTF Quasi-stationary H-modes with E ~ EIPB98(y,2) Normalised parameters achieved comparable with requirements of a Component Test Facility (CTF) N ~3, HH ~ 1, ne/nGr ~ 0.5 sustained for ~200ms (~ 4E) Ip = 0.73MA, BT = 0.46T Martin Valovic et al

  22. MAST H-mode transport close to ion neoclassical Ti=Ti(CXRS) r/a>0.5, ci~neoclassical r/a<0.5, ce~ion neoclassical Sensitivity test (strong e- D+ coupling) R<1.2m, Ti=Ti(CXRS) R>1.2m, Ti=Te r/a>0.5, ci~ ce ~ 6m2/s r/a<0.5, ci ~ ce ~ion neoclassical ci~ ce ~ 2-4m2/s in the mid radius region of the plasma (as required by STPP). R. Akers et al

  23. ce~3m2/s in the barrier region ci~2m2/s (comparable to Z-corrected Chang Hinton neoclassical value) in the barrier region cf~0.7m2/s in the barrier region (3x lower thanci) r*T(e-,D+)~0.14 (cf. r*ITB=0.014) - ITB criterion easily achieved in MAST ITBs with co-NBI - early heating, fast Ip ramp Rob Akers, Anthony Field et al

  24. Te, Electron ITB with counter-NBI High performance counter injection plasmas on MAST: Neutron rate down by ~2/3 but plasma energy comparable to co-injection. Slightly higher Zeff (~2-2.5, but no peaking of profile). Te profiles are broader, ne profiles more peaked than for co-injection. Maximum plasma energy ~120kJ in ELMing counter injection H-mode (no ears). Rob Akers, Anthony Field et al

  25. Te, Electron ITB with counter-NBI ce~1m2/s in the barrier region (~Chang Hinton ci) ci~10m2/s (no evidence of ion ITB) cf~2.0m2/s in the barrier region (5x lower thanci)

  26. Co-NBI Cntr-NBI Current Drive Two main options being pursued: NBCD Electron Bernstein Wave (EBW) current drive Initial NBCD studies promising - Preliminary studies at modest beam power, indicate a neutral beam driven current (~ 90kA) in approximate agreement with theoretical predictions (LOCUST). Rob Akers et al

  27. 5 degree tilt dZ=-0.1m dZ=-0.2m dZ=-0.3m dZ=-0.3m no tilt Off-axis NBCD Alternative beam geometries are under consideration for off-axis current drive. e.g. vertically offset beams, viz. RT=0.7m, E0=70keV, 2.65MW Peaked Te(r) Te0 = 4keV Broad ne(r) ne0 = 4.45 x 1019 m-3 INBI ~ 250 - 300kA for PNBI = 2.65MW (dZ = 0.3m) Rob Akers et al

  28. EBW current drive EBW current drive demonstrated in COMPASS-D (Shevchenko et al PRL 2002) 20 ~ 0.035 A/W/m2 MAST 60GHz EBW antenna (21 mirrors) for proof-of-principle O-X-B EBW tests An optimised low frequency system ( 20GHz) is under consideration for off-axis EBW current drive Vladimir Shevchenko et al

  29. Favourable divertor target power distribution in MAST Pup ~ Pdown in DND Pout >> Pin Glenn Counsell, Andrew Kirk et al

  30. ELM power efflux is to outboard targets Outboard strike points shift radially outwards by 2 - 3cm during low frequency ELMs but no significant broadening of target power deposition Andrew Kirk, Glenn Counsell et al

  31. Power loading in the STPP The divertor target loading depends on the SOL width, which is uncertain MAST data is compatible with a number of models (e.g. resistive interchange model) and incompatible with others. Compatible models predict power loads in the STPP to be in the range - outboard 5 - 10 MW/m2 - inboard 15 - 25 MW/m2 but there are a number of uncertainties. • More work is needed to reduce these • uncertainties and develop novel • divertor schemes (eg ‘biased’ divertor • or cascading pebbles) Andrew Kirk, Joon-Wook Ahn et al

  32. Divertor Biasing First experimental tests of toroidally asymmetric divertor biasing Ppeak reduced & slight broadening of power deposition x3 broadening of power deposition but Ppeak rises (NB. Pbias/P ~ 0.3) ExB drift shifts peak power several cm in opposite directions on live and grounded ribs - explicit prediction of theory G Counsell et al

  33. Plant Developments 2003 - 04 New centre column New divertor New error field correction system PF system improvements - e.g. P2 reversing switch

  34. NBI Upgrade 2003-04 NBI system upgrade for reliable long pulse operation at high power (5MW,  5s) - replacement of sources with JET-style PINIs - new calorimeters & residual ion dumps employing hypervapotrons Residual Ion Dumps Actively cooled calorimeter: Gate in closed position

  35. M3b Improved divertor, new centre column, NBI upgrade (beam line 1) Apr Mar M4 M5 restart NBI upgrade (2) Apr Mar MAST Schedule 2003/04 2004/05 (provisional) Oct Operations Engineering Break

  36. Summary The EU ST programme is centred on MAST at UKAEA Culham but there is widespread involvement of other organisations through collaboration. The inherent high beta capability of the ST ultimately offers the possibility of a compact power plant and/or component test facility - viable designs for these are being developed. However, there are many key issues which have to be addressed to bring such ideas to fruition, e.g. exhaust techniques, current drive, stability & confinement. Since construction of MAST (& NSTX), considerable progress has been made in tackling these key issues. Many of the problems are similar to those faced by the conventional tokamak community - there is a productive two-way relationship between STs and the conventional tokamak  STs are providing valuable input to ITER but at the same time can directly benefit from the extensive knowledge base of the conventional tokamak leading to rapid progress.

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