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An upgrade for MAST

An upgrade for MAST. G. Cunningham for the MAST team EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK. MAST stands on two legs. DEMO physics. CTF. Strategic objective number 1: ST based component test facility (CTF). Indicative ST-CTF parameters:.

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An upgrade for MAST

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  1. An upgrade for MAST G. Cunningham for the MAST team EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK. IAEA ST workshop 2008, Roma

  2. MAST stands on two legs DEMO physics CTF IAEA ST workshop 2008, Roma

  3. Strategic objective number 1: ST based component test facility (CTF) Indicative ST-CTF parameters: IAEA ST workshop 2008, Roma

  4. Strategic objective number 2: ITER and DEMO physics basis IAEA ST workshop 2008, Roma

  5. Features • Vacuum vessel and main PF coils retained, • Closed, and pumped, divertor for long pulse density control, • Extensive set of divertor coils for plasma shaping and divertor control, • Fatter centre column for increased TF and flux, • Increased NBI power, • Off-axis NBI and on-axis counter-current NBI for q profile and rotation control, • 1MW EBW heating and current drive • Continuous pellet injector IAEA ST workshop 2008, Roma

  6. NBI geometry (plan view) 1 on-axis counter-current PINI 2 double PINI boxes (1 on-axis, 1 off-axis PINI per box) IAEA ST workshop 2008, Roma

  7. Design process (and talk outline) Non-inductive steady state (>> resistive timescale), NB drive Bootstrap driven direct NB current drive High T High β Higher elongation Lower li Lower density Higher density Active pumping off-axis/counter NBI q profile control Reduce J0 Strong shaping () Substantial input power Adequate divertor power handling IAEA ST workshop 2008, Roma

  8. Divertor – magnetic design IAEA ST workshop 2008, Roma

  9. Elongation is a function of li, and control system • At suitable li (li(2)=0.65 here), and by optimising the divertor current, k>2.5 is achievable. • However such li is not readily sustained, • and vertical control is demanding • An additional 'X point' coil • increases triangularity, • improves vertical control, and raises βlimit. • The confinement region is • now satisfactory, but the divertor is problematic. IAEA ST workshop 2008, Roma

  10. New array of divertor coils • Array of divertor coils takes strike point out to larger radius • and gives good compensation for solenoid field. • Also, solenoid is enlarged to increase volt-seconds • ‘Startup’ (merging- compression) coil is removed • Radial field coil is relocated to admit off-axis NBI IAEA ST workshop 2008, Roma

  11. Wide range of divertor control • The divertor strike point location can be moved considerably, • allows higher power density operation for divertor target testing, • and low power density for long pulse. • The divertor is remote from the main plasma, allowing closed divertor operation. IAEA ST workshop 2008, Roma

  12. the same philosophy can be extended ... • Possible DEMO divertor solution, • Possible thanks to large MAST vessel, • Known in the US as 'super-X', • Ongoing development - collaborations welcome (simplification is no doubt possible!) IAEA ST workshop 2008, Roma

  13. Central shaping coil gives further versatility • Large centre column leaves space for a small central shaping coil • gives access to increased triangularity IAEA ST workshop 2008, Roma

  14. Divertor power handling • 10 MW m-2 considered the limit for 5s operation • Experiment has found most of the power goes to the outboard divertor ... • ... outboard/inboard ratio, Roi up to 30 IAEA ST workshop 2008, Roma

  15. Power deposition at the inner target (high ) • PNBI = 10 MW, PRAD = 3 MW, qtarget = 90°, lSOL = 1 cm, eflux = 4 INNER TARGET Even Rstrike ~0.3m is OK in DND IAEA ST workshop 2008, Roma

  16. ... outer target, declined plate • PNBI = 10 MW, PRAD = 3 MW, qtarget = 30°, lSOL = 1 cm, eflux = 4 OUTER TARGET Rstrike >0.8m is OK IAEA ST workshop 2008, Roma

  17. Divertor - active pump design IAEA ST workshop 2008, Roma

  18. Closed divertor with active (cryo-) pumping MAST, EXPERIMENT MAST MAST-U, PUMP OFF MAST-U, PUMP ON Closed divertor gives factor 5 reduction in vessel pressure, active pumping a further 50% decrease. (OSM/Eirene) Dα emission R(m) IAEA ST workshop 2008, Roma

  19. Model is validated against MAST data Observed Dα image Abel inverted image Model calculation INVERTED Da IMAGE IAEA ST workshop 2008, Roma

  20. Plasma scenario design • 8 scenarios developed (2 discussed here), • Transp (including Nubeam) is the main design tool (run until profiles are relaxed), • Non-inductive current fraction 40% to 100%, • βN from 3.7 to 6.7. IAEA ST workshop 2008, Roma

  21. Most scenarios have CTF-like shape • Three shapes in total • CTF like • high  (F) • MAST (E) • Most scenarios have CTF like shape. • A,B,C,D,G • Off-axis beams require high . IAEA ST workshop 2008, Roma

  22. Scenario A (CTF-like q profile) • Objective: Test the confinement and stability of a monotonic q-profile with q>2 • Avoid all low m/n MHD • Major problem in current STs • No sawteeth crashes • No 2/1 and 3/2 NTMs • Requires high TF to keep q > 2 • Irod = 3.2 MA • Scenario is ideally stable even without rotation. • Neoclassical transport increases with q • safe to use ITER scaling? • but recent ST results suggest stronger Bt and weaker Ip scaling, giving greater benefit from TF enhancement than has been assumed. A1 : □ A2 : + IAEA ST workshop 2008, Roma

  23. Two ways to scenario A A1: “High” density: n/nG= 0.5 Conservative approach. N ~ 3.7 > NITER=2.2 (Nth = 3.1) Balance on-axis Ohmic current with off-axis NBCD classical fast-ion diffusion Flat top: ft1.8s  3.7 R  37 E limited by TF to 2.2s A2: Low density:n/nG = 0.2 Almost non-inductive fNI = 0.8 N = 4.8, with high fast particle pressure Nfast=2.7 , Nth = 2.1 Dominated by NBI current anomalous fast-ion diffusion Dfi= 0.3 m2/s. Flat top: ft  1.8 s 2 R45E limited by TF to 1.8s IAEA ST workshop 2008, Roma

  24. Flexible machine - range of scenarios • A1,A2 : baseline, CTF-like q profile, 2 density variants • B : high fast particle content - confinement, fNI=0.9, βN=6, • C : long pulse, fNI>1, βN=6.7, reduced TF • D : high βT, Ip=2MA, q0~1, test fast particle β limit • E : 'touch-base', high li, low β • F : high =0.6, β limit and confinement scaling • G : high thermal βT (βN up to 7), Ip=2MA, ng=1, β limit testing • Common parameters: • Ip=1.2MA • κ=2.5 • A=1.6 • li(3)=0.5 • (except where stated otherwise) IAEA ST workshop 2008, Roma

  25. Main limiting instability: n=1 internal kink • Pressure driven internal n=1 kink in the low magnetic shear region (infernal mode) is the limiting instability in most scenarios (except G). • Mode is strongly stabilised by rotation. • Mode is stabilised by fast particles. • Wall has little influence on the stability. Effect of rotation on reference scenarios C C B D Growth rate / Alfven Toroidal rotation on axis (km/s) IAEA ST workshop 2008, Roma

  26. MHD stability - MISHKA analysis Once the internal mode is stabilised, the next least stable mode is an external kink, which is amenable to wall stabilisation. Stabilising plates are under consideration. IAEA ST workshop 2008, Roma

  27. Vertical position control • High elongation is a major objective • Little passive stabilisation close to plasma on LFS (NBI and diagnostic access) IAEA ST workshop 2008, Roma

  28. Vertical position control Rigid plasma analysis (RZIP) Growth rate (s-1) Resonant frequency (Hz) >1kHz mode stable operating zone proportional gain <150 Hz mode derivative gain IAEA ST workshop 2008, Roma

  29. Pulse length limitations IAEA ST workshop 2008, Roma

  30. ELM control coils Alternative ELM control coil geometries - both ex-vessel. (left, preferred) n=3 dipole (right) n=3,4,6 monopole Poloidal magnetic spectrum for the n=3 configuration of the dipole (left) and monopole (right) coil sets. Superimposed as the blue crosses are the q=m/3 rational surfaces of the CTF-like equilibrium. The dipole set is closer to resonance. IAEA ST workshop 2008, Roma

  31. 1MW 18GHz EBW heating/current drive Harmonic accessibility via O-X-B mode conversion. Midplane resonance topology with Doppler broadening. (shot #8694, Bφ = 0.55T at R=0.8m.) This diagram is calculated for present day MAST, will need to be scaled for higher TF. Shows optimum access at ~18GHz IAEA ST workshop 2008, Roma

  32. In conclusion, addressing CTF goals MAST today • Upgrade exceeds most physics parameters of CTF: Steady state Indicative values(not all simultaneous) Divertor Stability No r* extrapolation Transport step size Fast particles IAEA ST workshop 2008, Roma

  33. and remembering the synergy with ITER/DEMO physics priorities, Upgraded MAST is a key facility to progress fusion The MAST team welcomes international partners to help explore the exciting new physics we will reach with the upgrade IAEA ST workshop 2008, Roma

  34. This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission. IAEA ST workshop 2008, Roma

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  36. Physics basis of a long pulse ST High q0 Improves MHD stability Hollow j(R) High fBS High bN Low li Improves vertical stability High k Irod: limited by engineering and economics Ip : maximise for confinement ... but competes with fBS - required for off-axis current drive, and optimisation IAEA ST workshop 2008, Roma

  37. Main parameters • All the scenarios have 4 beams at 2.5 MW each (1 on-axis, 2 off-axis, 1 cntr.) • Scenarios A.2, B, and C differ mainly in Irod and the assumed fast ion diffusion. • “low” density  almost fully non-inductive. • Scenario D and G are high plasma current Ip= 2 MA. IAEA ST workshop 2008, Roma

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