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News from the ASDEX Upgrade Tokamak Programme

News from the ASDEX Upgrade Tokamak Programme. Hartmut Zohm for the ASDEX Upgrade Team MPI für Plasmaphysik, EURATOM Association. Introduction: The ASDEX Upgrade programme Scenario development with W-wall Disruption mitigation and avoidance Fast particle physics

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News from the ASDEX Upgrade Tokamak Programme

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  1. News from the ASDEX Upgrade Tokamak Programme Hartmut Zohm for the ASDEX Upgrade Team MPI für Plasmaphysik, EURATOM Association • Introduction: The ASDEX Upgrade programme • Scenario development with W-wall • Disruption mitigation and avoidance • Fast particle physics • Pedestal and ELM physics • Conclusions Talk given at PPPL, Princeton, USA, February 3rd, 2010

  2. Introduction: The ASDEX Upgrade Programme

  3. AUG Programme in Preparation of ITER and DEMO • Solving ‚immediate‘ questions aiding the design of details for ITER • guide ITER design in areas where input is still missing (ELMs, • NTMs, first wall materials…) Preparing ITER operation • develop operation scenarios that ensure baseline operation (Q = 10) • and make possible ‘advanced’ operation (Q > 10 or steady state) Developing and improving the physics base for DEMO • DEMO is a ‘point design’ – need first principles understanding • to build ‘numerical tokamak’ (strong interaction with theory) • Educating fusion plasma scientists • train and educate the generation that will run ITER

  4. The ASDEX Upgrade Tokamak • 'Medium size' tokamak • major radius 1.65 m • minor radius 0.5 m • plasma current  1.4 MA • magnetic field  2 - 3 T • pulse length < 10 s • heating power  24 MW Comprehensive set of diagnostic developed over the last ~20 years Versatile heating and fuelling systems open up wide operational space

  5. ASDEX Upgrade heating systems With the generator EZ4 back since mid-2009, we run high power discharges again Electron Cyclotron Resonance Heating: 2 MW @ 140 GHz 1(4) MW @ 140/105 GHz Neutral Beam Injection: 20 MW @ 70-100 kV NBCD by tang. beams Ion Cyclotron Resonance Heating: 8 MW @ 30-60 MHz

  6. ASDEX Upgrade and JET form a step ladder to ITER ASDEX Upgrade JET ITER Geometry similar to ITER, linear dimensions scale 1:2:4

  7. ASDEX Upgrade: operation with fully W-coated wall • Low Z-materials have too high erosion and T-retention for DEMO • demonstrate low retention with W-wall • demonstrate compatibility with high performance scenarios

  8. ASDEX Upgrade: operation with fully W-coated wall # 22974 • At high gas puff (high density operation), Gin = Gout • retention dominated by ramp-up/down  long pulse retention will be low

  9. Scenario development with W-wall

  10. The basic problem: termination by W-accumulation Electron density [10 m ] 19 -3 edge center time central W concentration: ~ 5 x 10-4 (inacceptable!)

  11. W concentration profile minor radius W content determined by sources and radial transport W peaking in the centre: neoclassical Inward pinch versus turbulent outward flux W influx across H-mode transport barrier: inward pinch versus ELM flushing W source profile (outside separatrix) net erosion versus retention

  12. W sources and their impact on the W-concentration • Influx of eroded W (spectroscopy) • highest flux from divertor (ELMs) • W content in plasma dominated • by main chamber source •  efficient divertor W-retention! • Flux from active ICRH antennae high • W sputtering from ICRH limiters • by sheath accelerated impurities • Low gas puff  low ELM frequency • W concentration can run away!

  13. Impurity scan gives stronger inward convection for higher charge Implication: W has strongest inward pinch – need ELM flushing Impurity transport in the pedestal is neoclassical

  14. A simple model for W influx in H-mode plasmas • In between ELMs: • neoclassical transport in barrier • causes strong W density gradient • During ELM: • rapid decrease of W density due • to (prescribed) increase of D • low-Z impurities flushd from confined • region erode W at the limiters • a fraction of the eroded W returns • immediately (prompt redeposition) • Time dependent multi-species STRAHL calculation solves particle balance • using neoclassical D and vin plus an artificial ‚ELM‘ D of 20 m2/s

  15. A simple model for W influx in H-mode plasmas • Scan of ELM frequency: • W influx increases • W edge density peaking decreases •  Net effect is a reduction cW (temporal averages over ELM cycle, parameter variation: Te at the limiter, taken to match W influx measurements)

  16. 2.4T 2.55T z(m) 0.8 0.4 Prad #22957 0 2.0 Position of ECRH very sensitive and has to be within rtor~0.2 to suppress W-accumulation with ECRH w/o ECRH 4.0s 6.0s -0.4 1.5 MW/m3 -0.8 R(m) 1.0 1.2 1.6 2.0 2.4 0.5 0 0 0.2 0.4 0.6 0.8 1 rtor Central ECRH suppresses accumulation efficiently Radiation profile Sensitivity of localisation suggests that physics other than Dturb contributes Modelling indicates that a-heating might play this role in ITER (AUG) problem: density peaking leads to ECRH cut-off – need O2/X3-mode!

  17. Tailoring of W content by Gas Puff • Lowering gas puff decreases ELM frequency, increases stored energy • W content also goes up – optimisation needed (no gas puff  accumulation) • increasing PECRH or Ptot allows to further decrease gas puff

  18. 2008 >7 days after Bor. 0-7 days after Bor. Operational space of the all-W ASDEX Upgrade • Recipe for standard H-mode • need central ECRH to avoid • impurity accumulation • need ELM frequency > 70 Hz • to periodically flush edge • restrict divertor heat flux to •  10 MW/m2 to protect tiles • Impact on: • density range (finite gas puff, • ECRH cut-off) • q95 range (ECRH on-axis) • confinement (gas puff • deteriorates pedestal) • heating power (protect tiles) 1.2 1.1 1.0 Wmhd [MJ] 0.9 0.8 MoreECRH or boronise Ptot = 9-12 MW 0.7 1MA, d0.3 0.6 0 2 4 6 8 10 12 2002-2006, D,puff = 0

  19. Extension of upper density limit: ECRH cut-off TORBEAM modelling Te,max = 2 keV; ne,max = 1.2·1020 1/m3 2.50 T 1.67 T Absorption along beam X2-Resonance serves as „beam-dump“ B=1.80 T 140GHz ECRH at schemes other than ‚classical‘ X2 have been explored: O2, X3 (cut-off densities: X2: 1.2 1020 m-3, X3: 1.6 1020 m-3, O2: 2.4 1020 m-3)

  20. Extension to low q95: central X3-ECRH at lower Bt Stationary type I ELMy H-mode at 1.8 T / 1 MA → q95 = 3.3 n/nG = 0.7, bN = 1.7, H = 0.9 – target for optimisation of ITER scenario 2

  21. Extension to higher heating power: impurity seeding • With reduction of C-content, divertor radiation strongly reduced • Impurity seeding applied to protect divertor in C-free environment • feedback controlled N2-seeding reliably protects tiles • sensor: thermocurrents between divertor plates ~ divertor heat flux

  22. Impurity seeding allows to go to high P/R AUG ITER #25844 Scen. 2 P[MW] 20 120 P/R [MW/m] 12 19 H 1 1 bN 2.7 1.8 ne [1020 m-3] 1 1 n/nG 0.7 0.85 q95 3.9 3.1

  23. Surprise: N2 seeding also improves confinement! With N2 seeding, energy confinement increases by 10-20% In addition, ELMs become smaller and more frequent (fELM 20-50%)

  24. Extension to better confinement: N2 seeding • Improvement mainly from Ti, ne essentially unchanged (not an RI mode!) • Ti gradient at half radius steeper – dilution reduces ITG growth rate (GS2) • Ti pedestal extends to higher values – dilution plus different ELMs? • (cN 2-4 %, DZeff 0.5 - 1)

  25. Extension to better confinement: N2 seeding • ELM energy loss drops (35 kJ → 20 kJ), shorter duration, less large filaments • not really consistent with increased pedestal temperature – role of SOL?

  26. Ptot = 9-12 MW 1MA, d0.3 Extension to better confinement: N2 seeding • Performance improved back to • previous level with C-wall • even in unboronised machine • and with finite gas puff • improved H-mode continues • to be a robust option for • improved performance • Still to come now that EZ4 back • exploration of benefit of higher • triangularity (d < 0.35 so far) • zero gas puff with more PECRH? 2008 >7 days after Bor. 0-7 days after Bor. N2 seeded, Bor. N2 seeded no Bor. (2009)

  27. Modelling for ITER Q=10 scenario

  28. Modelling for ITER Q=10 scenario W accumulation only for very low values of Dturb!

  29. Disruption avoidance and mitigation

  30. Avoidance of density limit disruption by ECRH @ q=2 • Target: ohmic density limit discharge, high q95 = 6.5 • MARFE formation triggers (3,1) and then (2,1) mode which disrupts plasma (collaboration CNR Milano /ENEA Frascati)

  31. Avoidance of density limit disruption by ECRH @ q=2 • Injecting ECRH (PECRH = 0.6 MW ~ POHM) significantly delays (2,1) onset • in this period, the density continues to increase (formally above nGW!) • not just a power balance effect – need to localise ECRH @ q=2!

  32. Avoidance of density limit disruption by ECRH @ q=2 • Turning off gas valve after ECRH onset allow to recover discharge • algorithm presently triggered by loop voltage increase (MARFE occurrence) • may be a route to operate close to nGW much safer than without

  33. Disruption avoidance by ECRH – high b case • Chose a more ambitious target: q95 = 3.9, bN=2.6 with early (2,1) NTM

  34. Disruption avoidance by ECRH – high b case • 1.5 MW of ECCD sufficient to avoid disruption, prepare safe landing • note: discharge never recovers performance – need to develop strategy • analysis of ‘scalability’ ongoing’

  35. Disruption mitigation by high pressure gas jet • ASDEX Upgrade routinely uses disruption mitigation • massive Ne puff from high pressure valve, triggered by locked mode • on ASDEX Upgrade, main aim is to reduce halo current forces • Additional research for ITER • mitigate power load by radiation • substantially increase density • to avoid generation of runaways • Need  two orders of magnitude • problem of fuelling efficiency ‘Slow’ valves 15 bar, 2 x 32 cm3 ‘Fast’ valve 60 bar, 80 cm3

  36. Disruption mitigation by Injection of Gas Jet • valve open within 1 ms • flight time ~ 0.1 ms • density rise and plasma cooling by radiation edge -> center • cooling of q=2 surface triggers thermal quench • m =1 structure of SXR profile at thermal quench • reduced spike or roll-over of plasma current starts current quench • substantial reduction of forces • and thermal loads demonstrated!

  37. thermal quench in-vessel valve AXUV vertical cameras  = 180o Radiation in mitigated disruptions is asymmetric Toroidal asymmetry in total radiated power can even reverse • propagation of radiating zone, but no symmetrisation

  38. Radiation in mitigated disruptions is asymmetric Radiation pattern has a ‘MARFE’ like structure during the energy quench • no ‘mixing’ in this phase, radiation mainly outside q=2 • later in the current quench, mixing occurs, but distribution of radiation • still not homogenous

  39. Radiation in mitigated disruptions is asymmetric He I line (He killer gas) Radiation pattern has a ‘MARFE’ like structure during the energy quench • no ‘mixing’ in this phase, radiation mainly outside q=2 • later in the current quench, mixing occurs, but distribution of radiation • still not homogenous

  40. neff / nc neff [m-3] E [V/m] (1.2 L dIp/dt) Substantial density increase towards nc • toroidal E field and hence nc tends to asymptotic value • neff /nc ~ 24 % with Ne and Eth < 0.45 MJ, but degrades with higher Wplasma • next: use more valves (linear superposition?)

  41. Fast particle physics

  42. Fast ion physics on ASDEX Upgrade • Aim is to obtain first principles understanding towards ITER • Needs strong collaboration with theory (available in-house ) • On the experimental side, we have to • characterise fast particle driven MHD and analyse its interaction • with the fast ions – comprehensive diagnostics available • (Mirnov, SXR, ECE-Imaging, Fast Ion Loss Detector) • characterise the fast particle distribution – just starting (CTS, FIDA)

  43. Collective Thomson Scattering CTS Scattering geometry gyrotron: blue, receiver: green Fast ion velocity distribution • Collective Thomson Scattering (Risø effort) now starting to produce results • first measurements of the distribution function during NBI successful • progress hampered by problems with gyrotron (needs 105 GHz) 

  44. Radial structure of a single TAE • ICRH beatwave from two ICRH antennae with slightly different frequency • beatwave frequency is swept ‚adiabatically‘ in the range 150-200 kHz • ICRH power level below that usually needed to excite TAEs • at df = 177 kHz, a mode is excited (corresponding to 155 kHz in rest frame)

  45. Radial structure of a single TAE • Comparison of radial eigenfunctions • allows to identify correct mode • pronounced minimum corresponds • to ‚odd‘ mode at 147 kHz • Note: this clearly shows TAEs can be • excited by ICRH beatwaves below • power level for fast particle excitation

  46. The ‘Alfvénic Zoo’ • n = 3, 4,5 TAEs localised at r 0.5, up-chirping RSAEs at r 0.3

  47. Fast particle losses due to a single TAE • Losses are in phase with mode • ‚convective‘ mechanism • note: ejected particles ≠ particles • that drive mode

  48. Fast particle losses due to a single TAE • Above certain amplitude, incoherent • losses observed as well • reminiscent of ‚diffusive‘ mechanism • need more analysis…

  49. Fast particle losses due to multiple AEs • In the phase with multiple TAEs and • Alfvén cascades, the level of • incoherent losses is high • note: different phase velocities, • need a ‚resonant‘ coupling

  50. Pedestal and ELM physics

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