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Current holes at ASDEX Upgrade

EURATOM Association. LT WS (W56) on Physics of Current Holes, Mito, Japan, 3-4 Feb 2004. Current holes at ASDEX Upgrade. Presented by O. Gruber for D. Merkl, J. Hobirk, P.J. McCarthy, E. Strumberger, ASDEX Upgrade Team. - hardware upgrades for improved control - integrated advanced scenarios

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Current holes at ASDEX Upgrade

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  1. EURATOM Association LT WS (W56) on Physics of Current Holes, Mito, Japan, 3-4 Feb 2004 Current holes at ASDEX Upgrade Presented by O. Gruber for D. Merkl, J. Hobirk, P.J. McCarthy, E. Strumberger,ASDEX Upgrade Team - hardware upgrades for improved control - integrated advanced scenarios - ion ITB with current hole: equilibrium, current diffusion - electron ITB with current hole - summary

  2. Electron Cyclotron Resonance Heating: 2 MW / 140 GHz / 2 s 4 MW / 105 -140 GHz / 10 s - on-line steerable mirrors ASDEX Upgrade R= 1.65 m a = 0.5 m Ip 1.4 MA Bt 3 T PNI  20 MW PICRH 8 MW PECRH 2 MW ASDEX Upgrade: flexible heating and fuelling systems Pellet Centrifuge: 240 - 1000 m/s repetition rate 80 Hz Neutral Beam Injection: - perp. heating 15 MW / 70-100 keV - tang. off-axis beams 5 MW/ 100 keV • Ion Cyclotron • Resonance Heating: • 8 MW / 30-60 MHz • - variable deposition

  3. recent hardware upgrades  improved control - extended pulse length to 10 s flattop (= 2-3tR even at Te(0) =10 keV) - extended PF coil operational window to run <d> = 0.55 discharges - developed ICRH to routinely deliver > 5 MW in ELMy H-mode - increasing W coverage of inner wall First wall materials need minimum erosion & low Tritium retention ●stepwise towards C-free interior - in 2004 campaign70% of first wall covered - W-divertor in upper SN C-divertor in lower SN ●Up to now: - all plasma scenarii still accessible usually W concentration below 10-5 - machine has been more ‚delicate‘ to run  central RF heating & ELM control by pellets suppresses impurity accumulation at improved core confinement W W W W W

  4. Next hardware extensions • Towards a C-free first wall: • - W coating of LFS poloidal limiters (actively cooled) 2004 • - W coated bottom divertor 2005 • Off-axis CD • - upgrading of ECRH started • (4 MW / 10 s / steerable mirrors / 105–140 GHz ) • - LHR system (3.7 GHz) in discussion

  5. Next hardware extensions MSE,... NI I YAG ECRH CX/ LHR passive shell DCN TNBI A9 current feeders ? • Stabilizing shell for external kinks & active RWM control shell time constant for n = 1 above 30 – 40 ms (d=3 cm, steel) 2 sets of 8 active toroidal coils (n = 1, 2)

  6. Next hardware extensions ECRH MSE,... YAG TNBI A9 passive shell CX/ LHR A9 DCN ECRH NI I current feeders ? • Stabilizing shell for external kinks & active RWM control CAS-3D (P. Merkel): - shell currents for n=1 kink - extension to resistive wall - benchmarking with resistive 2d-wall code

  7. Performance beyond H-mode: integrated "advanced" scenarios  More compact pulsed reactor / steady-state operation H89-P N / q95> 0.8  Aim to achieve these conditions in steady state: - energy and particle exhaust need nenGW - tolerable ELMs  bootstrap current fraction > 50%in stationaryadvanced H-mode: prime candidate for ´hybrid´ ITER scenario  BS fraction > 80% for continuous reactor operation: strong ITBs with reversed shear needed 2 H89-P ITER  bN

  8. ITBs produced in the current ramp-up with strong reversed shear • at JET (using LHCD) and JT-60U (using NBI) showed the existence • of an extended core region with zero toroidal current: current holes • open questions: equilibrium, stability, transport, sustainment • influence of size: duration limited by skin effect ? • ITB driven bootstrap current sufficient? • full non-inductive current drive needed for sustainmen t? (0.8 MA, 2.7 T) Motivation for current hole investigations

  9. Ion ITB discharge with current hole barrier extends over the qmin region  ITB driven bootstrap current and shear profile can be aligned

  10. Ion ITB discharge with current hole: MSE results current hole lost when third source switched off (ITB lasts longer) DTM magnetic axis geometry formula of MSE at ASDEX Upgrade

  11. current hole: equilibrium reconstruction • Cliste: • solves Grad-Shafranov equation using external magnetics and MSE data • cubic spines for basic functions  prevents sharp „current hole“ edge • uses poloidal flux y as main coordinate • for very low central current densities, rpol = y as a function of • spacial coordinates is poorly defined  convergence problems • new version: rmid rtor • weighted sum of previous solutions for y(R,z) and j(R,z) • during iteration to improve convergence • (successive ´over-relaxation´) • NEMEC: • modified 3-d stellerator equilibrium code (S.P. Hirshman) • energy minimizing fixed / free boundary code assuming nested flux surfaces • uses toroidal flux as main coordinate

  12. current hole: equilibrium reconstruction • good agreement of the q-profiles except of the current hole edge • measured position of the (2,1) DTM from SXR & ECE

  13. current hole: equilibrium reconstruction - colored points are the MSE observation points - shaded area labels the current hole Comparison of equilibrium reconstruction with MSE measurements

  14. current hole: current diffusion • ITB driven off-axis bootstrap current not sufficient to maintain current hole • initial current hole taken from CLISTE at 0.3 s vanishes within 100 ms • fast diffusion of beam current density ? • high fast particle content may contribute to BS current

  15. current hole: confinement - reversed magnetic and velocity shear improve heat insulation in core  T driven transport suppressed  internal transport barriers (ITBs) - stored energy of ion ITBs increases linearly with heating power

  16. ITB scenario with counter-ECCD pre-heating # 17542

  17. Electron and ion ITB Ctr-ECCD • no MSE available • sawtooth-like crasches in Te due to collapsing electron ITBs during ctr-ECCD: • indicates strong reversed shear (previous AUG results) or current hole (JET)

  18. Combined electron and ion ITB • early ctr-ECCD produces electron ITB • at delayed NBI onset, ion ITB develops •  combination of electron and ion ITB • foot of electron ITB sits at smaller radius

  19. Small current hole during on-axis ctr-ECCD (ASTRA) TORBEAM: IECCD=70 KA

  20. Summary • extended control tools for all scenarii: - operation at high shaping • -variable schemes for profile control (pressure, momentum, density, j, impurities) • - variety of methods for NTM suppression • - ELM control via shaping (type II ELMs), QH-mode and pellet pacemaking • - kink and RWM control envisaged • current holes in ion ITB discarges (early NI heating) observed: - current hole diameter up to 25% of minor radius - equilibrium reconstruction with CLISTE (convergence up to q0  40) and NEMEC (q0 > 1000) possible • -ASTRA current diffusion simulations show no sustainment by off-axis BS current • - anormal beam driven current diffusion & fast particle BS needed: • - off-axis co-CD supports: • current hole lost with switch-off of tangential off-axis beam • current holes with on-axis ctr-ECCD: • - electron ITBs • - combined electron and ion ITBs with both ECCD and NBI (Ti  Te  8 - 10 keV) • - ASTRA simulations indicate small current hole during central ctr-ECCD

  21. Ion ITB discharge with current hole: SXR results

  22. Advanced H-modes: performance Improved H-mode High bN Improved H-mode High bN q95 = 3.3 - 4.3 q95 = 3.3 - 4.3 1.5 4 H98(y,2) bN 3 1.0 2 bN = 1.8 1 0.5 0 0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 1.2 ne/nGW ne/nGW n* reactor relevant at medium densities :H89-P= 2.8, bN= 3.2 (IAEA1998) optimum exhaust close to Greenwald : H89-P= 2.4, bN = 3.5(H-mode WS 2001) (at q95= 3.5) - continuous transition

  23. Advanced H-modes: progress towards steady state & adv. performance advanced advanced  best combination of confinement, stability and density at high d > 0.4 and q95 3.5  higher q95 over-compensated by enhanced performance ITER ITER 2 N H98-P / q95 = 0.35 (0.2 in conv. ITER) Steady conditions for many current redistribution times: low n* - tripple product 1020 m-3keV s-1 - QDT(equivalent)  0.2 high Greenwald fraction

  24. ITBs: missing stationarity due to MHD events Decisive influence of scenario: H89-P • ITBs with early heating and RS - limited by coupling of infernal (at qmin 2) and extrnal kinks to bN< 2 • ITBs with delayed heating - highest performance achievable - high performance terminated by ELMs • Combined electron and ion ITBs - high performance terminated by central 2/1 MHD ITER  bN • sustained only with L-mode edge or poor H-mode edge • at better performance discharges short compared with current diffusion time • high control efforts required: p, j, MHD modes • a self-consistent scenario with reduced control requirements exists

  25. Can tokamaks be optimised towards continuous reactor? • high pressure gradient needed to get 80% bootstrap current fraction(Q >30) • reversed magnetic and velocity shear improve heat insulation in core •  T driven transport suppressed  internal transport barriers(ITBs) • ITB driven bootstrap current and reversed shear profile can be aligned • optimise MHD stability – high p-gradient at q(min) leads to global MHD modes • combination of electron and ion ITB scenarii needed - foot of ITB at r = 0.6

  26. Can tokamaks be optimised towards continuous reactor? • reversed magnetic and velocity shear improve heat insulation in core •  T driven transport suppressed  internal transport barriers (ITBs) • high pressure gradient needed to get 80% bootstrap current fraction (Q > 30) • ITB driven bootstrap current and reversed shear profile can be aligned • optimise MHD stability: high p-gradient at q(min) leads to global MHD modes • combination of electron and ion ITB scenarii needed • early ctr-ECCD produces electron ITB • at delayed NBI onset, ion ITB develops • foot of electron ITB sits at smaller radius

  27. Ion ITBs:barrier position and q profile aligned r(qmin) rpol - MHD modes trigger ITBs  relation with rational q values - strong barriers only in connection with reversed magnetic shear • barrier extends over the qmin region  ITB driven bootstrap current and shear profile can be aligned

  28. Ion ITBs: route to very high bootstrap fractions • ITB scenario with delayed heating: • - heating of 15 MW late in the current ramp • lower SN with high triangularity • transition to H-mode 0 1 t(s)

  29. Ion ITBs: route to very high bootstrap fractions • first large ELM destroys ITB ! • ITB scenario with delayed heating: • - heating of 15 MW late in the current ramp • lower SN with high triangularity • transition to H-mode 800 kA: ne/nGW=0.45 No-wall limit reached !?bN = 4.0 H89-P = 3.2 Tio = 14 keV 1 MA: Tio > 20 keV 1.5.1020 m-3keV s-1 ≥ 60 % BS current

  30. Motivation

  31. Can tokamaks be optimised towards continuous reactor? • early ctr-ECCD produces electron ITB • at delayed NBI onset, ion ITB develops • foot of electron ITB sits at smaller radius

  32. Summary (1) • Extended control tools for all scenarii: - 10 s flat-top pulses allow current profile relaxation - operation at high triangularities close to DN (d = 0.55 achieved) • -variable heating / CD schemes for profile control • (p, momentum, density, j, impurities) • Active MHD control: • - variety of methods for NTM suppression • - ELM control via shaping (type II ELMs), QH-mode and pellet pacemaking •  reduced target loads, impurity control • - kink and RWM control envisaged • - disruption mitigation (not covered)  Highest performance achieved in Ion ITBs with reversed shear - scenario extended to high confinement H89-P= 3.4 and high beta bN= 4 - Ti  Te  8 - 10 keVwith ctr-ECCD and NI - duration limited by strong ELMs, core and edge MHD modes - up to now transient max performance not sustainable - benchmark is advanced H-mode scenario

  33. Summary (2) • Advanced H-mode scenario: a basis for ITER hybrid operation (even steady-state or ignition possible) • - relaxed low shear q-profile (long sustainment compared to res. diffusion) • - control of density peaking & impurity accumulation with tailored heat dep. • - enhanced confinement H98-P= 1.1 - 1.5 and beta bN> 3 (up to no-wall limit) • over substantial operational range of q95 , d and density • - integration of type II ELMs close to Greenwald density and double null • - despite high densities, > 60% non-inductive current drive achieved • stepwise towards C-free interior (reduced erosion, T retention) • - all advanced plasma scenarii accessible with W concentration below 10-5 • - impurity accumulation at improved core confinement suppressed • with central RF heating & ELM control by pellets

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