DIII D UNDERSTANDING AND CONTROL OF TRANSPORT IN ADVANCED TOKAMAK REGIMES IN DIII D QTYUIOP C.M. GREENFIELD. Presented by

Transcription

1 UNDERSTANDING AND CONTROL OF TRANSPORT IN ADVANCED TOKAMAK REGIMES IN Presented by C.M. GREENFIELD for J.C. DeBOO, T.C. LUCE, B.W. STALLARD, E.J. SYNAKOWSKI, L.R. BAYLOR,3 K.H. BURRELL, T.A. CASPER, E.J. DOYLE, D.R. ERNST, J.R. FERRON, P. GOHIL, R.J. GROEBNER, L.L. LAO, M. MAKOWSKI, G.R. McKEE,5 M. MURAKAMI,3 C.C. PETTY, R.I. PINSKER, P.A. POLITZER, R. PRATER, C.L. RETTIG, T.L. RHODES, B.W. RICE, G.L. SCHMIDT, G.M. STAEBLER, E.J. STRAIT, D.M. THOMAS, M.R. WADE 3 AND THE TEAM Presented at the st Annual Meeting of the APS Division of Plasma Physics A UCL UCLA 3 5 ITY M OF W I ONSIN SC NIVER S A D I S ON November 5-9, 999 Seattle, Washington QTYUIOP 33-99

2 Abstract for an Invited Paper for the DPP99 Meeting of The American Physical Society Understanding and Control of Transport in Advanced Tokamak Regimes in C.M. GREENFIELD, General Atomics The program focuses on development of Advanced Tokamak regimes with sustained high fusion performance accomplished via pressure and current profile control. Ion thermal transport in these regimes can locally approach neoclassical values. Momentum, particle and, less often, electron thermal transport are also reduced. Optimization is approached through study of the transport processes governing formation, expansion, and sustainment of the low transport region. Internal transport barriers (ITB) often form in neutral beam (NBI) heated discharges with flat or hollow current profiles when the E B shearing rate ω E B becomes large enough to suppress turbulence and its associated transport. With co-nbi (rotation plasma current), the pressure gradient and toroidal rotation terms in the radial force balance E r =(Z i en i ) p i v θi B φ +v φi B θ oppose, requiring one to dominate for large E r. The central ω E B, dependent on the gradient of E r, is calculated largest with co-nbi. This may explain the absence of a distinct power threshold in co-nbi discharges, while those with counter- NBI exhibit unambiguous ITB formation only with P NBI MW (weak, transient barriers occur at lower power levels). Pellet injection can reduce or eliminate this threshold. ITB expansion can broaden the pressure profile, increase stability limits and improve bootstrap alignment. Results from other devices indicate the ITB radius, ITB, often follows the radius of the minimum safety factor, q min, implying that control of q min might be a technique to expand the ITB. A rapid initial current ramp and early, high power co-nbi was used to obtain q min.9 in. ITBs occurred at ITB..5, indicating little connection to q min. ITB often coincides with cancellation of opposing terms in the force balance, so that both E r and ω E B are small. With counter-nbi, there is no such cancellation, and larger ITB radii have been observed. If an L H transition is triggered during ITB formation, a reduced transport region can be formed encompassing the entire plasma, with no distinct barrier. Neoclassical ion transport throughout the plasma was previously obtained in such discharges with an ELM-free edge, terminated by edge MHD instabilities. A similar regime with an ELMing edge and ion diffusivities a few times neoclassical has been produced and sustained with β N H 89 9 for up to s. A key feature of these discharges is a significant bootstrap current (f BS 5%), with a flat bootstrap profile across most of the plasma. Additional current drive is required only near the half-radius for sustainment. Future plans include providing this current via ECCD. Supported by U.S. DOE Contracts DE-AC3-99ER563, DE-AC5-96OR6, W-75-ENG-8, DE-AC-76CH373. With L.R. Baylor, K.H. Burrell, J.C. DeBoo, T.C. Luce, B.W. Stallard, E.J. Synakowski, and the Team.

3 ADVANCED TOKAMAK EFFORTS IMPROVE ON THE TOKAMAK CONCEPT The tokamak has been very successful in achieving parameters near those required for ignition. The advanced tokamak seeks to further improve the concept: Improved fusion performance achieved through profile control. H mode: reduced transport at the edge. Core transport barriers. Combined core and edge barriers. Steady state operation requires noninductive current drive consistent with improved profiles. Bootstrap, beam driven, RF driven, Our challenge is to develop regimes with low transport (high confinement) which are consistent with MHD stability. Broaden core barriers and control edge gradients. Greenfield APS

4 EXPERIMENTS IN EXPLORE THE FORMATION, EXPANSION AND SUSTAINMENT OF ADVANCED TOKAMAK REGIMES FORMATION of internal transport barriers Power thresholds for creating an internal transport barrier (ITB) may depend on the balance of terms of the E B shearing rate. Pellets reduce threshold and trigger barriers with T i T e. EXPANSION of the internal transport barrier Core barriers are broadened with counter-neutral-beam injection. Larger or broader pressure gradients help E B shear stabilize microturbulence with counter-nbi and destabilize with co-nbi. SUSTAINMENT of Advanced Tokamak regimes A regime combining an ELMing H mode edge and a reduced transport core has been developed with high fusion performance (β N H 89 =9) sustained for up to 6 τ E. Confinement limited by relatively benign MHD instabilities rather than the usual transport processes. Flat bootstrap current profile makes this regime attractive for sustainment with off-axis ECCD. Greenfield APS

5 COUNTER NEUTRAL BEAM INJECTION ALLOWS ACCESS TO NEW REGIONS OF PARAMETER SPACE Standard recipe for internal transport barrier (ITB) formation in uses co-nbi (neutral beam injection parallel to the plasma current) applied early during the current ramp. ITB forms near magnetic axis at very low power. Fundamental limit to sustainment occurs when q min reaches unity. TFTR, JT 6U both report best performance with balanced or counter-nbi. However, these devices exhibit a power threshold. Recent experiment exploit the advantages of counter NBI. Counter neutral beam CD maintain an elevated q profile. Alignment of pressure gradient and rotation terms of the E B shearing rate is favorable with counter-nbi except near the magnetic axis. Greenfield APS

6 ITB FORMATION EXHIBITS POWER THRESHOLD WITH COUNTER-INJECTION Co-NBI: ITB forms with P NBI.5MW and expands with P NBI 5MW. Counter-NBI: Clear barriers formed only with P NBI ~9MW. Beam power losses occur in both co- and counter-nbi discharges. ~% lost for co-injection Shinethrough and charge exchange. ~3-% with counter-injection. Additional beam ion orbit losses. Difference between co and counter beam ion losses is too small to explain observed increase in threshold power. Threshold power similar to that in TFTR balanced injection at similar toroidal field. Greenfield APS

7 HIGHER POWER THRESHOLDS MAY BE EXPECTED FOR ITB FORMATION WITH COUNTER-NBI The Hahm-Burrell formula for the E B shearing rate: with ω RBθ Er E B= ( ) B ψ RBθ E r = (Z i en i ) - p i - v θi B ϕ + v ϕi B θ counter ω rot 5 s -.. ω p (carbon impurity) can be rewritten ω E B = ω p + ω rot co -..7s (L-mode).9s (L-mode).s (ITB).s (ITB) The sign of ω p is invariant for co or counter NBI. It can vary with location, due to the interplay between first and second derivatives. The sign of ω rot is different for co and counter NBI. Near the magnetic axis: ω p and ω rot oppose with counter-nbi and add with co-nbi. ω E B larger with co-nbi. Off-axis: ω p and ω rot add with counter-nbi and oppose with co-nbi. ω E B larger with counter-nbi (counter) Greenfield APS

8 CORE FLUCTUATIONS ARE REDUCED IN THE PRESENCE OF THE ITB T i (CER) Barrier begins forming soon after onset of high power neutral beam heating. No TFTR-like E r precursor seen. kev 8 χ i =.6 ELMing H mode phase interrupts ITB after MHD activity releases energy from core. m /s.. =.3 Core fluctuations drop following H L transition, in conjunction with ITB reformation. Profiles indicate resumption of ITB development. MW..5 BES δi/i (%) k θ cm - ( ms averaging) P ECH 9989 P NBI I P t (sec) D α (a.u.).6. MA Greenfield APS

9 THE CORE BARRIER BROADENS WITH INCREASING HEATING POWER T i 6 n e kev 5 T e 9 m -3 5 s - Ω 5 3.MW (9989.s) 9.3MW (9985.s) 7.MW (9985.s) DeBoo, CP.58 q Greenfield APS

10 TRANSPORT IS REDUCED IN ALL CHANNELS: LARGEST DECREASE IN ION THERMAL DIFFUSIVITY χ e χ i m /s. χ i neo.mw (9989.s) 9.3MW (9985.s) 7.MW (9985.s) D e χ ϕ m /s Greenfield APS

11 PELLETS CAN BE USED TO TRIGGER ITB FORMATION WITH DIFFERENT BARRIER CHARACTERISTICS Both deuterium and lithium pellets have been used to trigger formation of the ITB. No power threshold observed, even with counter-nbi. Similar to observations in other devices (JET, TFTR, ) Similar to PEP regimes previously observed in other devices. Barriers exhibit high gradients in both the density and temperature profiles. Lower temperatures and higher densities observed. T i / T e.5, much lower than observed with neutral beams alone. Further details of the pellet results will be presented by Baylor, UI. and Parks, UI.5. Greenfield APS

12 PELLETS INJECTED FROM THE HIGH FIELD SIDE DURING THE CURRENT RAMP CAN FORM AN INTERNAL TRANSPORT BARRIER MW 9 m <n e > P NBI H 89 Pellets () kev 9 m n e s (no pellet) T e 9978.s (PEP) T i.5..5 time (s) mm pellets injected during current rise from the new high-field-side guide tube produces peaked density profile for ITB studies with T i T e. MW of counter-nbi applied to produce ITB in L mode. Greenfield APS

13 EXPERIMENTS IN EXPLORE THE FORMATION, EXPANSION AND SUSTAINMENT OF ADVANCED TOKAMAK REGIMES FORMATION of internal transport barriers Power thresholds for creating an internal transport barrier (ITB) may depend on the balance of terms of the E B shearing rate. Pellets reduce threshold and trigger barriers with T i T e. EXPANSION of the internal transport barrier Core barriers are broadened with counter-neutral-beam injection. Larger or broader pressure gradients help E B shear stabilize microturbulence with counter-nbi and destabilize with co-nbi. SUSTAINMENT of Advanced Tokamak regimes A regime combining an ELMing H mode edge and a reduced transport core has been developed with high fusion performance (β N H 89 =9) sustained for up to 6 τ E. Confinement limited by relatively benign MHD instabilities rather than the usual transport processes. Flat bootstrap current profile makes this regime attractive for sustainment with off-axis ECCD. Greenfield APS

14 9989 (.7s): Counter-NBI W =.9 MJ P NBI =. MW (6.5 MW absorbed). COUNTER-NBI RESULTS IN BROADER PROFILES kev 5 5 T e counter co T i 9 m n e 873 (.8s): Co-NBI W =. MJ 5 s - P NBI = 9.6 MW (7.6 MW absorbed). 3 Ω 6 q Stallard, CP Greenfield APS

15 χ e THE REDUCED TRANSPORT REGION IS BROADER IN ALL CHANNELS WITH COUNTER-NBI χ i..8 Ion heating power m /s W/cm D e counter (9989.7s) co (873.8s). χ ϕ χ neo i m /s.. Possible causes of ITB broadening: Broader heating profile Enhanced E B shear Greenfield APS

16 COMBINATION OF p AND ROTATION EFFECTS IN ω E B NATURALLY BROADENS COUNTER BARRIERS 5 s ω E B p ω E B rotation ω E B Shearing rate ω E B separated into thermal main ion rotation and pressure gradient terms. Total calculated from CER impurity measurements. Main ion pressure term from profile measurements. Rotation term by subtraction. - ω E B p ω E B rotation ω E B 873.8s (co) s (counter) Greenfield APS

17 E B SHEAR TURBULENCE SUPPRESSION IS DOMINANT OVER A LARGER AREA WITH COUNTER-NBI Stability to drift ballooning modes calculated using a linear gyrokinetic stability (GKS) code. Non-circular, finite aspect ratio equilibria with fully electromagnetic dynamics. With counter-nbi: Linear growth rates smaller at at large, possibly due to higher Z eff near edge (core Z eff.5 in both cases). The shearing rate profile extends to larger radius. 5 s co (873.8s) ctr (9989.7s) ω E B γ max Greenfield APS

18 STATIONARY CENTRAL q PROFILES MAINTAINED BY COUNTER NEUTRAL BEAM CURRENT DRIVE. co-nbi counter-nbi..7s.6s..8s.97s.7s.8.7s.6 During ms period leading up to peak parameters: Co-NBI: q profile evolution accelerates. Counter-NBI: q profile evolution decelerates, becomes stationary with fully developed ITB. Counter-NBI provides central counter-current drive, which can maintain an elevated q profile. Alleviates an eventual limitation to ITB performance, as barrier terminates as q min Greenfield APS

19 EXPERIMENTS IN EXPLORE THE FORMATION, EXPANSION AND SUSTAINMENT OF ADVANCED TOKAMAK REGIMES FORMATION of internal transport barriers Power thresholds for creating an internal transport barrier (ITB) may depend on the balance of terms of the E B shearing rate. Pellets reduce threshold and trigger barriers with T i T e. EXPANSION of the internal transport barrier Core barriers are broadened with counter-neutral-beam injection. Larger or broader pressure gradients help E B shear stabilize microturbulence with counter-nbi and destabilize with co-nbi. SUSTAINMENT of Advanced Tokamak regimes A regime combining an ELMing H mode edge and a reduced transport core has been developed with high fusion performance (β N H 89 =9) sustained for up to 6 τ E. Confinement limited by relatively benign MHD instabilities rather than the usual transport processes. Flat bootstrap current profile makes this regime attractive for sustainment with off-axis ECCD. Greenfield APS

20 β N H89p SIGNIFICANT IMPROVEMENT IN LONG-PULSE ADVANCED TOKAMAK PERFORMANCE HAS BEEN ACHIEVED β N H 89 =9 for 6τ E, f bs.5, f NI Conventional Tokamak ELM-free H mode ELMy H mode L mode edge Advanced Tokamak Target τ duration /τ E Luce, GO Progress 96 ARIES- RS H 89 D α l i β N H 89 I P β N P NBI s. MA MW 3 Time (s) Greenfield APS

21 BETA SATURATION IS DUE TO BURSTING HIGH-FREQUENCY MHD Formation sequence similar to ELM free NCS H mode discharges [E. Lazarus, et al., IAEA 996]. Recent experiments done at lower plasma current and higher safety factor q. Transition to ELMing phase with little loss in performance. Performance limited by stability. High frequency MHD saturates confinement without x-event. Consistent with Alfvénic modes driven by fast ions. Usually terminated by resistive wall mode [Garofalo GO.]. % %/s T/s - β T D α db/dt dβ T /dt time (s) 9859 Greenfield APS

22 PROFILE EVOLUTION INDICATES A COMBINATION OF CORE AND EDGE BARRIERS kev 5 s T e Ω T i L-mode.6s H-mode.57s ELMing.97s 9 m q n e Greenfield APS

23 L mode: ITB is forming in core. ION THERMAL TRANSPORT REDUCED IN BOTH CORE AND EDGE χ e 9859 ELM free H mode: Reduced ion thermal transport throughout entire plasma volume. m /s. χ e χ i χi neo ELMing H mode: Ion thermal transport becomes elevated above neoclassical level. m /s. χ e χ i neo χ i Electron transport remains at L mode levels in the plasma interior. m /s χ i χ neo i Greenfield APS

24 SOMETHING OTHER THAN MICROTURBULENCE MAY LIMIT CONFINEMENT IN STEADY-STATE PHASE L mode and ELM free H mode ω E B exceeds calculated linear growth rate for drift-ballooning turbulence where reduced transport is observed. ELMing H mode: ω E B > γ max, but ion transport increases above neoclassical level. Some process other than what is usually considered transport may be controlling confinement here. MHD a likely candidate. 5 s - 5 s s (L-mode) ω E B.57s (ELM-free).97s (ELMing) γ max ω E B γ max s - 6 ω E B γ max Greenfield APS

25 CURRENT PROFILES ARE FAVORABLE FOR FUTURE SUSTAINMENT WITH ECCD 8 J A/cm 6 J OH J BS j, E determined from time history of magnetic reconstructions. f BS 5%, f NI 75% Central NBCD overdrives total current density ECCD at half-radius required for steady-state Greenfield APS

26 RECENT EXPERIMENTS HAVE INCREASED OUR UNDERSTANDING AND CONTROL OF TRANSPORT IN THE ADVANCED TOKAMAK FORMATION of internal transport barriers Appearance of ITB formation power threshold with counter NBI may be related to interplay between p and rotation terms of the E B shearing rate near magnetic axis. Pellets trigger formation of core barriers with T i T e (Baylor, UI. ). EXPANSION of the internal transport barrier Broader ITB produced with counter-nbi. Except in the vicinity of the magnetic axis, the pressure gradient and rotation terms of the E B shearing rate add, rather than cancel (as in co-nbi cases). Increased or broadened pressure profile aids E B shear stabilization of microturbulence. Broadened heating profiles may also impact barrier dynamics. Greenfield APS

27 RECENT EXPERIMENTS HAVE INCREASED OUR UNDERSTANDING AND CONTROL OF TRANSPORT IN THE ADVANCED TOKAMAK () SUSTAINMENT of Advanced Tokamak regimes Most promising AT regime combines ELMing H mode edge and low transport core. β N H 89 = 9 sustained for up to 6 confinement times. Performance appears to be controlled by relatively benign MHD instabilities rather than microturbulence. This allows the plasma to avoid more serious instabilities which could terminate the high performance. Ultimately, the regime is terminated by resistive wall modes. Work on controlling these instabilities is a major focus of the program. This regime is a promising candidate for sustainment with ECCD. Greenfield APS