INTERNAL TRANSPORT BARRIER THRUST

Transcription

1 INTERNAL TRANSPORT BARRIER THRUST by C.M. Greenfield Presented to Program Advisory Committee Meeting January, 3 /TST/wj

2 THE GOAL OF THRUST 7 IS TO ESTABLISH CONTROL OVER THE INTERNAL TRANSPORT BARRIER Increase spatial extent of barrier. Increased fusion performance. Control pressure gradient in barrier. Avoid MHD instabilities which can terminate ITB or disrupt discharge..5 a.u MHD limits.5 a.u. Counter-NBI Neon puffing Off-axis ECH Off-axis pellets Off-axis NBI.. Maintain elevated/reversed q profile. Avoid MHD instabilities when q min. Impacts ITB characteristics, especially in n e and T e profiles. Take advantage of favorable impact of counter-nbcd and bootstrap currents in broadened barriers. Improved stability.5 a.u. local p... Counter-NBI Modulated off-axis ECH Neon injection ITB Improved fusion performance?.6 a.u... Greenfield DAC

3 THRUST 7 EXPERIMENTS PERFORMED IN 999 Large qmin produced with fast current ramp and early, high power NB heating. No fixed relationship between details of the q profile and the ITB location identified. Counter-NBI conditioning and discharge development. Customers included RWM thrust, Confinement TSA and Heating and Current drive TSA. Impurity transport with counter-nbi. Particle source may be enhanced, but inward transport is not. Develop fast-ion free preheat using ECH. Produced NCS profiles similar to those using early beams. Electron transport barrier result shown in Confinement TSA talk result of this experiment. ITB formation and expansion with counter-nbi. Steady ELMless H mode shown in Confinement TSA talk from late in these discharges. ITB formation triggered by pellet injection. Greenfield DAC 3

4 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 DAC 4

5 99849 (.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 P NBI = 9.6 MW (7.6 MW absorbed). 5 s - 3 Ω 6 4 q Greenfield DAC 5

6 COMBINATION OF p AND ROTATION EFFECTS IN ω E B NATURALLY BROADENS COUNTER BARRIERS 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. 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 ª.5 in both cases). Shearing rate profile extends to larger. 5 s s (co) s (counter) s ω E B p ω E B rotation ω E B co (873.8s) ctr ( s) 4 ω E B ω E B γ max p ω E B rotation ω E B Greenfield DAC 6

7 5 4 IMPURITIES DO NOT PREFERENTIALLY BUILD UP NEAR THE MAGNETIC AXIS DURING AN ITB OF LIMITED DURATION co-nbi counter-nbi Zeff 3.6s.7s.8s.97s.7s.7s Z eff constant in the vicinity of the magnetic axis during ITB evolution. Counter-NBI results in little or no additional core accumulation. Significant impurity buildup near edge with counter-nbi. Greenfield DAC 7

8 DIII-D Pellet Injection Program Modifications to injector (that was installed on JET 987-9): All three guns fire.7 mm pellets Punch mechanism on one gun to reliably generate slower pellets (< m/s) Second gun has oversized barrel that allows moderate speed pellets independent guide tubes on inner wall (HFS) - one on midplane, one at 45 degrees - and one vertical. Can be connected to any of the pellet guns or a gas valve. Performance tests were performed on curved guide tubes. Initial experiments on the vertical injection port were performed in 998 and on the HFS ports in March-June 999. Aug LRB Greenfield DAC 8

9 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 ª T e. MW of counter-nbi applied to produce ITB in L mode. Greenfield DAC 9

10 THRUST 7 PLAN FOR The three-day plan will continue our attempts to address both expansion and pressure gradient control using counter-nbi. Optimization of Counter-NBI Generated ITBs ( days). Due to time constraints, we plan a limited attempt at exploration with one additional tool (TBD): Neon puffing to improve ITB (First of days). Off-axis ECH to expand radius of ITB ( day). Note that due to similar machine requirements, the transition from one experiment to the next may not be made at the beginning of a day. No explicit pellet experiments, but barriers triggered both with and without pellets may be included in experiments. An additional two days, if available, would be used to complete the above experiments. Greenfield DAC

11 OPTIMIZATION OF COUNTER-NBI GENERATED ITB ( DAYS) Background ITBs formed with counter-nbi, with (in at least some cases) nearly stationary q profiles. We did not have time to attempt barrier expansion and control. Proposed experiment Reestablish ITB with power stepdown. Ramp up neutral beam power (using PCS modulation). Scan ramp rate for optimum barrier growth without disruption. After barrier growth is established, reduce beam power later in discharge to attempt a steady phase. If successful, we can move on to a neutral beam feedback control attempt. What do we use as a feedback signal: neutron rate, stored energy, Er? MA MW 5 s I P S N P ECH (6 sources) (6 4 sources) (6 5 sources) 9986 (6 4 5 sources) P NBI time (s) Greenfield DAC

12 NEON IMPURITY INJECTION Reduced transport is a common feature of discharges with impurity injection (ISX B, Textor, JET,, ). Identified with decreased core fluctuations and calculated linear turbulence growth rates. Standard feedback loop: E B shearing rate increases as barrier forms. Improved energy (H89 ) and particle confinement. Produces broad ITBs with easily maintained L mode edge and lower Ti / Te. Previous experiments concentrated at low power and density and high q95. Remaining challenges: Access at low q95. Can elevated q profile be maintained? Feedback control of radiated power? Can low Zeff be maintained? Extensive modeling in progress to guide experimental development. T i (kev) Ion Temperature Profile t=.6 sec..4 χ i (m /s) Minor Radius () Neon Reference.8. χ i (=.65) Toroidal Rotation (rad/s).5x TIME (s).. no neon neon (.4V) neon (.8V) Carbon Rotation Profile t=.6 sec Minor Radius () Neon Reference. Greenfield DAC

13 EXPERIMENTS USING NEON INJECTION WILL OPTIMIZE AND CHARACTERIZE THE IMPACT OF IMPURITY-IMPROVED TRANSPORT Variation of impurity puff. Attempt feedback control of neon injection and pumping to control p rad and n e. Use neon as trigger in short burst, attempt to maintain ITB with E B shear. Attempt to use helium (lower Z) or nitrogen (better pumping?) as the impurity. Variation of q profile. Decrease q 95, either by increasing toroidal field or decreasing plasma current. Apply counter ECCD and/or counter NBCD to maintain elevated q profile. Variation of heating. Increase heating power. Momentum scan (reduce beam voltage at constant power). Increase density (gas puffing or HFS pellet injection). Use puff N pump to maintain higher concentration of impurity at edge. Greenfield DAC 3

14 Background Application of on-axis ECH increases transport in all channels. Transport in the ITB can become too good, becoming a challenge to MHD stability. Tools to make the barrier leaky may be desirable (AT Workshop). ECH may be a useful tool to limit the pressure gradient in the presence of an ITB, thereby extending the duration. ECH (heating) to limit or move ITB 9 kev/m 3 MW/m 3 4 A B C (a) Electron Pressure (c) 9 kev/m 3 A B C (b) Ion Pressure ECH Power Ion Thermal Density. B Diffusivity B C C A...4 r/a.8..4 r/a.8 m /s 8 4 (d) Control ITB position: Heating outside barrier predicted to cause barrier to move toward heating location expansion (Staebler). Control ITB strength: Modulated heating inside/outside barrier should regulate strength of gradient (Newman). Electron temperature (times 5eV) no transition and no perturbation transition and perturbation transition and no perturbation r/a Greenfield DAC 4

15 THRUST 7 GOALS FOR AND BEYOND Short term goal: : Open-loop tests of potential control mechanisms. Counter-NBI (started in 999). - Neon injection: combine previous results (G. McKee, APS invited talk, 999) with ITB techniques to extend good confinement region. Off-axis ECH to control barrier strength and position. -? Off-axis pellet injection. Barrier expansion through modification of rotation profile. Magnetic braking. Simulate successful balanced injection condition (TFTR, JT-6U) by using magnetic braking to remove momentum. Off-axis NBI. Broader heating profiles theoretically and experimentally (JT-6U) shown to broaden barrier. Target q profile modification with ECH/ECCD. Greenfield DAC 5

16 THRUST 7 GOALS FOR AND BEYOND (continued) Short term goals, continued All were proposed for, but limited time (three days) allows only the first -3 tools to be tested this year. Depending on future experimental time availability, may take -3 years to do all experiments. Longer term goal: Apply feedback control of promising (based on closed-loop tests) mechanisms to produce barriers with large spatial extent and gradients maintained below stability limits. Not necessarily synchronous with open-loop tests. Greenfield DAC 6

17 THRUST 7 EXPERIMENTS ADDRESS FESAC GOALS, AND 3 Advance fundamental understanding of plasma, the fourth state of matter, and enhance predictive capabilities, through comparison of experiments, theory and simulation. Core barriers provide a test bed for enhancing our understanding of turbulence and transport, as well as mechanisms which may reduce transport, including E B flow shear. These experiments allow us to probe these phenomena in more detail by changing the relationship of individual terms in the shearing rate as well as the instability growth rates. Interpretation of the results is done in active collaboration with the theory and modeling community, thus fostering comparison of experiments, theory and simulation. Resolve outstanding scientific issues and establish reduced-cost paths to more attractive fusion energy systems by investigating a broad range of innovative magnetic confinement configurations. Transport barrier control identified as key scientific issue at AT Workshop and Snowmass. Advance innovation and understanding in high-performance plasmas, optimizing for projected power-plant requirements; and participate in a burning plasma experiment. Improved understanding and control of core barriers is important for optimization of AT scenarios. Greenfield DAC 7

18 THRUST 7 EXPERIMENTS HAVE INCREASED OUR UNDERSTANDING AND CONTROL OF TRANSPORT BARRIERS 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 ª T e. 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. Thrust 7 results were reported in two invited talks at the APS-DPP conference in Seattle: C.M. Greenfield, Understanding and Control of Transport in Advanced Tokamak Regimes in, paper BI. L.R. Baylor, Improved Core Fueling with Pellets Injected from the High Field Side of the Tokamak, paper UI 4. Greenfield DAC 8

19 Additional slides

20 ITB FORMATION EXHIBITS POWER THRESHOLD WITH COUNTER-INJECTION Co-NBI: ITB forms with P NBI.5 MW and e xpands with P NBI 5 MW. Counter-NBI: Clear barriers formed only with P NBI ~9 MW. Beam power losses occur in both co- and counter-nbi discharges. ~% lost for co-injection Shinethrough and charge exchange. ~3-4% 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 DAC

21 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 4 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 P NBI I P t (sec) D α (a.u.).6.4 MA Greenfield DAC

22 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 ( s) co (873.8s). χ ϕ χ neo i m /s.. Possible causes of ITB broadening: Broader heating profile Enhanced E B shear Greenfield DAC

23 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 ª.5, much lower than observed with neutral beams alone. Greenfield DAC 3

24 STATIONARY CENTRAL q PROFILES MAINTAINED BY COUNTER NEUTRAL BEAM CURRENT DRIVE.4 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 DAC 4