Overview of Pellet Injection on DIII-D Tokamak from the High Field Side, the Low Field Side, and the Top.

Transcription

1 Overview of Pellet Injection on DIII-D Tokamak from the High Field Side, the Low Field Side, and the Top. T.C. Jernigan, L.R. Baylor, S.K. Combs, W.A. Houlberg (Oak Ridge National Laboratory) P. Gohil, P.B. Parks (General Atomics) Deuterium pellets have been injected into DIII-D from the inside wall, the outside, and the top. When the injection point is inside the magnetic axis of the plasma, the fueling efficiency has proven to be significantly higher (approaching 1% in some circumstances) than when the injection point is outside of the magnetic axis. This is attributed to the outward radial motion of the ablated plasma cloud, P.T.Lang, et. al,phys.rev.lett.79,178 ( 1997). A plasma deposition model by Parks, P.B. Parks Phys Plasmas 5, (), has been incorporated in the pellet code to compare with the experimental deposition. An additional slow injector (v pellet <= m/s) has been installed. This will allow comparison with inside launch injection near the mid-plane vs injection from higher on the inside wall (6 degree downward vs 56 degree downward). 1

2 DIII-D Pellet Injection System Recent Injector Modifications l All three guns now fire.7-mm pellets (1.8- and.-mm sizes also used in previous experiments) l New pellet punch mechanisms installed on two guns for reliable slow-pellet operation; these guns are also equipped with oversized gun barrels to further limit pellet speed (< m/s) New Pellet Punch (Not Shown) Initiates Pellet Motion; It Was Developed and Tested at ORNL before Installation on DIII-D

3 DIII-D Pellet Injection System Transport Guide Tubes l Straight guide tubes ( m)» Three independent standard low-field-side (LFS) injection tubes: 135º port l Curved guide tubes ( 1.5 m)» Two independent vertical injection tubes: º (V+3) and 6º (V+1) ports» Two independent HFS injection tubes on inner wall: 5º upper and lower ports» Special tube with tight bend radius for LFS injection experiments with fractured pellets l Performance tests were done at ORNL on mockups of DIII-D roller-coaster tube runs» Special ORNL components were developed for attaching multiple tube sections and size transitions» Pellet speeds are limited to m/s for survival in HFS tube runs and m/s for vertical installations» Combs, SOFE Proceedings, 1999 Any guide tube can be connected to any pellet gun (or a gas valve) 3

4 HFS Pellet Internal Injection Tubes on DIII-D ORNL 99-17C EFG

5 ORNL Pipe-Gun Facility Was Modified and Used for Testing Curved Guide Tubes and Mock-Ups of Inside Launch Pellet Injection on DIII-D, JET, and LHD 5

6 D Pellet at Gun Muzzle and Guide Tube Exit from Mockup Test of DIII D Lower HFS Pellet Injection ORNL EFG 6

7 Results from ORNL Mockup of DIII-D HFS Guide Tube Geometry Give Maximum Speed Allowed for Intact Pellets (.7-mm D Pellets) 1 1 Intact Pellets (%) 8 6 HFS mid HFS Pellet Velocity (m/s) Vmax HFS_5 = 5 m/s Vmax HFS_mid = 1 m/s 7

8 Alternative Injection Locations for Optimized Fueling Pellet penetration is well characterized, but deposition profile from LFS injection is anomalous ELMs triggered by LFS pellets are substantial and long lasting Alternative injection locations have been installed to investigate pellet fueling deposition from top ports and inner wall ports High field side (HFS) injection lines on DIII-D provide improved core fueling with HFS injected pellets HFS pellets have efficient fueling with minimized particle loss ELMs triggered by HFS pellets are similar to background ELMs Vertical injection inside magnetic axis provides improved fueling compared to LFS miplane or vertical LFS trajectory Vertical mounted injector may be optimal for reactor fueling 8

9 Pellet Penetration is Well Characterized, but Deposition Profile from LFS Injection is Anomalous Theroetical Penetration λ/a Penetration.5<T e <7.7 kev <v p <33 m/s JET Tore Supra DIII-D TFTR FTU Measured Penetration λ/a Ablation Emission (a.u.) DIII-D mm V p = 96 m/s Maximum Penetration depth agrees well with theory over a range of data from many devices, λ/a ~ T e -5/9 v p 1/3 (Baylor, et al., Nucl. Fusion 37, 5 (1997)) Mass deposition implies fast radial transport during the ablation process n e (1 19 m -3 ) ASDEX Upgrade first experiment to try HFS injection to test this hypothesis (Lang, et al., Phys. Rev. Lett. 79, 178 (1997.)) NGS Model. Deposition..6 t = 7. ms Measured H-mode.8MW NBI

10 High Field Side (HFS 5 ) Pellet Injection on DIII-D Yields Deeper Particle Deposition than LFS Injection 1.7 mm Pellets - HFS 5 vs LFS n e (1 19 m -3 ) 5 DIII-D measured n e HFS 5 v p = 118 m/s t = 5 ms Calculated Penetration LFS v p = 586 m/s t = 1 ms Fueling Efficiency: HFS - 95% LFS - 55% Net deposition is much deeper for HFS pellet in spite of the lower velocity Pellets injected into the same discharge and conditions (ELMing H-mode,.5 MW NBI, T e () = 3 kev) 1

11 Theoretical Model for Pellet Radial Drift ExB Polarization Drift Model of Pellet Mass Deposition (Rozhansky, Parks) Polarization of the ablatant occurs from B and curvature drift in the non-uniform tokamak field: W + W v = B B B 3 eb HFS R B 1/R E Pellet Ablatant (Cloud) LFS ExB The resulting E yields an ExB drift in the major radius direction The velocity of ablatant c s (L/R).5. For DIII-D this is km/s, i.e. faster than the pellet (dekloe, Mueller, Phys.Rev.Lett. (1999)) R stronger at higher plasma β Detailed model by Parks, P.B. Plasmas 1968, ().) 11 (Phys.

12 HFS 5 Pellet Injection Deposition Suggests Major Radius Drift of Ablatant Pellet Ablation =.8 Ablatant Drift =. n e (1 19 m -3 ) mm Pellet - HFS 5 DIII-D 9977 H-mode 7MW NBI HFS 5 v p = 153 m/s λ = 17cm t =.5 ms NGS Model (x.) Pellet D α The deposition shows deeper fueling than predicted Pellet D α emission agrees with ablation model (PELLET code) A radial drift of cm is inferred from the data - for comparison with detailed drift model. (Parks, P.B., Phys Plasmas ) 1

13 Preliminary HFS Comparison HFSmid Yields Deeper Deposition than HFS5 n e (1 19 m -3 ) HFSmid vs HFS 5 DIII-D HFSmid 153 m/s HFS5 3 m/s A direct comparison of same size.7mm pellets in the same shot shows shows slightly deeper fueling from HFSmid than from HFS5 trajectory 13

14 Both Vertical HFS and LFS Pellet Injection are Consistent with an Outward Major Radius Drift of Pellet Mass V+1 V+3 n e (1 19 m -3 ) 1.7mm Pellet - Vertical HFS vs Vertical LFS 5 V+1 HFS H-mode MW v p = 17 m/s Radius of Vertical Port V+3 LFS H-mode -.5 MW v p = m/s The net deposition profile measured by Thomson scattering - ms after pellet injection on DIII-D. V+1 HFS indicates drift toward magnetic axis while V+3 LFS suggests drift away from axis. 1

15 HFS Pellet Injection on DIII-D Yields Higher Fueling Efficiency than LFS Pellets Fueling Efficiency, η.8.6. Hmode V+1 Hmode. Hmode. Lmode V+1. Elm Free Hmode Hmode V+3 Elm Free Hmode Lmode Lmode LFS Lmode V+3 V HFS P NBI (MW) LFS Fueling Efficiency in H-mode ~ 5%.8.6. P NBI (MW) V+1 Fueling Efficiency in H-mode ~ 6-9 %.8.6. P NBI (MW) HFS Fueling Efficiency in H-mode ~ 9% HFS injection exhibits almost ideal fueling efficiency. Vertical injection provides higher fueling efficiency than LFS injection. LFS pellet fueling efficiency affected by major radius drift and by pellet ELM interaction. 15

16 Application of Pellets for Transport Barrier Formation: Internal Barriers: PEP-mode PEP-mode - overview and transport summary HFS pellets produce peaked density profile with T i T e Internal transport barriers can be produced both with strong Negative Central Shear (NCS) and weak internal shear (WS) depending upon pellet timing and beam timing and direction relative to the plasma current Pellets provide a tool to produce core transport barriers with parameters unobtainable by other methods thus providing a wider range to test physics of barrier formation and sustainment Baylor, et al, IAEA 16

17 HFS Pellets During Current Rise Lead to Internal Transport Barrier - PEP mode Ip (MA) ne (1 19 m -3 ) q 1 6 DIII-D PEP ITB P NBI (MW) H-mode non-pep ITB (99788) 8 q() 6 q min Time (s) HFS.7mm pellets injected during current rise produce highly peaked density profiles that develop PEP ITB with T i T e PEP survives transition to H-mode and can persist for > 1s Core collapse occurs as q min reaches 3/ 1 Collapse ne (1 19 m -3 ) Te, Ti (kev) PEP ITB non-pep ITB T e (TS) q DIII-D T i (CER) PEP Steepest n e, T e, T i gradients occur inside qmin 17

18 PEP-mode Ion Thermal Diffusivity in the Core Approaches Neoclassical Levels χ i (m /s) 1 1 DIII-D 9978 t = 1. s Weak Shear PEP ITB NCS PEP ITB χ e (m /s) 1 1 DIII-D 9978 t = 1. s NCS PEP ITB Weak Shear PEP ITB χ i neoc TRANSP calculation of thermal diffusivities shows ITB in core region out to =. as expected from the strong gradients in the kinetic profiles. ITB in NCS PEP case is stronger than weak shear PEP ITB, both approach neoclassical levels. χ e is lower in core of weak shear PEP. ω EXB becomes large enough to suppress ITG turbulence in the NCS PEP ITB plasmas. 18

19 NCS PEP-mode has Lower Electron Particle Diffusivity than Low Shear PEP Comparison De (m /s) DIII-D 11 WS PEP 9978 NCS PEP qmin TRANSP calculation of electron particle diffusivity shows reduced core particle transport in PEP just inside the minimum q region (=.). Both NCS PEP and WS PEP ITBs show reduced D e toward axis, but NCS case increases where profiles become flat. 19

20 ITG Modes are Stabilized in PEP-mode ITB Core Region 1 5 s ω ExB ITG Stable DIII-D 9978 t = 1.5s Trapped Electron Mode γ max Gyrokinetic Stability Calculation The ExB shearing rate exceeds the ITG growth rate inside the ITB ω ExB ( RBθ ) = B ψ Er RB Edge shearing rate is strong due to H-mode edge barrier θ

21 Application of Pellets for Transport Barrier Formation: Edge Barriers: H-mode PIH-mode - pellet induced H-mode overview Pellets have reduced H-mode power threshold by up to 33% Pellets enable test of transition theory Steep edge density gradient is necessary for H-mode transition However, injection location and extent of deposition not critical to the transition Gohil, et al, to be published in Phys Rev Lett 1

22 HFS Pellets have induced H-mode Transitions ne (1 19 m -3 ) D α (a.u.) ne (1 19 m -3 ) HFS Pellet Upper Divertor =.1 Dithering Phase =.9 ELMs Time (s) L-mode Target.8MW NBI Upper Single Null HFS pellet induces H-mode transition that is maintained H-mode power threshold reduced by.mw (up to 33%) using pellet injection (Gohil,P. submitted for publication)

23 Pellet Induced H-mode Transition Occurs at Lower Edge Temperature 6 DIII-D LFS DIII-D ne(1 19 m -3 ) - 1ms + ms After L-H ELM-free H Te (kev) 3 1-1ms + ms After L-H ELM-free H A critical edge temperature is not indicated in these H mode transitions Edge Te and Ti are reduced following pellet injection Pellet induced H modes have L-H transitions at plasma parameters far below theoretical predictions 3

24 The HFS Pellet Penetrates Much Further, But Still Produces A Significant Density Gradient At The Plasma Edge Pellet Time (ms) Upper Divertor Photodiode Shot Electron Density Profiles Electron Temperature Profiles 3 Electron Pressure Profiles Shot 117 ms before pellet (3598 ms) 1 ms after pellet (361 ms) ms after pellet (36 ms) After dither phase (368 ms) before ELM (3698 ms) /rs

25 α MHD Pellet Induced H-modes have Transitions at Plasma Parameters far Below Theoretical Predictions Rogers et al. Proc. 17th IAEA Fusion Energy Conf. Yokohama, Japan 1998, IAEA-CN-69/THP/1.6.. DIII-D ms before pellet ms after pellet After LH transition ELM free phase H-mode L-mode β/µ 1. Pogutse et al. Proc. th EPS Conf (P3-11). H-mode DIII-D ms before pellet ms after pellet After LH transition ELM free phase L-mode α MHD Wilson et al. Proc. 17th IAEA Fusion Energy Conf. Yokohama, Japan 1998, IAEA-F1-CN-69/TH3/.6.. H-mode DIII-D ms before pellet ms after pellet After LH transition ELM free phase L-mode After pellet After pellet After pellet α DIAM ν/µ.5 ν* Comparison of experimental edge local parameters with predictions from theories of H-mode transitions. 5

26 A Steep Edge Density Gradient is Necessary for the H-mode Transition Edge ne (1 1 m - ) Value of Edge ne is critical Peak Edge ne (1 1 m - ) No correlation with deposition extent Location of peak n e from pellet Time (s) Extent of the pellet deposition is not important for H-mode transition but a threshold gradient of 1X1 1 m - appears critical 6

27 Conclusions HFS injection ports installed on DIII-D and guns modified for slower pellet speeds (v pellet < m/s) to take advantage of the radial drift and lead to improved core fueling with HFS injection The pellet mass drifts in the plasma major radius direction on a fast (<1 µs) time scale during the redistribution process ExB polarization drift model is appears qualitatively correct, but there is insufficient data for a quantitative evaluation The new HFS pellet injection tool has been applied successfully for: PEP-mode ITB formation with T i T e, (unlike other ITB regimes) and first measurement of E r in PEP-mode Triggers for L to H-mode transitions for reduced power threshold HFS pellet injection is unique enabling technology that has led to several areas of new physics understanding on DIII-D 7