Taming Plasma-Materials Interface for Steady-State Fusion

Transcription

1 Taming Plasma-Materials Interface for Steady-State Fusion by H.Y. Guo, with H. Wang, J.G. Watkins, A.L. Moser, J. Boedo, L. Casali, B. Covele, B. Grierson, M. Groth, D.N. Hill, A.W. Hyatt, L.L. Lao, A.W. Leonard, A. McLean, C. Murphy, T. Osborne, C.F. Sang, P.C. Stangeby, D.M. Thomas and DIII-D Team SAS Presented at 2 nd IAEA-TM on Divertor Concepts Suzhou, China November 13 16, H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

2 Boundary/PMI Will Be a Critical Issue for Next-Step Devices Control of Divertor and Wall Heat and Particle Load is Needed Divertor target heat load: q t 10 MW/m 2 Divertor target temperature: T t 5 ev to suppress erosion (codeposition, tritium retention) Low Greenwald density fraction: n e /n GW down to ~0.5 ITER FNSF P/R (MW/m) ~20 30~45 B T (T) 5.3 ~5 Pulse length (s) 400 ~10 6 n/n GW ~1 ~ 0.5 β N to facilitate steady state operation (current drive, performance) 2 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

3 A New Small Angle Slot (SAS) Divertor Concept Is Developed to Provide Efficient Power Exhaust for Steady-State Fusion SOLPS Model-based design to obtain cold plasma over entire target at lower density 3 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

4 A New Small Angle Slot (SAS) Divertor Concept Is Developed to Provide Efficient Power Exhaust for Steady-State Fusion SOLPS DIII-D Shot n e ~ m -3 P NBI ~ 5 MW Model-based design to obtain cold plasma over entire target at lower density Experiments support key benefits predicted by models 4 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

5 Outline Major Approaches toward Divertor Optimization New Small Angle Slot (SAS) Divertor Concept Initial Tests on DIII-D Summary and Conclusions 5 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

6 Major Approaches for Divertor Optimization (1-f rad )P sep sin(θ div ) q target = 4 q f exp R st (Single-Null Divertor) Divertor Closure Enhance energy losses via controlling of recycling neutrals and radiation (f rad, θ div - poloidal field line angle) Magnetic Configuration Maximize poloidal flux expansion (f exp ), or increase strike point radius (R st ) We focus on optimizing divertor closure to maximize divertor power dissipation while minimizing divertor volume for core fusion 6 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

7 Advanced Divertors Require Effective Use of Neutral and Impurity Dissipation Processes Horizontal Flat target (DIII-D): Direct recycling neutrals toward upstream strong leakage out to sides 7 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

8 Advanced Divertors Require Effective Use of Neutral and Impurity Dissipation Processes Horizontal Flat target (DIII-D): Direct recycling neutrals toward upstream strong leakage out to sides Vertical Slant target (AUG, JET, ITER): Direct recycling neutrals toward separatrix reduce SOL leakage and enhance plasma cooling near strike point 8 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

9 Advanced Divertors Require Effective Use of Neutral and Impurity Dissipation Processes Horizontal Flat target (DIII-D): Direct recycling neutrals toward upstream strong leakage out to sides Vertical Slant target (AUG, JET, ITER): Direct recycling neutrals toward separatrix reduce SOL leakage and enhance plasma cooling near strike point Slot (C-Mod): Reduce neutral leakage via private flux region enhance neutral buildup inside divertor 9 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

10 New Small-Angle Slot Configuration Exploits and Extends a Number of Features of Well-Established Divertor Designs Progressive slot opening toward far- SOL: Spread neutrals in far SOL extending detachment across target 10 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017 Small target angle in near-sol: Directs recycling neutrals near the strike point to initiate detachment

11 SOLPS Analysis Finds a Critical Small Target Angle to Achieve Strong Cooling Near Strike Point θ Enhancing buildup of neutral density in a localized region near the strike point to promote cooling of plasma there Thus, facilitating onset of detachment 11 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

12 SAS Leverages Benefits from both Slot Structure and Small Target Angle to Achieve Detachment at Lower Main Plasma Density SOLPS Modeling SAS: Enabling detachment at very low upstream n u ; low T e over entire target Slant ITER-like target: Achieving detachment near strike point at a higher density; and T e remains high at far target Flat target: T e remains high near strike point until much higher upstream densities 12 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

13 A Prototype SAS Divertor Is Being Evaluated on DIII-D to Validate Model Predictions Prototype SAS (not pumped) 13 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

14 Initial Tests on DIII-D Show That SAS Can Significantly Enhance Plasma Cooling When Locating Strike Point near Small-Angle Target Langmuir probes show a distinct difference when strike point is placed near the small angle target Achieving low T e across entire divertor target, in contrast to the peaked target profile in the near-sol region for ITER-like vertical targets 14 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

15 SOLPS Shows That Reduced T e in Near-SOL of SAS Correlates with Enhanced Neutral Trapping SAS This shows critical importance of optimizing neutral distribution in the divertor to maximize neutral concentration in the near SOL 15 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

16 Increasing Divertor Closure Tends to Reduce Density at Onset of Detachment B drift Upper Detachment B drift LSN Previous DIII-D data show upper closed divertor detaches at ~20% lower main plasma density than lower open divertor, as indicated by J sat rollover 16 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

17 It Appears that SAS Enables Detachment at Lowest Density Achieved in H-modes, Based on Langmuir Probe Measurements B drift Upper B drift LSN Previous DIII-D data show upper closed divertor detaches at ~20% lower main plasma density than lower open divertor, as indicated by J sat rollover SAS appears to be detached at still lower densities, as indicated by very low J sat near strike point 17 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017 B drift SAS

18 Probe Data Indicate That SAS Achieves Cold Plasma Over Entire Target with Strong Reduction in Target Heat Flux Profiles at 5x10 19 q =7J sat kt e sin(α) 18 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

19 High Confinement Is Achieved with Divertor Detachment Using SAS Detachment SAS exhibits better confinement than LSN with a wider H-mode pedestal SAS can maintain good confinement while divertor is detached SAS appears to be detached at the beginning of density ramp, as indicated by very low level J sat near strike point LSN exhibits further confinement degradation when divertor starts to detach Onset of detachment at 4 s (n e ~6x10 19 m -3 ), as indicated by J sat rollover 19 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

20 DIII-D Will Take a Staged Approach toward Divertor Optimization and Model Validation Enhance diagnostic capability for model validation (pressure gauges, bolometer, heat flux, spectroscopy for D 0 /D 2 and impurities) Improve control of plasma density and impurities via pumping and gas injection in slot (SAS 2U) Improve power/particle balance between upper and lower divertor for AT (SAS 2L) SAS 1 SAS 2U SAS 2L 20 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017

21 Summary: New SAS Concept Has Been Developed Using SOLPS & Yielded Promising Results from Initial Tests on DIII-D A new SAS divertor concept was predicted to achieve cold/detached plasmas across target at lower main plasma densities by combining a closed slot with a shallow target angle Initial tests of SAS DIII-D have evidenced the key features predicted by SOLPS modeling: achieved cold divertor plasma at lower main plasma densities by directing recycling neutrals from the small angle target to the near-sol region with low T e across entire target by redirecting recycling neutrals toward the far-sol region SAS has also achieved good confinement with divertor detachment This may provide a means for simultaneous control of heat flux and erosion at divertor target for long pulse advanced tokamaks 21 H.Y. Guo/IAEA-TM on Div. Concepts/November 13-16, 2017