X-point Target Divertor Concept

Transcription

1 X-point Target Divertor Concept and Alcator DX High Power Density Divertor Test Facility Presented by Brian LaBombard for the Alcator Team Alcator DX Contributed Oral C Presented at the 55th Annual Meeting of the APS Division of Plasma Physics Denver, Colorado - November 11-15, 2013

2 MFE development is facing serious challenges; innovation and new experiments are required Four critical challenges must be overcome before steadystate, power-producing fusion reactors can be realized. (1) Safely handle extreme plasma exhaust power densities (2) Completely suppress material erosion at divertor targets (3) Do (1)+(2) while maintaining a burning plasma core (4) Develop robust, reactor-compatible current drive and heating techniques

3 Challenge #1: Power handling Results from a multi-machine database (JRT2010+ITPA) project to extremely narrow heat flux channel widths in a reactor (caveat: low divertor recycling, H-mode conditions) Heat flux power channel width appears to be independent of machine size depends only on B pol Eich, et al., NF 53 (2013) λ q ~ 1 mm in ITER? (!) 1/5 of planned value ITER Also see: R. Goldston, Invited Talk: YI Friday morning The Heuristic Drift Model of the Scrape-Off Layer: Physics Issues and Implications

4 Challenge #1: Power handling Results from a multi-machine database (JRT2010+ITPA) project to extremely narrow heat flux channel widths in a reactor (caveat: low divertor recycling, H-mode conditions) Heat flux power channel width appears to be independent of machine size depends only on B pol Eich, et al., NF 53 (2013) λ q ~ 1 mm in ITER? (!) 1/5 of planned value λ q ~ 1 mm also in DEMO? with x4 power exhaust! ITER, DEMO Also see: R. Goldston, Invited Talk: YI Friday morning The Heuristic Drift Model of the Scrape-Off Layer: Physics Issues and Implications

5 Challenge #1: Power handling Results from a multi-machine database (JRT2010+ITPA) project to extremely narrow heat flux channel widths in a reactor (caveat: low divertor recycling, H-mode conditions) Heat flux power channel width appears to be independent of machine size depends only on B pol Eich, et al., NF 53 (2013) λ q ~ 1 mm in ITER? (!) 1/5 of planned value λ q ~ 1 mm also in DEMO? with x4 power exhaust! ITER, DEMO Even if ELMs are eliminated, standard divertor solutions, employing high poloidal flux expansion, do not look credible for a reactor just on power density considerations alone. Also see: R. Goldston, Invited Talk: YI Friday morning The Heuristic Drift Model of the Scrape-Off Layer: Physics Issues and Implications

6 Challenge #2: Material erosion Even if the power handling challenge could be met with a conventional divertor, SS reactor requirements for material erosion/redeposition control (FNSF/DEMO) are nearly impossible to meet. ~10 MW/m 2 SS heat removal requires < ~5 mm thick armor plate. This restricts net tungsten erosion < 1 mm/year (Γ W ~ 2x10 18 /m 2 /s). But plasma ion flux is high (Γ i, ~ 1-2x10 24 /m 2 /s) requiring Γ W /Γ i, < D.G. Whyte, APS 2012; Stangeby and Leonard, NF 2011,

7 Challenge #2: Material erosion Even if the power handling challenge could be met with a conventional divertor, SS reactor requirements for material erosion/redeposition control (FNSF/DEMO) are nearly impossible to meet. ~10 MW/m 2 SS heat removal requires < ~5 mm thick armor plate. This restricts net tungsten erosion < 1 mm/year (Γ W ~ 2x10 18 /m 2 /s). But plasma ion flux is high (Γ i, ~ 1-2x10 24 /m 2 /s) requiring Γ W /Γ i, < D.G. Whyte, APS 2012; Stangeby and Leonard, NF 2011, Factor of < 10-6 net yield requires: - Efficient prompt redeposition > ~ 99% (redep. material is mixed ~ poor quality) or - Fully detached divertor with ion energies below sputtering threshold, i. e., T e < ~5 ev (with impurity ions) K. Krieger, JNM (1999) 207 Γ w /Γ i, vs T e,div measured in AUG maximum allowed T e

8 Challenge #2: Material erosion Even if the power handling challenge could be met with a conventional divertor, SS reactor requirements for material erosion/redeposition control (FNSF/DEMO) are nearly impossible to meet. ~10 MW/m 2 SS heat removal requires < ~5 mm thick armor plate. This restricts net tungsten erosion < 1 mm/year (Γ W ~ 2x10 18 /m 2 /s). But plasma ion flux is high (Γ i, ~ 1-2x10 24 /m 2 /s) requiring Γ W /Γ i, < D.G. Whyte, APS 2012; Stangeby and Leonard, NF 2011, G. Wright, NF 52 (2012) Factor of < 10-6 net yield requires: - Efficient prompt redeposition > ~ 99% (redep. material is mixed ~ poor quality) or - Fully detached divertor with ion energies below sputtering threshold, i. e., T e < ~5 ev (with impurity ions) Tungsten nanotendrils in C-Mod To avoid Fuzz: E He+ < 20 ev, T e < 7 ev

9 Q: Why not just operate with a conventional divertor in a fully detached regime? A: The thermal front of divertor detachment is unstable; it jumps to the X-point region leading to - reduced radiation in divertor volume - reduced screening of impurities - increased radiation in X-point region - cooling of LCFS and pedestal region - reduced plasma confinement - near thermal collapse (H-L transition, density limit disruption) Attached Divertor Alcator C-Mod Detached Divertor Lipschultz, FST 51 (2007) 369, Goetz, PoP (1996) 1908.

10 Q: Why not just operate with a conventional divertor in a fully detached regime? A: The thermal front of divertor detachment is unstable; it jumps to the X-point region leading to - reduced radiation in divertor volume - reduced screening of impurities - increased radiation in X-point region - cooling of LCFS and pedestal region - reduced plasma confinement - near thermal collapse (H-L transition, density limit disruption) Attached Divertor Alcator C-Mod Detached Divertor Lipschultz, FST 51 (2007) 369, Goetz, PoP (1996) Fully-detached conventional divertor operation must be avoided. Current approach (conventional divertors): - Produce a controlled, partial detachment using feedback on impurity seeding and gas puffing to reduce divertor heat fluxes to tolerable levels - Take the hit on core P rad increase, confinement degradation and Z eff increase This is the plan for ITER.

11 Q: Why not just operate with a conventional divertor in a fully detached regime? A: The thermal front of divertor detachment is unstable; it jumps to the X-point region leading to - reduced radiation in divertor volume - reduced screening of impurities - increased radiation in X-point region - cooling of LCFS and pedestal region - reduced plasma confinement - near thermal collapse (H-L transition, density limit disruption) Attached Divertor Alcator C-Mod Detached Divertor Lipschultz, FST 51 (2007) 369, Goetz, PoP (1996) Fully-detached conventional divertor operation must be avoided. Current approach (conventional divertors): - Produce a controlled, partial detachment using feedback on impurity seeding and gas puffing to reduce divertor heat fluxes to tolerable levels - Take the hit on core P rad increase, confinement degradation and Z eff increase This is the plan for ITER. Example of optimized conventional divertor: Reimold -- NO (Wednesday AM): Nitrogen-induced complete divertor detachment during stable H-Mode operation in ASDEX Upgrade

12 Q: Why not just operate with a conventional divertor in a fully detached regime? A: The thermal front of divertor detachment is unstable; it jumps to the X-point region leading to - reduced radiation in divertor volume - reduced screening of impurities - increased radiation in X-point region - cooling of LCFS and pedestal region - reduced plasma confinement - near thermal collapse (H-L transition, density limit disruption) Attached Divertor Alcator C-Mod Detached Divertor Lipschultz, FST 51 (2007) 369, Goetz, PoP (1996) Fully-detached conventional divertor operation must be avoided. Challenge #3: (for SS burning plasma) Develop robust, advanced divertor solutions that achieve full detachment with minimal impurity seeding Open up access to enhanced core plasma regimes, otherwise inaccessible

13 X-Point Target divertor (XPT) concept -- may offer a robust solution to challenges 1,2,3 Concept: Use a remote X-point to produce a fully detached, radiating plasma (X-point MARFE) as a virtual target.

14 X-Point Target divertor (XPT) concept -- may offer a robust solution to challenges 1,2,3 Concept: Use a remote X-point to produce a fully detached, radiating plasma (X-point MARFE) as a virtual target. Employ all advanced divertor ideas: - Increase major radius of target plate (~SXD 1 ) " n t! $ # R t R OMP % ' & 2 " T t! R OMP $ # R t % ' & 2 [1] Super-X divertor: P. Valanju, PoP 16 (2009) ; M. Kotschenreuther, NF 50 (2010)

15 X-Point Target divertor (XPT) concept -- may offer a robust solution to challenges 1,2,3 Concept: Use a remote X-point to produce a fully detached, radiating plasma (X-point MARFE) as a virtual target. Employ all advanced divertor ideas: - Increase major radius of target plate (~SXD 1 ) " n t! $ # R t R OMP % ' & 2 " T t! R % OMP $ ' # & - Set L to infinity (via target X-point adjust) on flux tube carrying peak heat flux. - Tight baffling for high neutral pressures - Feedback control for stable divertor MARFE R t 2 [1] Super-X divertor: P. Valanju, PoP 16 (2009) ; M. Kotschenreuther, NF 50 (2010)

16 X-Point Target divertor (XPT) concept -- may offer a robust solution to challenges 1,2,3 Concept: Use a remote X-point to produce a fully detached, radiating plasma (X-point MARFE) as a virtual target. Employ all advanced divertor ideas: - Increase major radius of target plate (~SXD 1 ) " n t! $ # R t R OMP % ' & 2 " T t! R % OMP $ ' # & - Set L to infinity (via target X-point adjust) on flux tube carrying peak heat flux. - Tight baffling for high neutral pressures - Feedback control for stable divertor MARFE R t 2 Potential Benefits: Fully detached divertor over large operational space; Core x-point MARFE avoided; PMI away from core plasma; Divertor heat flux & erosion problem solved. [1] Super-X divertor: P. Valanju, PoP 16 (2009) ; M. Kotschenreuther, NF 50 (2010)

17 X-Point Target divertor (XPT) concept -- may offer a robust solution to challenges 1,2,3 Concept: Use a remote X-point to produce a fully detached, radiating plasma (X-point MARFE) as a virtual target. Potential Benefits: Employ all advanced divertor ideas: - Increase major radius of target plate (~SXD 1 ) " n t! $ # R t R OMP % ' & 2 " T t! R % OMP $ ' # & - Set L to infinity (via target X-point adjust) on flux tube carrying peak heat flux. - Tight baffling for high neutral pressures - Feedback control for stable divertor MARFE Key Element: Large major radius of target X-point provides thermal front stability. Fully detached divertor over large operational space; Core x-point MARFE avoided; PMI away from core plasma; Divertor heat flux & erosion problem solved. [1] Super-X divertor: P. Valanju, PoP 16 (2009) ; M. Kotschenreuther, NF 50 (2010) R t 2

18 Goal: X-pt MARFE in divertor volume, not in core plasma q // Thermal fronts (a.k.a. MARFEs) accommodate a ~maximum value of incident q // Example Carbon 1 : 1/2 nt q //! 30 f e I m "3 ev [MW / m2 ] depends on plasma pressure, impurity fraction, q //! 0.6 GW / m 2 and! 4%;! m "3 ;! 100 ev [1] I. Hutchinson, NF 34 (1994) 1337.

19 Goal: X-pt MARFE in divertor volume, not in core plasma Large major radius of X-pt target provides stability/control q // q // Upstream q // decreases as ~ 1/R along divertor leg.

20 Goal: X-pt MARFE in divertor volume, not in core plasma Large major radius of X-pt target provides stability/control q // q // Upstream q // decreases as ~ 1/R along divertor leg. Thermal front position is stable to perturbations (n, T e, f I ).

21 Goal: X-pt MARFE in divertor volume, not in core plasma Put radiating, cold plasma where it belongs in the divertor! q // Emissivities up to 60 MW/m 3 have been seen in C-Mod, consistent with X-pt target divertor requirements.

22 Goal: X-pt MARFE in divertor volume, not in core plasma Put radiating, cold plasma where it belongs in the divertor! q // Critical Need for MFE: experimental facility to develop/test advanced divertor ideas under reactor-level q //, nt e and n div

23 Alcator DX Proven Alcator Technology: extremely strong super-structure sliding TF joints coaxial OH/PF coil feeds electro-formed terminals PF and OH coils supported by rigid vacuum chamber Reactor-relevant RF heating and current drive systems a concept for a high power density, advanced divertor test facility* Major/Minor Radius Alcator DX 0.73 / 0.2 m Elongation 1.7 Magnetic Field Plasma Current P AUX (net) Surface Power Density SOL Parallel heat flux Advanced Divertor Concepts Divertor and first-wall material Pulse Length 6.5 Tesla 1.5 MA 8 MW ICRF 2 MW LHCD ~ 1.5 MW/m 2 q ~ 2 GW/m 2 Vertical target; Snowflake; Super-X; X-point target; Liquid metal target Tungsten/ Molybdenum 3s, with 1s flat-top Key Elements: Alcator DX Demountable, LN 2 cooled, copper TF magnet Vertically-elongated VV Advanced divertor poloidal field coil sets (top and bottom) High power ICRF, 8MW Reactor-level P/S, SOL q and plasma pressures => same and higher than Alcator C-Mod Development platform for low PMI RF actuators: Inner-wall LHCD Inner-wall ICRF See poster TP , C-Mod Session, Thursday morning *

24 Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions Alcator DX Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets. Tame the plasma-material interface with plasma physics Double-null geometry Advanced divertors -- low-field side SOL Quiescent, low heat flux -- high-field SOL ~zero turbulent transport on high-field SOL 1 [1] Smick, et al., NF 53 (2013)

25 Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions Alcator DX Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets. ASDEX Grad-Shafranov equilibria obtained using ACCOME (Selene) 1 [1] Tani et al., J. Comp. Phys. 98 (1992) 332.

26 Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions Alcator DX Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets. Vertical Target Grad-Shafranov equilibria obtained using ACCOME (Selene) 1 [1] Tani et al., J. Comp. Phys. 98 (1992) 332.

27 Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions Alcator DX Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets. Long Leg Grad-Shafranov equilibria obtained using ACCOME (Selene) 1 [1] Tani et al., J. Comp. Phys. 98 (1992) 332.

28 Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions Alcator DX Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets. Super X Grad-Shafranov equilibria obtained using ACCOME (Selene) 1 [1] Tani et al., J. Comp. Phys. 98 (1992) 332.

29 Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions Alcator DX Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets. X-point Target Grad-Shafranov equilibria obtained using ACCOME (Selene) 1 [1] Tani et al., J. Comp. Phys. 98 (1992) 332.

30 Alcator DX -- a innovation platform for low PMI, reactor compatible RF actuators Splitter and multi-junction fabrication techniques produce compact LHCD launchers that can fit on the inside wall. High B-field side => lower n // => penetrating rays => higher CD efficiency High field side launch is highly favorable for LHCD, as noted in VULCAN study 2. [2] VULCAN: Podpaly, et al., FED 87 (2012) 215. Alcator DX Quiescent SOL => Low PMI => Excellent impurity screening 1 [1] McCracken, et al., PoP 4 (1997) 1681.

31 Alcator DX Challenge #4: (for SS burning plasma) -- a innovation platform for low PMI, reactor compatible RF actuators Splitter and multi-junction fabrication techniques produce compact LHCD launchers that can fit on the inside wall. High B-field side => lower n // => penetrating rays => higher CD efficiency High field side launch is highly favorable for LHCD, as noted in VULCAN study 2. [2] VULCAN: Podpaly, et al., FED 87 (2012) 215. Alcator DX Quiescent SOL => Low PMI => Excellent impurity screening 1 [1] McCracken, et al., PoP 4 (1997) Develop robust, reactor-compatible current drive & heating techniques

32 Summary Advanced divertors have the potential to solve four critical challenges for steady-state, power-producting fusion reactors: (1) Safely handle extreme plasma exhaust power densities (2) Completely suppress material erosion at divertor targets (3) Do (1)+(2) while maintaining a burning plasma core A new concept, the X-point Target divertor, aims to solve these challenges by producing a fully detached, X-point MARFE in the divertor volume as a virtual target. q // (4) Inner-wall launch (double-null) enables reactor-compatible current drive and heating techniques

33 Summary A concept for a high power density, advanced divertor test facility, Alcator DX, is being considered to test these ideas* Alcator DX Alcator DX Mission: Develop robust divertor solutions to power exhaust and material erosion challenges for steady-state plasma fusion reactors Explore core plasma performance in regimes otherwise inaccessible with conventional divertors Develop and test reactor relevant ICRF and LH drivers that minimize plasma-material interactions Inform the conceptual development and accelerate the readiness-for-deployment of next step devices (pre FNSF, FNSF, DEMO) We welcome your input and participation in this brainstorming activity. See poster TP (C-Mod session, Thursday morning): Critical need for MFE: the Alcator DX advanced divertor test facility *