x-point target divertor conceptlabombard/aps2013...labombard- x-point target divertor – aps2013...

Presented by Brian LaBombard for the Alcator Team

Contributed Oral C04.00002 Presented at the 55th Annual Meeting of the APS Division of Plasma Physics

Denver, Colorado - November 11-15, 2013

X-point Target Divertor Concept and

Alcator DX High Power Density Divertor Test Facility

AlcatorDX

LaBombard- x-point target divertor – APS2013 – Denver

(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

MFE development is facing serious challenges; innovation and new experiments are required

Four critical challenges must be overcome before steady-state, power-producing fusion reactors can be realized.

(4) Develop robust, reactor-compatible current drive and heating techniques


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)

Eich, et al., NF 53 (2013) 093031

Heat flux power channel width appears to be independent of machine size – depends only on Bpol

λq ~ 1 mm in ITER? (!) 1/5 of ‘planned’ value

ITER

Also see: R. Goldston, Invited Talk: YI2.00001 Friday morning “The Heuristic Drift Model of the Scrape-Off Layer: Physics Issues and Implications”



Eich, et al., NF 53 (2013) 093031



ITER, DEMO

λq ~ 1 mm also in DEMO? with x4 power exhaust!




Eich, et al., NF 53 (2013) 093031



ITER, DEMO

λq ~ 1 mm also in DEMO? with x4 power exhaust!

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.



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/m2 SS heat removal requires < ~5 mm thick armor plate. This restricts net tungsten erosion < 1 mm/year (ΓW ~ 2x1018/m2/s). But plasma ion flux is high (Γi,⊥ ~ 1-2x1024/m2/s) requiring ΓW/Γi,⊥ < 10-6.

Challenge #2: Material erosion

D.G. Whyte, APS 2012; Stangeby and Leonard, NF 2011, 063001


K. Krieger, JNM 266-269 (1999) 207

Γw/Γi,⊥ vs Te,div measured in AUG

maximum allowed Te


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., Te < ~5 eV (with impurity ions)





G. Wright, NF 52 (2012) 042003

Tungsten nanotendrils in C-Mod

To avoid Fuzz: EHe+< 20 eV, Te < 7 eV




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., Te < ~5 eV (with impurity ions)



A: The ‘thermal front’ of divertor detachment is unstable; it ‘jumps’ to the X-point region leading to

Q: Why not just operate with a conventional divertor in a fully detached regime?

- 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)

Lipschultz, FST 51 (2007) 369, Goetz, PoP (1996) 1908.

Alcator C-Mod

Attached Divertor Detached Divertor






Alcator C-Mod

Fully-detached conventional divertor operation must be avoided. Current approach (conventional divertors):

This is the plan for ITER.

- 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 Prad increase, confinement degradation and Zeff increase




Alcator C-Mod




Fully-detached conventional divertor operation must be avoided. Current approach (conventional divertors):

This is the plan for ITER.

- 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 Prad increase, confinement degradation and Zeff increase

Reimold -- NO6.00009 (Wednesday AM): “Nitrogen-induced complete divertor detachment during stable H-Mode operation in ASDEX Upgrade”


Example of optimized conventional divertor:




Fully-detached conventional divertor operation must be avoided.


Challenge #3: Develop robust, advanced divertor solutions that achieve full detachment with minimal impurity seeding

Open up access to enhanced core plasma regimes, otherwise inaccessible

(for SS burning plasma)


Alcator C-Mod



Concept: Use a remote X-point to produce a fully detached, radiating plasma (X-point MARFE) as a virtual target.

X-Point Target divertor (XPT) concept -- may offer a robust solution to challenges 1,2,3


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Employ all advanced divertor ideas: - Increase major radius of target plate (~SXD1)

[1] Super-X divertor: P. Valanju, PoP 16 (2009) 056110; M. Kotschenreuther, NF 50 (2010) 035003.


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- 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’




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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.






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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.

Key Element: Large major radius of target X-point provides thermal front stability.






Thermal fronts (a.k.a. MARFEs) accommodate a ~maximum value of incident q// depends on plasma pressure, and impurity fraction,

Goal: X-pt MARFE in divertor volume, not in core plasma

q// ! 30 fI1/2 nTe1020m"3eV

[MW /m2 ]

[1] I. Hutchinson, NF 34 (1994) 1337.

Example Carbon1:

q// ! 0.6 GW /m2

! 4%; !1020m"3; !100 eV

q//


Large major radius of X-pt target provides stability/control


Upstream q// decreases as ~ 1/R along divertor leg.

q//

q//


Large major radius of X-pt target provides stability/control


Upstream q// decreases as ~ 1/R along divertor leg. Thermal front position is stable to perturbations (n, Te, fI).

q//

q//



q//

Put radiating, cold plasma where it belongs – in the divertor!

Emissivities up to 60 MW/m3 have been seen in C-Mod, consistent with X-pt target divertor requirements.



q//

Put radiating, cold plasma where it belongs – in the divertor!

Critical Need for MFE: experimental facility to develop/test advanced divertor ideas under reactor-level q//, nTe and ndiv

AlcatorDX


a concept for a high power density, advanced divertor test facility*

*http://burningplasma.org/web/fesac-fsff2013/whitepapers/LaBombard_B.pdf

Alcator DX Major/Minor

Radius 0.73 / 0.2 m

Elongation 1.7

Magnetic Field 6.5 Tesla

Plasma Current 1.5 MA

PAUX (net) 8 MW ICRF 2 MW LHCD

Surface Power

Density ~ 1.5 MW/m2

SOL Parallel heat

flux q|| ~ 2 GW/m2

Advanced Divertor

Concepts

Vertical target; Snowflake; Super-X;

X-point target; Liquid metal

target

Divertor and first-wall material

Tungsten/Molybdenum

Pulse Length 3s, with 1s flat-top

•  Demountable, LN2 cooled, copper TF magnet

•  Vertically-elongated VV

•  High power ICRF, 8MW

•  Reactor-level P/S, SOL q|| and plasma pressures

•  Advanced divertor poloidal field coil sets (top and bottom)

•  Inner-wall LHCD •  Inner-wall ICRF

Key Elements:

•  Development platform for low PMI RF actuators:

•  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

Proven Alcator Technology:

Alcator DX

=> same and higher than Alcator C-Mod

See poster TP8.00033, C-Mod Session, Thursday morning

AlcatorDX


Alcator DX a facility to develop and test advanced divertor concepts at reactor conditions

“Tame the plasma-material interface with plasma physics”

~zero turbulent transport on high-field SOL1

[1] Smick, et al., NF 53 (2013) 023001.

•  Double-null geometry •  Advanced divertors -- low-field side SOL •  Quiescent, low heat flux -- high-field SOL

Idea: Configure internal PF coils to study a range of magnetic geometries and divetor targets.

AlcatorDX




‘ASDEX’

Grad-Shafranov equilibria obtained using ACCOME (Selene)1

[1] Tani et al., J. Comp. Phys. 98 (1992) 332.

AlcatorDX



Vertical Target




AlcatorDX



Long Leg




AlcatorDX



Super X




AlcatorDX



X-point Target




AlcatorDX


Alcator DX -- a innovation platform for low PMI, reactor compatible RF actuators

[2] VULCAN: Podpaly, et al., FED 87 (2012) 215.

Splitter and multi-junction fabrication techniques produce compact LHCD launchers that can fit on the inside wall.

High field side launch is highly favorable for LHCD, as noted in VULCAN study2.

� High B-field side => lower n// => penetrating rays => higher CD efficiency

� Quiescent SOL => Low PMI => Excellent impurity screening1

[1] McCracken, et al., PoP 4 (1997) 1681.

AlcatorDX


Alcator DX -- a innovation platform for low PMI, reactor compatible RF actuators

[2] VULCAN: Podpaly, et al., FED 87 (2012) 215.

Splitter and multi-junction fabrication techniques produce compact LHCD launchers that can fit on the inside wall.

High field side launch is highly favorable for LHCD, as noted in VULCAN study2.

� High B-field side => lower n// => penetrating rays => higher CD efficiency

� Quiescent SOL => Low PMI => Excellent impurity screening1

[1] McCracken, et al., PoP 4 (1997) 1681.

Challenge #4: Develop robust, reactor-compatible current drive & heating techniques (for SS burning plasma)


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.

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

q//

(4) Inner-wall launch (double-null) enables reactor-compatible current drive and heating techniques

AlcatorDX


•  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)

Mission:

Summary

A concept for a high power density, advanced divertor test facility, Alcator DX, is being considered to test these ideas*

*http://burningplasma.org/web/fesac-fsff2013/whitepapers/LaBombard_B.pdf

Alcator DX

See poster TP8.00033 (C-Mod session, Thursday morning): “Critical need for MFE: the Alcator DX advanced divertor test facility”

We welcome your input and participation in this brainstorming activity.

x-point target divertor conceptlabombard/aps2013...labombard- x-point target divertor – aps2013...

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