Modeling of plasma/lithium-surface interactions in NSTX: status and key issues J.N. Brooks, A. Hassanein, T. Sizyuk, J.P. Allain Purdue University 2 nd.

Modeling of plasma/lithium-surface interactions in NSTX: status and key

issues

J.N. Brooks, A. Hassanein, T. Sizyuk, J.P. Allain

Purdue University

2nd International Symposium on Lithium Applications for Fusion Devices, PPPL, April 27-29, 2011

J.N Brooks, PPPL April 2011

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Why analyze NSTX plasma/lithium interactions?

Lithium has been used extensively in NSTX*--will likely continue. Modeling can help explain device performance.

Flowing liquid lithium is a promising candidate for future devices (Fusion

Nuclear Science Facility, DEMO, etc.). Code validation & model improvement is possible, using NSTX data.

Concerns for NSTX analysis: Much more difficult to model than future devices

--- Transient conditions: ~1 second pulse

--- Small device–edge/boundary effects dominate

--- Lithium is not a flowing liquid (i.e., is static liquid or solid)

--- Other materials present (C, Mo, etc.)

--- Non-standard boundary conditions

*H.W. Kugel, “NSTX Plasma Response to Lithium Coated Divertor”, PSI-19 (2010), J. Nuc. Mat. to be published.

*C.H. Skinner et al., “Deuterium Retention in NSTX with lithium Conditioning”, ibid,. to be published.


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Lithium modeling issues

Lithium is the 2nd most complex surface material we have modeled (carbon is first due to chemical sputtering):

D trapping/pumping—highly dependent on surface content/structure

High vapor pressure

Temperature dependent sputtering & evaporation

Material-mixing issues: e.g. Li intercalation in carbon

Liquid vs. solid issues

Most (~2/3) sputtering is Li+ ions

Li+ ion redeposition in sheath and re-emission at surface


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Past & Current Work-Lithium/NSTX ModelingPast & Current Work-Lithium/NSTX Modeling

Liquid Lithium Divertor (LLD) plasma/surface interaction analysis [J.N. Brooks, J.P. Allain, T.D. Rognlien, R. Maingi., J. Nuc. Mat. 337-339(2005)1053]

[J.P. Allain, J.N. Brooks, Nuclear Fusion 51(2011)023002]

• static liquid lithium response, low-D recycle plasma

Lithium Inner Divertor (HIBD) • static (pure) liquid Li or solid Li surface, high D-recycle plasma

Mixed material analysis: Li + C on Mo inner divertor, high-D-recycle plasma

surface evolution: composition and sputtering

Li, C detailed analysis; e.g., sputtering of thin Li coatings on graphite


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REDEP/WBC LLD AnalysisREDEP/WBC LLD Analysis

REDEP/WBC code package simulation of NSTX Liquid Lithium Divertor (LLD). Full kinetic, [3-D, 3-V], full-process, sub-gyro-orbit analysis.

Plasma parameters from UEDGE/DEGAS solution, “0.65 D+ reflection coefficient” [D. Stotler, R. Maingi, et al.

(2008)]. Peak Te~250 eV; Ne~5x1017m-3. Includes LLD surface temperature profile, at t = 2 seconds [L. Zakharov]. (Tmax = 281° C).

Energy-dependent and surface-temperature-dependent sputter yields and sputtered Li atom velocity

distributions; for D, Li, C incidence, from TRIM-SP code runs for D containing Li.

Other models: Debye-only sheath, ionization rate coefficients, Li+ emission/redeposition & reflection, etc.

101 102 103 10410-2

10-1

100

Li S

putt

erin

g Y

ield

(par

ticle

s /

inci

dent

ion)

Incident particle energy (eV)

T = 380 C T = 270 C T = 200 C T = 420 C

angle of incidence = 45-degreesLi yield = ions + neutrals

D+ on liquid Li

TRIM-SP computed total LiTRIM-SP computed total Li sputter yields (ion+atom)sputter yields (ion+atom)


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Interesting physics for sputtered lithium transport in the NSTX low-recycle plasma regime:

Large sputtered atom ionization mean free path, order of 10 cm

Large Li+1 gyroradius ( ~5 mm), due to low B field (0.5T)

Low collisionality of Li ions with plasma; due to high Te, low Ne

Kinetic, sub-gyro orbit analysis required (i.e. WBC code)


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WBC Simulation of LLD sputtered lithium transport: 50 trajectories shown;

• Long mean free paths seen for ionization; subsequent long, complex, ion transport

UEDGE/NSTX GRID

2-D

3-D


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NSTX Liquid Lithium Divertor Analysis-resultsNSTX Liquid Lithium Divertor Analysis-results

Results are encouraging:--Moderate lithium sputtering; no runaway--Acceptable Li contamination: ~7% SOL, ~1% Core--Carbon (2%) flux to LLD appears acceptable--LLD could apparently handle higher heat flux


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• NSTX is replacing Row 1 Horizontal Inboard Divertor (HIBD) carbon tiles with molybdenum —To reduce carbon sputtering & core plasma carbon content.

•We are analyzing Mo, C, Li HIBD (“inner divertor”) sputtering erosion and plasma contamination, with high-recycle plasma.

REDEP/WBC NSTX Inner Divertor Analysis; with high-recycle plasmaREDEP/WBC NSTX Inner Divertor Analysis; with high-recycle plasma [with H. Kugel, R. Maingi, C. Skinner, et al.][with H. Kugel, R. Maingi, C. Skinner, et al.]


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NSTX Inner Divertor NSTX Inner Divertor High-RecyclingHigh-Recycling Plasma Solution Plasma Solution (J. Canik SOLPS code)(J. Canik SOLPS code)

R~1

Peak plasma values at divertorNe ~ 1x1020 m-3

Te ~ 60 eVstrike point


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Plasma Sheath Sheath affects transport of sputtered and evaporated Li, and other materials NSTX has non-standard tokamak boundary conditions—can affect sheath NSTX sheath analyzed with BPHI-3D code (w/o full turbulence model)

Parameter Outer divertor/

low-recycle plasma

Inner Divertor/

high-recycle plasma

Magnetic field 0.5 T 1.0 T

Field angle of incidence, from surface

5 3

Te

Ne

~250 eV

5x1017 m-3

~50 eV

1x1020 m-3

Normalized sheath potential

e/kTe

~3 ~3

Sheath structure Debye-only Magnetic + Debye


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Parameter Low-recyclePlasmaa

High-recyclePlasma

Location/surface-condition Outer divertor/ “Liquid Lithium Divertor” (LLD)

with high D pumping

Inboard divertor/nominally C and/or Mo, but with

assumed low-D-pumping Li coverageb

Plasma solution/D+ reflection coefficient

UEDGE R=0.65 [Stotler et al.]

SOLPS R~1 [Canik]

Peak electron temp. at divertor, eV 247 57

Peak electron density at divertor, m-3 5 x1017 3 x1020

Sheath structure Debye-only(~1 mm)

Magnetic + Debye(~2 mm)

Ionization mean free pathc, mm 64 0.77

Sputtered Li currente (atoms)/D+ ion current to divertor, s-1

1.43 x1020/1.98 x1021 6.12 x1021/4.31 x1022

Fraction redeposited on divertor .55 .99

Core plasma lithium contaminationpotential

~1% < 0.1%

WBC NSTX Lithium Divertor analysis: transport summary for two plasma cases (100,000

histories/simulation)

a Values from [J.P. Allain, J.N. Brooks, Nuclear Fusion 51(2011)023002]b 300 C surface assumed for D and Li on Li sputter yieldsc normal-to-surface; for sputtered Li atoms ionized in divertor region.d average for redeposited Li ions on respective divertor e includes sputtered atoms, and sheath-reflected sputtered ions re-emitted as atoms from surface.

• Major differences, but acceptable lithium erosion/redeposition in both cases


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WBC analysis: comparison of three surface materialsNSTX inner divertor, high-recycle regime

a with 1% C+3 and 1% Li+2 plasma impingementb normal-to-surface; for sputtered atoms ionized in divertor region.c numerical bound

• C and Li net sputter erosion is about 5-10 times higher than Mo erosion• No material highly contaminates core plasma

Parameter

Carbon Molybdenum Lithium

Ionization mean free patha, mm

5.3 0.72 0.77

Gross erosion rate, typical, nm/s

20 15 200

Net erosion rate, typical, nm/s

2 0.5 5

Core plasma contamination potentialb < 2 x10-3 < 5 x10-5 < 1 x10-3


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ITMC-DYN Integrated ModelsITMC-DYN Integrated Models

Dynamic evolution of mixed materials bombarded with multiple ion beams:Dynamic evolution of mixed materials bombarded with multiple ion beams: ITMC-DYN Computer Simulation Package

A. Hassanein, “Surface effects on sputtered atoms and their angular and energy dependence”, Fusion Technology 8 (1985) 1735.T. Sizyuk and A. Hassanein, "Dynamic analysis and evolution of mixed materials bombarded with multiple ions beams", J. Nucl. Materials, 40( 2010)60 T. Sizyuk and A. Hassanein “Dynamic analysis of mixed ion beams/materials effects on the performance of ITER-like devices“, to be published J. Nucl. Mat. (2010)


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ITMC-DYN analysis: time dependent sputtering of NSTX Mo inner ITMC-DYN analysis: time dependent sputtering of NSTX Mo inner divertor divertor (at strike point)(at strike point); with D, 1% C, 1% Li impingement; with D, 1% C, 1% Li impingement

• Substantial carbon and lithium sputtering occurs by end-of-shot


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ITMC-DYM Analysis: Spatial distribution of the deposited C and Li impurities in Mo substrate; NSTX Mo inner divertor (at strike point)

• C and Li surface contamination extend to ~10 nm• C and Li concentrations peak at ~ 5 nm depth and about equal the Mo concentration


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Conclusions-NSTX Lithium plasma/surface interactionsConclusions-NSTX Lithium plasma/surface interactions

Analysis of lithium erosion/transport in NSTX is important, but highly complex. (Results uncertain due to complexity of lithium/NSTX modeling, and general issues in plasma

predictive modeling.) Key focus is on mixed-material modeling.

The static liquid lithium divertor (LLD) with high-power, D-trapping plasma shots, is predicted to work well—from the (sputtering & evaporation) erosion standpoint.

A lithium surface—solid or liquid, for low or high D recycle plasma—has high erosion but low core plasma contamination potential (~0.1-1%).

A Mo surface may be substantially changed, in 1 second, by C and Li impingement. Mo core plasma contamination by sputtering appears low (<0.01%), in any event. (Not clear if Mo substantially reduces NSTX core plasma carbon content).

Continuing work: Other plasma solutions (e.g., inner Mo with outer low-recycle LLD), self-consistent material-mixing/evolution, data-calibrated model refinements.

{Supercomputing needed for more complete plasma/material interaction analysis.}

Modeling of plasma/lithium-surface interactions in NSTX: status and key issues J.N. Brooks, A. Hassanein, T. Sizyuk, J.P. Allain Purdue University 2 nd.

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