Lithium and Liquid Metal Studies at PPPL LTX

Lithium and Liquid Metal Studies at PPPL LTX R. Maingi, M. Jaworski, R. Kaita, R. Majeski, J. Menard, M. Ono PPPL High Power Devices NSTX-U Surface Analysis EAST Test stands IAEA TM on Divertor Concepts Vienna, Austria 1 -Oct-2015

Goal: cohesive program for liquid metals to be considered as PFC candidates for fusion devices Outline • Motivation: high steady power exhaust and high confinement scenarios with liquid metal PFCs • Options for liquid metal uses, and R&D needs • Excerpt of results from existing PPPL collaborations • Summary IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 2

Development of liquid metal PFCs for fusion devices: transformative area, deserving of effort • Motivation 1: power exhaust challenge harder than thought - Heat flux footprint decreases with Ip; no increase with R Ø Both steady and transient loads can exceed solid PFC limits • Motivation 2: Confinement difficult with bare high-Z PFCs - Good confinement is challenging with high-Z walls in e. g. JET • Evidence that liquid metal PFCs can exhaust higher power than solids, and lithium PFCs (liquid or solid) provide access to high confinement IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 3

ROSATOM Federal State Unitary Enterprise “Red Star” Lithium loaded targets withstood high steady and transient heat loads in plasma gun experiments • Steady operation with heat loads from 1 -11 MW/m 2 withstood for 3 hours • Heat loads < 25 MW/m 2 withstood with Li targets (5 -10 minutes, limited by Li inventory) Lithium Capillary Porous System (CPS) targets • Transient loads < 50 MW/m 2 withstood with Li targets without cooling (up to 15 s) IAEA Technical Meeting on Assessment of Atomic and Molecular Data Priorities 4

Lithium (solid and liquid) PFCs increase confinement NSTX LTX H 98~1 • 2 -4 x improvement over ITER 98 P(y, 2) (H-mode scaling) J. C. Schmitt, Phys. Plasmas 22 (2015) 056112 • H 98 y 2 increased from 0. 8 -> 1. 4 (H -mode scaling) D. P. Boyle, J. Nucl. Mater. 438 (2013) S 979 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 5

Goal: cohesive program for liquid metals to be considered as PFC candidates for fusion devices Outline • Motivation: high steady power exhaust and high confinement scenarios with liquid metal PFCs • Options for liquid metal uses, and R&D needs • Excerpt of results from existing PPPL collaborations • Summary IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 6

Liquid metals can be used in several ways to address part or all of the particle and power exhaust challenge • Provide particle exhaust, access to high H-factor for FNSF Ø Slow flow; may have to use advanced divertors for power exhaust Ø Open issue on liquid metal substrate material • Provide power exhaust: high PFC temperature solutions Ø Vapor-box configuration (Goldston, this conf. ), compatible with W Ø Low recycling may be achievable in low flux areas; will that allow access to high confinement? • Provide particle and power exhaust, access to high confinement: low PFC temperature solutions Ø Fast flowing liquid metal option (Majeski, this conf. ) Ø Use ferritic steel as option to W as substrate, which reduces needed R&D to develop neutron resistant W with low DBTT IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 7

Liquid metal PFCs are alternatives to solid PFCs, but have substantial R&D needs to assess viability • Advantages - Erosion tolerable from PFC view: self-healing surface - No dust; main chamber material and tritium transported to divertor could be removed via flow outside of tokamak - Liquid metal is neutron tolerant; protects substrate from PMI ü Liquid (and solid) lithium offer access to low recycling, high confinement regimes under proper conditions ü Very high steady, and transient heat exhaust, in principle (50 MW/m 2 from electron beam exhausted; also 60 MJ/m 2 in 1 msec) • Disadvantages and R&D needs Ø Need stable surface & flows to maintain a fresh surface Ø Liquid metal chemistry needs to be controlled Ø Temperature windows need optimization * Most of experience in fusion is with Li, but Sn and maybe Ga offer some promise in terms of broader temperature windows IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 8

R&D need: PFC erosion and temperature limit studies initiated in divertor simulators (MAGNUM-PSI) • Issue: what are PFC erosion rates for lithium targets? Abrams JNM 463 (2015) 1169 – Temperature-enhanced erosion seen with Li at low flux not well understood – High flux data (Abrams) showed substantial reduction in Li erosion, and agreement with adatom evaporation model (Doerner) • Issue: what is lithium PFC operating temperature range with Li? – Li melts at 180 o C, high evaporation rates > 400 o C – Hot target (> 700 o C) showed lithium emission localized near surface – Pre-loaded Li target lasted longer than gross evaporation time Neutral Li emission Plasma Neutral D emission Li Trapping Target T>700 C 10 mm MAGNUM-PSI: M. Jaworski, ISLA 2013/2015 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 9

R&D need: LM stability and hydrogen retention/release evaluated in confinement and surface science devices Theme: Horizontal layer of dense fluid over less dense fluid is unstable (drips): Rayleigh-Taylor instability • Rayleigh-Taylor analysis of NSTX Liquid Lithium Divertor showed droplet stability • DIII-D Di. MES expt. 2004 only marginally stable Jaworski et al. , Nucl. Fusion 53 (2013) 083032 Deuterium release in lithium • Temperature programmed desorption (TPD) shows oxygen inhibits formation of Li. D and reduces thermal stability of D in Li films Capece, J. Nucl. Mater. 463 (2015) IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 10

Goal: cohesive program for liquid metals to be considered as PFC candidates for fusion devices Outline • Motivation: high steady power exhaust and high confinement scenarios with liquid metal PFCs • Options for liquid metal uses, and R&D needs • Excerpt of results from existing PPPL collaborations • Summary IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 11

PPPL Lithium Program Elements: present, future • Liquid Li walls as a primary PFC (LTX) • Liquid Li as divertor PFC in long pulse tokamak (EAST) • Fundamental Li surface science studies (PU lab@PPPL) • Li deposition on divertor PFCs in high b ST (NSTX-U) • Li aerosol injection to probe pedestal physics and stability in high b advanced tokamaks (DIII-D) • Li granule injection for ELM control and possible conditioning (NSTX-U, DIII-D, EAST) • Plasma slow-flow test stand & fast-flow test facility (PPPL) • Li lifetime & trapping in divertor-like plasma (MAGNUM-PSI) • Liquid Li limiter (EAST) and divertor modules (NSTX-U) IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 12

NSTX: Liquid lithium divertor operation was compatible with reliable H-mode operation • Liquid lithium divertor on NSTX: Cu heat sink, SS permeation barrier, Mo mesh, with overhead Li ovens filling the LLD • Main result: good H-factor obtained with LLD Jaworski et al. , Nucl. Fusion 53 (2013) 083032 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 13

NSTX-U goal: Establish high t. E and b + 100% non-inductive, then assess compatibility with metallic PFCs Base: 2014 -18 5 year plan steps for implementation of cryo-pump + 2015 -16 high-Z PFCs + LLD 2017 -18 2019 -20 2022 2023 -24 High-Z tile row C BN Li High-Z Downward Li evaporator + Li granule injector Lower OBD high-Z row of tiles High-Z tile row Up + downward Li evaporator Cryo + full lower outboar d high-Z divertor Cryo + high-Z FW and OBD + liquid Li divertor (LLD) Bakeable high- Cryo-pump Z PFCs for liquid Li IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) All high-Z FW + divertor s + flowing LLD module or Flowing Li module 1 Oct 2015 14

EAST: Long-pulse ELM-free H-modes with constant radiated power enabled with lithium dropper (on loan from PPPL) Li dropper J. S. Hu et al, Phys. Rev. Lett. 114 (2015) 055001 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 15

EAST: Liquid lithium limiter developed at PPPL and inserted via midplane port • Designed and fabricated at PPPL • Used a DC EM pump, with BT of EAST, for steadystate recirculation • Implemented in EAST in Oct. 2014 J. Ren et al. , Rev. Sci. Instrum. 86 (2015) 023504 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 16

EAST: Liquid lithium limiter compatible with EAST H-mode discharges • Lithium light increased when current driven in limiter system • Performance improved in ohmic discharges • Da and impurities decreased in both divertors J. S. Hu et al. , Nucl. Fusion in preparation IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 17

Closing thought - development of LM PFCs is a transformative area • Motivations: power exhaust very challenging, and access to good confinement difficult with bare high-Z PFCs • Evidence that liquid metal PFCs can exhaust higher power than solids, and provide access to high confinement ü This is an area ripe for advancement IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 18

Thank you for your attention! IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 19

H-mode discharges maintained in EAST when liquid lithium limiter inserted past separatrix • Plasma-limiter interaction resulted in filamentary emission during ramp-up • Limiter emission uniform later in pulses • After exposure, limiter showed damage from plasma-wall interactions J. S. Hu et al. , Nucl. Fusion in preparation IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 20

W chosen for divertor PFCs & Be for wall PFCs of ITER • W advantages - Low physical sputtering yield & high energy threshold No chemical sputtering with hydrogen Low in-vessel tritium retention in most scenarios Reparable by plasma spray; good joining technology W PFCs can exhaust 5 -10 MW/m 2 steady heat flux • W disadvantages - Low allowable core concentration Melts under large transient loads High ductile-brittle transition temperature Recrystallizes & becomes brittles at temperatures >1500 K High activation Blisters and generates ‘fuzz’ under He bombardment Confinement reduced in tokamaks as compared with low-Z PFCs G. Federici, et. al. , Nucl. Fusion 41 (2001) 1967 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 21

Several thrusts to address knowledge gaps in LM program Ø LM PFC technology and science in flowing, self-cooled and externally cooled test systems - Flow rates from 1 mm/sec – 10 m/sec - Use capillary or j x B forces to overcome MHD forces that could cause mass ejection - Determine operating temperature windows - Assess hydrogenic species control and He entrainment Ø Fundamental LM surface science studies - Keep LM surface clean for reliable flow; understand PMI - Predict flow of LM, including wetting and de-wetting Ø Compatibility with attractive core/edge plasma - Plasma power and momentum exhaust; particle control - Applicability of low recycling regimes with excellent confinement: target H 98 > 2, enabled by LM resilience to transients and high peak heat flux exhaust IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 22

PPPL Lithium Program Elements • Liquid lithium walls as a primary PFC: effects on electron transport, fueling requirements (LTX) • Lithium as a divertor PFC in high b ST: power/particle exhaust and confinement improvement (NSTX-U) • Lithium lifetime and trapping in divertor-like plasma stream (MAGNUM-PSI) • Liquid lithium as primary PFC in long pulse tokamak (EAST) • Liquid lithium flowing loop R&D (PPPL test stand) • Fundamental lithium surface science studies (PU lab@PPPL) • Lithium injection in high b advanced tokamaks: confinement and stability changes (DIII-D, AUG) • Lithium granule injection for ELM control and possible conditioning: (DIII-D, EAST NSTX-U) IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 23

Pedestal performance and core confinement in JET scenarios was reduced with installation of ITER-like wall Beurskens PPCF 2013 • Partial performance recovery with N 2 seeding, which cannot be used in D -T campaign; less favorable results with Ne • Projected performance less than in 1990’s D-T experiment • Degradation and partial recovery seen in AUG, C-Mod (metallic PFCs) IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 24

In NBI heated H-modes, Li injection via dropper reduced ELM frequency but did not eliminate ELMs • Milestone: Compare access to ELM-free operation with active Lithium aerosol injection between lower hybrid heated and neutral beam heated H-modes • NBI H-mode • Stored energy decays slowly • Li dropper Li emission saturates ELM frequency drops ~25 -30% • • Zeff unaffected Courtesy of J. S. Hu & Z. Sun IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 25

Additional issues for NSTX-U LM deployment addressed by collaborations on international & domestic devices • Active discharge modification with Lithium injection - Li “droppers” for 18 sec H-modes in EAST (Maingi IAEA ’ 14, Hu, PRL ‘ 15) - Li dropper, Li granule injector in DIIID (Jackson, IAEA ‘ 14, Bortolon EPS ‘ 15) - Li pellet injector (and dropper) studies in AUG with W-PFCs (2015) • Flowing LM in EAST H-modes - Very slow flowing, midplane liquid lithium limiter tested in 10/2014: compatible with H-mode! - Next module being prepared for test Figure 1: Schematic of heated copper in FY 2016 plate and small liquid lithium - Research will inform design of flowing reservoir, mounted on an insertable probe for testing in EAST in 2014 LM modules for NSTX-U IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 26

Scenarios in tokamak discharges with High-Z PFCs can affect pedestal performance and core confinement C-Mod Beurskens PPCF 2013 Lipshultz, Po. P 2006 C-Mod Beurskens PPCF 2013 Kallenbach NF 2011 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 27

Impact of low lqmid studied for ITER IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 28

Cross-field transport can be reduced to get lower lqmid, but higher divertor neutral pressure Pn reduces qpeak IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 29

Update on gaps since Re. Ne. W: Steady heat flux exhaust is more challenging than projected at Re. Ne. W • Heat flux profile width lq measured in divertor ITER - lq projected to outer midplane with flux expansion • International effort found that lq varies inversely with Bpol, MP - No increase with R, PSOL - Low gas puff attached plasmas; some broadening and heat flux dissipation with detachment • Projected width in ITER ~ 1/5 previous value; operating window narrows Kukushkin, JNM 2013 • Much more challenging for reactors, due to higher Pfusion IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) Eich, NF 2013 1 Oct 2015 30

Operating window in ITER reduced with lower lqmid 20 Q P > PLH 5 Q > 5 Pa m < 0. 8 • Adjust cross-field transport to get desired lqmid (SOLPS) • Vary the divertor gas puffing to reduce qpeak < 10 MW/m 2 • Assess ratio of divertor neutral pressure Pn to critical value for detachment Pncrit; ratio m = Pn/Pncrit • Use boundary simulation as input to core simulation for Q, Pa, PLH, for operating window - m < 0. 8 - Q>5 - P > PLH IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 31

Operating window gets reduced with lower lqmid ITPA Projection ~ 0. 9 mm 20 15 Q Q 7 Q P > PLH 5 Q > 5 Pa 5 5 m < 0. 8 Pa Pa • Technology improvements (e. g. liquid metals) or complete elimination of ELM transients could increase qpeakmax by 50%, and partly restore operational window IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 32

Development of LM science and technology via dedicated test stands • Dedicated linear device with integrated liquid lithium loop can address physics and technology goals – Arc-source proposed to provide divertorrelevant heat fluxes – Material transport, recapture requires integrated lithium loop – Extensive water cooling to be avoided with lithium PFCs • Dedicated toroidal devices can demonstration basic stability – Similarity experiments with Ga. In. Sn could be restarted quickly – Dedicated lithium facilities will address low-density fluid and hydrogen cycle aspects directly UCLA MTOR Ga. In. Sn Experiment IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015

Lithium injection improves confinement on many machines, generally (but not always) via recycling reduction • NSTX: H-factor increased by 50% with lithium coatings - Reduction in recycling reduced the ne profile gradient and shifted it inward; very little Li penetrated inside the separatrix - Te gradient clamped by stronger driver for ETG - Pressure profile followed ne profile -> stabilizing to ideal MHD peeling/ballooning modes, so discharge went ELM-free • DIII-D: H-factor increased transiently by 60% with lithium injection of microscopic dust particles - No change in recycling, but change in ne, Te, pressure profiles nearly identical to NSTX when a newly observed instability occurs - Substantial Li penetration and ion dilution in edge - Profile inward shifts and ion dilution both stabilizing to ideal MHD • New ITPA multi-machine experiment to understand the effect of low-Z impurities on pedestal IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 34

Profile changes in DIII-D in ELM-free H-mode qualitatively similar to NSTX ELM-free H-mode with inter-shot Li evaporation • Shifting gradients away from separatrix improved edge stability in both DIII-D and NSTX DIII-D NSTX No Li With Li NSTX: Maingi, NF 2012 DIII-D: Maingi, PRL 2014 submitted Osborne, NF 2015 submitted R. Maingi – Effect of Li on DIII-D 35

Flow chart of how lithium results in ELM elimination similar for highly and weakly shaped discharges N from 0. 95 -1 (recycling region) Li coatings reduce recycling and core fueling (SOLPS) ne and ne reduced Te fixed ne, Pe gradient reduced e increased ETG (GS 2) Pi gradient unchanged Jbs, J|| reduced - stabilizing (ELITE) N from 0. 8 -0. 94 Deff reduced, most in ELM -free (SOLPS) NSTX-U ne and ne Te and Te increased e reduced strongly – m. T stable (GS 2) Edge Pe follows ne and Te; peak P’ shifts farther from separatrix IAEA FEC 2012 – Maingi EX/11 -2 Jbs, J|| increased but far from separatrix; improves stability to kink/peeling modes (ELITE) Oct. 8 -13, 2012 36

Lithium injection induces a bifurcation to higher pedestal pressure and width in DIII-D ne [m-3] PNBI [MW] Lithium injection Da [au] • H 98 y 2 <1. 8 here, 2. 0 in other discharges; ‘flat’ Pradtot HH 98 y 2 Pradtot [MW] • Teped nearly doubled during bifurcations Teped [ke. V] • Peped nearly tripled during bifurcations Peped [k. Pa] Pewidth [cm] • ELM-free bifurcated state can be seen in Da emission #159643 • Pewidth increased by 100% on very short time scale R. Maingi, PRL submitted; G. Jackson, IAEA FEC 2014 PD R. Maingi – Effect of Li on DIII-D 37

Steps and roles to ELM-free H-mode with Li Injection and Stimulated Bursty Chirping Mode BCM without Lithium (in <10% of natural ELM cycles) BCM turns on, and pedestal widths increase rapidly Li injection stimulates BCM more frequently Li too low ELM cycle unchanged; next ELM kills BCM Profiles flatten from 0. 98

Li injection in DIII-D, compared with NSTX & EAST DIII-D NSTX EAST Delivery method Dropper Inter-shot evaporation, (Dropper) Dropper, (Morning evaporation) Pedestal Width Increased ? Pedestal Height Increased ? H-factor Increased Unchanged Edge fluctuations Increased Decreased Increased Radiated power Steady during EF Ramp during EF Steady during EF Effect on ELMs Delayed Eliminated Recycling Unchanged Reduced R. Maingi – Effect of Li on DIII-D 39

Reactors heat exhaust more challenging when considering exhaust power normalized by device size (R, R 2, or R 3) • Required core radiation fraction 70 -90% for qpeak < 10 MW/m 2 Kotschenreuther, Po. P 2007 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 40

Reactors heat exhaust more challenging when considering exhaust power normalized by device size (R, R 2, or R 3) Kotschenreuther, Po. P 2007 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 41

Update on gaps since Re. Ne. W: ELM mitigation requirements are more challenging than projected at Re. Ne. W • For ITER: need 45 x ELM heat flux mitigation; previously ~ 20 x ITER – Set energy flux limit: DWELM < 0. 7 MJ (0. 2% Wp) – Project ELM frequency for ITER vs. Ip • Inter-ELM heat flux width lq ~ 1/Ip – Assess ELM damage limit for Be and needed freq. to keep core clean of W – Assess minimum ELM multiplier needed • Using DWELM f. ELM = a PSOL Loarte, NF 2014 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 42

ELM frequency increased in both JET and DIII-D; peak heat flux qpeak unchanged in JET but reduced in DIII-D JET q. Peak. IR (MW/m 2) 7 6 Natural DIII-D Outer Inner 5 4 f. ELM * qpeak= Const 3 2 Pellet 1 0 0 10 20 30 40 50 60 70 Pellet Frequency (Hz) Baylor IAEA 2012 Lang, NF 2013 • Is the difference related to metallic vs. carbon wall? IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 43

Several ‘new’ ideas developed since ALPS/APEX studies • Slow flowing (1 cm/sec) thin film (0. 1 mm) liquid lithium across SS plate, driven by j x B - Tested successfully in HT-7 in 2012 - First test in EAST as outboard limiter in 2014 • Continuous flow driven by thermoelectric effect: LIMITS (Liquid Metal Infused Trenche. S) - Tested successfully in HT-7 in 2012 - First test in EAST in 2014 • Surface tension balancing MHD forces: capillary porous systems - FTU, several Russian tokamaks, NSTX - Low recycling, low surface temperature scenario, and high recycling vapor-shielded scenario IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 44

‘Vapor-Box’ concept: exhaust chamber using evaporative LM cooling and radiation/convection heat exhaust Goldston, FESAC SP talk, July 2014 IAEA TCM on Innovative Divertors: PPPL LM Progam (Maingi) 1 Oct 2015 45