Concept for fast flowing liquid lithium walls and divertors Dick - - PowerPoint PPT Presentation

Supported by US DOE contract DE-AC02-09CH11466

Dick Majeski Princeton Plasma Physics Lab

with H. Ji, A. Khodak, T. Kozub, E. Merino, M. Zarnstorff

Concept for fast flowing liquid lithium walls and divertors

Liquid lithium PFCs offer a possible solution for reactor engineering issues

◆ Engineering features of liquid lithium:

– Renewable liquid surface – Neutron interactions only important for supporting substrate » Liquids not damaged by neutrons, fast particles – Convective heat removal (fast flow) permits use of low thermal conductivity substrates (steels) » Localized heat exchanger to remove plasma heat » Cycle coolant through hotter blanket to recover thermodynamic efficiency Ø Potential for control of in-vessel tritium inventory Ø PFC no longer needs BOTH neutron AND plasma tolerance Ø Admits low-pressure cooling

◆ Requires significant technology development

Liquid lithium PFCs offer alternative aproaches to physics issues

◆ Confinement and edge physics:

– Lithium PFCs shown to improve confinement – Solid and liquid lithium PFCs produce low core contamination – Lithium PFC compatible with a hot, low density edge Ø Smaller reactor scale size Ø Neutral beam fueling Ø Higher burnup fraction

◆ Development requires wider deployment of lithium PFCs in

confinement devices

τE,ITER-98P(y,2) (ms) 1 2 3 4

• Exp. τE (ms)

1 2 3 4 5 6 Cold shells 2 m2 liquid lithium

4 m2 liquid lithium PFC – 80% of LCFS

Passivated lithium

Lithium PFCs improve confinement

Good performance demonstrated with full liquid lithium wall

Energy Confinement Time (ms)

Pre-discharge lithium evaporation (mg)

• R. Maingi, et al., PRL 107 (2011) 145004

◆

Confinement improves in LTX

u Any lithium coating improves

performance relative to bare high-Z wall

u Improvements in coating quality

produce performance improvements

• Core Li concentration 1-3%

◆ Global parameters improve in NSTX

– H98y2 increases from ~0.9 à 1.3-1.4

Ø H98y2 up to 2 observed

– Core Li accumulation <1%

NSTX LTX

Flat electron temperature profile develops in LTX if edge gas load is removed

◆ Te profile initially

hollow, with strong fueling

Shot 1504291045 t = 464.0 ms

0.40 0.45 0.50 0.55 0.60 0.65 R [m] 100 200 300 Te [eV]

Shot 1504291543 t = 466.9 ms

0.40 0.45 0.50 0.55 0.60 0.65 R [m] 100 200 300 Te [eV]

◆ Peaked profile

develops

Shot 1504291634 t = 471.2 ms

0.40 0.45 0.50 0.55 0.60 0.65 R [m] 100 200 300 Te [eV]

◆ Te profile evolves

to flat or hollow, to LCFS

◆

All fueling (from centerstack) terminated at 462 msec ~3-4 msec required to clear gas from duct

◆

Lithium PFCs eliminate recycled neutrals

LCFS LCFS LCFS

◆

Edge electron temperature increases to 200 – 250 eV 464 msec 467 msec 471 msec

Hotter plasma edge is compatible with lithium PFCs

◆

Self-sputtering of Li on D-treated Li also drops with energy: – 24.5% at 700 eV – 15.8% at 1 keV

u Probability of direct reflection of incident H from lithium PFC also drops to

<10% for incident ion energy >500 eV

◆

Lithium sputtering peaks at ~ 200 eV impact energy – Li sputtering yield for D incident

• n deuterated Li, calculations

and IIAX measurements (Allain and Ruzic, Nucl. Fusion 42(2002)202). 45° incidence.

◆

At 700 eV the yield is 9% – Yield rises slightly for liquids to ~ 10%, just above the melting point – Yield is similar for H, D, T

◆

Liquid not structurally damaged by high energy ions

Liquid lithium wall concept

◆

Recirculate liquid lithium within the TF volume – Flow speed: 10 – 20 m/sec Ø 20 - 30 MW/m2 divertor heat load

◆

Integrates first wall with divertor

◆

Allows droplet or turbulent flow divertor – Further improve power handling

◆

J x B poloidal current to restrain free-surface liquid lithium PFC – Require 100 mA/cm2 to balance gravity in a 5T toroidal field

◆

Modest level of thermal isolation to maintain lithium surface below blanket temperature

◆

Fluid is returned to the torus top by inductive pumping (J×B force again)

◆

Power requirements for tokamak with 1-2 meter major radius appear modest – Require detailed analysis of duct flow

Low field side access for heating, diagnostics

Inductively driven flow in return ducts feeding HFS Small cross section for return ducts ➱Permit low field side NBI ➱RF launchers and other fueling High field side – axisymmetric, free surface flow Low field side – partial poloidal flow (axisymmetric, free surface) Lithium reservoir incorporates heat exchange system. Liquid salt?

Required in-vessel liquid lithium inventory 500 – 2,000 kg ➪ dominated by LFS system

Flowing lithium divertor concept would reduce required lithium inventory

◆

Nearer-term divertor test feasible in NSTX-U – Recirculating, electromagnetically driven and restrained flow – But: drag introduced by divertor fields

◆

Smaller scale » Lithium inventory ~20 kg for example of NSTX-U implementation – Startup, operation, shutdown may be feasible within timescale of NSTX-U toroidal field pulse

Free surface flow

Reservoir (cooled) Nonaxisymmetric return ducts Flow-forming nozzle

Two approaches to tritium removal under study

◆

Precipitation (M. Ono): – Solubility of hydrogenics in liquid lithium is 0.044 At. % at 200 °C » Order of magnitude increase at 300 °C – For a total PFC inventory of 0.5-2 metric tonnes, 0.5-2 kg of tritium corresponds to ~ 0.2% atomic – Approach: cool lithium PFC inventory to 190 – 200 °C » Lithium deuteride, tritide precipitates out as a solid » Remove by filtration

◆

Distillation: – Heat lithium stream (1-2 liters/minute) via electron beam » In this example, a 300 kW beam – similar to a modest e-beam welder – is required – Condense the lithium vapor, pump the liberated T,D – Multiple stages can be employed

Near-term plans

◆ Constructing ANSYS model for recirculating flow

– Estimate current, power requirements to drive return flow in ducts » Transition to axisymmetric in-duct flow – Thermal transfer in reservoir – Model both wall and divertor systems

◆ Engineering studies for toroidal test stand

– Toroidal field ~0.5T – Low aspect ratio coil set – Test free surface flow, recirculation in galinstan – Add normal (divertor) field components

◆ Combine test stand studies with renewed numerical modeling effort for

free-surface flows

Summary

◆ Confinement:

– Lithium PFCs offer improved confinement, low core impurity levels – Access to a hot edge for enhanced performance – ELM suppression in H-mode

◆ Engineering:

– Renewable surface » Not damaged by fast particles, neutrons – “Self-cooling” PFC possible » Plasma heat removed with the liquid metal » Allows localized heat exchange; use of liquid salts » Recover thermodynamic efficiency by routing coolant through hot blanket – Approaches to T,D removal appear feasible

◆ Testing in confinement devices promising ◆ Technological development lags far behind