Lithium Research in NSTX

Josh Kallman(with thanks to R. Kaita)

Student Seminar

PPPL

July 29, 2010

NSTXNSTX

College W&MColorado Sch MinesColumbia UCompXGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

Supported by

Advantageous properties of lithium demonstrated in PISCES-B steady-state plasma experiments at UCSD

Ion flux (m-2 s-1) 1023

Ion energy (eV) 20-300

Heat flux (MW/m2) 1-10

Te (eV) 2-40

ne (m-3) 1017-1019

Pulse length steady-state

Target materials C, W, Be, Li, etc.

and coatings

Plasma species H, D, He

• Ion flux determined from double Langmuir probe measurements of plasma parameters

– Cross-checked against total current to sample

• Deuterium retention measured after removing sample from PISCES-B and baking in vacuum furnace equipped with residual gas analyzer (i. e., Thermal Desorption Spectroscopy)

• Post-exposure outgassing of liquid lithium samples shows 100% retention of all incident deuterium plasma ions

– Retention is independent of sample temperature during exposure

• High recycling resumes once sample is fully converted to LiD

Liquid lithium samples exhibit low deuteriumrecycling until fully converted to LiD

10

221

021

10

20

Re

ten

tion

(ato

ms cm

-2)

1020 1021 1022

Ion Fluence (atoms cm-2)

2

National Spherical Torus Experiment atPrinceton Plasma Physics Laboratory

Goal is to reduce recycling in divertor region

• Poloidal field coil• Outboard divertor

• Inboard divertor

• Plasma

10 cm

• Capacity: 90 g Li

• Oven Temp: 600-680°C

• Rate: 1mg/min - 80mg/min

NSTX LIThium EvaporatoR (LITER) consists of heated reservoir inside stainless steel oven

• LITER central aiming axis to graphite divertor and gaussian angle at 1/e (dashed)

• Toroidal locations of LITER and Quartz Deposition Monitors (QDM)

Two LITERs oriented for coatingNSTX divertor region with lithium

Multipulse Thomson Scattering is primary electron temperature and density diagnostic

• Doppler broadening gives temperature

• Intensity of scattered light provides density

• Backscattering geometry permits high sensitivity and spatial resolution at outer edge, and nearly full radial profile

• 1 cm edge resolution, 3 - 5 cm at center, 8 - 10 cm on inner edge

• 2 Nd:YAG 30 Hz lasers

Lithium edge conditions flatten and broaden electron temperature profiles

Without lithium coatings

With lithium coatings

Confinement improves with lithium edge conditions

Stored energy in electrons

Total stored energy

• Lithium in porous molybdenum surface to be kept liquid by heated copper substrate

• Objective is to determine if liquid lithium can sustain deuterium pumping beyond capability of solid lithium coatings

Liquid Lithium Divertor (LLD) installed on lower divertor

Next step is to replace section of divertor region with fully-toroidal liquid lithium surface

Physics requirements for a Langmuir probe array to measure divertor profiles

• Heat flux profile at outer strike point has FWHM of 10 cm– current IR camera

resolution is 16 data points over this region

– higher spatial resolution could allow more accurate particle flux measurements

• ELMs occur on a time scale of several ms– temporal resolution should

be sufficient to operate during transient events (single tip probes would be limited by voltage sweep rate)

– triple probes would provide instantaneous data

0.455 0.460 0.465 0.470 0.475

Time (seconds)

ELMs in lower divertor, 129019

0.2 0.4 0.6 0.8 1.0 1.2 1.4

Radius (m)

2

4

6

Q [

MW

/m^2

]

Heat flux, 129019

2

4

6

t = .33st = .36st = .39s

Arb

itrar

y un

its

Strike pointlocation

0.45

Highly lithiated divertor conditions present materials challenge

• Elemental lithium is conductive, providing a possible path for shorting electrodes to each other or ground

• Lithium also reacts with carbon in the presence of oxygen (residually present in NSTX vacua) to form lithium carbonate, an insulator– this effect is beneficial in avoiding grounding and direct

conduction, but can provide barrier for incident electrons and ions

– previously installed Langmuir probes show no appreciable loss of signal with heavy lithium loading, but NSTX could deposit amounts an order of magnitude greater than previous years to fill LLD

• Strike point ablation can remove evaporated lithium, but large integral effect of continuous loading depositions is unknown– alternate cleaning methods, such as high current pulses to

electrodes, are under investigation

Probe array is located near LLD to provide local measurements

• Probe array begins just outboard of CHI gap and extends over roughly 1/3 of LLD radially

• Provides local measurements for plasma incident on both carbon and lithium PFCs

• As seen in next slide, edges of tile had to be sloped to accommodate height of as-built LLD without changing probe dimensions

LLD

Extent of probearray

Probe array in bay B gap

Downward view

16

33 radially arrayed triple-probes provide edge temperature and density characterization on a continuous basis− can also be operated as swept or

SOL current probesProbes based on MAST design

involving a macor cassette of closely spaced probes embedded in a carbon tile

– tile mount with radial coverage of divertor (Bay B)

– electronics provided by UIUC (see adjacent poster by M. Jaworski)

Close spacing of probes provides better resolution in high-gradient (strike point) regions

– each probe covers 3 mm radially, including spacing– probe heads are 2mm radial x 7mm toroidal

rectangles

Triple Langmuir probe array addresses edge diagnostic needs

10 cm

2.5 cm

Leadingedge

Liquid Metals Provide Possible Solution for “First Wall” Problem in Fusion Reactors

• Liquid metals can simultaneously provide: – Elimination of erosion concerns

• Wall is continuously renewed– Absence of neutron damage– Substantial reduction in activated waste– Compatibility with high heat loads

• Potential for handling power densities exceeding 25 MW/m2

Example: NAGDIS-II: pure He plasmaN. Ohno et al., in IAEA-TM, Vienna, 2006• Bombardment with 3.5x1027 He+/m2 at Eion = 11 eV for t = 36,000 s

• Structures appear on scale of tens of nm and reflect swelling due to “nanobubbles”

100 nm (VPS W on C) (TEM)

• Tungsten is only candidate for fusion reactors

– Tests involving long-term exposure to plasma reveals surface damage

Problematic nature of present solid materials motivates liquid wall research on NSTX

Future research beyond NSTX to focuses on requirements for “burning” D-T plasmas

• Power Balance: PH+P=PL

• PL proportional to nT/E and P proportional to n2

• PH =0 or “ignition” means n2 > nT/E or nE >T

• nE >1.5x1020m-3s for T=30keV

• Power amplification factor Q=5P/PH

– Q -> infinity as PH ->0

– New ITER device designed to achieve Q>10

Person

Plasma Major Radius 6.2 m Plasma Minor Radius 2.0 m Plasma Volume 840 m3

Plasma Current 15.0 MA Toroidal Field on Axis 5.3 T Fusion Power 500 MW Burn Flat Top >400 s Power Amplification >10

ITER to be next major step for addressing physics and engineering issues for MFE reactors

*Work supported in part by US DOE Contracts DE-AC02-09CH11466,DE-AC04-94AL85000, DE-AC52-07NA27344, and DE-AC05-00OR22725

H. Kugel 1), J-W. Ahn 2), J. P. Allain 7), M. Baldwin 2), M. G. Bell 1), R. Bell 1), J. Boedo 2), C. Bush 3), R. Doerner 2), R. Ellis 1), D. Gates 1), S. Gerhardt 1) T. Gray 1), J. Kallman 1), S. Kaye 1), B. LeBlanc 1), R. Maingi 3), R, Majeski 1), D. Mansfield 1), J. Menard 1),D. Mueller 1), C. Neumeyer 1), M. Ono 1), S. Paul 1), R. Raman 4), A. L. Roquemore 1),P. W. Ross 1), S. Sabbagh 5), H. Schneider 1), C. H. Skinner 1), V. Soukhanovskii 6),T. Stevenson 1), D. Stotler 1), J. Timberlake 1), W. R. Wampler 8), J. Wang 9), J. Wilgen 3), and L. Zakharov 1)

1) Princeton Plasma Physics Laboratory, Princeton, NJ 08543 USA2) University of California at San Diego, La Jolla, CA 92093 USA3) Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA4) University of Washington, Seattle, WA 98195 USA5) Columbia University, New York, NY 10027 USA6) Lawrence Livermore National Laboratory, Livermore, CA 94551 USA7) Purdue University, School of Nuclear Engineering, West Lafayette, IN 47907 USA8) Sandia National Laboratories, Albuquerque, NM 87185 USA 9) Los Alamos National Laboratory, Los Alamos, NM 97545 USA

Contributors and Acknowledgements*