Edge and Scrape-off Layer Diagnostics for the NSTX Liquid Lithium Divertor

1. NSTX Supported by Edge and Scrape-off Layer Diagnostics for the NSTX Liquid Lithium Divertor College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec J. Kallman, R. Kaita, H. Kugel, R. Ellis, S. Gerhardt, PPPL, M.A. Jaworski, UIUC, L. Roquemore, S. Zweben, PPPL APS DPP Meeting November 19, 2008 *Work supported by USDOE Contract DE-A C02-76-CH03073

2. Plasmas in machines with conventional walls face numerous issues • Secular density rise causes discharges to evolve towards the Greenwald density limit, leading to MHD instability • ELMs can cause loss of plasma stored energy and cause large transient heat fluxes to plasma-facing components (PFCs) • PFC-introduced high-z impurities can penetrate far into the plasma, leading to large radiative losses • Edge recycling provides large cold particle influx, leading to temperature gradients and gradient-driven instabilities

3. Two LIThium EvaportoRs (LITERs) used in 2008 to provide lithium wall coatings in NSTX • Design of LIThium EvaporatoR (LITER) Shown in Operating Orientation • Photo of LITER on probe & loaded with Li under argon H. Kugel,PPPL

4. Lithium coatings deposited on PFCs with LITER in NSTX have had success in improving plasma conditions Lithium acts as an absorbing wall, helping to control edge density Broader electron temperature profile leads to more stored energy in electrons Edge conditioning with lithium leads to reduction in ELMs Results summarized in:NP6.00084 : NSTX Experiments with Lithium Plasma-Facing Components -- Recent Results and Future Plans – Kugel Lower divertor D-alpha intensity Lower divertor D-alpha intensity 6 129019 – No Li 129038 – Fresh Li 4 Arbitrary units 2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time (seconds)

5. A liquid lithium surface has the potential to produce even more long-lasting performance gains • Benefits of evaporated lithium surfaces fade after a few shots without new evaporation as the top layer passivates, requiring reapplication of lithium coatings • Liquid lithium provides a greater volume for reacting with hydrogenic atoms than the surface coatings formerly deposited • Flowing liquid lithium also provides better power handling capabilities, allowing heat to be transported away from regions of high power deposition

6. NSTX plans to install a Liquid Lithium Divertor (LLD) module in FY 2009 • Location - lower outer divertor in four 90°sections. • Width - 20 cm starting 5 cm outboard of CHI gap. • Shape - replaces present graphite tiles. • Structure - 0.01cm Mo flame-sprayed on 0.02 cm SS brazed to 1.9 cm Cu. Resistive heaters and cooling lines maintain 200-400°C. • Li Loading - 2 lithium evaporators. LLD-1 90° SEGMENT GRAPHITE DIAGNOSTIC TILES • Each toroidal section electrically grounded to vessel at one mid-segment location to control eddy currents • Each toroidal section fastened at 4 corners to divertor copper baseplate with fasteners providing structural support, electrical isolation, and accommodating thermal expansion (design adopted from JET PPPL collaboration) • Narrow graphite tile transition regions between sections contain thermocouples, an array of Langmuir probes, and magnetic & current sensors H. Kugel, PPPL

7. Particle Balance and Recycling Model Used to Estimate 0-D LLD-1 Pumping Projections and Sensitivities Iterative Procedure • Convert measured D luminosity to particle flux using 20 ionizations per photon • Estimate LLD-1 flux intercept fraction from candidate discharge data for a given time slice • Vary RLLD in steps of 1 cm • Repeat for different WLLD, RPCORE and other input parameters Li surface particle sticking probability - 0.85 (LLD) * Red items estimated from CCD camera data (V. Soukhanovskii, LLNL) R.Maingi, ORNL

8. Results of 0-D analysis provide basis for LLD placement and width Low d : reduce ne by 50% High d : reduce ne by 25% LLD LLD CHI gap CHI gap R=0.65 R=0.84 Chosen LLD width (20 cm) and placement (R=.65-.84m) optimize pumping capability while minimizing programmatic risk Density Reduction 10cm 15cm 20cm 10cm 15cm Density Reduction 20cm R. Maingi, ORNL

9. Assessment of LLD effectiveness will first require diagnostics to measure effect on edge conditions • Higher edge temperatures and lower edge densities should lead to reduced edge collisionality, which in turn will work to stabilize ELMs and reduce transport • existing profile diagnostics such as Thomson scattering provide insufficient resolution in the edge • Knowledge of absolute particle fluxes is essential in determining recycling rates • High radial resolution necessary measure profile gradients in region near strike point • High temporal resolution necessary to observe conditions during transient events such as ELMs

10. Radial and temporal resolution conditions for edge measurements Heat flux, 129019 • IR profile data shows FWHM of strike point heat flux to be approx 10 cm • Current IR camera data gives approximately 16 data points over this range • a dense probe array could increase spatial resolution • ELMs occur on a time scale of several ms • an edge diagnostic such as a single-tip Langmuir probe would need a very fast sweep to fully characterize such events t = .33s t = .36s t = .39s 6 Q [MW/m^2] 4 2 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Radius (m) 129019 Lower divertor D-alpha intensity, 129019 6 4 Arbitrary units 2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time (seconds)

11. Triple Langmuir probe array will address many edge diagnostic needs • 33 radially arrayed triple-probes will provide edge temperature and density characterization on a continuous basis • Probes based on MAST design involving a macor cassette of closely spaced probes embedded in a carbon tile • tile mount will span both carbon tiles and LLD radially to provide edge plasma data across both surfaces • entire assembly can be secured with one end-plate • Close spacing of probes will provide better resolution in high-gradient (strike point) regions • each probe covers 3.5 mm radially, including spacing • probe heads are 2.5mm radial x 7mm toroidal rectangles

12. Cassette design will allow for ease of probe mounting and will include channels for wire transport • Macor cassette features grooves that will allow probes to slide in easily without necessitating screws to secure probes vertically • screwless design reduces mechanical stresses on the probes • Wiring channels allow for the wires from each group of probes to exit independently • wires will exit on sides of edge probes and through base of central probes

13. Probe design includes features to protect underlying surfaces and uses novel materials to facilitate assembly • Probes will be shaped so as to minimize direct exposure of macor to plasma or lithium • probe bend prevents direct line of sight for lithium or plasma down to cassette • probe widening at top allows for smaller gaps and greater shielding of surfaces below • Macor spacers between probes will provide insulation and structure to probe assembly • widened probe and spacer bases couple to cassette grooves • Probe fabrication investigations are in progress • likely material is extruded graphite cement to allow precise shaping • wires can also be attached with graphite cement, obviating need for mixed materials or additional screws

14. New probe electronics will permit continuous triple-probe operation • Triple probe interpretation will be supplemented by single-tip tile probe sweep characteristics • single tip probes can provide ion saturation current data • Initial implementation provides for 10 triple-probe sets • future upgrades will expand probe electronics • wiring scheme will allow external (to vessel) selection of which probes to operate a M. Jaworski, UIUC

15. Edge density profiles can be obtained with microwave reflectometry • Langmuir probes can only provide data in the vicinity of the PFCs • the density profile should be altered more deeply in the scrape-off-layer (SOL) as well • Reflectometry provides data within the mid-plane SOL • At this time, there is no reflectometry data from the divertor region • additional coverage in the future would allow more direct measurements closer to the region of particle pumping J. Wilgen, ORNL

16. Recycling measurements key to understanding of liquid lithium’s impact on edge physics • Liquid lithium’s role as a more effective particle sink will need to be verified with recycling diagnostics • Current measurements use D-α light, but liquid lithium presents a challenge to accurate measurements • the 655 nm line will be highly reflected by the liquid metal surface • The Lyman-α line, in the UV range, does not suffer such reflections, but is more difficult to use for absolute measurements • not in the visible spectrum, and detectors are harder to calibrate

17. Diode arrays will be used to measure Lyman-α light

18. IR cameras will diagnose steady state and transient LLD operation • In order to quantify the role LLD plays in altering the heat flux profiles in NSTX, a slow IR camera will view equilibrium conditions • To visualize how the LLD interacts with and suppresses ELMs, it is necessary to have a faster frame-rate IR camera to monitor transient heat fluxes • The emissivity of lithium complicates IR camera measurements • solutions such as two-color filters are being investigated R. Maingi, L. Roquemore R. Maqueda, D. LaBrie

19. Other diagnostics for LLD control and protection • In order to ensure the LLD is maintained at a temperature high enough for lithium to be liquefied and cold enough to not evaporate, thermocouples will be installed at several locations along each segment • Large halo currents during disruptions can lead to deleterious JxB forces on the LLD, and so monitoring these currents will provide data relevant to future concepts as well as inform on the present LLD design • Rogowski coils mounted on single-point grounding connector will measure these halo currents

20. Experimental plans • The first step is to compare LLD plasma discharges to those of plasmas with solid lithium PFC coatings by running similar equilibria under both types of conditions • by keeping fueling rate constant and running several shots without fresh lithium deposition, one can verify how solid and liquid lithium differ in passivation rate • glow discharge cleaning of LLD with He could also demonstrate surface revitalization without need for fresh lithium (verified in off-line testing) • Changes in plasma density will provide insight into the pumping efficiency of a liquid lithium surface • Other potential experiments involve sweeping the outer strike point over the LLD and observing the evolution in heat flux, particle flux, and plasma temperature and density profiles • Purposefully triggering ELMs with resonant magnetic perturbations could also provide a means for studying how they interact with a liquid lithium surface and how they affect the edge and H-mode bulk plasma densities