# Presentation: 506911

## Recent Lithium Evaporative Coating and Electron Beam Heating Results in CDX-U

- Presented by Dick Majeski
- R. Kaita, T. Gray, H. Kugel, J. Spaleta,
- J. Timberlake, L. Zakharov
- PPPL
- R. Doerner, R. P. Seraydarian
- UCSD
- V. Soukhanovskii
- LLNL

## Outline

- Experiments with evaporated lithium layers on CDX-U
- Electron beam implementation
- Effects of evaporated lithium coatings
- Plasma discharges with solid lithium wall coatings
- Solid lithium wall coatings are effective at gettering oxygen
- Lowered recycling, but not as low as with liquid lithium.
- Observations on high power density e-beam heating of thin layers of lithium
- Demonstrated power handling of 40 MW/m2 on *static lithium*

## E-beam coating experiments

- Electron gun first  installed in CDX-U in March
- Differentially pumped Wilson seal - long stroke to position over tray
- Interferes with plasma; must be removed
- TF + VF used to guide beam (~70G ea, typ.)
- Lithium tray fill used as target.

- Deposition monitor view
- (Inficon XTM/2)

- Window (camera view)

- Axial e-beam (phase II)

- (March install)

## Radial e-beam

- Converted Thermionics e-gun
- Very simple beam “optics”
- 4 kV, 300 - 350 mA typ.
- 5 min. operating cycle, run at up to 50% duty factor
- Uncooled (Tantalum, Macor, SS)

- Charging of probe tip
- insulator disturbs beam

- Gaussian,
- width = 3 mm

## Electron beam evaporation run from 4/07/05Third 240 sec. cycle at 1.2 kW40 MW/m2Produced 1000Å coating on deposition monitor at 0.9m distanceViewing windows acquired opaque, metallic coating

## Plasma operations with evaporated coatings

- Procedure:
- E-beam evaporation to produce a 1000 Å coating of lithium
- Measured at 0.9m with a quartz crystal deposition monitor
- Retract e-beam, switch magnet power supplies
- Setup for tokamak discharges
- Total elapsed time ~15 min. until first discharge
- Time for many monolayers of surface coating on the fresh lithium
- Strong effect on vacuum conditions
- Water disappears from the RGA
- Base pressure drops by 2 (to 6-7  10-8 Torr)
- Good impurity reduction
- Recycling is indeterminate
- Fueling requirement is higher
- No particle pumpout (?)

## Fueling comparison: bare tray, hot lithium, solid coatings

- Time (sec)

- Particle input
- (from puffing).
- Prefill only here.

- ne> (1012 cm-3)

- ne> (1012 cm-3)

- Time (sec)

**Pre-lithium**
**(bare SS tray)**

**Post-lithium**
**(liquid lithium at >300C)**

- Evaporated, solid
- Li wall coatings

- ne> (1012 cm-3)

- No. of deuterium atoms

- 3.5 1019

## Peak density does respond to solid lithium coating

- Includes both discharges with prefill only and those with fueling during the shot

- Tim Gray

## Beam source design modified to permit plasma operations with beam installed

- Axial beam orientation to allow mounting in upper port
- Beam inserted ~ 5cm past upper vessel wall
- 5 cm behind upper rail limiter
- Guide beam to lithium with vertical field only
- 4 kV, 300 mA

## Tray temperatures

- Movie

**Lithium**

## Electron beam evaporation run from 5/04 Third 240 sec. cycle at 1.3 kW40 MW/m2~10Å coating on deposition monitor at 1.0m distanceNo visible coatings on any windows

## Slide 13

## Tray power balance

- Mass of lithium in south tray half: 140g
- Mass of tray underlying lithium: 2.1 kg
- 528 J/°C to heat the lithium
- 1,050 J/°C to heat the tray
- 1,578 J/oC to heat the whole schmear
- Beam input for 240 sec: 288 kJ
- Losses (conduction, radiation) estimated from cooldown following the beam cycle: 70 kJ/240 sec
- IF beam power input is uniformly distributed over lithium + tray, predicted temperature rise should be 140 oC
- Actual temperature rises 225 - 375  or 150°C
- Note temporal resolution of measurements is only 1 minute with present system

## Summary

- Electron beam evaporation of lithium to produce wall coatings was far more difficult than expected
- Entire lithium inventory is heated
- Suggests that convective heat flow completely dominates
- Leonid has modeled this; defer discussion to his talk
- Wall coatings *were *obtained with successive heating cycles
- 1000Å at ~85 cm was selected as a “standard coating”
- Lithium “gettering” produced robust, high current discharges
- Recycling lower; no quantitative estimate
- Evaporation experiments have demonstrated 40 MW/m2 power handling capability of thin (3-4 mm) static (i.e. no forced flow) lithium films
- Tests limited only by available power density

## Issues for static liquid metal divertors

- What is the effect of a high magnetic field?
- CDX-U coils can only operate up to 200 - 300 Gauss for long pulse
- Testing at 5T is desirable, with divertor-like field geometry
-  Is this power handling capability limited to lithium?
- What about tin, gallium?
- What is the peak surface temperature?
- Surface temperature distribution?
- IR camera highly desirable (slow is ok)
- How thin/thick can the layer be?
- What is the power handling limit for ~100 sec pulses?
- Would a thermally controlled substrate allow for steady state operation?
- Can we mitigate the effect of plasma-driven currents?

## Control of static lithium

- CDX has used a tray design which forces currents in the lithium to flow in the toroidal direction
- Sufficient to prevent J  B induced motion (in CDX-U)
- Backup design utilized active switching; never implemented

- CDX-U tray grounding
- Passive system
- Toroidal current flow
- Tray is an approx. flux surface

- IGBT

- IGBT

**J**

- Active switching: ground path is switched between tray ends every 10 - 100 sec
- Inertia prevents lithium motion
- Stable with finite vertical **B**
- Divertor application

- Electrical
- breaks