# Presentation: 134226

## High-current measurements in flowing liquid Li 

- D.N. Ruzic, B.J. Schultz, P.W. Brenner
*Plasma Material Interaction Group*
*University of Illinois at Urbana-Champaign*

## Outline

- FLiRE design
- Previous results
- Ion source
- Design Testing
- Measurements
- Latest results
- Current status
- Summary and future work

## FLiRE redesign

- Simplified flow path
- Increased reliability
- View of lithium flow down second chamber

## Previous results have qualitatively shown He retention in flowing Li

- Ion source on ramp until start of flow

- Shutter closed throughout flow

- Ion Source on ramp before and during flow

- Open and close shutter during flow

## Source has been designed to increase ion flux

- Previously used SPECS ion source with limited results
- Low current (μA) and low flux
- Switched to plasma source design (mA)

## Old Ion Source Design

- Anode inside of alumina tube
- Grounded cap flush against end of tube above ramp
- Screen placed inside of cap to accelerate ions into Lithium stream through sheath

- +5000

## Original source design

- Created using alumina tube and high voltage feed-through
- Grounded cap with screen

## Flaws in the design were noticed

- No gas flow inside of tube led to high He pressure inside of the chamber (>250 mTorr)
- At higher pressures He blow through was sometimes seen during runs
- Cap design led to unstable plasmas
- Could not adjust anode distance inside of source

## New Source has been designed

- Allows for gas flow inside of tube
- Anode can be removed and adjusted
- Lower breakdown pressures (<100 mTorr)
- Cap can accept different screens

## Flux measurements were conducted outside of FLiRE chamber

- Limit exposing FLiRE chamber to atmosphere between tests
- Easy access to plasma source and calibration materials

## Test ramp

- Same width and depth as ramps inside of FLiRE but shorter length
- Grounded inside of chamber
- Copper strip on top of ceramic insulator inside flow channel
- Ammeter connected to copper strip to measure current and find minimum value of ion flux
- Actual ion flux may be larger since whole ramp not always hit by plasma

## Current measured in test chamber correlates to ion gun power

- Current collected on copper strip correlates linearly with supply power
- No current measuring diagnostics when flowing in FLiRE
- Current during runs approximated using linear fit
- Measuring power gives current to ramp inside FLiRE

## Current was seen to drop over time

- FLiRE requires constant reproducible current during flows
- Previous FLiRE experiments have shown He signals only with a high current          ( > ~1 mA)  on ramp
- Ion induced secondary emission changes
- Temperature
- Oxidation removed by sputtering

## Many designs were attempted to maintain constant high current

- Different screen materials
- Stainless Steel
- Copper
- Tungsten
- Nickel
- Different orientations
- Insulator with screen
- Varying opening sizes

## Stainless steel screens showed promise

- Tests showed adequate current on test ramp but decreased over time
- New screens showed best results but could not regain full potential after first use
- Screens became discolored on ramp facing side

- new

- After 10 min.

## Hollow cathode design was implemented

- Cap was moved down from end of tube
- Higher current was seen from more ionizations created in hollow cathode
- Also attempted to create hollow cathode inside of tube using shim-stock
- In both cases we could not obtain stable current measurements on the test ramp

- +5000

- 1 to 2 cm instead of flush (1 mm)

## Hollow cathode was switched to be the ramp itself !

- Extended alumina tube inside of ramp through smaller diameter tube attached to end of original tube
- Ions are accelerated through the sheath formed inside the ramp
- Have seen:
- “Instant on” capability – current immediately jumps to a stable level
- Much higher current reading on test ramp (>2 mA vs <1 mA)

- + 5000

## Ramp hollow cathode source

- Test ramp current measured still approximated as a linear correlation to supply power at various pressures

## Current stays constant with time

## Latest experiments

- Flows with source at lower pressures (< 100 mTorr)
- Monitor mass three on RGA during runs
- Static He runs in order to determine background noise and look for possible He blow through
- Runs with plasma source on before flow to look for any He transferred to the lower chamber

## Mass three has been monitored on RGA

- It has been suggested that the mass four signals seen on the RGA during runs could have been the result of a wide mass three (HD) peak or a D2 signal.
- This data shows that is not likely.

- Between Flows

## Looking at previous measurements

- Mass four does not appear to follow mass three in the earlier measurements shown before.

## Latest measurements:static He runs

- However, mass three and mass four signals appear to increase together now indicating dirty lithium

## Runs with plasma source on ramp prior to start of flow

- Mass four again seems to follow mass three
- No He retention
- Same as static He runs

## Possible explanations

- Temperature too high (~300 oC vs 250 oC), diffuses faster back to top chamber
- Previous measurements dealt only with 250 oC temperatures for retention measurements
- Li vapor from surface being ionized evidenced by Li atoms ionizing inside hollow cathode.
- Overflow of neck, Li with He implanted does not reach lower chamber – it spills into top chamber – not intended – see movie!

## Flow velocity alone can explain it...

- Lower flow velocity (15 cm/s vs 44 cm/s), all may escape back to top chamber
-  = ~1/2
- v = velocity
- m = slope of measured
- 	He partial pressure vs energy
- Implies that
- R (44 cm/s) = ~1.75* R (15cm/s)

## Ramp hollow cathode design
Copper tape holding on extending tube
Tube extends within 0.7 cm of ramp
Plasma quickly shifts to top of ramp (where lithium enters) before flow starts
Plasma changes colors as Li flow begins
Lithium covers tube tip! Gas starts to escape through the copper tape
Pooling in neck – to prevent this is why we are flowing slower
Overflow of Li onto spill trays (not the first time)

- Copper tape holding on extending tube
- Tube extends within 0.7 cm of ramp
- Plasma quickly shifts to top of ramp (where lithium enters) before flow starts
- Plasma changes colors as Li flow begins
- Lithium covers tube tip! Gas starts to escape through the copper tape
- Pooling in neck – to prevent this is why we are flowing slower
- Overflow of Li onto spill trays (not the first time)

- Movie of the last time the plasma source operated in FLiRE

## Plasma source operating in FLiRE

## Current status of FLiRE

- Undergoing cleaning of neck – flow channel likely constricted with oxides and nitrides. After cleaning, higher velocities can again be achieved.
- Lithium overflowed spill trays and destroyed copper gaskets
- First cleaning in 360 runs (since June 2005, a long time for FLiRE)
- Will load with fresh Li – no hydrogen exposure.

## Summary and Future Work

- Many designs attempted to increase ion current onto lithium stream at a constant rate
- Hollow cathode design inside of ramp seems most suited
- Performed measurements at low pressures in top chamber (<10-5 Torr) in order to monitor mass three and mass four.
- The notion that the mass four may be a result of a wide mass three (HD) peak or D2 has been debased.  A baseline can be recorded and He can be assumed to be the cause of any mass 4 peak greater than ~10-8 Torr
- Li is lkely contaminated with hydrogen at the moment from extensive use and will be replaced.
- Increased He flow rate into the plasma gun should eliminate Li ionization
- Lower temperature and higher flow velocity easy to obtain once neck passage is cleared
- FLiRE currently undergoing cleaning and Li replacement.  On track for definitive results at PSI meeting in China.

## Acknowledgements

- US Department of Energy
- Graduate assistants:
- Matt Coventry
- Travis Gray
- Undergraduate assistants:
- Michael Sabor
- Patrick Mangan
- Dan Lossie