# Presentation: 409763

## Argon Purification: Lessons from Icarus and Design of a Purification System for  a Very Large Detector

- Adam Para, Fermilab
- FLARE Workshop, November 3

## Outline

- Liquid Argon Time Projection Chamber: a mature technology
- Neutrino oscillations opportunities with the NuMI beam
- Liquid Argon detector for the NuMI off-axis experiment
- ‘Other’ physics with the LAr detector
- FLARE: Fermilab Liquid Argon Experiments. Letter of Intent P942,  http://www-off-axis.fnal.gov/flare
- FLARE Workshop:  ‘All about Liquid Argon Detectors’, Fermilab, November 4-6

## Many years of intense R&D

## Drifting electrons over long distance (3m)?

- Vdrift = f(E). Use E= 500 V/cm
- HV across the drift gap = 150 kV
- Vdrift = 1.55 mm/sec
- tdrift = 2msec
- Number of collisions/sec ~1012
- 2x109 collisions along the longest path
- ‘none’ of them must ‘eat’ an electron
- Concentration of electronegative (O2) impurities < **10****-10**
- Rule of thumb: 1 ppb 250 sec, 10 msec  25 ppt

## Measuring argon purity below 0.1 ppb ?

- Best commercial O2 gauge: least count 0.2 ppb (not bad at all, but nut good enough)
- How do you know that there are no other impurities, not detectable with your purity ,monitors, which absorb electrons (remember MarkII? )
- Electron lifetime detector
-     Carugno,Dainese,Pietropaolo,Ptohos
-     NIM A292 (1990) 580:
- Extract electrons from a cathode
- Drift over a certain distance
- Measure charge along the path

## Argon purification: early attempts

- Use commercial filters/getters designed to purify gaseous argon inserted into a corcuit for argon boil-off recovery
- Works, albeit slowly
-  A major breakthrough: (at least some of) these can be applied to purify liquid (need  cryogenic cartridges/lines)
- Much larger mass can be purified, much shorter time constant.

## Argon purification: liquid and gas phase

- Re-circulate liquid/gaseous argon through standard Oxysorb/Hydrosorb filters (R20 Messers-Griesheim GmBH)
- ICARUS T600 module:
- 25 Gar m3/hour/unit
- 2.5 Lar m3/hour

## Argone purity/electron lifetime in real life ?

- Impurities concentration is a balance of
- Purification speed c
- Leaks in(t)
- Outgassing A, B
- For a T600 module: asymptotic purity/lifetime > 13 msec

## Argon purity, ctnd.

- Q:Oxisorb R20 filters have design purity level of <5 ppb. How come that the results are so good (<0.1ppb)?
- A: Specs refer to gaseous argon at NTP.
- In a liquid phase impurities ‘freeze out’ at the vessel walls. The natural purification speed is limited by diffusion speed. (Related: B. Kephart, E706)

- Electron lifetime improvement in ‘regular argon’

- Electron lifetime  in ultra-pure argon doped with oxygen

- Degradation of argon purity is consistent with diffusion time

## Argon purity, lessons for a very large detector

- Long electron lifetimes (~10ms)/drift distances (>3m) appear achievable with commercial purification systems
- The main source of impurities are the surfaces exposed to the gaseous argon
- Increasing the ratio of liquid volume to the area of gaseous contact helps (dilution)
- Increasing the ratio of cold/warm surfaces helps (purification)
- Role of the walls must be well understood, though, if they provide the critical step in argon purification (capacity, saturation, treatment, evacuation..)

## A toy model for a very large tank operation

- Argon is delivered to small holding tanks. It is purified there to ensure its ‘purifiability’  and it is subsequently transferred to the main tank at the rate of 8t/hour.
- At the end (after 270 days):
- total area of warm surface is 1700 m2
- Volume of gaseous argon is 4500 m3

## Argon pre-purification?

- Assume recirculation system in the main tank with capacity of 100 t/hour
- Oxygen level during the fill is an equilibrium between the  new oxygen introduced with the newly pumped argon and the oxygen absorbed by a purifier. This level corresponds to ~7% of the purity of the incoming argon. After the fill is completed the purity leve improves by a order of magnitude every 50 days

## More realistic model 

- Liquid argon  recirculation through a purifying system: 100 t/hour
- Gaseaus argon recirculation through a purifier: 1000 m3/hour
- Oxygen balance in a liquid volume:
- Inflow:
- Oxygen coming with the initial fill of argon
- Oxygen diffused from the gas phase
- Outflow:
- Oxygen absorbed by a purifier
- Oxygen balance in a gas volume
- Inflow:
- Outgassing from warm tank surfacs
- Outgassing from cables
- Inflow from leaks
- Outflow:
- Oxygen absorbed by filtering system
- Oxygen transferred to a liquid phase

## Some important facts

- In equilibrium at the liquid/gas interface oxygen concentration (per unit mass) is the same (within 10%) in the liquid the gas phase (R. Schmitt). Owing to the difference in densities practically all oxygen ends up in the liquid.
- Mixing of the ago in a tank is quite rapid. Vertical movement of the liquid is about 1 m/hour near the surface (Z. Tang) Liquid acts as a ‘sponge’.
- Outgassing is reduced with time like 1/t. Only ~1% of the outgassed material is oxygen (somewhat time dependent).
- Surface of warm walls is 50% bigger than that of cables. Outgassing of walls depends on the treatment/cleaning procedure. In the following we will assume that the steel outgasses as much as cables.

## Argon purity vs outgassing?

- Given the time scales of filling large tank the outgassing of cables and/o walls does not represent any problem with a recirculation system

## An impossible mishap ?

- An example: total volume of argon above the liquid level is replaced by air.
- The recovery time is inversely proportional to the capacity of the recirculation/purification system
- With 100 t/hour pumping speed it takes ~ 9 month to reastablish working conditions

## Leaks(?)

- Assume leak rate 0.001 cm3/sec (air entering the tank). With 100 t/hour recirculation an equilibrium is reached at 10 ppt level.
- Tank is run at some overpressure: prevents outside gas from entering tha argon volume.
- There is no air outside the tank/pipes. All lines are vacuum lines, argon fills the volume between tank walls.
- Welded steel tanks DO NOT LEAK

## Conclusions

- A relatively modest liquid argon purification/recirculation system with capacity of 100 t/hour provides a significant overkill capacity to ensure argon purity level exceeding the requirements even under unrealistically pessimistic assumptions
- Fast mixing of argon within the tank makes the gas purification system of very limited use, probably not worth the complication of the installation and operation of such a system

## Thermal analysis of a 50 kT liquid argon tank

- Zhijing Tang, PPD

- Rough analogy: big boiling pot
- Vapor bubbles at the surface only (hydrostatic pressure)
- Total heat leak: 49 kW
- Maximal temperature difference Tmax = 0.1oC
- Temperature difference over most of the volume 0.01oC
- Maximum flow velocity: 7.7 cm/s
- Heat leak through a signal feed-through chimney 48W/chimney