Transcript Modular: Argon purification and handling Claudio Montanari INFN - Sezione di Pavia

Modular: Argon purification and
handling
Claudio Montanari
INFN - Sezione di Pavia
Outline
 Basic considerations
 Standard procedures from ICARUS
experience
 Large volumes challenges
 Dimensioning and design of purification
systems for ModuLAr
 Conclusions
Claudio Montanari - Criodet 2 - LNGS June 14, 2007
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Basic Considerations
 Considerable experience of the Icarus Collaboration has shown
that free electron lifetimes of several milliseconds are currently
realised with commercial purification systems based on
OxysorbTM and molecular sieves.
 In order to ensure a free electron lifetime for the longest ≈ 3 ms
fly path a vigorous purification of the LAr must be kept at all
times:
 In analogy with what is currently performed with T600 and all
previously constructed detectors, the purification must be
performed both in the liquid and in the gaseous phase.
 The reference schemes developed so far within the Icarus
Collaboration can be rescaled straightforwardly up to volumes of
some thousands cubic meters.
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Attainable Free Electrons Lifetime
Free Electron Lifetime evolution during
recirculation of 50 liters chamber
Free electron lifetimes
in excess of several
milliseconds are
routinely achieved with
the standard
procedures developed
within the Icarus
Programme.
Present best result is
in the range of 10 ms
actually limited by our
capability to measure
longer lifetimes due to
the limited size of the
detector.
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Basic Requirements
Required Lifetime for 4m drift ≈ 10 ms  ≈0.03 ppb (O2 equiv)
Required Lifetime for 2m drift ≈ 5 ms  ≈0.06 ppb (O2 equiv)
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Basic Requirements
Use of larger drift fields brings several advantages, including a reduction in the
requirements for the free electrons lifetime. 1 kV/cm could be attained rather
easily for 2 m maximum drift corresponding to 0.5 kV/cm for 4 m maximum drift.
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The Standard Icarus Procedure

The “standard” Icarus procedure for purification and handling
LAr consists of 5 steps:
1. Use ultra high vacuum standards for detector components
design, construction, cleaning and assembly;
2. Removal of air and outgassing of surfaces by evacuating the
argon container volume to the molecular vacuum level (< 10–
3 mbar);
3. Fast cooling (to reduce pollution from outgassing) and filling
with argon ultra-purified by means of chemical filters and
molecular sieves;
4. Recirculation of the gas phase to block the diffusion of the
impurities coming from the hot parts of the detector and from
micro-leaks on the openings (typically located on the top of
the device) in the bulk liquid;
5. Recirculation of the bulk liquid volume to further reduce the
impurities concentration up to the required level.
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Large volume challenges

Extrapolation of the standard Icarus procedure to volumes potentially
very large (up to 10000 m3 or more) is almost straightforward except
for step 2 (evacuation to molecular vacuum of the detector volume).

A detector of several thousand m3 is very hard to evacuate and a
new method has to be applied. The idea is to perform successive
flushing in the gaseous phase in order to attenuate the presence of
gases other than Argon with an approximate exponential chain.

An improvement in the liquid purification system is also needed to
enlarge in a significant way the TPC volume. New purification
devices have to be implemented, possibly operating near or directly
inside the detector. They should be simple, robust and without
moving parts, to guarantee total reliability.
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Large volume challenges (II)

Uniformity in the free electrons lifetime is a major issue for very large
volume detectors. Convective motions provide a natural mechanism
for mixing the liquid volume. For such a mechanism to be effective in
providing uniformity of the liquid purity, liquid motions and the effect
of the presence of the detector structures have to be carefully studied
at the design level. Trapped volumes have to be avoided; the
distribution network of the purification and recirculation system have
to be designed accordingly.

In running conditions, monitoring of the uniformity of the liquid
properties can be easily performed by means of cosmic muon
tracks.

A number of dedicated devices (purity monitors) have to be
installed in the non active zones (behind the wires and the race
tracks) to complete the information and to check the purity during
the initial filling and the startup phases of the detector.
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Alternatives to vacuum (I)



Standard procedures, adopted to pre-condition industrial storage cryostats
before first filling, reduce air concentration at the level of ppm in the gas
phase, corresponding to ppm level in the liquid phase, after filling. These
procedures are based on high rate flushing of the connecting pipes and of
the container inner volume from the bottom to the top.
Tests have been made by the ICARUS Collaboration to avoid high vacuum
before filling:
1. With the 10 m3 prototype, empty, no structures inside: high rate flushing
of the connecting pipes plus compression / expansion cycles in the main
volume:
 about 100 cycles from 1.0 to 1.1 bar, corresponding to 10 volumes
exchange  5 ppm residual air concentration for perfect gas
diffusion results from computation;
 Free electron lifetime after filling ≈ 30 µs.
For these procedures to be effective, the geometry of the internal detector
structures have to be carefully studied in order to eliminate trapped volumes
(tubular elements, preferential flow paths, etc.). This matches with the
requirement for the uniformity of the liquid purity.
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Simulations
 A detailed simulation of the basic ModuLAr design has been
performed using the FemLabTM code (a professional tool to solve
gas transport problems).
 Several options have been considered:



Gas injected on one side of the volume and extracted from the
opposite side;
Gas injected on the bottom of the volume and extracted from the
top;
Different flow rates.
 Results of the simulation indicate that the best solution consists
in injecting the argon on the bottom of the container and
extracting it from the top (due to the larger weight, argon
concentrates on the bottom, pushing the air to the top):

few volumes (≈ 6) of gas flow are required to reach a residual
air concentration < 1 ppm (< 1 ppb in the argon liquid phase).
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Filling procedures
T600 Purification Unit
• Filling rate should match the maximum supply rate of
liquid Argon. Reasonable estimate for supply speed
from a single dealer is ≈ 100 m3 / day (5 trucks). This
leads to a filling rate ≈ 4 m3 / hour (6 at the design
level); about 40 days will be necessary to fill a 4000
m3 detector.
• Single HydrosorbTM / OxysorbTM cartridge (type R20,
maximum size commercially available) like the ones
used for the T600 (see right):
• Purification rate: 0.5 m3 / hour
• Adsorption capacity: 40 normal liters of O2
• The number of purification cartridges required for the
initial purification of the liquid volume is correlated
with the purity of the argon provided by the supplier.
Very low Oxygen (and other contaminants)
concentrations (small fractions of ppm or less) can
be obtained at the production level and can be part
Claudio Montanari - Criodet 2 - LNGS June 14, 2007
of the requirements of the supply contract.
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T600 purification Units
Liquid recirculation pump
Purification unit
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A “self cleaning” cryostat
The mass of LAr moved is potentially
very large.
It is an improvement that will allow to
avoid an external circulation.
cleaning
With a good control of the input heat and
then of the convective motions, it could
be possible to realize a “self cleaning”
liquid circuit, collecting the LAr inside a
number of “cleaning boxes” placed on
the walls.
For a 4000 m3 liquid argon volume, a rate
240 m3/day (6%/day or purification cycle
of 16.6 days) would allow to reach in 60
days the target purity (0.03 ppb) if the
initial e-negative concentration is at the
level of 1 ppb (O2 equiv.).
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Gas Recirculation System
Recirculation of the gas
phase is used to prevent
diffusion of impurities
coming from to hot parts
of the detector and from
openings (almost all
located on the top) in the
liquid volume.
In this case the T600
scheme can be used
with no modifications.
Gas recirculation scheme
Required gas recirculation rate scales like the top surface of the liquid
volume. Extrapolating from the T600 case we have that, for an exposed
surface of 9 x 51 m2 (4000 m3 ModuLAr) there is a factor 6 surface increase
with respect to the T600. A gas recirculation rate of about 600 Nm3 / hour will
be required .
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LAr Purity Monitor: working principle
Lifetime
Determination
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T600 purity
monitor
ReadOut
Electronics
R&D is in progress for new photocatodes
to improve the sensitivity.
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Conclusions

Free electron lifetimes of several milliseconds are required for the
operation of 4.5 kton or 9 kton ModuLAr detectors. Such purities are
routinely achieved using standard procedures developed within the
ICARUS programme.

Extrapolation of the standard ICARUS procedures for the basic ModuLAr
designs is almost straightforward with one relevant exception: removal of
air from the main argon volume and pre-conditioning of the internal
surfaces before filling with LAr has to be done by high rate flushing of pure
Argon gas.

For such a procedure to be effective, geometry of the internal detector
structures must not contain trapped volumes.

Standard procedures used to pre-condition cryogenic storage dewars are
able to remove air to the level required for the filling of the experiment.

Our simulations also demonstrate that a residual concentration of air in
the argon volume of less than 1 ppm can be reached in less than one
month with a rather modest gas flow rate.
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