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Control of Contamination in The Cryostat
Rafe SchindlerSLAC
Internal Camera ReviewOctober 14, 2008
LSST Internal ReviewOctober 14, 2008
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Philosophy To Control Contamination Starts With Understanding Each Individual Material Allowed In Cryostat
• Establish A Database of Permissible Materials and How They Must Be Processed & Stored in Advance of Integration into Cryostat
– Materials Test Facility Provides The Input Data• Provides outgassing species, outgassing rates (incl. temperature
dependence), condensability, evaporability and impact on optical transmission
– Program:• Study all Potential Materials (eg: NASA Database) & Their Preparation• Study Coatings for Non-vacuum Friendly Materials (eg: use of Parylene-C and HT on epoxy and electronics)• Develop cleaning and handling procedures
• Create Database For Tracking All Components In Cryostat– Individual materials tests and verification– Sub-assembly tests and verification – Tests and verification after shipping but before I&T at SLAC– Tests during the buildup of the cryostat no suprises since
disassembly and cleaning would be a long process
LSST Internal ReviewOctober 14, 2008
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Where Are We?
• Material Test Facility Started Commissioning In July
– Developing procedure for operating the chambers and protocol for measurements.
– Started making preliminary ROR measurements on some of the critical components:
• circuit board materials, • epoxies in feedthroughs and connectors, • effect of Parylene-C coating on adsorption
– Still assembling the cryogenic parts, so we have no data from the QCM or optical transmission chamber yet.
• Started to Assemble the Database For Measuring Materials and Tracking All Components In Cryostat
– Ultimately tells you what to expect from each subcomponent in cryostat --- and in particular, during the sub-integrations
LSST Internal ReviewOctober 14, 2008
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Material Contamination Test Facility Schematic
LSST Internal ReviewOctober 14, 2008
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Cryostat Material Preparation & Test Facility
- Samples Exposed to 50%rh 24 hrs. (mass)
- Samples Enter in A1- Baked & Pumped in C1
- Sample moved to C2- Outgasses in C2 - Measure Species and
Rate of Rise with RGA vrs temperature
- Condensable Products Deposited on cold Quartz Microbalance
- Condensible Products deposited on cold optical glass disk
- Sample exits thru LL
- Glass disk moves to C3- Light Transmittance
versus wavelength thru disk measured in C3
- Re-evaporated condensables measured with RGA in C3
- Glass disks enter and exit thru A3
Turbo pump
“A1”
“C1”
Transport arm
“C2”
“C3”
“A3”
Ion pump
Inert Gas Delivery
Wobble stick
“LL”
Scroll pump
TC and InstrumentsHeater
Power supplies
RGA-1 RGA-2
Transport arm
LSST Internal ReviewOctober 14, 2008
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Rear view
“LL”
Turbopump manifold
RGA units
Quartz Crystal micro balance unit
CC Vac gauges
LSST Internal ReviewOctober 14, 2008
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water
H2
N2 CO2
Snapshot: RGA during Outgassing of Polyimide/E-Glass sample at 84° C
Rate Of Rise (H2O) & Bkd Curve at 79 0C
Empty Chamber: H2O 8x10-12 Torr/sec
Sample In: H2O 1.4x10-9 Torr/sec
9 cm2 Polyimide E-Glass Sample
400 Seconds
4x10-8
1x10-8
2x10-8
3x10-8
2x10-9
LSST Internal ReviewOctober 14, 2008
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Example: Preliminary Test of Epoxy In Custom Made Douglas Signal Feedthrough*
Outgas rate 55°C(torr-liters/sec/cm2)
Outgas rate 90°C(torr-liters/sec/cm2)
H2 6.2x10-8 3.7x10-7
H2O 1.6x10-8 1.4x10-7
N2 1.7x10-8 1.5x10-7
CO2 5.6x10-9 6.7x10-8
Surface area of epoxy Sample : 4.2 cm2
*These Feedthroughs Are on the Back Flange and Operate Close to Ambient Temperature
LSST Internal ReviewOctober 14, 2008
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Example of Sensitivity: Airborn Connector Body
• First Look At Our Baseline Connector Body Used Everywhere --- Between Sensors, FEE and RCC.
• Shows The Usual H2, H2O and N2 and CO2
• But Also a set of spikes – Possibly Sulphur Dioxide (64+66)
H2
H2O
N2
CO2
LSST Internal ReviewOctober 14, 2008
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Materials Spreadsheet – Started To Build This
LSST Internal ReviewOctober 14, 2008
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Current Vacuum Design of Cryostat
• All seals in the cryostat are double O-ringed with pumpouts
• Divide cryostat into two vacuum regions (i) focal plane & (ii) everything else
• Connect them thru the lowest conductivity seals we can obtain
• The GRID, GRID Shrouds, Cryoplate and FEE run at coldest temperatures.
• The sensor surface is 10200C warmer than almost everything around it – except L3. Its temp. is controlled by the raft-tower makeup heaters.
• Focal plane region sees L3, colder GRID walls, GRID Shroud and the pumping chimneys. A 400l/sec TMP at rear gives ~130 l/sec at CCD’s
• Everything else (BEE, Cables, MLI) is pumped by 2nd TMP at ~400 l/sec
LSST Internal ReviewOctober 14, 2008
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Cryostat Vacuum System Design
Pump pathConductance: 195 l/secEff. Pump speed: 130 l/sec
F.P. chimney connects front end vacuum region to pumping plenum—this is attached to the Cryo Plate
Sheet metal F.P. pumping plenum drops into Feedthrough Plate
400 l/sec turbo pump
LSST Internal ReviewOctober 14, 2008
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Cooldown/Warmup• The CCD surface should never be the coldest surface in the cryostat
• During operation The CCD sensors will always be warmer (~100C) than the surrounding materials that are most likely to outgas (FEE electronics, connectors, and cables)
• Before cooldown we envisage a long period of N2 flushing the cryostat and pumping (at somewhat elevated temperatures) to reduce H20 in the cryostat. Additional pumping capacity during the initial pumpdown might be used.
• Cooldown requires the Cryo plate be cooled with the FEE off but the makeup heaters on -- to keep the CCDs warmer than the FEE and GRID. We cool only within the allowed “survival range” of the FEE, before turning them on…and completing the cooldown.
• If FEE electronics in a tower fails, the CCD & Raft Plate temperature drifts down in the tower. The makeup heaters should be adequate (and redundant) to avoid the CCD being over-cooled
LSST Internal ReviewOctober 14, 2008
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What Else ?
• All material will be vacuum baked and stored in inert atmosphere prior to I&T. During I&T we will work in as dry/inert an atmosphere as is safe.
• We are selecting, processing and coating materials in the cryostat to reduce the uptake of H20, CO/CO2 during assembly and the subsequent desorption during operation
• Assuming we can reduce or eliminate all materials that produce other heavier condensables or contaminants that react chemically with the optical coatings, we will still be left with some residual slow - desorption of the usual condensibles molecules onto the focal plane. (Water etc…..)
• We are looking into the use of more passive pumping in the focal plane region to provide backup to the TMP’s, and/or mitigate the need to run them during observing.
LSST Internal ReviewOctober 14, 2008
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Options
• Two Options Exist: – “cold” evaporable getters that trap gases (zeolytes and charcoals)
– Need to be tied to the GRID or Cryo-Plate – Problem of containment and dusting in FP region
(eg: people have used 0.2 m PTFE mesh)
– Desorption during warmup – need temp. control, isolatation or pumped
– “warm” non-evaporable getters that chemically break down gases (NEG Pumps)
– commonplace in accelerators and electronics packaging– Need to be kept “warmer” for better performance:
…. Eg: tied to the cryostat wall
• Both Options have (not insurmountable) problems– Space in cryostat Focal Plane region – Access to insert and replace materials near focal plane
• We want to understand from our materials testing, what we really will need, but are looking into these other options in parallel
LSST Internal ReviewOctober 14, 2008
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Example: Small SAES NEG Pump
LSST Internal ReviewOctober 14, 2008
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A VERY OLD ESTIMATE OF OUTGASSING RATESBASED ON EARLIER DESIGN AND MATERIALS
ZONE MATERIALSEXPOSED
AREAOUT GASSING
PRESSURE (10 Hrs @500 L/s)
m2 10-6 Torr-L/sec Torr
Metals/Glass/ Ceramic 4.7 907 1.8E-06
Focal Plane Coated PCB's/ Plastics 0.89 3
Metals/Glass/ Ceramic 18 10602 3.4E-05
GRID and FEE Super Insulation 263 293
Cu Thermal Straps 11280 122
Coated PCB's/ Plastics 7.9 5761
BEE Metals/Glass/ Ceramic 12 0.28 5.4E-06
Super Insulation 178 220
Coated PCB's/ Plastics 7.3 2468
PICTURES
LSST Internal ReviewOctober 14, 2008
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C1 inside view
Samples come infrom A1 on mag.transport arm
Heaters (2)
Samples proceed to C2
Thermocouples (2)
LSST Internal ReviewOctober 14, 2008
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C2 inside view
Samples enterfrom C1
Sample boxesenter and exitthru A2
Glass disks comein from C3 cleanand go back dirty
RGA
Glass diskstage
Thermocouples (3)
Heaters (2)
refrigerant loop
Cold strap
Quartz balance(crystal is under stage)
“Wobble stick”
Sample box platform
LSST Internal ReviewOctober 14, 2008
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C3 inside view
Glass disks come in and go out thru A3To C2
Light beam comesup thru bottom,passes thru disksand is detected above
“wobble stick”
Refrigerantloop
Actuator piston moves basket back and forth
Thermocouple (1)
double glass disk stage
Cold strap
Cold strapbent down in “U”to allow motion
LSST Internal ReviewOctober 14, 2008
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Glass Disk Holder and Sample Box
Glass Disk Holder(clamps glass and sits snuggly in baskets)
Sample box(sliding lid and
2 holes for outgassing)
LSST Internal ReviewOctober 14, 2008
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Optical setup overview
Light source
detectordiodemounted here
Beam goesthrough glassdisks in C3
Repeated witheach of 6band passes:- 400 nm- 500 nm- 600 nm- 750 nm- 850 nm- 1000 nm
LSST Internal ReviewOctober 14, 2008
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Optical setup box
Filter wheel
Beam splitter
Reference diode
aperture
TO C3
LSST Internal ReviewOctober 14, 2008
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Final picture: some peripherals
microbalance50% RH environmentVacuum oven
LSST Internal ReviewOctober 14, 2008
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INSTRUMENTATION FOR 3rd OPTICAL TRANSMISSION CHAMBER
Contaminated & Reference Samples Moved (Cold) From 2nd Chamber
White Light Monitor Diode & Window Filter Wheel (LSST) Cold Sample
Slide or Uncontaminated Slide Window Precision Photodiode/PM. Diode
Temporal Stability of Light Source Calibrated Out Using Monitor Diode
Piston Moves Samples Back & Forth In Vacuum Thru Same Optical Path to Interleave Measurements (Avg. Out Instabilities in Light Output & Light Path)
Observe <0.1% sensitivity: comparable to photometric target
Measurements Sequenced By Computer (~0.1 Hz) and Recorded and Analyzed
LSST Internal ReviewOctober 14, 2008
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RESULTS FROM TESTS OF OPTICAL TRANSMISSION CHAMBER
400nm
850nm750nm
600nm500nm
1000nm
DISTRIBUTION OF MEANS OF 120 EXPERIMENTS (@1000 samples) TAKEN OVER ~20 MINUTES
ERROR AS A PERCENT OF MEAN (DURING 20min)
TARGET < 0.1%
0.02%