Control of Contamination in The Cryostat

1. Control of Contamination in The Cryostat Rafe Schindler SLAC Internal Camera Review October 14, 2008

2. 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 Review October 14, 2008

3. 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 Review October 14, 2008

4. Material Contamination Test Facility Schematic LSST Internal Review October 14, 2008

5. Cryostat Material Preparation & Test Facility Transport arm “LL” • Samples Exposed to 50%rh 24 hrs. (Dmass) • 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 Transport arm RGA-1 RGA-2 “A3” “A1” “C2” “C1” “C3” TC and Instruments Heater Power supplies Ion pump Inert Gas Delivery Turbo pump Scroll pump Wobble stick LSST Internal Review October 14, 2008

6. Rear view CC Vac gauges RGA units “LL” Quartz Crystal micro balance unit Turbopump manifold LSST Internal Review October 14, 2008

7. Snapshot: RGA during Outgassing of Polyimide/E-Glass sample at 84° C Rate Of Rise (H2O) & Bkd Curve at 79 0C 9 cm2 Polyimide E-Glass Sample water 4x10-8 Sample In: H2O 1.4x10-9 Torr/sec 3x10-8 H2 2x10-8 Empty Chamber: H2O 8x10-12 Torr/sec 1x10-8 N2 CO2 2x10-9 LSST Internal Review October 14, 2008 400 Seconds

8. Example: Preliminary Test of Epoxy In Custom Made Douglas Signal Feedthrough* *These Feedthroughs Are on the Back Flange and Operate Close to Ambient Temperature Surface area of epoxy Sample : 4.2 cm2 LSST Internal Review October 14, 2008

9. 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) H2O H2 N2 CO2 LSST Internal Review October 14, 2008

10. Materials Spreadsheet – Started To Build This LSST Internal Review October 14, 2008

11. 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 10200C 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 Review October 14, 2008

12. Cryostat Vacuum System Design Pump path Conductance: 195 l/sec Eff. 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 Review October 14, 2008

13. 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 Review October 14, 2008

14. 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 Review October 14, 2008

15. 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 mm 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 Review October 14, 2008

16. Example: Small SAES NEG Pump LSST Internal Review October 14, 2008

17. A VERY OLD ESTIMATE OF OUTGASSING RATESBASED ON EARLIER DESIGN AND MATERIALS LSST Internal Review October 14, 2008

18. PICTURES

19. C1 inside view Samples come in from A1 on mag. transport arm Heaters (2) Samples proceed to C2 Thermocouples (2) LSST Internal Review October 14, 2008

20. C2 inside view Quartz balance (crystal is under stage) RGA Samples enter from C1 Cold strap Heaters (2) Glass disk stage Sample boxes enter and exit thru A2 refrigerant loop “Wobble stick” Glass disks come in from C3 clean and go back dirty Sample box platform LSST Internal Review October 14, 2008 Thermocouples (3)

21. C3 inside view Actuator piston moves basket back and forth Cold strap bent down in “U” to allow motion Cold strap Thermocouple (1) Refrigerant loop Glass disks come in and go out thru A3 To C2 Light beam comes up thru bottom, passes thru disks and is detected above “wobble stick” double glass disk stage LSST Internal Review October 14, 2008

22. 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 Review October 14, 2008

23. Optical setup overview detector diode mounted here Beam goes through glass disks in C3 • Repeated with • each of 6 • band passes: • 400 nm • 500 nm • 600 nm • 750 nm • 850 nm • 1000 nm Light source LSST Internal Review October 14, 2008

24. Optical setup box Filter wheel TO C3 aperture Beam splitter Reference diode LSST Internal Review October 14, 2008

25. Final picture: some peripherals Vacuum oven 50% RH environment microbalance LSST Internal Review October 14, 2008

26. INSTRUMENTATION FOR 3rd OPTICAL TRANSMISSION CHAMBER Contaminated & Reference Samples Moved (Cold) From 2nd Chamber White LightMonitor Diode & WindowFilter Wheel (LSST)Cold SampleSlide 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 Review October 14, 2008

27. RESULTS FROM TESTS OF OPTICAL TRANSMISSION CHAMBER 400nm 500nm 600nm 750nm 850nm 1000nm 0.02% DISTRIBUTION OF MEANS OF 120 EXPERIMENTS (@1000 samples) TAKEN OVER ~20 MINUTES ERROR AS A PERCENT OF MEAN (DURING 20min) TARGET < 0.1% LSST Internal Review October 14, 2008