Control of Contamination in The Cryostat

Control of Contamination in The Cryostat

Rafe SchindlerSLAC

Internal Camera ReviewOctober 14, 2008

LSST Internal ReviewOctober 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


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


4

Material Contamination Test Facility Schematic


5

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


6

Rear view

“LL”

Turbopump manifold

RGA units

Quartz Crystal micro balance unit

CC Vac gauges


7

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


8

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


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)

H2

H2O

N2

CO2


10

Materials Spreadsheet – Started To Build This


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 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


12

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


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


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.


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 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


16

Example: Small SAES NEG Pump


17

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


19

C1 inside view

Samples come infrom A1 on mag.transport arm

Heaters (2)

Samples proceed to C2

Thermocouples (2)


20

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


21

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


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)


23

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


24

Optical setup box

Filter wheel

Beam splitter

Reference diode

aperture

TO C3


25

Final picture: some peripherals

microbalance50% RH environmentVacuum oven


26

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


27

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%

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Control of Contamination in The Cryostat

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