Requirements
- Phase
Noise Limits on Partial Pressures (Outgassing Rates):
Reference: T040001-00,
Vacuum Hydrocarbon Outgassing Requirements
|
Gas Species
|
LLO
Actual
(torr)
|
LHO
Actual
(torr)
|
Initial LIGO
Requirement
(torr)
|
Adv. LIGO
Requirement
(torr)
|
Goal
(torr)
|
|
|
TBD
|
TBD
|
|
|
|
|
H2O
|
TBD
|
TBD
|
1 x 10-7
|
1 x 10-10
|
1 x 10-10
|
|
N2
|
TBD
|
TBD
|
6 x 10-8
|
6 x 10-11
|
6 x 10-11
|
|
CO
|
TBD
|
TBD
|
5 x 10-8
|
5 x 10-11
|
5 x 10-11
|
|
CO2
|
TBD
|
TBD
|
2 x 10-8
|
2 x 10-11
|
2 x 10-11
|
|
CH4
|
TBD
|
TBD
|
3 x 10-8
|
3 x 10-11
|
3 x 10-11
|
|
AMU 100 Hydrocarbon
|
TBD
|
TBD
|
7 x 10-10
|
2 x 10-12
|
7 x 10-13
|
|
AMU 300 Hydrocarbon
|
TBD
|
TBD
|
5 x 10-11
|
2.2 x 10-13
|
5 x 10-14
|
|
AMU 500 Hydrocarbon
|
TBD
|
TBD
|
1 x 10-11
|
9 x 10-14
|
1 x 10-14
|
- Optical
Loss Limits, Scatter and Absorption, Due to Deposition
Reference: TBW
|
Optical Loss
|
Initial LIGO
Requirement
(ppm/yr)
|
Adv. LIGO
Requirement
(ppm/yr)
|
|
Absorption
|
< 2 ¨
|
< 0.1
|
|
Scatter
|
< 10 ¨
|
< 10
|
¨E960022-B,
section 1.1 states < 0.5 ppm/yr/optic absorption and < 10 ppm/yr/optic
for initial LIGO.
Materials & Components Qualification
Processes
The basic reference for the procedures by which materials
and components are qualified for service in the LIGO vacuum system is:
E960022,
LIGO Vacuum Compatibility, Cleaning Methods and Qualification Procedures
Pending Changes for Rev. C:
- integrate
in the cleaning/air bake/FTIR sampling and evaluating that was performed
on the large parts for Initial LIGO
- put
in a qualified increase in the bake temperatures for 6061-T6 aluminum
- deal
with the issue of safety of ultrasonic cleaning with methanol
- deal
with the issues of safety of gross cleaning with acid, trichloroethane
and acetone
- switching
from a callout of "Ameristat poly sheet", an obsolete term, to
a generic description of an acceptable bag material
- section
6.1.1 on material exposure tests in a high irradiance optical cavity
refers to 150 kW/cm2 which is the initial LIGO test level. A
note to the effect that a higher level (value?) is now employed for adv.
LIGO qualification
- add
the high irradiance cavity test equipment/procedure reference, P990032
 Guidance on Qualification Material Quantity
& Preparation
For polymer materials, or other high rate outgassing
elements, which are to be tested for acceptability in the LIGO vacuum
system:
·
Materials must be prepared in accordance with their
proposed processing, i.e. it is not just the inherent material, but also any associated
surface treatment and chemical processing which must be qualified.
- Testing
composite material assemblies is much preferred over testing individual
materials (e.g. best to test a kapton, adhesive, copper flexible circuit
assembly than to test each material individually).
- For
RGA testing, the amount of material tested should be the larger of
either:
- ~
½ the intended amount in the vertex vacuum volume (single IFO), or
- ~
2 times the intended amount in the end station vacuum volume
The above suggested amounts
are based on scaling from test conditions to the LIGO in situ conditions assuming
that the RGA background is limited to 2e-11 torr-liters/sec, the RGA chamber
pump rate is 10 liters/sec and the target partial pressure for the
hydrocarbon sum mass is 6e-14 torr. See T04001 for allocated budget for a
more precise estimate.
- For
optical contamination exposure cavity testing, the surface area of the
material tested should be at least 0.1 times the surface area of the
amount intended in a LIGO vacuum volume, preferably 0.3 times this
amount (if size and costs allow).
The suggested amount above is based on the
measured uncertainty (standard deviation) in the rate of absorption change in
time, after one month of testing, 0.4 ppm/yr, compared to the requirement of
< 0.1 ppm/yr, accounting for the approximate test pump rate of 1/50th
of the LIGO pump rate.
Approved Materials and Components
List
The list of approved materials
for advanced LIGO is given in:
E960050,
LIGO Vacuum Compatible Materials List
A proposed form for documenting the results of the vacuum
qualification testing:
UHV
Preparation and Part QA
Every part which goes into the LIGO vacuum system must be
prepared (cleaned, handled and wrapped/packaged) in an approved manner. In
addition every part has some measure Quality Assurance (QA) such as process
travelers which include results from a vacuum cleanliness measurement such as
mass spectrometry (aka Residual Gas Assay, RGA) or Fourier Transform
Infra-Red (FTIR) results. The basic reference for the procedures for part UHV
preparation and QA is:
E960022,
LIGO Vacuum Compatibility, Cleaning Methods and Qualification Procedures
New Approaches to Achieve the Adv. LIGO Vacuum Requirements
Optical Contamination Exposure Cavity
The optical contamination cavity
testing appears capable of meeting Adv. LIGO requirements, with an the
increase in the irradiance and a longer run time. CHECK THIS STATEMENT!
Filtered Air Bake and FTIR
Testing for Large Components
Ideally all parts would be placed into a test
vacuum chamber and their outgassing rates directly measured. However for
large parts it is impractical to get large enough UHV bake chambers. In these
cases we employ air baking (with filtered air) and subsequent FTIR testing of
solvents which sample the surfaces of the part. However we have lost the
traceability on the FTIR testing that we had in Initial LIGO for the Beam
Tubes and for the Seismic Isolation System (SEI). We had established a
cross-correlation between RGA results and FTIR results. The FTIR testing
house has changed their equipment and a new cross-correlation factor must be
established. In addition, we have learned that sample preparation and FTIR
testing varies considerably between testing services. Larry Jones and Ken
Mailand had embarked on a "Cleaning Pathfinder" effort to establish
new methods and sources for large part cleaning in the So. California area.
This was motivated by the pending SEI prototype parts, which have been
delayed for other reasons.
Large, Uniform, High Temperature Vacuum Bake Oven
The current LIGO vacuum bake oven background
levels are to high to qualify to the Adv. LIGO requirement (see T040001).
In addition we have a large suspension structure, which cannot fit into the
current LIGO vacuum bake ovens. Given the proximity of this structure to the
optics, it seems prudent to plan for a vacuum bake and RGA measurement rather
than rely upon an air bake and FTIR test. Furthermore, we know that added
bake oven capacity is needed for AL. Mike Zucker and Larry Jones proposed an
alternative vacuum bake oven approach where the temperatures can be higher
and more uniform (no cold spots for contamination to condense; some notes from Mike
are here).
Larry Jones and Oddvar Spjeld have pursued this
alternative with industry to get cost and schedule information for a large
vacuum bake oven. The requirements are defined in the following documents:
·
E040392-02 for
the Chamber
·
E040393-02 for
the Oven
·
D040502-00,
Assembly
·
D040503-00,
Chamber
·
D040504-00,
Oven
Although the oven appears to be a close fit in the
D040502 drawing, the response from the oven manufacturers is to make it
larger, essentially a walk-in oven. The project has been proposed (see CR040017)
to the LIGO Configuration Control Board (CCB) and the Vacuum Review Board
(VRB). A decision is pending. (BTW this oven, if available, would be used for
the quadruple controls prototype suspension due to be UHV prepared in ~April,
2005)
Thermal Desorption Mass Spectrometry (TDMS)
Rai Weiss has proposed using Thermal Desorption
Mass Spectrometry (TDMS) techniques, wherein the surface is heated and the
evolved gas is collected and measured by a mass spectrometer. In principal
the crude (~0.1 torr) vacuum required could be generated locally and scanned
across surfaces of the part. Unfortunately this method would not easily allow
measurement of the outgassing from trapped holes, welded regions and complex
geometries – where most of the outgassing problems likely reside. More details here …
Extrapolating RGA Results
from Elevated Temperature Outgassing Measurements
Dennis Coyne determined the doubling temperature
for outgassing from some polyimides (see T040032). If
the chemical kinetics, or desorption dynamics, involved in the outgassing do
not change over modest temperatures (a single rate equation applies), then
one can measure the outgassing rate at a few higher temperatures and then
extrapolate to room temperature conditions. This might enable current LIGO
Lab vacuum bake ovens to meet the AL requirements. Thoughts?
|