LIGO Document T2200011-v2

LIGO optical absorption limits on hydrocarbon partial pressures

Document #:
LIGO-T2200011-v2
Document type:
T - Technical notes
Other Versions:
Abstract:
High molecular weight hydrocarbons (AMUs > 100) can be deleterious to the LIGO interferometer by either a) causing phase noise due to scattering if their density is high enough in the 4 km long arms, or by b) deposition onto sensitive optics resulting in increased optical loss by absorption and/or by scattering. The required (and goal) hydrocarbon partial pressures were established for the Advanced LIGO (aLIGO) project; see T040001. As a practical matter we have only been able to achieve a HC partial pressure of ~3 pTorr and (fortunately) we’ve found this quality of vacuum to be sufficient … so far. While this is close to the aLIGO required partial pressures As noted in the Advanced LIGO (aLIGO) Acceptance Document, the theoretical model connecting hydrocarbon (HC) partial pressure to an optical absorption rate increase due to a putative molecular film deposition, has not been experimentally confirmed. The limit on the partial pressure of high mass hydrocarbon (HC) partial pressure was speculated to be ~2 x 10^-15 Torr in order to limit the accumulation to 0.1 monolayer per year (albeit using conservative parameters; see E1000731). This memo attempts to: 1) provide some possible explanations for the apparent discrepancy between the speculated partial pressure limit for deposition onto sensitive optics and performance to date, 2) establish a modeling framework to use for establishing a revised HC partial pressure limit (for LIGO vacuum Review Board (VRB) approval), and 3) make recommendations for further study/work.

This analysis uses the LMA proprietary coating designs (ETM HR, ETM AR, ITM HR, ITM AR) together with estimates of contaminant complex refraction indices. This coating analysis uses a modified version of the Matlab code jreftran_rt.m which is provided in the *.zip file uploaded at C1000251. The calculated electric field for the ETM HR coating is also compared with the LMA calculation in the *-Efield compare.xlsx file uploaded at C1000251. Access to C1000251 is restricted since the coating designs are LMA proprietary.
The complex refractive index spectroscopy results from the referenced publications (available as supplementary material from the publishers) are plotted and compared in one of the Mathematica files. The source data files are uploaded here as *.zip files for convenience.
Authors:
Notes and Changes:
v2 includes an added section noting carbon growth on the mirrors of a LIGO optical resonator at 514 nm wavelength.
Referenced by:

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