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Hanford Observatory

Mirror Change at LIGO Hanford

- Contributed by Dale Ingram

Recent commissioning work at LIGO Hanford Observatory has included the replacement of an H1 large optic. Nineteen staff members and visitors executed a precise and highly choreographed replacement procedure on June 29, minimizing the exposure of the pristine interior of the vacuum while precisely aligning the incoming mirror to match the position and angle of its outgoing counterpart.
[tools_1] Members of the vent team transfer
sterilized tools outside the vacuum chamber
while inside a mobile clean room

LIGO scientists realized the need to replace the mirror, ITMX, by noting the interferometer’s thermal response to increasingly higher laser powers employed over the past year. High light power is desirable to minimize shot noise, but higher power also increases the energy absorption in the mirrors, producing greater changes in the shape of the mirror face. Detector mirrors were fabricated with slight curvatures to offset these thermally-induced curvatures. H1 should perform optimally at 6 watts of input laser power as the design curvatures and thermally induced curvatures balance each other’s effects. However several measurements indicated that the detector’s performance was peaking at 2.4 watts, suggesting the possibility of excess absorption.

Two groups of scientists undertook independent investigations in the spring of 2005 to pinpoint the source of the absorbtion. The two methods differed in detail but shared the general strategy of evaluating an aspect of the interferometer’s output that depended on mirror curvature (hence absorption) as the instrument transitioned through several thermal states. The results converged to suggest abnormal absorption in ITMX with some excess in ITMY as well.
A portion of a mirror face. The copper coil is part of the control system. The screw has a viton earthquake stop at its tip to protect the mirror against extreme motions (more than about 0.5 micron)
Staff members worked through a six-page checklist of staging activities in advance of the vent. Gate valves came down on the morning of June 29 to isolate the vacuum region where the replacement would occur. Admission of purge air brought this region to a normal atmosphere, at which time the chamber door was removed. Technicians inside the chamber carefully assessed the resident mirror’s alignment as control room personnel monitored the outputs of position sensors on the suspension cage. The mirror’s earthquake stops were tightened to prevent excess motion as the cage was ushered out of the chamber. Installation of the replacement followed, and careful alignments restored it to a satisfactory operating position. The door went back on the chamber about 18 hours after the start of the vent, and the long pump-down of the enclosure began (to be completed five weeks later). The vent team then departed, many having been on site for the entire procedure.
LIGO Livingston supplied two staff members who entered a second chamber to clean the surface of its optic, ITMY. The mirror procedures should help LIGO meet its H1 inspiral range goal of 30 million light years during the upcoming S5 Science Run, scheduled to start in the fall of 2005.
[prep_1] [door_1]
Preparing for the removal of the chamber door. A dust monitor on the floor lets the team know when the particle count is low enough for door removal One of the two doors that were removed
on 6/29 with its dust cover
[switch_4] [switch_5]
Removing the baffle from the face of the mirror suspension. The suspension cage is visible inside the open chamber (upper left portion of photo) The resident mirror is on its way out of the chamber
onto a cart at the center of the photo
[switch_7] [switch_6]
Removing hardware from the cage that has just come out of the chamber The mirror is visible in the large opening at the bottom of the suspension cage
[new_optic] [mirror_face]
Meanwhile the replacement ITMX rests under a shroud, waiting for installation. A close up of the mirror face. The mirror is 25 cm wide, 10 cm thick and has a mass of 10 kg. It touches only its suspension wire inside the cage and is controlled by the combination of tiny magnets glued to its face along with the copper coils that are visible in the photo
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