LIGO's Commissioning Break Ends, O3 Resumes
News Release • November 4, 2019
On November 1st at 15:00 UTC, the LIGO and Virgo gravitational wave detectors resumed their search for gravitational waves after taking a planned month-long break to perform some maintenance and upgrades. All three sites halted operations for the entire month of October (read LIGOs Commissioning Break Commences for information on some of the bigger projects that were planned at LIGO's Livingston and Hanford locations).
Staff, technicians, and engineers at both LIGO sites were extremely busy during the break. Combined, over 500 tasks were completed at Hanford and Livingston by October 31st! Two items mentioned in the pre-break story, namely LHO's wind fences and the leak search at Livingston, are still ongoing. Meanwhile at Virgo, engineers focused on increasing the laser input power from 19 W to 26 W, a task that required a complete re-tuning of the interferometer. Effort was also devoted to studying selected noise sources. Lessons learned from that study will improve Virgo's future operation. Virgo was also able to find the sources of some limiting noises and remove a few of them. To learn more about what Virgo accomplished in October, visit Virgo's story on their commissioning-break work.
The extent to which this work has improved the instruments’ sensitivities will be known in the weeks to come. The second half of LIGO's third observing run will conclude on April 30th, 2020.
Below are some photos taken of the work performed at all three facilities.
LIGO Livingston completed over 300 tasks, small and large, during the break. Quite a feat! Among those, three of the biggest were inspecting and cleaning the mirrors at the ends of LIGO's arms ("test masses"), installing a metal shroud around a device that is used to help LIGO operators lock the interferometer, and installing light baffles of various shapes and sizes in several locations to eliminate scattered light. Livingston engineers were also tasked with finding and sealing a small leak in one of its vacuum tubes; that search continues. Below are a few pictures showing some of the bigger projects that were completed.
Test Mass Inspection and Cleaning
LIGO Livingston inspected and cleaned the mirrors (test masses) at the end of each arm. It is delicate, meticulous work, made all the more difficult by the fact that engineers need to wear ‘full bunny suits’ to work inside the chambers. The bunny suits make sure that any dust or skin cells or hair or other bits of stuff that humans are constantly shedding doesn’t contaminate the inside of the chamber or stick to the mirrors themselves (something that could have disastrous consequences if LIGO's powerful infrared laser were to vaporize such a mote of dust).
Inspecting the optic revealed one point absorber in the coating on the mirror. GariLynn Billingsley, LIGO’s optics expert, explained why it is important to find them,
“[A point absorber] heats up in the presence of the laser beam. As you know, things get bigger when they heat up, so this causes a little mountain on our otherwise perfectly flat optic. When we get mountains on the optic, the interferometer beam resonates differently and causes excess loss [of photons] in the arms…thus making the detector less sensitive.”
Unfortunately, in this case, there is not a lot LLO can do to fix the situation, but knowing where they are is still important. It will also help LIGO and its coating vendors better-understand how to prevent the occurrence of such features in the future.
Shroud Installation and Light Baffles to Reduce Scattered Light
LIGO makes detections by examining the interference pattern generated by merging the laser beams coming from each arm. We use the laser beam like a ruler, telling us exactly how long the arms are at any given moment. This is why LIGO's laser must be incredibly stable (emitting one wavelength of light at all times) and that the waves of light comprising the beams in each arm remain properly oriented with respect to one another (their ‘phase’). If the light changed in any way, the resulting interference pattern could mask a gravitational wave.
Shroud Around TMS
When LIGO Livingston discovered that there was some stray light bouncing around inside one of the BSCs housing the Transmission Monitor System, or TMS, they knew they had to do something about it. The TMS helps LIGO operators lock the interferometer so the instrument can sense gravitational-wave signals. A green laser is used during this locking process, but LIGO's invisible main near infrared laser beam also travels through this device. Since the IR beam is invisible, the scattered green light indicates where the IR beam is also scattering inside the chamber. A problem arises when light (even infrared) bouncing off the walls of the chamber makes its way back into the main-beam path. This is because the chamber itself is not isolated from ground vibrations, so IR light reflecting off that 'vibrating' surface that makes its way back into the main-beam path will carry those vibrations along with them. These vibrations show up as noise in LIGO's output signal and can sigificanly decrease LIGO's ability to detect gravitational-wave vibrations. To remedy this problem, LIGO Livingston installed a shroud around one of their TMS devices to keep the light from scattering off the walls of the chamber.
LIGO Livingston also installed stray light baffles in several different places inside the instrument, all designed to eliminate scattered infrared laser light, which degrades LIGO's ability to detect gravitational waves. The photos below show a large baffle panel being installed on the photon-calibrator (P-Cal) periscope in one of the vacuum tubes. The frame of the periscope is made of shiny metal, and it tends to move when the ground underneath the vacuum tube moves. Any scattered light bouncing from the surfaces of the periscope could also make its way into the path of LIGO's main laser beam. Just like the light bouncing from the chamber walls near the TMS, scattered light from the P-Cal periscope could transmit those vibrations into the instrument, and reduce our sensitivity.
Alena Ananyeva, who has led the baffle production effort and who helped install these baffles, explains how these baffles solve the problem of scattered light:
"The P-Cal baffles are made from a super-polished stainless steel coated with diamond-like carbon, an absorptive black coating. One can call such a baffle a black mirror. Imagine some scattered light comes in contact with one of these black mirrors. The black coating absorbs a significant fraction of infrared scattered light. Additionally, the baffles are formed or/and tilted in order to guide any light that is not absorbed by the black coating away from the test mass. The super polished surface of the baffles is essential to control direction of the reflection.”
LIGO Hanford completed over 200 individual tasks, small and large, during the October break. Among those, some of the biggest were replacing some aging vacuum equipment, amputating an entire segment of the vacuum system that was not in use, and replacing a window between two HAM chambers that was scattering the laser beam that passed through it. LHO also replaced a fiber-optic cable in its light squeezer device, and began construction of two wind fences, one at the end of each arm. Below are a few pictures showing some the work that was done during the break.
New Turbomolecular Pump
Since LIGO's vacuum system is so critical, it is important that all the systems designed to keep LIGO's vacuum as pristine and secure as it is are maintained at high levels. At LHO, that meant replacing some ageing turbomolecular pumps (see photo at right). At 20-years old, the pumps had reached the end of their expected lifetimes, so replacements were ordered to make sure that LIGO Hanford's vacuum system will be secure for another 20 years.
Also at LIGO Hanford, a large part of an unused portion of the vacuum system, called H2 (which was originally expected to house a second interferometer which will instead be sent to India) was separated from the main vacuum system. Engineers removed two 'expansion spools', which connected the arms of H2 to BSC7 and BSC8 (see diagram) to the extra arm segments. BSC7 and 8 then had solid 'flat-plate' doors installed to seal those chambers. When O3 ends, the individual HAM chambers and the BSC chamber will be completely separated from the tube segments and repurposed at LLO and LHO.
Septum Window Replacement
One additional task that LHO engineers weren’t sure they’d get to was replacing a ‘septum window’ between two HAM chambers (HAM 5 and HAM 6). LIGO’s laser beam passes through this window, and the (now) old one was scattering light, sending photons off in different directions. To eliminate this source of noise, LHO decided to replace the entire window assembly. The replacement went smoothly and the new window is scattering much less light than the old one. This should help to improve LHO’s overall sensitivity. Note in the images how shiny the metal interiors of the HAM chambers are. These shiny surfaces can be problematic if light reflects off of them. So reducing scattering anywhere along the laser’s path is important to LIGO’s ability to detect gravitational waves.