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Advanced LIGO News

LIGO Hanford's H1 Achieves Two-Hour Full Lock

February 2015

LIGO Hanford's H1 detector began achieving lock stretches of more than two hours early in February 2015 with all of the detector's core control systems engaged. LIGO commissioners will now undertake a program of optimization and noise-hunting to move H1 forward in sensitivity as LIGO continues to plan for the first data run of the advanced detector era in Fall 2015, a run named O1 (Observation Run 1). The two hour benchmark is significant since it represents the performance threshold that permits acceptance of H1 -- the completion of the Advanced LIGO project in relation to this detector. Meanwhile the L1 detector at LIGO Livingston continues to progress, currently performing with an in-commissioning estimated sensing range of 60 Megaparsecs (nearly 200 million light years) for a signal from a neutron star inspiral. The phased approach to Advanced LIGO installation and testing called for L1 to be locked about six months before H1 while other detector work was underway at Hanford. Going forward LIGO will leverage the knowledge acquired from L1 commissioning as fully as possible to speed H1's progress on the path to O1.

During the second week of February commissioners moved H1 from a state of partial resonance to full locks of more than two hours that included the DC readout technique. After just one week with access to the full compliment of control signals that a locked detector provides, the team had reached a level of sensitivity that was roughly 40% as good as the best performance ever achieved by the first-generation H1. Much of the credit goes to the Advanced LIGO seismic isolation systems that are delivering superb stability of the beams inside the interferometer. Shown below are some members of the commissioning team on a typical late night in the LIGO Hanford (LHO) control room.

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The February 2015 successes at LHO came on the heels of two challenging months of work. On the evening of December 3, 2014 the commissioning team for the first time brought all of H1's optical cavities into resonance on infrared light from H1's pre-stabilized laser, the main laser system. This breakthrough came after several months of painstaking tuning of various alignment servos. As soon as H1 came under full control, it became apparent that the buildup of light power in the interferometer was less than half of the expected value on full resonance. In principle the deficit could arise from one or more of several sources -- excess absorption, excess scatter, beam misalignment, inadequate mode matching and more. Several experiments were undertaken to narrow the list of suspects.[CR] Evidence pointed to excess absorption, potentially because of surface contamination on Y end test mass. In mid-December both of H1's end station vacuum chambers were vented to allow a small team to enter these chambers to re-clean the mirror surfaces. Pump-down of these chambers then occurred over the winter holiday break and by mid-January commissioners again had access to the full interferometer. The work at the end stations was greatly aided by the presence of high-resolution cameras that peer at the end test masses through viewports in the vacuum chambers. These cameras will play an important role in the Advanced LIGO photon calibrator system (pcal) by photographically tracking the tiny pcal laser beams on the test mass surfaces. In the absorption circumstance, the pcal cameras were of great help in augmenting data from optical cavity measurements to identify and mitigate the excess absorption. The photo above shows a pcal camera view of an end test mass that's reflecting the green laser light used by the arm length stabilization system to bring the long arms of the detector under servo control.

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The photo above shows commissioners who worked in the LHO control room on the night of December 3. This group is a subset of a larger team consisting of individuals in the LIGO laboratories at Caltech, MIT, both observatory locations and at several institutions across the LIGO Scientific Collaboration.

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