L1 Interferometer Sets New Performance Standard
LIGO Livingston’s L1 interferometer now reaches beyond 45 million light years in the search for gravitational waves from inspiraling neutron stars, a sensing range that exceeds its previous best by more than 15%. This recent improvement marks the plus side of a bad news-good news story and brings L1 alongside Hanford’s H1 in sensitivity as the S5 science run continues.
The bad news began on the morning of May 2 when LLO operators realized that one of L1’s 10-kg mirrors, ITMY (above left), had become immobilized on its suspension mount, rendering the interferometer uncontrollable. A large seismic impulse apparently lodged the mirror on several of its viton-tipped earthquake stops. Staff members began a series of interventions from the control room that became more serious over the following two days as the mirror refused to swing free. On May 5 the method of last resort was employed – a full vent of the chamber, removal of the chamber door and entry into the chamber by garbed personnel (above right) who manually dislodged the mirror. Following chamber closure and a 72-hour pump down, the gate valves that isolated the vented region were re-opened and interferometer operations were restored.
Upon returning to science operations the interferometer immediately manifested a sharp improvement in sensitivity that corresponded to a drop in the noise floor between 40 and 100 Hz. How did the noise reduction occur? One hypothesis suggests that the sticking and dislodging of ITMY, along with the resetting of several of its earthquake stops, may have changed the static electric fields between the mirror and the stops, removing a subtle source of dissipation. The photo at left shows three of the stops and a through-mirror view of one of the control system’s voice coils.
L1’s sensitivity increase followed two weeks of early April commissioning. The commissioning team produced significant improvements in the angular control of L1’s optics, yielding a more stable and robust interferometer that holds lock for longer periods and maintains a more consistent sensing range from one minute to the next. Project scientists and engineers are now pushing to improve the duty factors of all three interferometers to maximize LIGO’s “triple coincidence” observing time as the S5 run moves forward.
Last modified June 29, 2006
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