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The Fringes are Back! Earthquake Repairs Completed at Hanford

The Fringes are Back! Earthquake Repairs Completed at Hanford

- Contributed by Fred Raab

In May, repairs of the damage wrought by the February 28 Nisqually Earthquake were completed. The magnitude 6.8 shaker on the opposite side of the Cascade Mountain range caused our suspended mirrors to move too far in their suspension cages, resulting in unbalanced mirrors and broken actuator attachments. We quickly marshalled spare components and mobilized people to respond to this setback. Within a week we were venting chambers for repairs and inspecting the optics component by component. Even as casualty mirrors were brought to the optics lab for the removing of broken attachments and re-cleaning, design began on adding improvements to the earthquake stops. The LIGO interferometers were designed to withstand a quake of this caliber more mightily than they did in practice. (After all the Caltech interferometer test bed in Southern California typically experienced severe earthquake activity every year or two.) A number of modifications were made to the stops and to the procedure for installing them, all of which should give us a most robust system when the next quake hits.

Pictured Below: At left, the fully-repaired optics within the HAM9 chamber at Hanford. At right, note the new sensor/actuator heads surrounding the mirror in BSC4 chamber.

Repaired optic in HAM9. New sensor/actuator heads in BSC4.

We also took advantage of this venting to correct some shortcomings in the initial design that had been uncovered in early commissioning work. The most important fix was to the sensor/actuator heads used to sense and damp small motions of the suspended mirrors. The shadow sensors in these heads pick up the shadow of a small magnet attached to the mirror. When the mirror moves, the sensor detects motion of the shadow and then our electronic damping system sends small currents to coils in the sensor head that attract or repel the magnet to damp the motion. Unfortunately, the original shadow sensors also picked up changes in scattered light when the mirrors misaligned slightly under intense illumination. This would be misinterpreted by the damping system and the mirror would be kicked out of resonance. Obviously this placed an unwanted limit on the amount of laser we could use in the interferometer. We continued our commissioning activities at low laser power while a new shadow sensor was designed, tested and produced in sufficient quantities to retrofit the interferometer. By the time of the Nisqually Earthquake we had managed to get just enough units to do the retrofit on the same vent as the earthquake repairs. The new sensor/actuator heads can be clearly seen (each has a wire entering the cylindrical head) surrounding the mirror in the photo at right above.

Concurrent with earthquake repairs on the two-kilometer interferometer (WA2K), teams worked steadily in neighboring vacuum chambers to complete the installation of four-kilometer interferometer (WA4K) parts in the corner station. The momentous pumpdown of the corner station vacuum chambers occurred before the end of May. We allowed enough time for water vapor pressure in the chambers to drop sufficiently so that we could briefly open the gate valves between the corner station and mid stations to verify whether our alignment of the repaired and newly-installed mirrors was on the mark. On June 8, Stan Whitcomb--the same man who surveyed the early earthquake damage--was able to quickly recover beams traversing both interferometers, and the welcome sound of fringes was once more heard in the control room at Hanford!

This brings us back to commissioning a WA2K interferometer with much better capabilities. One mirror more remains to be installed for the initial LIGO detector--the end mirror on the Y-arm of the WA4K interferometer. Work has already begun on commissioning the laser and input optics systems for WA4K. So commissioning will once again dominate Hanford Observatory activities.