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Special Report on Virgo

First VIRGO Engineering Run at Cascina

First VIRGO Engineering Run at Cascina

- Contributed by Raffaele Flaminio

On Friday, September 21, there was a lot of excitement in the VIRGO control room here at Cascina, Italy (notice the happy faces abounding in Figure 1 below). The first VIRGO engineering run was about to begin, and would encompass three days of continuous data taking. This was a major milestone in the commissioning of the Central Interferometer, and an important step in the construction of the VIRGO detector.

Figure 1.

This first phase of the VIRGO commissioning began at Cascina last April. During this time, the Fabry-Perot input mirrors are substituted by totally reflecting mirrors, and the so-called Central InTerFerometer (CITF) is operated in the recycled Michelson configuration.

In this phase of commissioning, the CITF is operated in the simple Michelson configuration (shown in schematic outline in Figure 2 below). The interferometer uses a low power Nd:YAG laser as light source since the final laser is still being commissioned. The input beam as well as all the interferometer mirrors and optical benches were aligned during the spring. The interferometer was locked on the dark fringe for the first time in June 2001. Locking was acquired and maintained by controlling the optics position using electro-magnetic actuators suspended behind the mirrors and attached to the same seismic isolator from which the mirrors are suspended. Achieving lock was an important achievement for the fully digital control architecture used in VIRGO.

Figure 2.

The goal of the run on 21-23 September was to collect a "long" stream of data with the interferometer operated in a well-defined configuration. Then the data would be used to help the detector characterization effort and to start a real data analysis activity.

The run was a big success. The interferometer was in operation for three days, 24 hours a day. More than 30 people, representing most of the VIRGO labs, spent a part of their weekend on shift at Cascina.

Most of the software for interferometer control and data collection performed well. The interferometer was kept locked on the dark fringe for most of the time. Only four losses of lock occurred between Friday evening and Saturday morning (and three of these were generated by human activity). Starting from Saturday morning, the Michelson interferometer was kept locked on the dark fringe until the end of the run for more than 51 hours later, indicated by the chart in Figure 3 at left below. The end of the run was a good occasion for opening a few bottles of excellent French champagne, as can be seen in Figure 4 at right.

Figure 3. Figure 4.

More than 1 TB of raw data was acquired. Trend data were produced online. Post-processing to produce 50 Hz rate frames has just been completed. Several investigations using this data are ongoing. Several strange (and thus interesting) behaviours have been detected and are being studied.

More information about the engineering run can be found at http://wwwcascina.virgo.infn.it/commissioning/E0. Additional information on the CITF commissioning is also available from http://wwwcascina.virgo.infn.it/commissioning.