Interferometer_sensitivity_studies (General)swinkels - 20:30 Tuesday 01 April 2008 (20239)
Print this reportMagnetic line injection, LIGO styleToday, I did some magnetic line injections in the way they do at LIGO. The idea is not to put the coil close the mirror and then use a dipole model to calculate the injected field, but to put the coil as far away as possible and have the magnetometers as close as possible to the mirrors. In this way, the magnetometers see the same field as the mirrors and you can directly measure a transfer function between magnetometers and dark fringe. The obvious disadvantage of placing the coil further away is that the injected fields are several orders of magnitude lower.
In the central building, the coil was placed in the middle of the balcony on top of the DAQ room. Note that, in this way, you inject a field on all mirrors at the same time. At the NE, the coil was placed in the north-east corner. Lines were visible in the dark fringe below 70 Hz. At higher frequencies, the lines were injected for 10 minutes, so it might still be possible to retrieve something from the data.
At the NE, the magnetometers are normally placed against the wall. I moved them about 2 meter next to the tower (Em_MABDNE02 was probably saturating). Closer was not possible due to the length of the cables. I placed them back after the measurement. The preferred configuration would be next to the towers, but this would require some work on the cables and has a danger of saturating at 50 Hz. In the central building, the ideal location for the magnetometers would be in the corner between NI and WI, since those are the most sensitive.