Poster excerpt 15: The Ginger Project and Status of the Ring-Laser at LNGS
(The text below was provided by A.Di Virgilio)
GINGER (Gyroscopes IN GEneral Relativity) is a project to measure the LenseThirring effect with a ground based detector, based on an array of ring-lasers. The LenseThirring effect, due to the fact that the Earth is rotating, implies a non reciprocal effect on the path of two beam of light, travelling inside a closed path. The ring-laser measures the absolute angular velocity, and as well is sensitive to the LenseThirring effect, so comparing the Earth angular velocity measured by IERS and the measurement done with the GINGER array is it possible to evaluate the Lense Thirring effect. GINGER is a proposal supported by the very high level of accuracy obtained by the GrossRing G of the laser ranging station of Wettzell, but it is already very clear what it is necessary to do in order to realise GINGER:
1)The Gross Ring is based on a monolithic structure which cannot be extended to form an array; it is necessary to develop a suitable strategy based on electronic feedbacks in order to enhance the stability of the ring-lasers based on mechanical structures. Not only the geometry of the ring must be controlled, but as well external metrology will be necessary to monitor the relative angles between rings and for diagnostics.
2)The installation of Wettzell has as well shown that superficial installations are not suitable for very high accuracy apparatus, since environmental disturbances, as rain, wind etc., usually spoil the resolution. Underground installation is a possible solution.
G-GranSasso is an experiment of the INFN Commission II, which is working in order to make GINGER a reality. Two different prototypes have been realised: GP-2, located inside the INFN Pisa section, is a prototype designed to test the control strategy, and GINGERino is a 3.6m side ring laser, built inside the LNGS underground laboratory, at the moment under commissioning. GINGERino will show if LNGS would be a suitable location for GINGER. In Figure GINGERino is shown, all our prototypes are based on 4 boxes containing the four mirrors, the boxes are connected by pipes, the whole volume is vacuum tight and filled with a mixture of Helium and two isotopes of Neon, between two mirrors is inserted the discharge to create the plasma and to generate the laser.
Our prototypes, especially GINGERino, are of importance for geodesy, since the Earth angular velocity is measured with high rate, and for geophysics, since they can measure the rotational component of tele seismic events coming from strong earthquakes. Seismometers and tilt meters are located around GINGERino, data are already acquired.
Our Poster is giving a pictorial view of GINGER and our prototypes, and the two test stand GP-2 and GINGERino are shown, very recent data of the tiltmeters, and a teleseismic events recorded by GP-2 are shown as well.
There are no conclusions about the measurements, it will be necessary to take few months of data, and the analysis will take time, but the poster gives a taste of what will be the outcome of this two prototypes.