Determination of High Precision Underground Equipotential Profiles for the Alignment of a Future Linear Collider

Abstract

The alignment challenge presented by the Compact Linear Collider (CLIC) project requires us to look closely at the ultra-high frequencies ( < 1 km) of the gravity field and our ability to model or determine underground equipotential profiles at a very high level of precision. This is of particular importance in the context of an alignment system based on Hydrostatic Levelling System (HLS) and other instruments dependent on gravity. In the first part of this paper, the theoretical formulation of the gravity field, Astronomical Levelling and a misalignment operator are presented. Then, the error propagation model of Astronomical Levelling is revisited and adapted to the specifications of accelerator alignment. Afterwards, numerical gravity field simulations, based on sinusoidal anomalies of varying geometry and density, give the first orders of magnitude of the signals in the equipotential and in the observation space which can be expected at ultra-high frequencies. Finally, a measurement campaign based around a tunnel, 850 m in length, at a depth of 80 m, that include deflections of the vertical and gravimetric measurements is presented.