Development of a magnetic measurement bench for insertion devices of a free electron laser

Abstract

Light sources such as free electron lasers and synchrotrons are very useful tools for the imaging of atomic sized structures and capturing fast processes. These sources use magnetic structures known as insertion devices to produce high brilliance X-Ray light. Measurement and precise adjustment of the magnetic fields produced in these insertion devices is critical to achieve high quality X-ray beams. These measurements are taken through the use of dedicated measurement systems known as magnetic measurement benches. One main feature of these measurement benches is the traversal of a measurement head such as a Hall probe through the centre of the magnetic field of interest. The system must ensure that the probe is kept perfectly stable to prevent any positional offsets during measurement from distorting the magnetic field profile obtained. Advances in insertion device technology have led to the production of invacuum insertion devices, where the magnetic structures are placed inside a vacuum chamber. The development of a measurement bench capable of taking precise measurements of insertion devices inside a vacuum chamber has thus become a design problem of high interest. In a collaborative project with the Paul Scherrer Institute in Switzerland, a new magnetic measurement bench for insertion devices is presented. The measurement bench integrates a Hall probe stabilisation system as well as a magnet pole optimisation instrument inside the vacuum chamber along with all motion control electronics. The probe is mounted to a base plate which is traversed longitudinally by means of a linear motor. Position stabilisation is achieved through the use of laser sources and position sensor devices which track the movement of the probe during measurement. Position corrections are made through the use of compact motors with absolute linear encoders which are controlled by a position feedback loop. Communications with a fixed station from which the bench is operated are all done wirelessly. The details of the design of this measurement bench as well as its performance are explained in detail in this dissertation. Encouraging results were achieved, both in terms of Hall probe stabilisation as well as in reproducibility of the magnetic field profile obtained by the bench.