21The design, realization and operation of a prototype or "demonstrator" version of an active target and time projection chamber (ACTAR TPC) for experiments in nuclear physics is presented in detail. The heart of the detection system features a micromegas gas amplifier coupled to a high-density pixelated pad plane with square pad sizes of 2×2 mm 2 . The detector has been thoroughly tested with several different gas mixtures over a wide range of pressures and using a variety of sources of ionizing radiation including laser light, an α-particle source and heavy-ion beams of 24 Mg and 58 Ni accelerated to energies of 4.0 MeV/u. Results from these tests and characterization of the detector response over a wide range of operating conditions will be described. These developments have served as the basis for the design of a larger detection system that is presently under construction.1
The tolerance in the alignment of the quadrupoles in the linacs and in the undulator lines of the Swiss Free Electron Laser (SwissFEL), the next project at the Paul Scherrer Institute (PSI), will be about 1 . This accuracy will be reached using beam alignment techniques. To minimize the commissioning time, it is also requested to install the quadrupoles with a precision of . To achieve this goal, the vibrating wire technique will be used in association with other systems to determine the position of the magnetic axis of the quadrupoles with respect to external fiducials. After a general introduction to the project and to the instrumentation principle, the novel approach developed at PSI is presented. The main innovation consists in the use of a phase lock loop (PLL) for maintaining the wire in its resonance condition. This approach simplifies the operation of the system in case the wire is moved or replaced and increases substantially the reliability of the measurement outcomes in the frame of a series test campaign.Index Terms-Magnetic axis determination, magnetic measurements, phase lock loop, single stretched wire, vibrating wire technique.
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