Examination of RF Signal Processing for Cavity BPMs

Accelerators for charged particles, e.g. electrons, protons, ions, are used for fundamental physics research (high energy physics, HEP), applied science with synchrotron light, medical applications and many other research areas. Beside the fundamental technical components, i.e. an evacuated beam pipe, magnets providing the guide fields, and RF-driven resonators for the accelerating field, all accelerators and beam-lines require some beam instrumentation devices to quantify the characteristics of the beam, e.g. beam intensity, beam orbit, and emittance, as well as some fundamental machine parameters, like the so-called tune. The measurement of the beam orbit is based on beam position monitors (BPM), which are electromagnetic detectors with appropriate read-out electronics, distributed along the beam-line of the accelerator. While the basic function is the measurement of the beam orbit, based on the transverse displacement of the beam detected at the BPMs, it enables many additional measurement features, e.g. measurement of beam energy, chromaticity as well as fault finding and analysis. The operation principle of most BPMs is based on the detection of the image charge of the beam. In this thesis cavity BPMs are discussed. They take a different approach. The BPM pickups are resonant RF cavities, where the amplitude of an eigenmode excited by the beam depends on the beam position. Nowadays, only a few linear accelerator based free-electron laser (FEL) light sources, e.g.\ at DESY/XFEL (Hamburg, Germany) and at SLAC/LCLS (Stanford, USA) use cavity BPMs for routine operation. This type of BPM offers a resolution in the submicrometer regime for a single passage of the bunched beam. At the European Organization for Nuclear Research (CERN) in Switzerland the construction of a linear electron collider with a center of mass energy of up to 3$\,\mathrm{TeV}$ is object of several studies. The project is called Compact Linear Collider (CLIC). To achieve an acceleration gradient of 100$\,\mathrm{MV/m}$ a two-beam concept is proposed, where the so called drive beam is used to accelerate the main beam. The presented BPMs are planed for the main beam line of CLIC. The BPM pickups are designed to provide resolution better than 50$\,\mathrm{nm}$. A test setup with three of these CLIC cavity BPMs is installed at the CERN Linear Electron Accelerator for Research (CLEAR). Due to the availability of three BPMs the resolution of these BPMs can be obtained by comparing the position determined by one cavity with the ballistic trajectory defined by the two other BPMs. High resolution BPMs, which are able to determine the position of single bunches, could also be used to determine single particle properties. A particular interesting -- and challenging -- application is related to the measurement of the electron dipole moment (EDM). Here, one idea is to fill equidistant electron bunches in a storage ring, with the spin of every even (or odd) bunch flipped, while precisely monitoring the change of the beam position as asymmetry in the transverse beam spectrum in a magnetic field to access the electric dipole moment of electrons. This however would require one or more cavity BPMs operating in a ring accelerator, which is a major challenge as of the high beam coupling impedance of a cavity BPM that tends to cause beam instabilities. The focus of this thesis is an analysis of the cavity BPM read-out electronics, in particular the RF analogue system used to downconvert and postprocess the 15$\,\text{GHz}$ signal content generated by the bunched beam passing the cavity BPM pickups. Additionally, the operating principle of the CLIC cavity BPMs is presented and the performance of the setup located in the CLEAR tunnel is analyzed. For the presented beam measurements this setup was able to achieve a single bunch, single-pass resolution of 2.60$\,\mathrm{\mu m}$

Status of the CLEAR electron beam user facility at CERN

The CERN Linear Electron Accelerator for Research (CLEAR) has now finished its second year of operation, providing a testbed for new accelerator technologies and a versatile radiation source. Hosting a varied experimental program, this beamline provides a flexible test facility for users both internal and external to CERN, as well as being an excellent accelerator physics training ground. The energy can be varied between 60 and 220 MeV, bunch length between 1 and 4 ps, bunch charge in the range 10 pC to 2 nC, and number of bunches in the range 1 to 200, at a repetition rate of 0.8 to 10 Hz. The status of the facility with an overview of the recent experimental results is presented