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| Dissertation / PhD Thesis | GSI-2022-00795 |
2021
RWTH Aachen University
Please use a persistent id in citations: urn:nbn:de:101:1-2022052603254453552868 doi:10.18154/RWTH-2021-10393
Abstract: The JEDI collaboration is currently performing a direct measurement of the Electric Dipole Moment (EDM) of charged particles with beams of polarized protons and deuterons, using the COoler SYnchrotron (COSY) at the Forschungszentrum Jülich in Germany. For the precision EDM search, the JEDI collaboration is aiming for an experimental target with systematic errors of the same order of magnitude as the statistical sensitivity of10−29 e cm per year of data acquisition. The Magnetic Dipole Moment (MDM) is many orders of magnitude larger than the EDM. If the beam trajectory in the storage ring deviates from the nominal trajectory, this can cause the particles to move through a radial magnetic field. As a result of the interaction with the MDM, this leads to spin precessions which are indistinguishable from a true EDM signal. Therefore, it is important that the beam orbit be very close to the magnetic center ofthe focusing elements in the ring. The RMS deviation of the orbit from the nominal orbit should be as close to zero as possible. This means that the Beam Position Monitors(BPMs) must have a suitable sensitivity that meets the goals of the EDM experiment up to the desired accuracy level.In this work, a novel compact inductive BPM based on a segmented toroidal coil (Rogowski coil) has been developed. Various aspects of this Rogowski BPM as a non destructive monitor for measuring the transverse beam coordinates were investigated. The theoretical model describing the induced voltages of the Rogowski BPM was extended using the 'lumped model' approximation to obtain a more realistic description ofthe measurement of the induction signals. In this extended model, the coupling between individual quadrant coils was considered. Further theoretical studies were performed to investigate the effect of the angular overlap of the winding on the electrical response. Also, the sensitivity of the beam positions to the specifics of the windings of the segments was investigated. Electromagnetic simulations performed in COMSOL Multiphysics were used to investigate a simple 3D model for the Rogowski BPM. Simulations were performed in bothtime and frequency domains. Different boundary conditions were applied on individual quadrants to investigate the electrical properties of the coils. Another simplified model describing a single quadrant coil in the Rogowski BPM was developed assuming a 2Daxisymmetric geometry. In this analysis, the impedance and the inductance of the single coil were studied over a frequency range of 10MHz.Several hardware developments and improvements, starting from a simple ring coilup to the complete BPM with its feed throughs, flanges and connectors, were achieved. Developments on the laboratory test bench enabled more accurate calibrations. Thenewly developed cylindrical knife-edge structure was used to study the behavior ofthe stepper motor drives with a laser tracker, and absolute BPM calibration (with anaccuracy of a few micrometers) is possible after the BPMs are installed in the accelerator. Calibration measurements for the Rogowski BPM were performed in the test bench and compared with the theoretical model. A good agreement between the measurement results and the theoretical expectations was achieved. Various experimental investigations were carried out in the laboratory, such as theme asurement of the frequency response, the measurement of the position response, the measurement of the stability of the resonance curves, as well as the tuning of there sonance curves with shunt capacitors, the measurement of the time variations of the electrical signals, as well as the effect of the wave form of the excitation source on the BPM calibration, the possibility of using the BPM as a BCT, and the distribution of the Signal to Noise Ratio (SNR). In addition, the effect of the geometric specifications of the windings, the way the whole circuit is connected, and the system’s Self Resonant Frequency (SRF) on the position resolution at different frequencies were also investigated. Two Rogowski BPMs with different geometrical and electrical characteristics were developed and successfully installed and tested in COSY. They were installed at the entrance and exit of the Radio Frequency Wien Filter (RF WF). The results of local orbit bumps introduced around the RF WF showed reasonable and linear position responses measuredby the two BPMs. A resolution of up to 400 nm was achieved with the Rogowski BPMs (for a single position measured at 750 kHz during a sampling time of 1s, a system’s SRF of 3.229 MHz and aparticle intensity of 5 × 109). This corresponds to an order of magnitude improvementin resolution compared to typical capacitive BPMs used in COSY. Moreover, with thenewly proposed method for absolute instrument calibration, and taking into account the accuracy of the stepper drives used for calibration, as well as the errors caused by temporal drifts, the accuracy of the Rogowski BPMs is expected to be about 20 µm (errorof three standard deviations, corresponding to a beam off-centered by 1 cm), which constitutes an improvement by a factor of five compared to typical BPMs.
Keyword(s): BPM , Rogowski , inductive coupling , EDM , storage rings , JEDI
Note: Dissertation, RWTH Aachen University, 2021
Contributing Institute(s): Research Program(s): Experiment(s):
Appears in the scientific report 2022
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