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Title: Design and analysis techniques for cavity beam position monitor systems for electron accelerators
Author: Joshi, Nirav Yashvantray
Awarding Body: University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2013
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Beam position monitors are required in all accelerators for the measurement and optimization of the beam parameters. Cavity beam position monitors (CBPM) offer the possibility of measurement of beam centroid positions at the nanometer scale. These devices can be and typically are used at electron accelerator facilities, both existing light sources and test facilities proposed for future linear colliders, such as the International Linear Collider (ILC) and Compact Linear Collider (CLIC). The requirements for the CLIC main linac are to measure the beam position using approximately 5000 beam position monitors (BPM) with 50 nm resolution, at every 50 ns. The high resolution, enormous scale of the system and the small bunch separation of 0.5 ns present many challenges and demand innovative approaches for the design and operation of the CBPM system. A cylindrical cavity BPM system has been designed in collaboration with the Diamond Light Source, in the C-Band frequency region. The design ideas, which will be beneficial to CLIC BPM and other designs, such as the deliberate separation of modes coupled to the x and y position measurements and the cavity operation without mechanical tuning are tested in the design. The major resonance modes of the cavity are simulated using Eigenmode simulation. The coupling and isolation characteristics are simulated using S-parameter simulations, while the beam coupling is studied through time domain simulations. Four cavities were fabricated according to the design discussed in this this. Their coupling and isolation were tested through S-parameter measurements. The dipole modes are separated by more than 5 MHz in frequency. The values of the quality factors were measured using the impedance method. The field orientation of the dipole and quadrupole modes were measured using the bead-pull perturbation technique and found to be rotated by 12° and 30 from x-axis respectively. The initial beam studies were carried out at the Diamond Light Source and at the ATF2 beam line, and are presented in this thesis. The techniques for position determination of temporally closely spaced bunches are studied. A method was developed to remove the errors in the position determination, due to the overlap of the signals from the previous bunches, by subtracting the decayed phasors from the previous bunch. The method is applied to the signals from the CBPM system on the ATF2 beam line, in the two and three bunch mode operation. The overestimation in position determination of the second bunch is reduced from more than 67% to less than 2%. Position resolution of better than 3 um is demonstrated for the second bunch. The observed phase difference between the consecutive bunches is studied for different bunch spacing. The performance of the code is verified against simulated data.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available