Development of feedback algorithms for future linear colliders

Thesis

The Feedback On Nanosecond Timescales (FONT) intra-train feedback system has been designed to provide beam stabilisation at the interaction point (IP) of a future linear collider, such as the International Linear Collider (ILC). A prototype system based around cavity Beam Position Monitors (BPMs) is installed in the final focus system at the Accelerator Test Facility (ATF2) at KEK, and is designed to demonstrate nanometre-level stabilisation using low-latency beam-based feedback. One focus of this thesis is the development of this prototype system, including the optimisation of the BPM resolution and the improvement of the beam stabilisation performance. The feedback system was tested on trains of two bunches with a separation of 280 ns, for which the position of the first bunch was measured and the subsequent bunch was stabilised. The correction was implemented using a stripline kicker, with a custom power amplifier, and the feedback calculations were performed on a FONT5A digital board built around a Field Programmable Gate Array (FPGA).

Studies of the BPM resolution are presented, highlighting the importance of sample integration of the BPM waveforms in improving the resolution from ˜ 40 nm to ˜ 20 nm. Recent improvements to the FPGA firmware allow for the use of waveform integration during feedback operation. The feedback loop can be configured to include either input from a single BPM to provide local beam stabilisation, or to use signals from two BPMs to stabilise the beam at an intermediate location. Stabilisation to 50 nm and 41 nm have been demonstrated for 1-BPM and 2-BPM feedback respectively.

The ATF2 extraction-line feedback system has demonstrated the latency, resolution and correction range required for an ILC IP feedback system. Simulations were performed to show that a similar feedback system could be used to provide the required level of luminosity recovery and stabilisation for the ILC. The beam transport was modelled using PLACET and the beam-beam interactions were simulated in GUINEA-PIG. A bunch-by-bunch IP feedback system was modelled in Octave, for which various feedback algorithms were investigated. Ground motion, the jitter of the damping-ring extraction kicker and wakefields were modelled and preliminary studies suggested that the proposed feedback system could help achieve ˜ 95% of the design luminosity.

Authors: Ramjiawan, R

• DOI: 10.5287/ora-yxy59nnmx
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford