CLIC Drive Beam Stabilisation Author: Alexander Gerbershagen Supervisors: Prof Philip Burrows (University of Oxford) Dr Daniel Schulte (CERN) What is CLIC Drive Beam? The Compact Linear Collider (CLIC) is a proposed multi-TeV e+e- collider being developed at CERN, which is based on two-beam technology. One of the beams, so called Drive Beam, with low energy but high current, is used to deliver RF power for the accelerating structures of the high energy, low current beam, called Main Beam. This scheme has been chosen, since the CLIC main beam frequency (~12 GHz) is too high for any conventional RF source. Fig. 1: Layout of the CLIC collider at 3 TeV How does Drive Beam recombination scheme work? The Drive Beam is produced in 240ns long trains with 180° phase shifted bunches at 0.5 GHz frequency. Acceleration power is provided by standard 1GHz RF source, so that all bunches are accelerated equally (Fig. 4). The frequency is then increased from 0.5 GHz to 12 GHz by recombining bunches in the delay loop And the combiner rings. The trains overlap during this recombination process and the errors from different trains come together in one train (Fig. 5). Why do we need to stabilize it? First, bunches pass the recombination scheme of the Drive Beam. Then the RF power is extracted from the Drive Beam in so called Power Extraction and Transfer Structures (PETS) and the RF waves are led from there to the accelerating structures of the Main Beam. The stability of the Drive Beam is a critical issue, since its jitters and instabilities would lead to unstable power supply to the main beam. Some tolerances (e.g. phase, gradient and energy error tolerance) are very strict. The filling time of the RF structures is ~ 60ns, hence we integrate the errors over intervals of same order of magnitude (10ns) to receive following distribution of error as function of frequency (Fig. 3). Fig. 2: Two beams scheme of CLIC Fig. 4: Layout of the CLIC RF complex Fig. 3: Charge error as a function of frequency Fig. 5: Train errors overlap How can we stabilise it? In order to avoid the main beam RF jitter one has to compensate the errors of the beam before the PETS e.g. with help of a feedback system. Dependent on jitter frequency, the feedback system can reduce or increase the error (Fig. 6). To optimise the total impact of feedback, we have to integrate error over frequencies and minimise the value as function of feedback latency and gain (Fig. 7). We realize that if t(latency) + t(gain) ≈ 240ns (which corresponds to the train length) the feedback is optimal. Conclusion: Cool, we can even reduce the white noise errors with a feedback system! This is in general not possible, but it becomes possible in this case due to recombination scheme. What is a feedback system? Ideal feedback system: 1. Measures error value 2. Compensates error on the next bunch For CLIC the bunch spacing (0.08 ns) is much smaller then the latency time of the feedback and gain time of the correction kicker (~ 10 ns). Hence, the correction will look like on Fig. 9. But if the jitter frequency is anti-resonant to feedback latency, the feedback system corrects the measured value too slowly, and only increases the error (Fig. 10). The standard feedback system used in our simulations, is called proportional-integral- derivative controller (PID controller). Fig. 10: Resonant (top) and anti-resonant (bottom) feedback Fig. 6: Charge error as a function of frequency (without and with feedback) Fig. 9: Example of feedback correction with long latency and gain time and small bunch spacing What will we do in the future? Include more realistic effects in the simulation Perform the simulations for possible future feedforward systems Use the results as input for the main beam studies Make predictions and test them at CLIC Test Facility (CTF3) Simulate usage of Feedback On Nanosecond Timescale (FONT) system Fig. 7: Current error integrated over frequencies as a function of latency and gain length Applying the feedback before the recombination and integrating over 10ns thereafter puts the error at the blue bunch and its correction on the red bunch in one interval, hence reducing the total error (Fig. 8). Special thanks for providing support and material: Prof Philip Burrows Dr Daniel Schulte Dr Franz Tecker Dr Guido Sterbini Dr Oleksiy Kononenko Fig. 8: Scheme of bunch recombination and integration over 10ns intervals For more information see: http://clic-study.web.cern.ch/clic-study/ http://www-pnp.physics.ox.ac.uk/~font/index.html