Prototype tests of a wide-band Synchrotron-Light based Beam Pattern Monitor for measuring charge-couple bunch instabilities in High-Energy Particle Accelerator Beams Sophie Dawson2,3, Mark Boland2,4, David Peake2,4, Roger Rassool2,3, Ralph Steinhagen1 1 Beam Instrumentation Group, CERN ACAS, Australian Collaboration for Accelerator Science 3 School of Physics, University of Melbourne 4 Accelerator Group, The Australian Synchrotron 2
Δp/p Motivation Head-Tail Oscillations T T T E ΔQ > 0 Tail H H H 2 s Head 1 ωs τ 20 ns/div transverse longitudinal ΔQ < 0 e.g. J. Gareyte, Head-Tail Type Instabilities in the PS and Booster, CERN, 1974 PS: 120 ns bunch length less demanding in terms of bandwidth SPS/LHC: bunch length down to 1 ns requires GHz analog bandwidth
Head-Tail Instabilities m=1 m=2 m=5 sum signal sum signal Simulated instabilities for SPS m=1 m=2 m=5
Electro-Magnetic Pickups BPCL.421, the 60cm stripline pickup used in SPS
Electromagnetic Pickups SPS and LHC both have striplines in place for transverse BPM measurements. Both experience signal drop-offs at ~6GHz, so there are limitations to the detection of higher order instability modes. Aiming to use synchrotron radiation to measure with bandwidths high enough to be able to view the higher order instabilities as well EM pickup draws power from beam, which can cause beam instabilities and is an issue w.r.t. signal processing Synchrotron light pickups use power which is in any case emitted by the beam. (Less/No impedance issues) S21: Σ & Δ Σ-Δ Insulation
ACAS
Australian Synchrotron beam lines x-ray sources A 3rd generation light source Accelerates and stores 3GeV electrons Beam stored in a storage ring 216m in circumference 8 beamlines in operation, capacity for 30+ Facilities are very accessible Users include medical researchers, biologists electron ring
ASLS Simulated power spectrums Visible light region Using visible light for BPM ASLS ODB beamline an accessible source of visible light to test before switching to optical fbre delivery system
Fill Pattern Monitor (FPM) Equipment The simplest experimental setup for FPM involves: - Synchrotron light (extracted from the beam pipe with a mirror) - A slit to select a given fraction of light - An MSM photodetector - A scope for viewing the photodetector signal
FPM The new FPM being built extends the behaviour of the current FPM at the ASLS. FPM is a longitudinal beam pattern monitor that measures and controls the top-up operation of the ASLS. D. Peake, M. Boland, R. Rassool, et al., Measurement of the real time fll-pattern at the Australian Synchrotron, NIMA, 2008
MSM Photodetector Photodetector housing is the size of an SMA connector. Interleaved metal contacts on a semiconductor base.
Photodetector Characteristics Hamamatsu G4096-03 (G7096-03) 30ps rise time Typical dark current of 100pA at 25 Maximum S/N ratio of ~150dB (lower with cooling) Active area = 0.2*0.2 mm2 We chose a commercial product for reasons of accessibility, cost effectiveness, robustness.
Alternative Photodetectors Y. Chen, S. Williamson, T.Brock et al., 375-GHz-bandwidth Photoconductive Detector, Appl. Phys. Lett. 59, 1991
Maintaining Bandwidth -1.5 db at 12GHz (0.5m semi-rigid cable). -3.2 db at 12GHz (0.5m semi-rigid cable and MiniCircuits bias tee).
Beam Position Measurement Principle I 2 I 1 = Beam position measured with X I 1 I 2 I1 I2 Quadrant of four detectors. 1 2 3 4 e.g. Vertical position = 1 2 3 4 PD1 PD2
Delta-Sum Detection Scheme MSM PDs in balanced-detection mode when bias voltages are opposed. +10V I1 Δ ~ I1-I2 Common-mode occurs when PDs are biased with identical voltages. Balanced-detection mode reduces noise. I2 DC bias voltage can be used to recentre steady-state beam signals. -10V/+10V Phase compensation through wideband optical delay lines (e.g. squeezing fbres, EO-crystals). Common mode is independent of phase of source.
Pros and Cons of Fibre Signal Transport + Each detector used in the system can be kept separate, so fabrication of all PDs onto a single die is not required. + Can place electronics in a location where radiation and EMC is less of an issue + Fibre transport bandwidth is much higher than RF cables + Optical path path is more fexible (don't need to deal with vibration-stabilised optics, lenses, mirrors...) - Signal degradation will still occur on the km scale Thus aim to keep fbre length short (~25-200m) - Coupling losses need to be addressed carefully
ASLS Experimental Setup
Optical Fibre Types Cisco, Fibre Types in Gigabit Optical Communications, 2008
Synch Light BPM Experimental Results of Prototype Tests 1. Light Levels is the light collected enough to work with a scope or other electronics? 2. Bandwidth Can we achieve a response with >12GHz bandwidth? 3. Do we have enough sensitivity to resolve small beam position changes?
Light Power Measurements CTF3, ASLS and LHC were measured and compared across a range of wavelengths. Beam spot much larger than PD active area. Peak power derived from detector sensitivity: ASLS: ~667uW (from measurements) CTF3: ~5mW (from measurements) LHC: ~1W (for protons) Fisher, A, LHC Perfomance Note-014, Expected Performance of the LHC Synchrotron-Light Telescope (BSRT) and Abort-Gap Monitor(BSRA), 2009 Courtesy of Jan Kovermann
Beam Size Measurements ASLS CTF3 FWHM: 2.03mm
ASLS and CTF3 Raw Signal Examples CTF3 ASLS Note: Lower CTF3 plot is a low bandwidth/integrated signal.
Bandwidth Tests Using a ps Laser Pulse Pulsed laser shone directly onto a PD with bias tee and amps connected. Direct digitisation using a fast sampler from Guzik. (13GHz BW, 40GS/s, 64GB buffer)
CTF3 Bunch Spectra Simulation Time Domain signal: Four different repeated bunch patterns Frequency domain signal: 1 batch signal has 3, 6, 9 and 12GHz components. 4 batch signal has only 12GHz component
Measured CTF3 Bunch Spectra FFT shows a similar relationship between the frst and fourth batches as in the simulation. FFT Also displays a bandwidth exceeding 12GHz Bandwidth opens up diagnostic possibilities such as: Fill Pattern Monitoring (as at ASLS): Shot-by-shot Combination effciency measurements
Beam Position Sensitivity To take direct bunch-by-bunch measurements, need to use a scope Scope measurements are limited by ADC resolution and noise Turn-by-turn oscillations using direct diode detection (similar to LHC's BBQ system) Turn-by-turn oscillations, comparison between MSM and BPM
Beam Position Sensitivity - ASLS
Beam Position Sensitivity - ASLS
Tune Shift Measurement - ASLS
Beam Position Sensitivity CTF3 Beam and PD stationary CTF3 Beam with moving PD
Summary Creating a robust and cost effective Beam Position Monitor that operates with a very high bandwidth to measure higher-order beam instabilities. This BPM uses synchrotron light naturally produced by circular electron and high-energy hadron accelerators during operation. Prototype BPM was used to: Measure beam signals with >12GHz bandwidth at ASLS and CTF3 Detect beam position and other properties with presently available light levels Detect changes in beam position Future Plans: Integration of coupling into a gradient index fbre to simplify the photon beam delivery to the MSM detector. Installation of system onto ASLS (CTF? LHC?).
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