Time-Correlated Single-Photon Counting for versatile longitudinal diagnostics at the MAX IV Laboratory storage rings

TL;DR

The paper demonstrates how Time-Correlated Single-Photon Counting (TCSPC) can be used as a precise, non-destructive diagnostic for both filling patterns and longitudinal bunch profiles in MAX IV storage rings. It details two TCSPC setups with diverse detectors (PMA, SPAD, and PMA Hybrid) and a Tango-based live analysis, enabling per-bunch monitoring of bunch length, profiles, and phase. A core contribution is a deconvolution-based method to remove system transit-time spread (TTS) by determining a system TTS from low-current Gaussian references and applying it to recover true intra-bunch distributions, validated against streak-camera measurements. The approach yields robust, continuous monitoring for standard operation and accelerator-physics studies, including inhomogeneous fills and Landau-cavity effects, and complements streak cameras for fourth-generation light sources.

Abstract

Precise diagnostic on the electron beam parameters is a very valuable tool and essential in the operation of synchrotron light sources. One possible option is to employ the emitted synchrotron radiation for non-destructive measurements. A tool, which has been used in many ring based synchrotron light sources is Time-Correlated Single-Photon Counting (TCSPC). It allows to measure the arrival time distribution of the emitted photons and by that reveals the filling pattern, i.e., the charge distribution onto the electron bunches stored in the storage ring. At MAX IV, two TCSPC setups were installed and the analysis was developed further to also allow for the measurement of the longitudinal profiles of the individual bunches. The analysis is available as a Tango device in the accelerator control system and continuously provides, for example, the bunch length of each bunch as well as the bunch profiles and phases. This improved the diagnostic capabilities significantly, for example, in the presence of Landau cavities, which are becoming increasingly more common in new fourth-generation synchrotron light sources.

Figures (15)

• Figure 1: Example measurement of time-correlated single-photon counting (TCSPC) at the 1.5 GeV storage ring. The histogram shows the correlation between arrival time and number of detected synchrotron light photons from the 32 stored electron bunches.
• Figure 2: TCSPC setup shown as sketch (left) including all the timing and jitter contributions and a photo of the optical setup (right) at the 1.5 GeV ring visible light diagnostic beamline showing a filter wheel with neutral density (ND) filters and the PMA and the SPAD detectors.
• Figure 3: Filling patterns measured with the SPAD detector (without additional focusing onto the detector) without (blue) and with (orange and green) tune excitation in different individual bunches show a temporary and reversible reduction by about 10% for the excited bunch.
• Figure 4: Raw histograms of a short (39 ps rms) Gaussian bunch measured at the 3 GeV ring showing the differences in the transit-time spreads caused by the three different detector types Photomultiplier Assembly (PMA), Single-Photon Avalanche Diode (SPAD), and Hybrid-Photomultiplier Assembly (HYB).
• Figure 5: Intrinsic jitter from the PicoHarp 300 and PicoHarp 330 measured by splitting the external trigger to the trigger input and one input channel. The time resolution is the highest possible (PH300: 4 ps, PH330: 1 ps).