Fast mapping of terahertz bursting thresholds and characteristics at synchrotron light sources

Dedicated optics with extremely short electron bunches enable synchrotron light sources to generate intense coherent THz radiation. The high degree of spatial compression in this so-called low-${\alpha }_c$ optics entails a complex longitudinal dynamics of the electron bunches, which can be probed studying the fluctuations in the emitted terahertz radiation caused by the microbunching instability (“bursting”). This article presents a “quasi-instantaneous” method for measuring the bursting characteristics by simultaneously collecting and evaluating the information from all bunches in a multibunch fill, reducing the measurement time from hours to seconds. This speed-up allows systematic studies of the bursting characteristics for various accelerator settings within a single fill of the machine, enabling a comprehensive comparison of the measured bursting thresholds with theoretical predictions by the bunched-beam theory. This paper introduces the method and presents first results obtained at the ANKA synchrotron radiation facility.

Figure 1

The fluctuating THz signal (color coded) of each of the 184 rf-buckets is shown for the first 130 thousand consecutive turns, out of the total recorded of 2.7 million in one second. The bursting behavior differs for bunches with different currents. On the right-hand side the filling pattern, consisting of three trains, is indicated by the bunch current (adapted from [22]).

Figure 2

THz signal (color coded) of one bunch as a function of turn number during the decrease of the bunch current. Both the temporal evolution and the repetition rate of an outburst show a strong dependence on bunch current. For better visibility the THz signals were shifted in time so that the first bursts are aligned.

Figure 3

The spectrogram shown in (a) was obtained from measurements of a single bunch for slowly changing beam current lasting two and a half hours, while the spectrogram in (b) was obtained from a snapshot measurement lasting just one second by using the FFT of THz signals similar to those shown in Fig. 1. For the snapshot measurement the bunch current resolution is limited by the number of bunches and their current distribution. Compared to this, the standard measurement during a slow current decrease results in a higher current resolution. Despite the limited resolution of the snapshot spectrogram, the dominant bursting frequencies and the thresholds between different bursting regimes are clearly visible.

Figure 4

Standard deviation of the THz signal of each bunch as a function of the corresponding bunch current, revealing the onset of bursting emission (“bursting threshold”) at 0.2 mA. The individual bunch currents (shown on the right) were patterned to equally sample the whole bunch current range (adapted from [22]).

Figure 5

Standard deviation of the THz signal as a function of the bunch current for a constant rf-voltage of 1047 kV and different settings of the magnet optics, resulting in different synchrotron frequencies and hence different bursting thresholds (adapted from [24]).

Figure 6

Measured bursting thresholds from three sets of measurements as a function of the synchrotron frequency. Here “combined sweep (2)” refers to the same measurements taken at 11 months apart from “combined sweep (1).” For “combined sweep (1),” measurement with the same magnet optics are connected by dashed lines. The different colors indicate the different acceleration voltages. The horizontal error bars indicate the systematic error on the measured synchrotron frequency. The vertical error bars include the error of the algorithm for the automated threshold detection, the bunch current measurement as well as the expected spread of the threshold due to multibunch effects. The solid lines show the theoretical expectation according to Eq. (2) from the bunched-beam theory. The dotted lines indicate the one standard deviation uncertainties on the theoretical predictions. Within uncertainties all measurements agree with the bunched-beam theory.