Benjamin Kehrer

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-αc 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 micro-bunching 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 multi-bunch 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.

4-Channel Single Shot and Turn-by-Turn Spectral Measurements of Bursting CSR

The test facility and synchrotron radiation source ANKA at the Karlsruhe Institute of Technology (KIT) in Karlsruhe, Germany, can be operated in a short-pulse mode. Above a threshold current, the high charge density leads to microwave instabilities and the formation of sub-structures. These timevarying sub-structures on bunches of picosecond duration lead to the observation of bursting coherent synchrotron radiation (CSR) in the terahertz (THz) frequency range. The spectral information in this range contains valuable information about the bunch length, shape and sub-structures. We present recent measurements of a spectrometer setup that consists of 4 ultra-fast THz detectors, sensitive in different frequency bands, combined with the KAPTURE readout system developed at KIT for studies requiring high data throughput. This setup allows to record continuously the spectral information on a bunch-by-bunch and turn-by-turn basis. This contribution describes the potential of timeresolved spectral measurements of the short-bunch beam dynamics.

Influence of Filling Pattern Structure on Synchrotron Radiation Spectrum at ANKA

We present the effects of the filling pattern structure in multi-bunch mode on the beam spectrum. This effects can be seen by all detectors whose resolution is better than the RF frequency, ranging from stripline and Schottky measurements to high resolution synchrotron radiation measurements. Our heterodyne measurements of the emitted coherent synchrotron radiation at 270 GHz reveal discrete frequency harmonics around the 100 000th revolution harmonic of ANKA, the synchrotron radiation facility in Karlsruhe, Germany. Significant effects of bunch spacing, gaps between bunch trains and variations in individual bunch currents on the emitted CSR spectrum are described by theory and supported by observations. INTRODUCTION With the approximation that the signal of every revolution in a storage ring is the same, the beam spectrum can be written as the convolution of an infinite pulse train, separated by the revolution time T0, the filling pattern signal sF (t) and the single pulse signal sp(t) [1]: s(t) =XT0 (t) ∗ sF (t) ∗ sp(t), (1) where XT0 (t) denotes the Shah distribution [2]: XT0 (t) = ∞ ∑ n=−∞ δ(t − nT0) . (2) The filling pattern signal consists of dirac delta peaks at the position kTRF of the k-th bunch and height Vk corresponding to the bunch charge