Simultaneous Detection of Longitudinal and Transverse Bunch Signals at Anka

The ANKA storage ring offers different operation modes including the short-bunch mode with bunch lengths tuned down to a few picoseconds. This can lead to the occurrence of micro-bunching instabilities coupled to the emission of coherent synchrotron radiation (CSR) in so-called ’bursts’. To study this CSR instability we use several turn-by-turn enabled detector systems to synchronously measure both the THz signal as well as bunch profiles. The different detectors are placed at different locations around the storage ring. Here we discuss the experimental setup and calibration of the various systems’ synchronisation. OVERVIEW During the short bunch mode offered at the ANKA storage ring the momentum-compaction factor αc is lowered and thus the bunch length is reduced as well (’squeeze’). This is also coupled to the emission of coherent synchrotron radiation (CSR) and the occurrence of micro-bunching instabilities as long as the bunch current is above the instability threshold. The study of the CSR sets some requirements to the diagnostics: The response time and repetition rate must be sufficient to study a single bunch in a multi-bunch environment on a single-turn base. The acquisition time range should be long enough to cover the relevant time scales (up to some milliseconds). During the last years several detector systems have been installed and commissioned. They recently have been enabled to measure the bunch signatures synchronously. Here we focus on three different systems: A fast-gated intensified camera (FGC) to measure the horizontal bunch profile, the KAPTURE system to record signals from fast THz detectors and the electro-optical bunch profile monitor (EOSD) to record the longitudinal bunch profile. Fast-gated Intensified Camera (FGC) The fast gated camera setup [1] that is used to monitor the horizontal bunch profile is located at the visible light diagnostics port [2]. It consists of a fast-gated intensified camera and a galvanometric mirror. This mirror is used to sweep the incoming incoherent synchrotron radiation over the CCD sensor before it is read out. The image intensifier in front of the CCD sensor is gated and thus allows to pick one bunch out of a multi-bunch fill. In case of ANKA the repetition rate of the gate allows to monitor up to every 6th turn. The voltage ramp for the mirror driver and the corresponding trigger signals for the camera are controlled ∗ [email protected] † now at PSI, Villigen, Switzerland ‡ now at DESY, Hamburg, Germany Figure 1: Sample picture from the fast-gated intensified camera (FGC). Top: raw data showing images of the same bunch for every 14th turn. Middle: centroid positions from 2D Gaussian fits plotted as a function of turns showing a synchrotron oscillation with f s (synchrotron frequency). Bottom: Corresponding spot size versus time. The turn and time axes have their origin at the first revolution trigger after the measurement trigger. via an arbitrary waveform generator. With this setup it is possible to place up to 65 spots (each spot corresponds to one single turn) on the sensor. The raw data from the CCD is processed by applying a 2D Gaussian fit to each spot to determine the horizontal bunch position and size, a sample picture is shown in Fig. 1. KAPTURE The KAPTURE system is a FPGA-based read-out system to sample fast (THz-) detectors on a turn-by-turn and bunchby-bunch base [3]. It consists of four ADCs that allow either to sample one detector with a local sampling rate of more than 300 GS/s or four detectors in parallel once per bunch and turn. For the investigation of the CSR intensity we use several detector systems such as Schottky diodes or a hot electron bolometer. Figure 3 shows an example of data taken with this systemwhere we used two Schottky diodes to detect the CSR and one avalanche photo diode (APD) to measure the incoherent synchrotron radiation in the visible range. Electro-optical Spectral Decoding (EOSD) At ANKA we use a streak camera and the principle of electro-optical spectral decoding (EOSD) to measure the longitudinal bunch profile. The EOSD setup is based on Proceedings of IPAC2016, Busan, Korea MOPMB014 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T03 Beam Diagnostics and Instrumentation ISBN 978-3-95450-147-2 109 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s