Patrik Schönfeldt

Parallelized Vlasov-Fokker-Planck solver for desktop personal computers

The numerical solution of the Vlasov-Fokker-Planck equation is a well established method to simulate the dynamics, including the self-interaction with its own wake field, of an electron bunch in a storage ring. In this paper we present Inovesa, a modularly extensible program that uses opencl to massively parallelize the computation. It allows a standard desktop PC to work with appropriate accuracy and yield reliable results within minutes. We provide numerical stability-studies over a wide parameter range and compare our numerical findings to known results. Simulation results for the case of coherent synchrotron radiation will be compared to measurements that probe the effects of the microbunching instability occurring in the short bunch operation at ANKA. It will be shown that the impedance model based on the shielding effect of two parallel plates can not only describe the instability threshold, but also the presence of multiple regimes that show differences in the emission of coherent synchrotron radiation.

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.

Simulation and measurement of the dynamics of ultra-short electron bunch profiles for the generation of coherent THz radiation

The shape of an electron bunch has a tremendous impact on its emission of synchrotron radiation. Especially the formation of sub-structures can increase the yield in the THz region. This thesis investigates the micro-bunching instability, a mechanism where structures form due to self-interaction of the electrons with their own wake-field. The methods include simulation and measurements. On the simulation side, the thesis describes the optimization of simulation algorithms to increase numerical stability as well as computational performance. On the experimental side, an optimized monitor for single-shot bunch profile measurements was designed to allow continuous bunch profile measurements with high signal-to-noise ratio and a sub-ps resolution at 2.7 MHz repetition rate.

Studies of Longitudinal Dynamics in the Micro-Bunching Instability Using Machine Learning

The operation of synchrotron light sources with short electron bunches increases the emitted coherent synchrotron radiation (CSR) power in the THz frequency range. However, the spatial compression leads to complex longitudinal dynamics, causing the formation of micro-structures in the longitudinal bunch profiles. The fast temporal variation and small scale of these micro-structures put challenging demands on their observation. At the KIT storage ring KARA (Karlsruhe Research Accelerator), diagnostics have been developed allowing direct observation of the dynamics by an electro-optical setup, and indirect observation by measuring the fluctuation of the emitted CSR. In this contribution, we present studies of the micro-structure dynamics on simulated data, obtained using the numerical Vlasov-Fokker-Planck solver Inovesa. To deal with generated data sets in the order of terabytes in size, we apply the machine learning technique k-means to identify the dominant micro-structures in the longitudinal bunch profiles. Following this approach, new insights on the correlation of the CSR power fluctuation to the underlying longitudinal dynamics can be gained.

Unveiling relativistic electron bunch microstructures and their dynamical evolutions, using photonic time-stretch

The photonic time-stretch technique allows electric field pulse shapes to be recorded with picosecond resolution, at megahertz acquisition rates. Using this strategy, we could directly record spatial patterns that spontaneously appear in relativistic electron bunches, and follow their dynamical evolution over time. We present recent results obtained using two strategies. At SOLEIL, we present the shapes of the THz pulses which are emitted by the structures, and detected far from the emission point, at the end of a beamline. At ANKA, we present how it has been possible to monitor directly the electron bunch near-field. These new types of single-shot recordings allow direct and stringent tests to be performed on electron bunch dynamical models in synchrotron radiation facilities.

Unveiling the complex shapes of relativistic electrons bunches, using photonic time-stretch electro-optic sampling

Photonic time-stretch enables to record electric field pulse shapes with picosecond resolution, at megahertz acquisition rates. Using this strategy, we could record the complex spatiotemporal evolution of relativistic electron bunches circulating in storage ring accelerators. We tested two complementary approaches. At SOLEIL, we monitored the THz pulses of coherent synchrotron that are emitted by the electrons. At ANKA, we monitored directly the electron bunch near-field.

An ultra-fast linear array detector for MHz line repetition rate spectroscopy

We developed a fast linear array detector to improve the acquisition rate and the resolution of Electro-Optical Spectral Decoding (EOSD) experimental setups currently installed at several light sources. The system consists of a detector board, an FPGA readout board and a high-throughput data link. InGaAs or Si sensors are used to detect near-infrared (NIR) or visible light. The data acquisition, the operation of the detector board and its synchronization with synchrotron machines are handled by the FPGA. The readout architecture is based on a high-throughput PCI-Express data link. In this paper we describe the system and we present preliminary measurements taken at the ANKA storage ring. A line-rate of 2.7 Mlps (lines per second) has been demonstrated.

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