Bernd Steffen

MicroTCA.4-Based RF and Laser Cavities Regulation Including Piezocontrols

In this paper, we present an universal solution for radio frequency (RF) and laser cavities regulation, including piezocontrols and drivers based on MicroTCA.4 electronics. The control electronics consists of an Analog to Digital Converter-advanced mezzanine card (AMC) with an analog rear transmission module (RTM) to downmix and measure the RF signals and a low cost AMC-based Field Programmable Gate Array mezzanine card carrier for fast data processing and digital feedback operation connected to an RTM piezodriver. For the RF cavity regulation, the piezodriver includes additional inputs to use piezoelements as active force sensors. The fine tuning of the laser is carried out using a cavity fiber stretcher. The coarse tuning of the supported optics is done using a piezomotor-driven linear stage. Both channels can be operated using digital feedback controllers. First, results from continuous wave operation of the RF field controller and the cavity active resonance control with the piezotuners are demonstrated. The laser lock application performance using both fine and coarse channel feedbacks is shown and briefly discussed.

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.

Fast intra bunch train charge feedback for FELs based on photo injector laser pulse modulation

Bunch charge variations in free electron lasers such as the free electron laser (FEL) in Hamburg (FLASH) or the European X-ray FEL (E-XFEL) impact the longitudinal phase space distribution of the electrons resulting in different bunch peak currents, bunch durations, and bunch shapes. The electron bunches are generated by short ultraviolet (UV) laser pulses impinging onto a photocathode inside a radio frequency (RF) accelerating cavity. At FLASH, bursts of bunches up to 800 pulses with an intra train repetition rate of 1 MHz are used and even higher repetition rates (up to 4.5 MHz) are planned at the E-XFEL. Charge variations along these bunch trains can be caused by variations of the laser pulse energies, instabilities of the accelerating fields in the RF cavity, and time-dependent effects in the photoemission process. To improve the intra bunch train charge flatness and to compensate train-to-train fluctuations, a dedicated digital control system, based on the Micro Telecommunication Computing Architecture (MicroTCA.4) standard, was designed, implemented, and successfully tested at FLASH. The system consists of a bunch charge detection module which analyzes data from a toroid system and provides the input signal for the controller which drives a fast UV Pockels cell installed in the optical path of the photocathode laser. The Pockels cell alters the laser polarization and thus the transmission through a polarizer. The modulation of UV laser pulse energy with an iterative learning feedforward minimizes the repetitive errors from bunch train to bunch train. A fast feedback algorithm implemented in a field-programmable gate array allows for fast tuning of bunch charge inside the bunch train. In this paper, a detailed description of the system and the first preliminary measurement results are presented.