CLASSE, Ithaca, New York
A need for femtosecond resolution beam arrival time measurements has arisen with the transition from many-picosecond-long bunches in ring-based accelerators to a few femtosecond-long bunches in high- gain free-electron lasers. Here we present an electro-optical detection scheme that uses the signal of a beam pick-up to modulate the intensity of a femtosecond laser pulse train. By detecting the energies of the laser pulses, the bunch arrival time can be deduced. We tested this scheme by distributing a laser pulse train to two locations in the FLASH linac, separated by 60 m, using length-stabilized optical fibers. By measuring the arrival times of the same electron bunches at these two locations, we determined an rms bunch arrival time resolution of 6 fs. This unprecedented monitor resolution allowed us to reduce the beam arrival time jitter from almost 200 fs down to 25 fs with an intra-bunch train feedback. Alternatively, the same detection scheme can be used for large dynamic range micrometer-resolution beam position measurements by using a stripline-type pickup mounted perpendicularly to the beam path, and then detecting the arrival time difference of both pick-up signals.
ELETTRA, Basovizza
Several diagnostic instruments for the FERMI@Elettra FEL, among them the Bunch Arrival Monitor (BAM) and the Cavity Beam Position Monitor (C-BPM), require accurate readout, processing, and control electronics. All systems must be also integrated within the main machine control system. The back-end platform, based on the MicroTCA standard, provides a robust environment for accommodating such electronics, including reliable infrastructure features. Two types of Advanced Mezzanine Cards (AMC) had been developed in-house and manufactured for meeting the demanding performance requirements. The first is a fast (160 MSps) and high-resolution (16 bits) Analog to Digital and Digital to Analog (A|D|A) Convert Board, hosting 2 A-D and 2 D-A converters controlled by a large FPGA (Virtex-5). The FPGA is also responsible for service and host interface handling. The latter board is Analog to Digital Only (A|D|O) Converter, derived from the A|D|A, with an analog front side stage made of four A-D converters. The overall systems’ architectures, together with the specific AMCs’ functionalities, are described. Results on performance measurements are also presented.
SLAC, Menlo Park, California
ANL, Argonne
CERN, Geneva
The Linac Coherent Light Source (LCLS) is a free-electron laser (FEL) at SLAC producing coherent 1.5 angstrom x-rays. This requires precise and stable alignment of the electron and photon beams in the undulator. We describe construction and operational experience of the beam position monitor (BPM) system which allows the required alignment to be established and maintained. Each X-band cavity BPM employs a TM010 monopole reference cavity and a single TM110 dipole cavity detecting both horizontal and vertical beam position. The processing electronics feature low-noise single-stage three-channel heterodyne receivers with selectable gain and a phase-locked local oscillator. Sub-micron position resolution is required for a single-bunch beam of 200 pC. We discuss the specifications, commissioning and performance of 36 installed BPMs. Single shot resolutions have been measured to be about 200 nm rms at a beam charge of 200 pC.
LBNL, Berkeley, California
Stabilized optical fiber links have been under development for several years for high precision transmission of timing signals for remote synchronization of accelerator and laser systems. In our approach, a master clock signal is modulated on an optical carrier over a fiber link. The optical carrier is also used as the reference in a heterodyne interferometer, which is used to precisely measure variations, mainly thermal, in the fiber length. The measured variations are used to correct the phase of the transmitted clock signal. We present experimental results showing sub-10 fsec relative stability of a 200 m link, and sub-20 fsec stability of a 2.2 km link.
GSI, Darmstadt
TUD, Darmstadt
In this paper, a method for measuring the absolute signal delays of active optical transmission lines will be presented. This measurement method is an essential part of the timing system for FAIR (Facility for Antiproton and Ion Research). To prevent interference of the timing signals whose delays are to be measured with the measurement signal sequence, the latter is transmitted on a separate optical carrier in the same fibre. By using a wavelength selective mirror at the end of the transmission line, the optical measurement signals are reflected and lead back to the measurement unit. The measurement sequence consists of a number of sinusoidal signals with different frequencies that are modulated one by one on the optical carrier. For each frequency a phase comparison of the outgoing and returning signal is performed. In the last step, the absolute delay is calculated from the obtained phase values by using an algorithm. It will be shown that this method enables cost efficient delay measurements with an accuracy of better than 100 fs.
JLAB, Newport News, Virginia
The advent of the energy recovering linac (ERL) brings with it the promise of linac-quality beams generated with near storage ring efficiency. This potential will not, however, be fulfilled without overcoming a number of technical and operational challenges. We will review the basics of ERL dynamics and operation, and give examples of idiosyncratic ERL behavior and requirements posing particular challenges from the perspective of diagnostics and instrumentation. Beam performance parameters anticipated in next-generation ERLs will be discussed, and a “wish list” for the instrumentation of these machines presented.