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.
DESY, Hamburg
By using magnetic chicane bunch compressors, high-gain free-electron lasers are capable of generating femtosecond electron bunches with peak currents in the kilo-ampere range. For accurate control of the longitudinal dynamics during this compression process, high-precision beam energy and arrival-time monitors are required. Here we present an electro-optical detection scheme that uses the signal of a beam pickup to modulate the intensity of a femtosecond laser pulse train. By detecting the energies of the laser pulses, the arrival-time of the pickup signal can be deduced. Depending on the choice of the beam pickup, this technique allows for high-resolution beam position measurements inside of magnetic chicanes and/or for femtosecond-resolution bunch arrival-time measurements. In first prototypes we realized a beam position monitor with a resolution of 3 μm (rms) over a many-centimeter dynamic range and a bunch arrival-time monitor with a resolution of 6 fs (rms) relative to a pulsed optical reference signal.
LBNL, Berkeley, California
SLAC, Menlo Park, California
The scientific potential of femtosecond x-ray pulses at linac-driven FELs such as the LCLS is tremendous. Time-resolved pump-probe experiments require a measure of the relative arrival time of each x-ray pulse with respect to the experimental pump laser. In order to achieve this, precise synchronization is required between the arrival time diagnostic and the laser, which are often separated by hundreds of meters. We describe an optical timing system based on stabilized fiber links which has been developed for the LCLS to provide this synchronization. Preliminary results show synchronization of the synchronization signals at the sub-10 fsec level and overall synchronization of the x-ray and pump laser of <40 fsec. We present details of the implementation and LCLS and potential for future development.
Fermilab, Batavia
Optical transition radiation (OTR) imaging for transverse beam-size characterization is a well-established technique at many accelerators including the Fermilab A0 photoinjector (A0PI) facility. However, there is empirical evidence for gamma greater than 1000 beams that the utilization of the polarization component orthogonal to the dimension of interest resulted in a smaller projected image profile. Generally, at the A0PI low beam energies of 14-15 MeV and emittances of 3 mm mrad, one encounters beam sizes of 0.8 to 1.5 mm (σ). However, the use of 50-micron wide slits to sample the beam’s transverse phase spaces significantly alters the required resolution of the converter screen and imaging system. In this case, we dealt with slit-image sizes of about σ 100 microns and less, depending on drift distance and beam divergence. In the course of our study of the slit images, we have found that the OTR polarized component orthogonal to the narrow beam dimension of interest systematically gave us ~20-micron smaller projected image sizes than with the total OTR intensity. This is one of the first reports of this polarization effect at such a low-gamma regime (~30).
STFC/RAL/ASTeC, Chilton, Didcot, Oxon
Royal Holloway, University of London, Surrey
STFC/RAL/ISIS, Chilton, Didcot, Oxon
The front end test stand FETS is an R&D project hosted at Rutherford Appleton Laboratory RAL with its aim to demonstrate a high power, fast chopped H- ion beam and will consist in final stage of ion source, low energy beam transport LEBT, RFQ and a transport line including a chopper system at 3MeV output energy. Possible candidates of applications are Isis upgrade (RAL neutron source), future spallation sources or the Neutrino factory. The high beam power may cause problems due to its thermal power deposition on diagnostics parts introduced into the beam so non-interceptive beam instruments are highly preferred to avoid those problems. Diagnostics for H- beams can benefit of laser light where photons with suitable energy are able to detach the additional electron. This method is applied to a beam profile monitor close to the ion source of the FETS beam line and the paper gives a status report of the ongoing process of commissioning and provides a detailed discussion of problems and recent changes including first "proof-of-principle" measurements.
BNL, Upton, Long Island, New York
A beam profile and energy monitor for H- beams which measures electrons stripped from the beam by a laser has been installed in the high energy beam transport (HEBT) line at the Brookhaven National Lab linac. Our 100mJ/pulse, Q-switched laser neutralizes 70% of the beam during its 10ns pulse. Also electrons are stripped by the residual gas at a rate of ~1.5 x 10-8/cm at 1 x 10-7torr. Beam electrons have the same velocity as the beam and so have an energy of 1/1836 of the beam protons. There is a chamber in which the laser light passes through the ion beam followed by a dipole magnet which deflects the electrons by 90° through a biased retarding grid (V<125kV) into a Faraday cup detector. To measure beam profiles, a narrow laser beam is stepped across the ion beam removing electrons from the portion of the H- beam intercepted by the laser. To measure the energy spectrum of the electrons, we use either the gas-stripped or laser-stripped signal. The total current is measured as the voltage on the grid is raised in small steps. We deduce the energy spread of the H- beam by deconvolving the electron spectrum into components from beam energy and from space-charge fields.
Cockcroft Institute, Warrington, Cheshire
The University of Liverpool, Liverpool
The introduction of Silicon Photomultipliers (SiPMs) as single photon sensitive detectors represents a promising alternative to traditional photomultiplier tubes. This is especially true in applications in which it is compulsory to attain magnetic field insensitivity, low photon flux detection, quantum efficiency in the blue region that is comparable to standard photomultipliers, high timing resolution, dimensions comparable to the dimensions of an optical fiber diameters, and low costs. The structure of the SiPM is based on an array of independent avalanche photodiodes (APDs) working in Geiger-mode at a low bias voltage with a high gain. The output signal is proportional to the number of pixels "fired" by impacting photons. The detection efficiency for state-of-the-art devices is in the order of 20% at 500 nm. In this contribution, the measured dark count rates of different SiPMs are compared and the influence of this noise on the real signal is presented. These results are then used to correct the photon count and determine the optimized working parameters for a future beam loss monitor at CTF3/CLIC.
ANL, Argonne
As the Advanced Photon Source (APS) prepares for a large-scale upgrade, many of the fundamental limitations on beam stability have to be identified. Studies have been conducted to measure thermal mechanical effects of both the water and air handling systems impacting insertion device vacuum chambers (IDVES). Mechanical stability of beam position monitor pickup electrodes mounted on these small-gap IDVES places a fundamental limitation on long-term x-ray beam stability for insertion device beamlines. Experiments have been conducted on an ID vacuum chamber indicating that the BPM blocks are moving with water temperature cycles at the level of 10 microns/degree C. Measurements and potential engineering solutions will be described.
NewPath, Salt Lake City, Utah
We have derived algebraic expressions for the open-circuit voltage that is induced on a Rogowski coil, having imperfectly spaced turns, by a time-dependent current which passes through the aperture of the coil. The derivation of these expressions requires that the layer of the winding is thin, the number of turns is large, and the cross-section of the winding is uninform and rectangular. Examples are given in which these expressions are applied to determine the effects of the gap between the ends of the coil, as well as other irregularities in the spacing of the turns, on the position sensitivity–defined as the dependence of the induced voltage on the coordinates of the current. The results in these examples agree with those that others have obtained by numerical methods or measurements with Rogowski coils. This technique may be used to quickly define the design specifications which are required to satisfy a specific upper limit for the positional sensitivity.
MEPhI, Moscow
DESY Zeuthen, Zeuthen
The Photo Injector Test Facility at DESY, Zeuthen site (PITZ) develops electron sources of high brightness beams, required for linac based free electron lasers (FELs) like FLASH or the European XFEL. One of the key issues in electron beam optimization is the minimization of the transverse emittance. The main method to measure emittance at PITZ is a single slit scan technique, implying local beam divergence measurement by insertion of the slit mask at a definite location within the beam and measurements of the transmitted beamlet profile downstream of the slit station. “Emittance Measurement Wizard” (EMWiz) is the program used by PITZ operators for automated emittance measurements. EMWiz combines an acquisition program for beam and beamlet image recording and a postprocessing tool for the analysis of the measured transverse phase space of the electron beam. It provides a way to execute the difficult emittance measurements in an automatic mode and to get a calculated emittance result.
RIKEN/SPring-8, Hyogo
Hamamatsu Photonics K.K., Hamamatsu-city
JASRI/SPring-8, Hyogo-ken
The SCSS test accelerator, which was constructed to check feasibility of XFEL/SPring-8, is operated for user experiments using stable EUV SASE. This accelerator provides a high quality electron beam with parameters suitable for power saturation of the EUV SASE, such as a bunch length of 300 fs and a peak current of 700 A. Evaluating the parameters is very important to ensure the stable generation of the SASE. The bunch length measurement systems to evaluate the parameters have been developed. The systems use the rf zero phasing method, the EO sampling method with temporal decoding and an 800 nm laser, and the method of observing OTR (near infrared region) by a FESCA-200 streak camera, which are mature technologies. All the measured bunch lengths were about 300 fs (FWHM), which is consistent with the individual methods. The most important result is that the streak camera with optimum tuning directly measured the temporal structure with femtosecond resolution. This presentation briefly introduces a development plan for a higher-resolution streak camera and a beam monitor system for the XFEL, as well as the experimental set-up and results.
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.
LBNL, Berkeley, California
High precision phase measurement is important for many areas of accelerator operation. In a heterodyne digital receiver, one source of phase error is due to the thermal variation of the input stage. We have developed a technique to calibrate this drift. A CW calibration signal is sent through the same components together with the RF signal to measure and compensate the component drift. At intermediate frequency (IF), we use FPGA based digital signal processing to measure and reconstruct the RF signal after applying appropriate correction. Using this technique, we can measure the phase of a 2856 MHz signal with an accuracy of 15 mdeg. We describe how this is approach is applied to the femto-second timing distribution system.
ELETTRA, Basovizza
The bunch arrival monitor (BAM) for the IV generation synchrotron light source FERMI@Elettra is presented. It is based on an original idea developed at FLASH/DESY, specifically designed and built in-house for FERMI@Elettra. Each BAM station consists of a front-end module, located in the machine tunnel, and of a back-end unit located in the service area. It makes use of the pulsed optical phase reference along with the stabilized fiber link. The front end converts the bunch arrival times into amplitude variations of the optical phase reference pulses distributed over the link. The analogue signal is generated at the e-beam's passage in a broadband pick-up and is sent to the modulation input of an electro-optical modulator (EOM). The back end acquires, synchronously, the amplitude modulated pulses, using a broadband photodiode and a fast analog-to-digital converter. The digitized data is sent to the machine control system for further processing. The dedicated analog-to-digital, conversion processing and communication board, part of the monitor back end, is briefly described. The BAM measurements performed on FERMI@Elettra at 10 Hz are presented.
DESY, Hamburg
TEMF, TU Darmstadt, Darmstadt
DESY Zeuthen, Zeuthen
INRNE, Sofia
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
INFN/LASA, Segrate (MI)
RAS/INR, Moscow
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin
Uni HH, Hamburg
FNRF, Chiang Mai
MBI, Berlin
The Photo Injector Test facility at DESY Zeuthen site (PITZ) is dedicated to developing and optimizing high brightness electron sources for short wavelength Free-Electron Lasers (FELs) like FLASH and the European XFEL, both in Hamburg (Germany). Since October 2009 a major upgrade has been ongoing with the goal of improving the accelerating components, the photocathode drive laser system, and the beam diagnostics. The previously operated and fully characterized gun was brought to FLASH and will go into operation soon. A gun of the same type is installed now in PITZ. The conditioning has started and the gun will be characterized throughout 2010. A new booster cavity, Cut Disk Structure (CDS), was developed and will be mounted at PITZ in spring 2010. The booster cavity will be able to accelerate electrons above 20 MeV/c and will be suitable for long RF pulses. The most important upgrade of the diagnostics system will be the implementation of a phase space tomography module (PST) consisting of three FODO cells each surrounded by two screen stations. The results of commissioning, gun and booster conditioning and the very first measurements will be reported.
DESY, Hamburg
DESY Zeuthen, Zeuthen
The stability of the photo injector is a key issue for the successful operation of linac based free electron lasers. Several types of jitter can impact the stability of a laser driven RF gun. Fluctuations of the RF launch phase and the cathode laser energy have significant influence on the performance of a high brightness electron source. Bunch charge measurements are used to monitor the stability of the rf gun phase and the cathode laser energy. A basic measurement is the so called phase scan: the accelerated charge downstream of the gun is measured as a function of the launch phase, the relative phase of the laser pulses with respect to the RF. We describe a method which provides simultaneous information on rms jitters from phase scans at different cathode laser energies. Fluctuations of the rf gun phase together with cathode laser energy jitter have been measured at the Photo Injector test facility at DESY in Zeuthen (PITZ). Obtained results will be presented in comparison with direct independent measurements of corresponding instability factors. Dedicated beam dynamics simulations have been done in order to optimize the method performance.
PSI, Villigen
JLAB, Newport News, Virginia
Electro Optical (EO) sampling is a promising non-destructive method for measuring ultra short (sub-ps) electron bunches. The FEMTO slicing experiment at the Swiss Light Source modulates about 1 pC of the 4-nC electron bunch longitudinally. The coherent synchrotron radiation (CSR) emitted by this substructure was sampled by 100 fs long pulses from an Yb fiber laser in EO crystals of different materials (GaP, ZnTe). The broadening of this ps long structure over several turns of the synchrotron could be measured with sub-ps resolution.
UMD, College Park, Maryland
MPI-K, Heidelberg
Cockcroft Institute, Warrington, Cheshire
We have developed a technique that employs a digital micro-mirror array to produce an image of the halo of an electron beam with enhanced dynamic range. Light produced by the beam intercepting a phosphor screen is first imaged onto the array; an adaptive mask is created and applied to filter out the beam core; and the result is reimaged onto an intensified CCD camera. We describe the optics used, the masking operation and preliminary results of experiments we have performed to study beam halo at the University of Maryland Electron Ring (UMER).