RadiaBeam, Marina del Rey
Purdue University, West Lafayette, Indiana
UCLA, Los Angeles, California
Funding: This work is supported by the US Defense Threat Reduction Agency SBIR contract HDTRA1-08-P-0035.
Conventional X-ray sources used for medical and industrial imaging suffer from low spectral brightness, a factor which severely limits the image quality that can be obtained. X-ray sources based on Inverse Compton Scattering (ICS) hold promise to greatly improve the brightness of X-ray sources. While ICS sources have previously been demonstrated, and have produced high-peak brightness X-rays, so far experiments have produced low average flux, which limits their use for certain important commercial applications (e.g. medical imaging). RadiaBeam Technologies is currently developing a high peak- and average-brightness ICS source, which implements a number of improvements to increase the interaction repetition rate, as well as the efficiency and stability of the ICS interaction itself. In this paper, we will describe these improvements, as well as plans for future experiments.
RadiaBeam, Marina del Rey
UCLA, Los Angeles, California
The fixed-field alternating-gradient (FFAG) betatron has emerged as a viable alternative to RF linacs as a source of high-energy radiation for industrial and security applications. RadiaBeam Technologies is currently developing an FFAG betatron with a novel induction core made with modern low-loss magnetic materials. The principle challenge in the project has been the design of the magnets. In this paper, we present the current status of the project, including results of the magnet design and testing.
UCLA, Los Angeles, California
BNL, Upton, Long Island, New York
Coherent radiation emitted by relativistic electron bunches traversing the edge regions of dipole magnets in a chicane bunch compressor was extracted and transported for measurement, using a dedicated terahertz beamline at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory (BNL). Measurements include frequency spectrum and polarization of the radiation. The measurements are compared to predictions from QUINDI, a new simulation code developed at UCLA to model radiation emitted by charged particles in bending systems. Simulations and measurements indicate that because of interference of radiation from the two magnet edges, the edge radiation is suppressed at long wavelengths. In addition to being a source of broadband terahertz radiation, the system is also used as a non-invasive, single-shot, relative bunch length diagnostic to monitor compression in the chicane.
Rome University La Sapienza, Roma
RadiaBeam, Marina del Rey
UCLA, Los Angeles, California
INFN/LNF, Frascati (Roma)
We present here the design, including RF modelling, cooling, and thermal stress and frequency detuning, of an S-band RF gun capable of running near 500 Hz, for application to FEL and inverse Compton scattering sources. The RF design philosophy incorporates many elements in common with the LCLS gun, but the approach to managing cooling and mechanical stress diverges significantly. We examine the new proprietary approach of RadiaBeam Technologies for fabricating copper structures with intricate internal cooling geometries. We find that this approach may enable very high repetition rate, well in excess of the nominal project this design is directed for, the SPARX FEL.
*C.Limborg et al.,“RF Design of the LCLS Gun”,LCLS Technical Note LCLS-TN-05-3
**P. Frigola et al.,“A Novel Fabrication Technique for the Production of RF Photoinjectors”,published in EPAC08.
UCLA, Los Angeles, California
University of Tel-Aviv, Faculty of Engineering, Tel-Aviv
PSI, Villigen
The resonant harmonic interaction of an electron beam (e-beam) with an EM input field in a helical undulator is explored. The e-beam is coupled to the input radiation field at frequency harmonics through transverse gradients in the EM field, and helical micro-bunching of the e-beam is shown to occur naturally at the higher harmonics with the injection of a simple gaussian laser mode onto a cylindrically symmetric e-beam. This approach is under investigation as a method to generate a strongly pre-bunched e-beam seed for superradiant emission of light that carries orbital angular momentum in a downstream free-electron laser.
UCLA, Los Angeles, California
A helical undulator, utilizing permanent-magnet of cylindrically symmetric (Halbach) geometry has been developed at UCLA’s Neptune Facility. The initial prototype is a short 10 cm, 7 periods long helical undulator, designed to test the electron-photon coupling by observing the micro-bunching has been constructed and is currently being tested in the Neptune facility. An Open Iris-Loaded Waveguide Structure (OILS) scheme which conserves laser mode size and wave fronts throughout the undulator, is utilized to avoid Gouy phase shift caused by focusing of the drive laser. Coherent Transition Radiation and Coherent Cherenkov Radiation is used for micro-bunching diagnostic. Currently the undulator has been built, magnets were calibrated via pulsed wire method.
UCLA, Los Angeles, California
BESSY GmbH, Berlin
LMU, Garching
The fundamentals of insertion device physics demand that to have access to ever higher photon energies either the beam energy must increase or the undulator period must decrease. Recent advances in accelerator technology have increased beam energies and at the same time insertion device technology has developed creative ways of producing light of the desired energy, characteristics and quality. This paper describes the simulation work for the design of a 9 mm period in-vacuum planar undulator using a new rare-earth magnetic material.
UCLA, Los Angeles, California
BNL, Upton, Long Island, New York
It is possible to obtain spectral and angular information of inverse Compton sources using only an x-ray imaging device and various foils with K-edges in the many keV energy range. Beam parameters are chosen such that on-axis photons are above the K-edge for a given material, where absorption is strong and there is relatively zero transmission. Photons observed off-axis are red-shifted and fall below the K-edge, therefore being transmitted and creating a “donut” pattern, or "lobes" in the ideal case for a circularly or linearly polarized laser, respectively. We present simulation and experimental results of the double differential spectrum (DDS) for angle and energy of Compton photons generated at the BNL ATF.
UCLA, Los Angeles, California
Funding: Work supported by DOE.
We report experimental observation of narrow-bandwidth pulses of coherent Cherenkov radiation produced when a sub-picosecond electron bunch travels along the axis of a hollow circular cylindrical dielectric-loaded waveguide. For an appropriate choice of dielectric structure properties and driving electron beam parameters, the device operates in a single-mode regime, producing radiation in the THz range. We present measurements showing the emission of a narrowly-peaked spectrum from a fused silica tube 1 centimeter long with sub-millimeter transverse dimensions. We discuss the agreement of this data with theoretical and computational predictions, as well as possibilities for future study and application.
INFN/LNF, Frascati (Roma)
UCLA, Los Angeles, California
Istituto Nazionale di Fisica Nucleare, Milano
INFN-Roma II, Roma
ENEA C.R. Frascati, Frascati (Roma)
University of Tehran, Tehran
INFN-Roma, Roma
One of the main goals of the SPARC high brightness photoinjector is the experimental demonstration of the emittance compensation process while compressing the beam with the velocity bunching technique, also named RF compressor. For this reason, the first two S-band travelling wave accelerating structures downstream of the RF gun are embedded in a long solenoid, in order to control the space charge induced emittace oscillations during the compression process. An RF deflecting cavity placed at the exit of the third accelerating structure allows bunch length measurements with a resolution of 50 μm. During the current SPARC run a parametric experimental study of the velocity bunching technique has been performed. The beam bunch length and projected emittance have been measured at 120 MeV as a function of the injection phase in the first linac, and for different solenoid field values. In this paper we describe the experimental layout and the results obtained thus far. Comparisons with simulations are also reported.
RadiaBeam, Marina del Rey
UCLA, Los Angeles, California
NCSU, Raleigh, North Carolina
INFN/LNF, Frascati (Roma)
Electron beam based additive fabrication techniques have been successfully applied to produce a variety of complex, fully dense, metal structures. These methods, collectively known as Solid Freeform Fabrication (SFF) are now being explored for use in radio frequency (RF) structures. SFF technology may make it possible to design and produce near-netshape copper structures for the next generation of very high duty factor, high gradient RF photoinjectors. The SFF process discussed here, Arcam Electron Beam Melting (EBM), utilizes an electron beam to melt metal powder in a layer-by-layer fashion. The additive nature of the SFF process and its ability to produce fully dense parts are explored for the fabrication of internal cooling passages in RF photoinjectors. Following an initial feasibility study of the SFF process, we have fabricated a copper photocathode, suitable as a drop-in replacement for the UCLA 1.6 cell photoinjector, with internal cooling channels using SFF. Material analysis of the prototype cathode and new designs for a high duty factor photoinjector utilizing SFF technology will be presented.
SLAC, Menlo Park, California
UCLA, Los Angeles, California
Funding: Work supported by U.S. DoE Grant No. DE-FG03-92ER40693.
The first successful attempt to generate ultrashort (1-10 picosecond) relativistic electron bunches characterized by a ramped current profile that rises linearly from head to tail and then falls sharply to zero was recently reported.* Bunches with this type of longitudinal shape may be applied to plasma-based accelerator schemes as an optimized drive beam, and to free electron lasers as a means of reducing asymmetry in microbunching due to slippage. We will review the technique used to generate these bunches, which utilizes a sextupole-corrected dogleg compressor to manipulate the longitudinal phase space of the beam, and examine its potential application in a realistic plasma wakefield accelerator scenario, the proposed FACET project at SLAC.
* R. J. England, J. B. Rosenzweig, G. Travish, Phys. Rev. Lett. 100, 214802 (2008).
UCLA, Los Angeles, California
Funding: This work was supported by DOE grants DE-FG03-92ER40727 and DE-FG03-92ER40693
Many proposals and ongoing national projects exist worldwide to build a single-pass X-ray FEL amplifier in which a high-brightness, multi-GeV electron beam has a resonant energy exchange with radiation in an undulator. Because of the practical limit on the undulator period, the electron beam energy represents one of constraints on the shortest reachable wavelength. Recently the high-order harmonic FEL/IFEL interactions were considered theoretically as a technique that would allow the reduction of the beam energy without corresponding decrease in the undulator period and the magnetic field strength. We demonstrate microbunching of the 12.3 MeV electrons in a 7th order IFEL interaction, where the seed radiation frequency is seven times higher then the fundamental frequency. Strong longitudinal modulation of the beam is inferred from the observation of the first, second and third harmonics of the seed radiation in a Coherent Transition Radiation spectrum. The level of seed power is comparable to that required for microbunching at the fundamental frequency of the ten-period-long undulator. The implications of these results for the next generation of FELs will be explored.
RadiaBeam, Marina del Rey
UCLA, Los Angeles, California
A Coherent Optical Transition Radiation (COTR) arising from the photo-injector electron beams spontaneous microbunching at optical frequencies has been recently observed in a number of experiments. This effect can lead to an undesirable optical background for OTR beam profile measurements at these facilities. A method to resolve this problem is proposed, based on selectively suppressing the back-scattered COTR using multiple scattering in the insertion foil. An analytical treatment of COTR dependence on the angular divergence in the radiating beam is presented, and the efficacy of the approach is illustrated with the numerical examples.
UCLA, Los Angeles, California
INFN/LNF, Frascati (Roma)
ENEA C.R. Frascati, Frascati (Roma)
It has been suggested that an ultra-short, very low charge beam be used to drive short wavelength single-spike operation at the SPARC FEL. This paper explores the development and construction of a longitudinal diagnostic capable of completely characterizing the radiation based on the Frequency-Resolved Optical Gating (FROG) technique. In particular, this paper explores a new geometry based on a Transient-Grating (TG) nonlinear interaction and includes studies of start to end simulations for pulses at the SPARC facility using GENESIS and reconstructed using the FROG algorithm. The experimental design, construction and initial testing of the diagnostic are also discussed.
UCLA, Los Angeles, California
INFN/LNF, Frascati (Roma)
Rome University La Sapienza, Roma
A hybrid photoinjector, which we present here, consists of a 6-cell traveling wave structure with a standard 1.6-cell RF gun attached to the one end and a 3-m long linac following for further acceleration. With this structure, no reflection observed at the input port. This enables to build the accelerator without a circulator which limits the power and the frequency of RF. From the beam dynamics point of view, the beam is produced as the normal RF guns and gets short by velocity bunching in the traveling wave section right after the gun. The peak current can reach more than 1 kA, with about 2 mm.mrad of the emittance at 20 MeV. We discuss more details about the beam dynamics as well as the RF structure.
UCLA, Los Angeles, California
The velocity bunching is a hot topic in normal conducting photoinjectors to generate high-brightness beams instead of magnetic chicanes in the low energy region. We apply this technique to the superconducting photoinjectors. The linac considered here consists of several 9-cell TESLA cavities, the standard 1.6-cell normal conducting RF gun is assumed, though. In the case of 1.1 nC injection, the peak current increases to 1 kA with 2.6 mm.mrad of the emittance. The peak current can be higher but the emittance becomes worse in that case, and vice versa. We discuss more details on the spot.
UCLA, Los Angeles, California
Rome University La Sapienza, Roma
We present the status of development of a 1.5+0.5 cell photoinjector run in the blowout regime. LANL Parmela simulation results indicate a near uniform beam of slice energy spread on the order of 500 eV when neglecting thermal effects. We examine the use of an extra half cell to control longitudinal beam growth and compare the system in development with previous 1.6 cell photoinjector designs.
INFN/LNF, Frascati (Roma)
Istituto Nazionale di Fisica Nucleare, Milano
CEA, Gif-sur-Yvette
INFN-Roma II, Roma
ENEA C.R. Frascati, Frascati (Roma)
SOLEIL, Gif-sur-Yvette
INFN-Roma, Roma
University of Tehran, Tehran
UCLA, Los Angeles, California
The SPARC project foresees the realization of a high brightness photo-injector to produce a 150-200 MeV electron beam to drive 500 nm FEL experiments in SASE, Seeding and Single Spike configurations. The SPARC photoinjector is also the test facility for the recently approved VUV FEL project named SPARX. The second stage of the commissioning, that is currently underway, foresees a detailed analysis of the beam matching with the linac in order to confirm the theoretically prediction of emittance compensation based on the “invariant envelope” matching , the demonstration of the “velocity bunching” technique in the linac and the characterisation of the spontaneous and stimulated radiation in the SPARC undulators. In this paper we report the experimental results obtained so far. The possible future energy upgrade of the SPARC facility to produce UV radiation and its possible applications will also be discussed.