High field classical and quantum electrodynamical phenomena can now be studied experimentally, as new techniques to produce ultrahigh intensity laser pulses and synchronized relativistic electron beams are becoming more readily available. These techniques include chirped pulse amplification and photoelectron bunch acceleration in a high gradient rf gun. Applications range from x-ray free-electron lasers and next generation light sources to advanced laser acceleration concepts and (lambda) -(lambda) collider physics. The basic physics of nonlinear electron-photon scattering processes in vacuum is revived, within the framework of classical electrodynamics. 3D vacuum propagation and diffraction effects and their relation to the paraxial approximation are first considered, and the axial electromagnetic field components required to satisfy the gauge condition are derived. The nonlinear Lorentz dynamics and ultrahigh intensity Compton scattering, as well as radiative corrections described by the Dirac-Lorentz equation, are then discussed in detail. The nonlinear Doppler shift induced by laser radiation pressure during Compton scattering is analyzed and shown to yield chaotic optical spectra at ultrahigh intensities for linearly polarized light; a correction scheme, relying laser pulse shaping, is also briefly outlined. For circularly polarized light, an exact expression for the nonlinear Compton backscattered spectrum is derived.
The ability to nondestructively image living organisms, examine biological materials, and inspect packages without the necessity of removing the contents are applications well suited to x-ray radiography. We have developed a portable flash x-ray source having adjustable peak x-ray photon energy, which delivers a 34 milliroentgen x-ray dose at 30 cm in a 60-ns full-width at half-maximum pulse using less than 5 joules of stored energy. This technology has generated considerable interest in the biomedical imaging community and promises to replace older x-ray system that were not readily portable.
The Regenerative Amplifier Free-Electron Laser is a new FEL approach aimed at achieving the highest optical power from a compact RL-linac FEL. The key idea is to re-inject a small fraction of the optical power into a high-gain wiggler to enable the FEL to reach saturation in a few passes. The use of large outcoupling increases the FEL output efficiency and reduces the risk of optical damage to the feedback mirrors. This paper summarizes the design of the high-power IR regenerative amplifier FEL and describes the initial experimental results. The highest optical energy achieved thus far at 15.5 micrometers is 0.5 J over a train of 1000 micropulses. We infer a pulse energy of 0.5 mJ in each 10 ps micropulse, corresponding to a peak power of 50 MW.
Results obtained on the ID19 beamline at ESRF, where particularly high coherence is associated with the long source-to-sample distance and the small size of the x-ray source, illustrate the possibilities of imaging using coherence. These features make the imaging of phase objects extremely simple, since a 'propagation' technique, similar to the defocusing mode of electron microscopy and to in-line Gabor holography in optics, can be used. The physical principle involved is Fresnel diffraction. We used this 'propagation' technique both to measure, via the figures obtained from a fiber and a periodic grating, the source size, and to image objects with negligible absorption for hard x-rays but appreciable variations in optical path length. Examples of the latter ar two or three-dimensional images of light natural or artificial materials. The 3D reconstruction can be performed either with a filtered back- projection algorithm designed for attenuation tomography, which was shown to be a good approximation in some cases, or with a phase reconstruction procedure similar to that used for electron microscopy. The spurious images associated with beam line components, and the conditions for coherence preservation are also briefly discussed.
The OK-4/Duke storage ring free electron laser (FEL) was commissioned in November, 1996 and demonstrated lasing in the near UV and visible ranges. During one month of operation we performed preliminary measurements of the main parameters of the OK-4 FEL: its gain, lasing power and temporal structure. In addition to lasing, the OK-4/Duke FEL generated a nearly monochromatic 12.2 MeV (gamma) -ray beam. In this paper we describe the status of the main subsystems including the injector system and the ring itself, and discus future and in-progress upgrades to these systems. We also describe the parameters measured to date of the injector, the storage ring, the generated optical laser beams, and the backscattered (gamma) -ray beam.
Free space propagation or Fresnel diffraction is an effective method to sense the phase modulation of hard x-ray beams. We use the variation with propagation distanced of the diffraction pattern both to study periodic objects and to handle the inverse problem, i.e. to extract information on the phase and amplitude of the transmitted wave from the intensity distribution in the defocused images. The 'Talbot effect', a special manifestation of Fresnel diffraction, implies that the intensity distribution of coherent light transmitted through a periodic object is periodic both in the direction of the object periodicity and in the propagation direction. We have performed a first investigation of the Talbot effect for hard x-rays on two phase gratings. Apart from a lateral shift of a/2, identical intensity distributions are ideally expected in two recording planes a distance a2/(lambda) apart. Actually, a decrease of contrast occurs as a result of the partial coherence of the incident beam, providing a measure of the degree of coherence. Focus variation, as applied in transmission electron microscopy, consists in recording at different d a series of images which are combined through a suitable algorithm to reconstruct the phase modulation introduced by a polymer fiber from the experimental data.
Results of first operation of the Israeli Electrostatic- Accelerator Tandem Free-Electron Laser (EA-FEL) are reported. This EA-FEL utilizes a 1.4 Amp electron beam obtained from a parallel flow Pierce-type electron gun. The e-beam is transported through a resonator located inside a plane Halbach configuration wiggler, both located at the high voltage terminal of the van de graaf accelerator. The high voltage terminal is charge to a positive plates waveguide and two Talbot effect quasioptical reflectors. It exhibited a quality factor of Q approximately equals 30,000. Millimeter wave radiation pulses of 2 microsecond(s) ec duration were obtained at a frequency of 100.5 GHz, as predicted, at a power level above 1 kW.
There are a number of codes that can calculate the performance of a free-electron laser (FEL) in 3D with nonideal electron beams, wigglers, and optics. Unfortunately, these codes can be very computationally intensive. So, given the large number of parameters associated with an FEL, it is often impractical to utilize such a large-scale code to develop a preliminary design. To overcome this problem we developed a reduced FEL model that is sufficiently accurate to provide a realistic performance estimate, while, at the same time, the algorithm is sufficiently efficient to investigate a large number of parameter variations. The low-level algorithms design the wiggler, determine the effective energy distribution of the electron beam inside the wiggler, determine the small-signal gain, determine the saturation gain and efficiency, and design the optics.
It is commonly assumed that a matched electron beam optimizes free-electron laser (FEL) performance; however, this assumption has not been proven for FELs operating in the high gain regime. We test this hypothesis using a 3D multimode analysis which is capable of modeling matched and unmatched electron beams and for design parameters suitable for a SASE experiment at Duke University using the PALADIN wiggler. The gain length predicted for a matched beam is in good agreement with analytic theory. Further, the simulation indicates that while the gain length is optimized for a matched beam the saturated power is not necessarily optimized.
We propose to use techniques of pulse modulation applied in visible light laser optics to the x-ray domain. In a first step we study the emission process of synchrotron radiation in terms of the time dependence of energy and phase for individual electrons showing that the prerequisite for bunch compression would be provided. Secondly, we indicate a possible x-ray optical scheme consisting of pairs of crystals or multilayers to delay, superimpose or compress x- ray pulses. The proposed layout should permit bunching and pulse-shaping of synchrotron x-rays down to about 1 picosecond which would significantly improve the present time resolution in x-ray experiments conducted at modern synchrotron radiation facilities. However, it remains to be shown what is the result for collective emission of synchrotron radiation for a bunch of many electrons.
Preliminary calculations using the computer code PARMELA indicate that it is possible to achieve peak currents on the order of 1 kA using a thermionic-cathode rf gun and ballistic bunch compression. In contrast to traditional magnetic bunching schemes, ballistic bunch compression uses a series of rf cavities to modify the energy profile of the beam and properly chosen drifts to allow the bunching to occur naturally. The method, suitably modified, should also be directly applicable to photo injector rf guns. Present work is focusing on simultaneously compressing the bunch while reducing the emittance of the electron beam. At present, the calculated normalized rms emittance is in the neighborhood of 6.8 (pi) mrad with apeak current of 0.88 kA, and a peak bunch charge of 0.28 nC from a thermionic-cathode gun.
In recent years, comprehensive design studies have been initiated on angstrom-wavelength free-electron laser (FEL) schemes based on driving highly compressed electron bunches from a multi-GeV linac through long undulators. The output parameters of these sources, when operated in the so-called self-amplified spontaneous emission mode, include lasing powers in the 10-100 GW range, full transverse and low-to- moderate longitudinal coherence, pulse durations in the 50- 500 fs range, broad spontaneous spectra with total power comparable to the coherent output, and flexible polarization parameters. In this paper we summarize the status of design studies of the x-ray optics system and components to be utilized in the SLAC linac coherent light source, a 1.5-15 angstrom FEL driven by the last kilometer of the SLAC three kilometer S-band linac. Various aspects of the overall optical system, selected instrumentation and individual components, radiation modeling, and issues related to the interaction of intense sub-picosecond x-ray pulses with matter, are discussed.
Recent developments in electron-gun and injector technologies enable production of short, high-charge bunches. In this parameter regime,the curvature effect on the bunch self-interaction, by way of coherent synchrotron radiation (CSR) and space-charge forces as the beam traverse magnet bends, may cause serious emittance degradation. In this paper, we study an electron bunch orbiting between two infinite, parallel conducting plates. The bunch moves on a trajectory from a straight path to a circular orbit and begins radiating. Transient effects, arising from CSR and space-charge forces generated from source particles both on the bend and on the straight path prior to the bend, are analyzed using Lienard-Wiechert fields, and their overall net effect is obtained. The influence of the plates on the transients is contrasted to their shielding of the steady- state radiated power. Results for emittance degradation induced by this self-interaction are also presented.
An experimental set-up for the observation of intensity fringes of x-rays propagating in vacuum between two separated crystals is reported. In this experiment a Bragg reflection near 90 degrees was used to observe the oscillations of the measured intensity between the monochromator and mirror crystals. The x-ray intensity profiles; were collected as a function of the angular position of the crystal-mirror in the vicinity of a Si(800) reflection at a synchrotron radiation energy of about 9.1344 keV. Interference contrast in the experimentally observed intensity profiles proves the phase-sensitive nature of the intensity fringes. Novel approach to interpretation of synchrotron radiation coherence is discussed. The experimental observations open new possibilities for novel applications in high-energy radiation optics.
We describe a new radiographic technique using quasi- monochromatic (gamma) -rays. Conventional radiography of dense or thick objects is severely limited by the broad energy spread of bremsstrahlung (gamma) -rays. The development of intense, tunable, near-monochromatic (gamma) - rays sources in the 4-30 MeV range affords the opportunity to develop a new type of radiographic tomography. Such (gamma) -rays will shortly be available from inverse Compton free-electron laser sources. The expected narrow energy spread and energy tunability will allow not only the structure and distribution of material in an object to be determined but also the specific elemental composition of the material. This is because each element has a slightly different absorption cross section minimum. Furthermore, the quasi-monochromatic nature of the incident (gamma) -ray beam will allow discrimination between scattered and unscattered photons exiting the test object, and result in reliable composition and density data. In this paper, we present an overview of the radiographic process and some early computer simulation results.
Scattering experiments with coherent x-rays depend crucially upon the availability of high-brilliance x-ray sources. We summarize the coherence properties of high-brilliance undulator beams and briefly describe their use for x-ray photon correlation spectroscopy.
The quantitative imaging of phase object using 16 keV x-rays is reported. The theoretical basis of the techniques is presented along with its implementation using a synchrotron x-ray source. We find that our phase image is in quantitative agreement with independent measurements of the object.
Commissioning of a Fourier transform soft x-ray spectrometer is under way at the advanced light source, Lawrence Berkeley National Laboratory, as a branch of beamline 9.3.2. The spectrometer is a novel soft x-ray interferometer designed for ultra-high resolution spectroscopy in the photon energy region of 60-120 eV with a theoretical resolving power E/(Delta) E-106. This instrument is expected to provide experimental results which sensitively test models of correlated electron processes in atomic and molecular physics. The design criteria and consequent technical challenges posed by the short wavelengths of x-rays and desired resolving power are discussed. The fundamental and practical aspects of soft x-ray interferometry are also explored.
High energy x-ray phase contrast experiment with an unprecedent resolution is shown. The coherent and divergent submicron beam from an x-ray waveguide is used to realize a lensless microscope and to magnify spatial variations in optical path length 500 times or more. The defocused image of a nylon fiber with a resolution of 0.14 micron is presented. Exposure times as short as 0.1 seconds gave already visible contrast, opening the way to high resolution, real time studies.
A ne method to record x-ray in-line holograms with quasi- equal-path is proposed in this paper, the key is using a micro zone plate. This method has several advantages over the traditional x-ray in-line holography. First, the requirement of the temporal coherence for the x-ray beam is very low. Second, it reduces the requirement of the resolution of recording media, and last it weakens the disturbance of the twin image. The method can also be used to measure the spatial coherence of x-ray with a single exposure.
We have implemented in the undulator first-optics enclosure of the Massachusetts Institute of Technology-McGill University-IBM Corporation Collaborative Access Team Sector at the Advanced Photon Source an x-ray beamline and a spectrometer optimized for performing small-angle, wide- bandpass, coherent x-ray scattering experiments. We describe the novel features of this set-up. The performance of the beamline and the spectrometer has been characterized by measuring static x-ray speckle patterns form isotopically disordered aerogels. Statistical analysis of the speckle patterns has been performed from which we extract the speckle width sand contrast versus wave-vector transfer and sample thickness. The measured speckle widths and contrast are compared to direct numerical evaluations of the intensity correlation function. The calculated widths are in poor agreement with the measurements but the calculated contrast agrees well with the measured contrast.