We present thresholds for optical breakdown in bulk transparent solids and water with 10-fs laser pulses. In solids, we used microscopy and scattering techniques to determine thresholds for plasma formation and permanent damage in a wide variety of materials. Transmission measurements show that damage occurs at energies where there is little absorption of the laser pulse. In water, we used scattering and acoustic techniques to measure the breakdown threshold for 100-fs pulses. In contrast to solids, transmission measurements in water indicate that there is no plasma or bubble formation unless there is significant absorption. For comparison, we also measured breakdown thresholds for 200-ps pulses.

Pulsed x-ray sources have been used in transient structural phenomena investigations for over fifty years; however, until the advent of synchrotron sources and the development of table-top picosecond lasers, general access to high temporal resolution x-ray diffraction was relatively limited. Advances in diffraction techniques, sample excitation schemes, and detector systems, in addition to increased access to pulsed sources, have led to what is now a diverse and growing array of pulsed-source measurement applications. A survey of time- resolved investigations using pulsed x-ray sources is presented and research opportunities using both present and planned pulsed x-ray sources are discussed.

Optical pump, x-ray diffraction probe experiments have been used to study the lattice dynamics of organic materials using a laser-produced plasma x-ray source. The x-ray source is generated from a 10 Hz, 26 mJ, 120 fs laser beam focused on a silicon wafer target. The emitted K_(alpha ) x-ray radiation is used to probe a cadmium arachidate Langmuir-Blodgett film and a TlAP crystal optically perturbed at laser fluences from 1.8 J/cm² to 27 J/cm². Ultrafast disordering inside the lattice -- within a time scale below 600 fs to few tens of picoseconds -- is clearly observed and produce a drop of the probe x-ray diffracted signal.

Laser-plasma X-ray sources can reach peak brilliances comparable to synchrotron sources, with pulse durations up to 3 orders of magnitude shorter. Future applications of these sources in the investigation of femtosecond phenomena are particularly attractive because of their low cost and wide accessibility. Even modest laser systems can be used for single-shot experiments when combined with optimized crystal optics and sensitive detectors. Crystals are considered here for ultrafast absorption spectroscopy, in which both the variation of the penetration depth of x-rays and expansion of the source have to be taken into account. For real-time diffractometry, a combination of a toroidally-bent crystal together with a thin flat sample-crystal with low dispersion is proposed. Such arrangements can also be used to measure X- ray pulse duration or to shorten synchrotron pulses in the picosecond region.

The generation of ultra-short x-ray pulses from femtosecond electron bunches, produced with laser driven plasma based accelerators, is discussed. The proposed accelerator relies on standard laser wakefield excitation by a high power ultra- short laser drive pulse in a plasma. Femtosecond duration electron bunches are produced through laser triggered injection using the colliding laser pulse scheme. Numerical simulations of test particles, with prescribed plasma and laser fields, indicate that for a 5 TW laser drive pulse, the wakefield acceleration scheme has the capability to produce relativistic (10s of MeV's) electron bunches over an acceleration length equal to the diffraction distance of the laser beam. The colliding pulse offers the possibility to produce femtosecond long electron bunches with peak currents on the order of a kilo Ampere, fractional energy spread of order a few percent and normalized transverse emittance less than 1 mm-mrad using 1 TW injection pulses. An example of the performance of this electron beam source for the generation of femtosecond X-rays using Bremsstrahlung and laser scattering will be discussed.

High-contrast energy modulation (HCM) of high-brightness electron bunches, made possible by recent experimental or conceptual advancements in ultra-short-pulse terawatt IR/visible/UV laser and storage ring-based insertion device technologies, makes it possible to consider schemes for generating ultra-short radiation pulses based on the highly non-linear bunching induced by the modulation. In prior publications, the single-particle interaction with the laser and insertion device fields was analyzed and shown to lead to the appropriate initial conditions for bunching. In the present paper we report on continuing studies of the collective (i.e., multi-particle) dynamics of the modulated bunch region. In addition, we describe preliminary analyses that indicate that the coherent power generated by HCM sources could be made comparable to or greater than that of corresponding single-pass, gain-saturated Free Electron Lasers (FELs). Selected differences between the dynamics and performance of conventional high-harmonic FELs and Optical Klystrons (OKs) and the analyzed high-contrast modulation/bunching system are discussed. Some possible applications of HCM to the flexible tailoring of particle beam phase space parameters are mentioned.

We report on the MBI User Facility at BESSY II, presently under construction, which is dedicated to study the dynamics of photo-induced processes by combining laser and synchrotron pulses. In this paper we focus on the synchronization of a modelocked ultrafast Ti:sapphire laser to the Berlin electron storage ring for synchrotron radiation (BESSY). Two different techniques have been applied -- one based on a digital phase comparator and the other based on analog high-harmonic mixing. Both schemes may be easily adjusted to either single, multi- or hybrid-bunch operation of the synchrotron. Moreover, the temporal accuracy of the synchronization unit suitably matches the widths of the synchrotron pulses (some ten picoseconds) to be expected at BESSY II. Therefore, the currently performed test experiments at BESSY I provide the basis for time- resolved photon-induced experiments which combine laser and SR-undulator pulses in a pump-probe scheme at BESSY II. This facility will be available within the first half of 1999.

For storage ring based synchrotron radiation sources which deliver high flux coherent X-ray pulses at a high repetition rate, the figure of merit is the brilliance. Third generation synchrotron radiation sources provide a brilliance in the 10²⁰ range, in the usual units, for 10 keV photon beams. Such a performance approaches the ultimate limit imposed by the diffraction limit of synchrotron radiation emission. A gain of up to 6 orders of magnitude, with respect to the brilliance of second generation sources, was obtained by reducing transverse electron beam emittances and by optimizing undulator radiation. The horizontal emittance was dramatically decreased by reducing the dispersion function in the bending magnets. As a consequence, the dispersion of the electron revolution time around the ring, as a function of energy, was reduced. Under this quasi-isochronous condition, electron bunch lengths were naturally shortened and reached the few tens of ps range. The quasi-isochronous tuning of storage rings was further tested on third generation rings to evaluate the possibilities of reaching the subpicosecond range and improving the present peak brilliance of 10²³. Unfortunately, the nature of the electromagnetic environment, with which the electron bunch interacts, makes the bunch lengthen with increasing current. Best predictions of peak brilliance performances achievable on storage rings will leave them far behind linac driven FEL sources if the principle of Self Amplified Spontaneous Emission works as predicted.

In an earlier paper the emission process of synchrotron radiation was studied theoretically for the current induced by a single electron interacting with a bending magnet showing that a chirp of the hard x-ray could be expected that could be used for a compression of x-ray bunches using dispersive optics. In this paper we will present a detailed analysis of the results of an experimental study that was conducted on a bending magnet beamline (BM5) at the ESRF to learn if such a chirp exists for an electron bunch as a function of the x-ray energy and of the emission angle below and above the electron orbit plane. Fourier transforms of the observed time signals were performed that indicated a phase shift of the x-rays with energy and emission angle. Using a classical method to describe the propagation of quasi-Gaussian beams and introducing partial time and space derivatives of the wave packet drift, we established a new relationship to describe analytically this particular behavior of synchrotron x-rays that involves a relativistic Doppler effect.

In an earlier paper the emission of synchrotron radiation was studied theoretically for the current induced by a single electron interacting with a bending magnet showing that a chirp of the hard x-ray could be expected that could be used for a compression of x-ray bunches using dispersive optics. In this paper we will present results of an experimental study that was conducted on a bending magnet beamline (BM5) at the ESRF to learn if such a chirp exists for an electron bunch as a function of the x-ray energy and of the emission angle below and above the electron orbit plane. Our results for the pulse shape show that such a chirp could not be clearly detected at a level of several tens of picoseconds FWHM. This means that the phase relationships of all the electrons producing the x- ray bunch are averaged out leading to the absence of a net chirp. However, a fast Fourier transform of the observed signal showed that there was a phase lag. This will be described in a companion paper. Here, the details of the experimentally observed time structure will be given.

Time resolved detection in both detector and synchrotron resolution mode are presented. Using this technique the response of the Bragg profile in perfect Silicon to a MHz ultrasonic wave field has been studied on the high resolution triple crystal diffractometer at the ESRF high energy beamline ID15A. High photon energies up to 500 keV have been used for the analysis of a longitudinal acoustic wave in the 10 mm bulk. First, conventional time averaged rocking curves show intensity gains by up to a factor of 50, and a reciprocal space mapping reveals full information on the acoustic wave vector. Secondly, time and space resolved reflection curves have been taken. They give detailed insights into the properties of the acoustic wave field in space and time. In particular, they allow us to identify uniquely true standing waves without parasitic strain components or higher harmonic excitation. Time averaged and snapshots of the time resolved rocking curves represent different aspects of the density of states for the lattice parameter distribution. They are understood analytically by the elliptic integral K and the inverse circle function, respectively.

After a brief review of the research and development activities on the control of the time structure of x-ray beams conducted at the European Synchrotron Radiation Facility, we present first results of the performance of an optical chopper based on a vibrating crystal device. With this system we were able to select time windows from about 200 ns to infinite with a repetition rate of 20 kHz and the high throughput provided by the high reflectivity of perfect single crystals.

The diffraction of an X-ray beam on an ultrasonic wave propagating at the surface of a crystal gives rise to diffraction satellites whose temporal structure is correlated to the one of the ultrasonic wave. Two different applications of this phenomenon are presented in this paper: By matching the temporal structure of the surface acoustic wave (SAW) with the filling mode of the storage ring, it is possible to pick or reject the incident X-ray pulses and thus to build a chopper working in the MHz range. The second application deals with binary data transfer by an X-ray beam. The experiment consists in modulating the signal fed to the acoustic transducer with a signal carrying digital data, which is a way of transmitting this information into the X-ray beam satellite.

With the advent of new x-ray sources, the detection of radiation with picosecond temporal resolution becomes increasingly important. We used the time structure of the radiation at the European Synchrotron Radiation Facility (ESRF) to characterize fast detectors developed for x-ray diagnostics. The detectors tested were neutron pre-irradiated Gallium Arsenide (GaAs) detectors and chemical-vapor-deposited (CVD) diamond detectors. The experiments were performed on beamline BM5, both in single-bunch and multi-bunch mode. The response of the detectors to the hard x-ray pulses was recorded using a single shot 7 Ghz oscilloscope or using a digital oscilloscope in sampling mode. For a current of the electrons in the storage ring of typically 10 - 15 mA in monomode-bunch, the pulse length was between 100 ps and 150 ps (FWHM). In multi-bunch mode, the X-ray pulse length was independent of the current and shorter. We measured FWHM pulse duration values down to 50 ps. Comparison with data obtained with a streak camera showed that the time resolution of our detectors was at best 25 ps.

The advent of CPA femtosecond lasers has opened the way to a new regime of interaction with atoms and molecules. In some experiments like time-resolved x-ray diffraction of laser- excited samples, the signal to be measured can contain very few photons and repetition rates up to 1 kHz are required. The laser-triggered x-ray streak camera system is therefore a promising tool for the study ultrashort x-ray events. We present the results of the characterization tests performed on our subpicosecond x-ray streak camera at the University of Michigan and at the European Synchrotron Radiation Facility. This new ultrafast diagnostic is triggered by a short laser pulse and can acquire images at rates up to 1 kHz and features a subpicosecond time resolution along with a 40 micrometer spatial resolution. We discuss the different issues related to the interaction between the laser pulse and photo-conductive switches, the synchronization of the detector.

We provide a general introduction to the use of silicon avalanche photo-diodes (APDs) for x-ray timing measurements. We describe (and compare) some devices available from various manufacturers. In general, time resolutions of approximately 1 ns are typical, and pulse widths from such devices are in the range of 1 to 10 ns, allowing high count rates to be reached, easily in excess of 10⁷ Hz. We include a table of different devices and detailed references.

Single-pass free electron lasers (FEL) based on the self- amplification of spontaneous emission (SASE) do not require optical resonators and may therefore be used in the vacuum ultraviolet (VUV) and X-ray region down to wavelengths around 0.1 nm. Due to the start-up from noise, however, there is only little longitudinal coherence, and the photon pulses consist of many uncorrelated narrow lines. In this paper we present the conceptual design of two different optical schemes for the VUV SASE-FEL at DESY, aiming at the production of fully coherent, tunable VUV and soft X-ray radiation with good pulse-to-pulse stability. At long wavelengths it is planned to install a narrow-band optical feedback using a blazed grating in Littrow mounting. This scheme is based on the high repetition rate of the accelerator and combines the light pulse coming from the first electron bunch with the fifth electron bunch which amplifies it to saturation. At shorter wavelengths it is proposed to divide the undulator into two parts and put a high-resolution grazing-incidence monochromator in between. This scheme works for any pulse timing because the seeding pulse is combined with the same electron bunch at the entrance of the second undulator driving it to saturation.

In recent years, substantial exploratory studies have been conducted on the source properties of idealized Angstrom- wavelength Free-Electron Lasers (FELs) operating in the Self- Amplified Spontaneous Emission (SASE) regime. With the more recent advent of dedicated design studies aimed at developing SASE FEL R&D facilities, attention has focused on developing more realistic descriptions of the FEL source taking into account various anticipated departures of the electron beam from ideality. In this paper we extend prior statistical descriptions of the SASE FEL source to enable the systematic inclusion of the effects of correlated variations of the electron beam's phase-space parameters vs, e.g., longitudinal position in the bunch. Selected practical consequences of incorporating the extended source parameter space in the development or design of coherence-related methods and instruments are discussed.

We describe the proceedings of the Workshop on the Uses and Generation of Femtosecond Radiation, held at the E. O. Lawrence Berkeley National Laboratory (LBNL), in February 1998, and some of the ideas that were generated subsequent to the workshop. The motivation for this workshop was to bring together accelerator physicists interested in the generation of ultra-short (less than 200 fs) pulses of XUV and x-ray radiation, and scientists interested in using them. The primary purpose of the workshop was to educate the accelerator physicists about the source characteristics necessary to carry out specific experiments, and to inform the user community of ideas currently being explored by the accelerator community. A second objective was to develop a set of parameters and requirements that could form the basis for a broad-based femtoscience user facility. In this paper we describe some of the ideas and techniques that accelerator physicists are pursuing to fulfill the diverse requirements of this expanding community.

A beam chopper together with the temporal structure of x-rays emitted by a synchrotron storage ring can be utilized to generate x-ray bursts of variable length and time separation. A Si cube, cut for diffraction from the (220) planes, was mounted to a low-speed motor to produce a beam chopper based upon the Darwin width of the crystal. An x-ray pulse, consisting of an envelope of individual pulses characterizing the loading pattern of the storage ring, was transmitted. The width of the transmitted pulse and the time between pulses was varied by varying the rotation frequency of the Si cube. Pulses as short as approximately equal 75 ps or as long as approximately equal 4 microseconds were transmitted with pulse separation spanning from 4 ms to 167 ms.

We present the concept of an experiment that aims at probing in real-time photoinduced structural modifications in a large class of media ranging from biological systems to solid state materials using time-resolved X-ray spectroscopies (Extended X-ray Absorption Fine Structure-EXAFS or X-ray Absorption Near-Edge Structure-XANES) in the picosecond time domain. The principle of the experiment is based on the pump-probe scheme, where an ultrashort laser pulse induces a structural modification in a chemical, physical or biological system. The picosecond hard X-ray pulse from a synchrotron probes the evolution of the structure in real-time by means of an adjustable time delay between the pump and the probe pulses. The paper discusses the case of iodine in liquid solvents as a test bench of the technique.