A. Zholents

A recirculating linac-based facility for ultrafast x-ray science

We present an updated design for a proposed source of ultra-fast synchrotron radiation pulses based on a recirculating superconducting linac, in particular the incorporation of EUV and soft x-ray production. The project has been named LUX - Linac-based Ultrafast X-ray facility. The source produces intense x-ray pulses with duration of 10-100 fs at a 10 kHz repetition rate, with synchronization of 10s fs, optimized for the study of ultra-fast dynamics. The photon range covers the EUV to hard x-ray spectrum by use of seeded harmonic generation in undulators, and a specialized technique for ultra-short-pulse photon production in the 1-10 keV range. High-brightness rf photocathodes produce electron bunches which are optimized either for coherent emission in free-electron lasers, or to provide a large x/y emittance ration and small vertical emittance which allows for manipulation to produce short-pulse hard x-rays. An injector linac accelerates the beam to 120 MeV, and is followed by four passes through a 600-720 MeV recirculating linac. We outline the major technical components of the proposed facility.

Accelerator physics challenges of the fs-slicing upgrade at the ALS

The goal of the Femtoslicing project at the ALS is to provide 100-200 fs long pulses of soft and hard x-rays with moderate flux and with a repetition rate of 10-40 kHz for experiments concerning ultrafast dynamics in solid state physics, chemistry and biology. The femtoslicing principle employs a femtosecond laser beam to interact resonantly (inverse FEL interaction) with the electron beam in the ALS. The induced energy spread over the femtosecond duration is converted to a transverse displacement by exploiting the dispersion of the storage ring. The displaced femtosecond electron pulse then radiates and produces femtosecond synchrotron radiation. To achieve the necessary spatial separation of the energy modulated slice from the rest of the bunch, a sizeable local vertical dispersion bump in the undulator used as radiator is required. This presents challenges in terms of the nonlinear dynamics and control of the vertical emittance.

A recirculating linac for ultrafast X-ray science

Ultrafast X-ray science is an important new research frontier that is driving the development of a novel source for generating femtosecond X-ray pulses. Femtosecond X-rays will open new areas of research in physics, chemistry and biology by enabling direct measurements of structural dynamics in matter on the fundamental time scale of a vibrational period. Modem synchrotrons and X-ray techniques have dramatically advanced our scientific understanding of the static or time-averaged structure of matter on the atomic scale. However, the structure of matter is not static, and to understand the behavior of matter at the most fundamental level requires structural measurements on the time scale on which atoms move. The evolution of structure is dictated by the making and breaking of chemical bonds and the rearrangement of atoms, and this occurs on the time scale of a vibrational period, -100 fs. Atomic motion on this time scale ultimately determines the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes. A thorough understanding of such dynamic behavior is a first step to being able to control structural evolution and will have important scientific applications. At present, our ability to measure and understand fundamental structural dynamics is limited by lack of appropriate experimental tools. Femtosecond lasers provide the requisite time resolution and have been applied to study ultrafast processes in numerous fields of research. The award of the 1999 Nobel Prize in Chemistry for the application of femtosecond lasers to the study of chemical dynamics underscores the fundamental significance of this time regime. In condensed matter systems, however, visible photons from lasers are not effective as structural probes since they interact with electronic states extending over multiple atoms. On the other hand, X-ray photons can provide the requisite structural information via interaction with localized core levels. This capability is driving the emergence of ultrafast X-ray science as a new field of research in which X-ray techniques are used in combination with femtosecond lasers and applied in a time-resolved manner to probe structural dynamics, as illustrated in Figure 1. Recently, a number of important advances have been made in the generation of ultrashort X-ray pulses using a variety of techniques, including high-order harmonic generation from femtosecond optical pulses [ 1-41, laser-plasma-based X-ray sources 15-71, and laser-driven X-ray diodes [8]. However, the average X-ray flux (and brightness) from these sources is many orders or magnitude below what is required for sensitive structural measurements. Such performance is pi,esently available only in long pulses &om modem synchrotrons. The longstanding effort to develop ultrafast X-ray science at Lawrence Berkeley National Laboratory (LBNL) began with the development of a source of femtosecond X-rays based on Thomson scattering between relativistic electrons from the injector linac at the Advanced Light Source (ALS) and a terawatt femtosecond laser pulse [9,10,1 I]. The capabilities of this source were sufficient for studies of ultrafast ordeddisorder transitions in crystalline solids using Bragg diffraction [ 121. A parallel effort in time-resolved diffraction at the ALS made use of an ultrafast X-ray streak-camera detector and longpulse X-rays from a bend-magnet beamline [13,14]. More recently, we have demonstrated the generation of femtosecond X-rays directly from the ALS storage ring using a technique in which a femtosecond laser pulse modulates the energy of a thin slice of a stored electron bunch [ 15-16]. This approach can provide substantially higher flux and brightness in comparison with sources based on Thomson scattering (with a pulse duration of -100 fs) [ 171. Presently, a bend-magnet beamline is being commissioned at the ALS which makes use of this technique (flux and brighmess shown in Figure Z), and an undulator beamline has been proposed which will provide -1 O3 improvement in flux and -lo4 improvement in brightness in 200-fs X-ray pulses in the 0.5 to 8-keV range. The performance of these beamlines represents the practical limit of what can be achieved at existing third-generation synchrotron light sources. Nevertheless, the femtosecond X-ray flux and brightness is nearly seven orders of magnitude below what is typically available for static X-ray measurements. While initial time-resolved X-ray experiments have been performed using the sources mentioned earlier, they suffer from significant limitations in one or more critical parameters, including (a) X-ray

A proposal for the generation of ultra-short X-ray pulses

In this paper it is shown that optical stochastic cooling in a 150 MeV electron storage ring will allow production of a beam with longitudinal emittance 4.5 {times} 10{sup {minus}6} MeV{center_dot}m. Such a small emittance accompanied with a bunch compression technique based upon the transformation in the longitudinal phase space will allow achieving a bunch length 30 {mu}m. This bunch could then be used for the generation of ultra-short x-ray pulses by the Compton scattering of laser photons.

A high repetition rate VUV-soft X-ray FEL concept

We report on design studies for a seeded FEL light source that is responsive to the scientific needs of the future. The FEL process increases radiation flux by several orders of magnitude above existing incoherent sources, and offers the additional enhancements attainable by optical manipulations of the electron beam: control of the temporal duration and bandwidth of the coherent output, reduced gain length in the FEL, utilization of harmonics to attain shorter wavelengths, and precise synchronization of the X-ray pulse with seed laser systems. We describe an FEL facility concept based on a high repetition rate RF photocathode gun, that would allow simultaneous operation of multiple independent FELs, each producing high average brightness, tunable over the VUV-soft X-ray range, and each with individual performance characteristics determined by the configuration of the FEL. SASE, enhanced-SASE (ESASE), seeded, harmonic generation, and other configurations making use of optical manipulations of the electron beam may be employed, providing a wide range of photon beam properties to meet varied user demands.

Numerical study of coulomb scattering effects on electron beam from a nano-tip

Nano-tips with high acceleration gradient around the emission surface have been proposed to generate high brightness beams. However, due to the small size of the tip, the charge density near the tip is very high even for a small number of electrons. The stochastic Coulomb scattering near the tip can degrade the beam quality and cause extra emittance growth and energy spread. In the paper, we present a numerical study of these effects using a direct relativistic N-body model. We found that emittance growth and energy spread, due to Coulomb scattering, can be significantly enhanced with respect to mean-field space-charge calculations.

Methods of Attosecond X-Ray Pulse Generation

We review several proposals for generation of solitary attosecond pulses using two types of free electron lasers which are envisioned as future light sources for studies of ultra-fast dynamics using soft and hard x-rays.

Techniques for synchronization of X-ray pulses to the pump laser in an ultrafast X-ray facility

Accurate timing of ultrafast X-ray probe pulses emitted from a synchrotron radiation source with respect to the signal initiating a process in the sample under study is critical for the investigation of structural dynamics in the femtosecond regime. We describe schemes for achieving accurate timing of femtosecond X-ray synchrotron radiation pulses relative to a pump laser, where X-rays pulses of <100 fs duration are generated from the proposed LUX source based on a recirculating superconducting linac. We present a description of the timing signal generation and distribution systems to minimize timing jitter of the X-rays relative to the experimental lasers.

Simulation of the microbunching instability in beam delivery systems for free electron lasers

In this paper, we examine the growth of the microbunching instability in the electron beam delivery system of a free electron laser (FEL). We present the results of two sets of simulations, one conducted using a direct Vlasov solver, the other using a particle-in-cell code Impact-Z with the number of simulation macroparticles ranging up to 100 million. Discussion is focused on the details of longitudinal dynamics and on numerical values of uncorrelated (slice) energy spread at different points in the lattice. In particular, we assess the efficacy of laser heater in suppression of the instability, and look at the interplay between physical and numerical noise in particle-based simulations.

Re-circulating linac vacuum system

The vacuum system for a proposed 2.5 GeV, 10/spl mu/A re-circulating linac synchrotron light source is readily achievable with conventional vacuum hardware and established fabrication processes. Some of the difficult technical challenges associated with synchrotron light source storage rings are sidestepped by the relatively low beam current and short beam lifetime requirements of a re-circulating linac. This minimal lifetime requirement leads directly to relatively high limits on the background gas pressure through much of the facility. The 10/spl mu/A average beam current produces very little synchrotron radiation induced gas desorption and thus the need for an ante-chamber in the vacuum chamber is eliminated. In the arc bend magnets, and the insertion devices, the vacuum chamber dimensions can be selected to balance the coherent synchrotron radiation and resistive wall wakefield effects, while maintaining the modest limits on the gas pressure and minimal outgassing.