J. Arthur

X-ray free-electron lasers

In a free-electron laser (FEL) the lasing medium is a high-energy beam of electrons flying with relativistic speed through a periodic magnetic field. The interaction between the synchrotron radiation that is produced and the electrons in the beam induces a periodic bunching of the electrons, greatly increasing the intensity of radiation produced at a particular wavelength. Depending only on a phase match between the electron energy and the magnetic period, the wavelength of the FEL radiation can be continuously tuned within a wide spectral range. The FEL concept can be adapted to produce radiation wavelengths from millimetres to Angstroms, and can in principle produce hard x-ray beams with unprecedented peak brightness, exceeding that of the brightest synchrotron source by ten orders of magnitude or more. This paper focuses on short-wavelength FELs. It reviews the physics and characteristic properties of single-pass FELs, as well as current technical developments aiming for fully coherent x-ray radiation pulses with pulse durations in the 100 fs to 100 as range. First experimental results at wavelengths around 100 nm and examples of scientific applications planned on the new, emerging x-ray FEL facilities are presented.

The LCLS: A fourth generation light source using the SLAC linac

Recent technological developments make it possible to consider use of the Stanford linear accelerator to drive a linac coherent light source (LCLS)—a laser operating at hard x‐ray wavelengths. In the LCLS, stimulated emission of radiation would be achieved in a single pass of a high‐energy, extremely bright electron beam through an undulator, without the optical cavity resonator normally used in storage ring‐based free‐electron lasers. The x‐ray laser beam would be nearly diffraction limited with very high transverse coherence, and would exhibit unprecedented peak intensity and peak brightness, and sub‐picosecond pulse length. Such an x‐ray source offers unique capabilities for a large number of scientific applications.

A Multicast Tree Router for Multichip Neuromorphic Systems

We present a tree router for multichip systems that guarantees deadlock-free multicast packet routing without dropping packets or restricting their length. Multicast routing is required to efficiently connect massively parallel systems computational units when each unit is connected to thousands of others residing on multiple chips, which is the case in neuromorphic systems. Our tree router implements this one-to-many routing by branching recursively-broadcasting the packet within a specified subtree. Within this subtree, the packet is only accepted by chips that have been programmed to do so. This approach boosts throughput because memory look-ups are avoided enroute, and keeps the header compact because it only specifies the route to the subtrees root. Deadlock is avoided by routing in two phases-an upward phase and a downward phase-and by restricting branching to the downward phase. This design is the first fully implemented wormhole router with packet-branching that can never deadlock. The designs effectiveness is demonstrated in Neurogrid, a million-neuron neuromorphic system consisting of sixteen chips. Each chip has a 256 × 256 silicon-neuron array integrated with a full-custom asynchronous VLSI implementation of the router that delivers up to 1.17 G words/s across the sixteen-chip network with less than 1 μs jitter.

Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses

Secondary electron cascades were measured in high purity single-crystalline chemical vapor deposition (CVD) diamond, following exposure to ultrashort hard x-ray pulses (140 fs full width at half ma ...

Microchannel water cooling of silicon x‐ray monochromator crystals

The use in silicon x‐ray monochromator crystals of water cooling channels with dimensions optimized for efficient heat transfer from silicon to water has been investigated. Such channels are typically about 40 μm wide and 400 μm deep. Procedures have been found for reliably producing microchannel‐cooled crystals with very small amounts of residual strain. These crystals have been tested at a high‐power wiggler beam line at the Cornell High Energy Synchrotron Source, using an x‐ray beam having total power in excess of 250 W and normal‐incidence power density greater than 5 W/mm2. Under these conditions, the surface‐temperature rise of a typical microchannel‐cooled crystal was less than 5u2009°C, and degradation of the (111) rocking curve at 12 keV was very slight. The cooling efficiency is consistent with analytic calculations.

Chirped-beam two-stage free-electron laser for high-power femtosecond x-ray pulse generation

A method for generating femtosecond-duration x-ray pulses with a free-electron laser is presented. This method uses an energy-chirped electron beam propagating through an undulator to produce a frequency-chirped x-ray pulse by self-amplified spontaneous emission. A short temporal pulse is created by use of a monochromator to select a narrow radiation bandwidth. A second undulator is used to amplify the short-duration radiation. The radiation characteristics produced by a chirped-beam two-stage free-electron laser are calculated, and the performance of the chirped-beam two-stage option for the Linac Coherent Light Source is considered.

Status of the LCLS x-ray FEL program (invited)

The Linac Coherent Light Source (LCLS) program involves a collaboration of several U.S. National Laboratories and universities with the goal of designing and building the first fourth-generation hard x-ray source, an x-ray free-electron laser (FEL). This FEL will utilize extremely short, intense, low-emittance electron pulses created by the high-energy linear accelerator at the Stanford Linear Accelerator Center. The FEL radiation produced will feature unprecedented peak brightness, short pulse length, and spatial coherence, tunable over an energy range of 0.8–8 keV. With favorable funding, major construction will begin by 2004 and the LCLS will be operating late in 2006. The LCLS facility will include experimental stations for carrying out groundbreaking experiments in several scientific fields. Current research and development efforts are directed at experimentally studying the physics of high-gain FELs, and refining the details of the plan for the LCLS facility. The FEL experiments, at Argonne and Broo...

Future possibilities of the Linac Coherent Light Source

A study of the potential for the development of the Linac Coherent Light Source (LCLS) beyond the specifications of the baseline design is presented. These future developments include delivery of X-ray pulses in the 1 fs regime, extension of the spectral range, increase of the FEL power, exploitation of the spontaneous emission, and a more flexible time structure. As this potential is exploited, the LCLS can maintain its role as a world-leading instrument for many years beyond its commissioning in 2008 and initial operation as the worlds first X-ray free-electron laser.

Chirped-beam two-stage SASE-FEL for high power femtosecond X-ray pulse generation

Abstract A method for generating femtosecond duration X-ray pulses using a single-pass free-electron laser (FEL) is presented. This method uses an energy-chirped electron beam propagating through an undulator to produce a frequency-chirped X-ray pulse through self-amplified spontaneous emission (SASE). After the undulator, we consider passing the radiation through a monochromator. The frequency is correlated to the longitudinal position within the pulse; therefore, by selecting a narrow bandwidth, a short temporal pulse will be transmitted. The short pulse radiation is used to seed a second undulator, where the radiation is amplified to saturation. In addition to short pulse generation, this scheme has the ability to control shot-to-shot fluctuations in the central wavelength due to electron beam energy jitter. We present calculations of the radiation characteristics produced by a chirped-beam two-stage SASE–FEL, and consider the performance of the chirped-beam two-stage option for the Linac Coherent Light Source.

Progress report on the LCLS XFEL at SLAC

The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >1012 photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100 m and 350 m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in the middle of 2010. The project design permits straightforward expansion of the LCLS to multiple undulators.