An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. SLAC-PUB-12162 Submitted to Applied Physics Letters 2Our understanding about dynamical processes in chemistry, materials science and biology on the picosecond and sub-picosecond time scale stems almost exclusively from time-resolved spectroscopy. Structural changes, on atomic length scales, can only be inferred indirectly from the analysis of spectra. Both x-ray and electron diffraction share the goal of 'imaging' molecular structures with a time resolution that captures the motions as systems evolve, whether they be solids, liquids or gases. Lab scale experiments in both electron diffraction 1,2 and x-ray scattering 3 have produced impressive results. Recently, in anticipation of the construction of the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), an experiment using the electron bunch from the SLAC Linac to produce spontaneous undulator radiation 4 has shown the possibilities for ultrafast x-ray scattering from condensed systems with 100 fs time resolution. 5 This has encouraged us to approach ultrafast electron diffraction (UED) using experimental techniques based on electron sources developed for particle accelerators, with the aim of obtaining single-shot diffraction patterns on a 100 fs time scale.Electron diffraction is complementary to x-ray scattering, but features much larger cross sections that allow the study of surface phenomena, the bulk structures of thin foils and membranes, as well as molecular structures of gas phase samples. 6 As with linac based x-ray sources there has been significant development of electron sources for UED based on the use of photocathodes. 7 Unfortunately, the space-charge interactions of the electrons within a pulse, and the initial kinetic energy distribution with which the electrons are generated, have made it difficult to obtain pulses much shorter than 1 ps 8,9,10 ,in 'conventional' UED experiments using ≈30 keV electron beams. To improve the time resolution one could use fewer electrons per pulse, but that requires longer data acquisition times to obtain the necessary signal-to-noise ratio. 11 Alternatively, it is possible to increase the electric field inside the electron gun, while reducing the flight distance between the gun and the target. 12 Both tend to reduce the time of flight of the electron pulse, thereby giving the electron pulse less time to spread. Even so, this 3 approach is limited because the maximum DC and pulsed electric fields are 12 MV/m and 25 MV/m, respectively. 13,14 In the present work we take a fresh approach to ultrafast time-resolved pump-probe diffraction by using MeV electron be...

Purpose: MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. Methods: Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified. Results: For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-widthtenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields. Conclusions: Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility. C 2016 American Association of Physicists in Medicine. [http://dx

No abstract

No abstract

The very-high-frequency gun (VHF-Gun) is a new concept photo-injector developed and built at the Lawrence Berkeley National Laboratory (LBNL) for generating high-brightness electron beams capable of driving X-ray FELs at MHz-class repetition rates. The gun that purposely uses established and mature radiofrequency and mechanical technologies, has demonstrated over the last many years the capability of reliably operating in continuous wave mode at the design accelerating fields and required vacuum and mechanical performance. The results of VHF-Gun technology demonstration were reported elsewhere [F. Sannibale, et al., Phys. Rev. ST-Accel. and Beams 15, 103501 (2012)], here in this paper we provide and analyze examples of the experimental results of the first high-brightness beam tests performed at the Advanced Photo-injector EXperiment (APEX) test facility at LBNL that demonstrated the gun capability of delivering the beam quality required for driving high repetition rate X-ray FELs.

The Linac Coherent Light Source (LCLS) at SLAC requires the rf photo-injector to produce a beam with a normalized, projected emittance of 1 micron in a 10 ps long bunch with a charge of 1nC. In addition, a small longitudinal emittance is needed to attain the desired 3 kiloamperes peak current after compression in two chicane bunchers. To achieve this excellent beam quality, we are performing systematic studies of both the transverse and longitudinal beam properties from the rf photocathode gun at the SLAC Gun Test Facility (GTF). Time resolved emittances (slice) are determined by using a bunch with a linear energy chirp which is dispersed by a magnetic spectrometer. By varying the strength of a quadrupole lens upstream of the spectrometer allows measurement of the individual slice emittances. Spectrometer images at the various quadrupole settings are binned in small energy/time windows and analyzed for the slice parameters. Our measurements indicate a temporal resolution of approximately 100 femtoseconds. In addition, the longitudinal phase space distribution is determined by measuring the energy spectrum over a range of linac phases. The correlated and uncorrelated components of the phase space distribution are determined by fits to the energy spectra analogous to a quad scan in the transverse dimension. The combined analysis of the transverse and longitudinal data gives not only the slice and longitudinal emittances, but also any correlations due to wakefields or other effects.

No abstract

The design of the Linac Coherent Light Source assumes that a low-emittance, 1-nC, 10-ps beam will be available for injection into the 15-GeV linac. The proposed rf photocathode injector that will meet this requirement is based on a 1.6-cell S-band rf gun equipped with an emittance compensating solenoid. The booster accelerator with a gradient of 25 MV/m is positioned at the beam waist coinciding with the first emittance maximum, i.e., the "new working point." The uv pulses required for cathode excitation will be generated by tripling the output of a Ti:sapphire laser system. Details of the design and the supporting simulations are presented.