High-brightness beam tests of the very high frequency gun at the Advanced Photo-injector EXperiment test facility at the Lawrence Berkeley National Laboratory.

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 free electron lasers (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 [Sannibale et al., Phys. Rev. Spec. Top.-Accel. 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 test facility at LBNL that demonstrated the gun capability of delivering the beam quality required for driving high repetition rate X-ray FELs.

The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction.

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.

Beyond a phenomenological description of magnetostriction.

Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction-the underlying magnetoelastic stress-can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the sub-picosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization.

Publisher Correction: Beyond a phenomenological description of magnetostriction.

"The technical support from SLAC Accelerator Directorate, Technology Innovation Directorate, LCLS laser division and Test Facility Division is gratefully acknowledged. We thank S.P. Weathersby, R.K. Jobe, D. McCormick, A. Mitra, S. Carron and J. Corbett for their invaluable help and technical assistance. Research at SLAC was supported through the SIMES Institute which like the LCLS and SSRL user facilities is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-76SF00515. The UED work was performed at SLAC MeV-UED, which is supported in part by the DOE BES SUF Division Accelerator & Detector R&D program, the LCLS Facility, and SLAC under contract Nos. DE-AC02-05-CH11231 and DE-AC02-76SF00515. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515."and"Work at BNL was supported by DOE BES Materials Science and Engineering Division under Contract No: DE-AC02-98CH10886. J.C. would like to acknowledge the support from National Science Foundation Grant No. 1207252. E.E.F. would like to acknowledge support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES) under Award No. DE-SC0003678."This has been corrected in both the PDF and HTML versions of the Article.

Stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2 probed with MeV ultrafast electron diffraction.

Transitions between different charge density wave (CDW) states in quasi-two-dimensional materials may be accompanied also by changes in the inter-layer stacking of the CDW. Using MeV ultrafast electron diffraction, the out-of-plane stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2 is investigated for the first time. From the intensity of the CDW satellites aligned around the commensurate l = 1/6 characteristic stacking order, it is found out that this phase disappears with a 0.3 ps time constant. Simultaneously, in the same experiment, the emergence of the incommensurate phase, with a slightly slower 2.0 ps time constant, is determined from the intensity of the CDW satellites aligned around the incommensurate l = 1/3 characteristic stacking order. These results might be of relevance in understanding the metallic character of the laser-induced metastable "hidden" state recently discovered in this compound.

A direct electron detector for time-resolved MeV electron microscopy.

The introduction of direct electron detectors enabled the structural biology revolution of cryogenic electron microscopy. Direct electron detectors are now expected to have a similarly dramatic impact on time-resolved MeV electron microscopy, particularly by enabling both spatial and temporal jitter correction. Here we report on the commissioning of a direct electron detector for time-resolved MeV electron microscopy. The direct electron detector demonstrated MeV single electron sensitivity and is capable of recording megapixel images at 180 Hz. The detector has a 15-bit dynamic range, better than 30-µm spatial resolution and less than 20 analogue-to-digital converter count RMS pixel noise. The unique capabilities of the direct electron detector and the data analysis required to take advantage of these capabilities are presented. The technical challenges associated with generating and processing large amounts of data are also discussed.

Electron-lattice energy relaxation in laser-excited thin-film Au-insulator heterostructures studied by ultrafast MeV electron diffraction.

We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels.

A novel system for measurement of the transverse electron momentum distribution from photocathodes.

The transverse momentum of electrons produced by a photocathode contributes significantly to the performance of several different types of accelerator-based light sources, such as Free Electron Lasers, as well as systems designed for ultrafast electron diffraction and dynamic transmission electron microscopy. Minimization of the transverse emittance from photocathodes is the subject of intensive research, and therefore measurement of this parameter is of great importance. Here, we describe a simple system that offers real time measurements of transverse emittance and can be easily integrated into the photocathode fabrication process.

Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory.

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.

Use of a buried nasolabial flap to fill a depression over the lower jaw. Case report.

A viable local flap is not plagued by the problems of resorption and dissolution. If it is to be denuded and buried, however, great care must be taken to select a donor area that is as hairless as possible. Otherwise, there may be cyst formation, drainage, and infection.

Construction of the cupid's bow.

The creation of the cupid's bow, philtral dimple, and tubercle can greatly improve the central upper lip. The white roll is dissected free as a bucket handle. A crescent of central vermilion is excised and planted as a composite graft to simulate a tubercle. The white roll is sutured to the vermilion, and a full-thickness skin graft is sutured to the new philtral dimple.

Reconstruction of the oral mucocutaneous junction.

An inset free skin graft from the postauricular area was used to create a natural appearing "white roll" mucocutaneous junction in a lip repair.

Treatment of temporal scalp wound separation in the rhytidectomy patient.

A method of approaching the separated, granular temporal scalp wound in a rhytidectomy patient is presented. The time-honored principles of wound healing are discussed with reference to the first skin graft described.

Glabellar frown lines: direct excision, an evaluation of the scars.

The direct excision of glabellar frown lines is a method that is not new, but neither has it been well evaluated. This paper describes the method, discusses the reason for the problem, and suggests why this method should have long-lasting results. Several examples of the scars are demonstrated. In selected patients, this may be a reasonable alternative to the myriad surgical possibilities in our quest for a solution to this problem.

Rhytidectomy flap necrosis in Raynaud's disease.

A case of flap necrosis after rhytidectomy in two patients with Raynaud's symptomatology are presented. The pathology and complications are explained and reasons postulated for the necrosis and wound-healing problems. Clinical descriptions of the fingertips and skin may help give a cautious warning for similar cosmetic surgical candidates.

Reconstruction and/or salvage of nipple projection.

A simple method of creating nipple projection in flattened nipple-areola complexes is presented. The results required, on average, 14 months of follow-up, with very good projection and general appearance in all patients. This technique eliminates the unnecessary surgical gymnastics of other methods when obtaining nipple projection.

Closure of scalp defects by using multiple flaps in a pinwheel design.

A multi-flap design for closure of defects in the inelastic scalp is presented. Two cases are reported, and the advantages of the technique are described. The engineering principle is elaborated and discussed.

Hidradenitis suppurativa of the breast areolae. Case report.

A case of hidradenitis suppurativa of the areolae of the breasts is presented. The treatment was excision and split-skin grafting, and this was successful.

Further experiences with the arytenoid-epiglottic flap for chronic aspiration pneumonia.

We present our experiences with 7 patients in whom we did arytenoid-epiglottic flaps for the treatment of chronic aspiration pneumonia. The indications for the operation are outlined, and some technical suggestions are made.

Myofibroblasts and free silicon around breast implants.

Fibrous tissue capsules, from around silicone breast implants, were examined by light microscopy, and by transmission and scanning electron microscopy, the latter combined with energy dispersive X-ray analysis. Contractile fibroblasts (myofibroblasts) were found in most of these fibrous capsules, and especially in those recently formed or manipulated. The element silicon was positively identified by X-ray analysis in half the tissue specimens. This material was found only around gel-filled implants. In the specimens studied, the clinical hardness of the breast was unrelated to the presence of silicon in the capsule.