Explore the vast range of titles available.

The terahertz gap is a region of the electromagnetic spectrum where high average and peak power radiation sources are scarce while at the same time scientific and industrial applications are growing in demand. Free-electron laser (FEL) coupling in a magnetic undulator is one of the best options for radiation generation in this frequency range, but slippage effects require the use of relatively long and low-current electron bunches to drive the terahertz FEL, limiting amplification gain and output peak power. Here we use a circular waveguide in a 0.96-m strongly tapered helical undulator to match the radiation and electron-beam velocities, allowing resonant energy extraction from an ultrashort 200-pC 5.5-MeV electron beam over an extended distance. Electron-beam spectrum measurements, supported by energy and spectral measurement of the terahertz FEL radiation, indicate an average energy efficiency of ~10%, with some particles losing >20% of their initial kinetic energy.A single-pass free-electron laser based on a waveguide in a tapered helical undulator is developed. The energy conversion efficiency from a relativistic electron beam to terahertz waves at 0.16 THz is ~10%.

In this paper we present a detailed discussion of the helical undulator system developed for high extraction efficiency experiments in the tapered‐enhanced stimulated superradiant amplification regime. The design is based on permanent magnet technology and comprises two Halbach arrays orthogonally oriented and shifted by 90° with respect to each other. When used in low energy beamlines for THz generation, the electron beam trajectory and transport are particularly sensitive to the undulator off‐axis fields so that it becomes important to complement on‐axis field measurements with analysis and tuning of the higher‐order field components. Here we describe how the pulsed wire measurement technique can be effectively used to retrieve on‐ and off‐axis magnetic field characteristics of the undulator. A simple two‐dipole model is developed to guide the final adjustments to the permanent magnet positions along the array to tune the quadrupole and sextupole components of the field. In this work the transport of low energy electrons through helical magnetic undulators is discussed with application for THz generation in waveguide free‐electron lasers. To optimize beam transmission, the off‐axis fields sampled by low energy beams are tuned with a 3D pulsed‐wire measurement technique that reveals localized errors in the quadrupole and sextupole field moments.

High conversion efficiency between electrical and optical power is highly desirable both for high peak and high average power radiation sources. In this paper we discuss a new mechanism based on stimulated superradiant emission in a strongly tapered undulator whereby a prebunched electron beam and focused laser are injected into an undulator with an optimal tapering calculated by dynamically matching the resonant energy variation to the ponderomotive decelerating gradient. The method has the potential to allow the extraction of a large fraction (∼50%) of power from a relativistic electron beam by converting it into coherent narrow-band tunable radiation, and shows a clear path to very high power radiation sources of EUV and hard x-rays for applications such as lithography and single molecule x-ray diffraction. Finally, we discuss a technique using chicane delays to suppress the sideband instability, improving radiation generation efficiencies for interaction lengths many synchrotron wavelengths long.

In this study, for the first time, we demonstrated the presence of microRNAs and extracellular vesicles in human blastocoel fluid. The bioinformatic and comparative analyses identified the biological function of blastocoel fluid microRNAs and suggested a potential role inside the human blastocyst. We found 89 microRNAs, expressed at different levels, able to regulate critical signaling pathways controlling embryo development, such as pluripotency, cell reprogramming, epigenetic modifications, intercellular communication, cell adhesion and cell fate. Blastocoel fluid microRNAs reflect the miRNome of embryonic cells and their presence, associated with the discovery of extracellular vesicles, inside blastocoel fluid, strongly suggests their important role in mediating cell communication among blastocyst cells. Their characterization is important to better understand the earliest stages of embryogenesis and the complex circuits regulating pluripotency. Moreover, blastocoel fluid microRNA profiles could be influenced by blastocyst quality, therefore, microRNAs might be used to assess embryo potential in IVF cycles.

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m−1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, p x /m e c) or (y, p y /m e c) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine-including the imaging of virus particles-which may profit from this new model of performing XFEL science.

This paper discusses the extension to different electron beam aspect ratio of the Child-Langmuir law for the maximum achievable current density in electron guns. Using a simple model, we derive quantitative formulas in good agreement with simulation codes. The new scaling laws for the peak current density of temporally long and transversely narrow initial beam distributions can be used to estimate the maximum beam brightness and suggest new paths for injector optimization.

We describe an inverse free electron laser (IFEL) interaction driven by a near single-cycle THz pulse that is group velocity-matched to an electron bunch inside a waveguide, allowing for a sustained interaction in a magnetic undulator. We discuss the application of this guided-THz IFEL technique for compression of a relativistic electron bunch and synchronization with the external laser pulse used to generate the THz pulse via optical rectification, as well as a laser-driven THz streaking diagnostic with the potential for femtosecond scale temporal resolution. Initial measurements of the THz waveform via an electro-optic sampling based technique confirm the predicted reduction of the group velocity, using a curved parallel plate waveguide, as a function of the varying aperture size of the guide. We also present the design of a proof-of-principle experiment based on the bunch parameters available at the UCLA PEGASUS laboratory. With a \\(10\\,\\mathrm{MV}\\,{{\\rm{m}}}^{-1}\\) THz peak field, our simulation model predicts compression of a \\(6\\,\\mathrm{MeV}\\) \\(100\\,\\mathrm{fs}\\) electron beam by nearly an order of magnitude and a significant reduction of its initial timing jitter.

Advances in ultrafast laser technology and nanofabrication have enabled a new class of particle accelerator based upon miniaturized laser-driven photonic structures. However, developing a useful accelerator based on this approach requires control of the particle dynamics at field intensities approaching the damage limit. We measure acceleration in a fused silica dielectric laser accelerator driven by fields of up to 9 GV m −1 and observe a record 1.8 GV m −1 in the accelerating mode. At these intensities the dielectric is driven beyond its linear response and self-phase modulation changes the phase velocity of the accelerating mode, reducing the average gradient to 850 MeV m −1 . We show that free-space optics can be used to compensate this dephasing and demonstrate that tailoring the laser phase and amplitude can facilitate optimization of the beam dynamics. This could enable MeV scale energy gain in a single stage and pave the way towards applications in scientific, industrial, and medical fields. In recent years, photonic structures that mediate the transfer of energy from a laser to a particle beam have gained interest as a way to access more compact accelerations techniques for use in a wide variety of applications. The authors investigate by numerical calculations and experimentally the effect of nonlinear pulse distortions on the operation of dielectric laser accelerators.

Recent studies of the performance of radio-frequency (rf) copper cavities operated at cryogenic temperatures have shown a dramatic increase in the maximum achievable surface electric field. We propose to exploit this development to enable a new generation of photoinjectors operated at cryogenic temperatures that may attain, through enhancement of the launch field at the photocathode, a significant increase in five-dimensional electron beam brightness. We present detailed studies of the beam dynamics associated with such a system, by examining an S-band photoinjector operated at250MV/mpeak electric field that reaches normalized emittances in the 40 nm-rad range at charges (100–200 pC) suitable for use in a hard x-ray free-electron laser (XFEL) scenario based on the LCLS. In this case, we show by start-to-end simulations that the properties of this source may give rise to high efficiency operation of an XFEL, and permit extension of the photon energy reach by an order of magnitude, to over 80 keV. The brightness needed for such XFELs is achieved through low source emittances in tandem with high current after compression. In the XFEL examples analyzed, the emittances during final compression are preserved using microbunching techniques. Extreme low emittance scenarios obtained at pC charge, appropriate for significantly extending temporal resolution limits of ultrafast electron diffraction and microscopy experiments, are also reviewed. While the increase in brightness in a cryogenic photoinjector is mainly due to the augmentation of the emission current density via field enhancement, further possible increases in performance arising from lowering the intrinsic cathode emittance in cryogenic operation are also analyzed. Issues in experimental implementation, including cavity optimization for lowering cryogenic thermal dissipation, external coupling, and cryocooler system, are discussed. We identify future directions in ultrahigh field cryogenic photoinjectors, including scaling to higher frequency, use of novel rf structures, and enabling of an extremely compact hard x-ray FEL.