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Vacuum radio-frequency (rf) breakdown is one of the major factors that limit operating accelerating gradients in rf particle accelerators. The occurrence of rf breakdowns was shown to be probabilistic, and can be characterized by a breakdown rate. Experiments with hard copper cavities showed that harder materials can reach larger accelerating gradients for the same breakdown rate. We study the effect of cavity material on rf breakdowns with shortX-band standing wave accelerating structures. Here we report results from tests of a structure at cryogenic temperatures. At gradients greater than150MV/mwe observed a degradation in the intrinsic cavity quality factor,Q0. This decrease inQ0is consistent with rf power being absorbed by field emission currents, and is accounted for in the determination of accelerating gradients. The structure was conditioned up to an accelerating gradient of250MV/mat 45 K with108rf pulses and a breakdown rate of2×10−4/pulse/m. For this breakdown rate, the cryogenic structure has the largest reported accelerating gradient. This improved performance over room temperatures structures supports the hypothesis that breakdown rate can be reduced by immobilizing crystal defects and decreasing thermally induced stresses.

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.

In this paper, a novel mathematical—physical model of the generalized elasto-thermodiffusion (hole/electron interaction) waves in semiconductor materials is studied when the hyperbolic two-temperature theory in the two-dimensional (2D) deformation is taken into account. Shear (purely transverse) waves are dissociated from the remainder of the motion and remain unaffected by external fields. The coupled system of partial differential equations of the main interacting fields has been solved. Using the Laplace transform method, the governing equations of motion and heat conduction can be formulated in 2D. The hole charge carrier, displacement, thermal, and plasma boundary conditions are applied on the interface adjacent to the vacuum to obtain the basic physical quantities in the Laplace domain. The inversion of the Laplace transform with the numerical method is applied to obtain the complete solutions in the time domain for the main physical fields under investigation. The effects of thermoelastic, the phase-lag of the temperature gradient and the phase-lag of the heat flux, the hyperbolic two-temperature parameter, and comparing between silicon and germanium materials on the displacement component, carrier density, hole charge carrier, and temperature distribution have been discussed and obtained graphically.

In the context of the two-temperature thermoelasticity theory, a novel mathematical–physical model is introduced that describes the influence of moisture diffusivity in the semiconductor material. The two-dimensional (2D) Cartesian coordinate is used to study the coupling between the thermo-elastic plasma waves and moisture diffusivity. Dimensionless quantities are taken for the main physical fields with some initial conditions in the Laplace transform domain. The linear solutions are obtained analytically along with unknown variables when some conditions are loaded at the surface of the homogenous medium according to the two-temperature theory. The Laplace transform technique in inversion form is utilized with some numerical algebraic approximations in the time domain to observe the exact expressions. Due to the effects of the two-temperature parameter and moisture diffusivity, the numerical results of silicon material have been introduced. The impacts of thermoelectric, thermoelastic, and reference moisture parameters are discussed graphically with some physical explanations.

Investigated is a novel model in the photo-thermoelasticity theory that takes into account the impact of porosity and initial stress. A generalized photo-thermoelastic that is initially stressed and has voids is taken into consideration for the general plane strain problem. The solutions for the fundamental variables in two dimensions are obtained using the Laplace–Fourier transforms method in two dimensions (2D). Physical fields such as temperature, carrier concentration, normal displacement, and change in volume fraction field can all be solved analytically. The plasma of electrons, thermal load, and mechanical boundary conditions at the porosity medium’s free surface are used to show certain illustrations. The context of the Laplace–Fourier transformation inversion operations yields complete solutions. To complete the numerical simulation and compare several thermal memories under the influence of the porosity parameters, silicon (Si), a semiconductor porosity material, is used. The main physical variables are described and graphically displayed with the new parameters.

A design for a compact x-ray light source (CXLS) with flux and brilliance orders of magnitude beyond existing laboratory scale sources is presented. The source is based on inverse Compton scattering of a high brightness electron bunch on a picosecond laser pulse. The accelerator is a novel high-efficiency standing-wave linac and rf photoinjector powered by a single ultrastable rf transmitter at X-band rf frequency. The high efficiency permits operation at repetition rates up to 1 kHz, which is further boosted to 100 kHz by operating with trains of 100 bunches of 100 pC charge, each separated by 5 ns. The entire accelerator is approximately 1 meter long and produces hard x rays tunable over a wide range of photon energies. The colliding laser is a Yb∶YAG solid-state amplifier producing 1030 nm, 100 mJ pulses at the same 1 kHz repetition rate as the accelerator. The laser pulse is frequency-doubled and stored for many passes in a ringdown cavity to match the linac pulse structure. At a photon energy of 12.4 keV, the predicted x-ray flux is 5×1011photons/second in a 5% bandwidth and the brilliance is 2×1012photons/(secmm2mrad20.1%) in pulses with rms pulse length of 490 fs. The nominal electron beam parameters are 18 MeV kinetic energy, 10 microamp average current, 0.5 microsecond macropulse length, resulting in average electron beam power of 180 W. Optimization of the x-ray output is presented along with design of the accelerator, laser, and x-ray optic components that are specific to the particular characteristics of the Compton scattered x-ray pulses.

This paper investigates an uncommon technique by using the influence of the random function (Weiner process function), on a two-temperature problem, at the free surface of the semiconducting medium, by using the photo-thermoelasticity theory. Using the Silicon material as an example of a semiconducting medium under the influence of a magnetic field, the novel model can be formulated. To make the problem more logical, the randomness of the Weiner process function is aged to the governing stochastic equation. A combining stochastic process with the boundary of the variables is studied. In this case, the stochastic and deterministic solutions were obtained for all physical quantities. The additional noise is regarded as white noise. The problem is investigated according to a two-dimensional (2D) deformation. The normal mode method can be used mathematically to obtain numerically the deterministic, stochastic, and variance solutions of all physical quantities. Three sample paths are obtained by making a comparison between the stochastic and deterministic distributions of the field variables. The impacts of adding randomization to the boundary conditions are highlighted. The numerical results are shown graphically and discussed in consideration of the two-temperature parameter effect.

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.

A unique methodology due to the effect of stochastic heating is utilized to study the Moisture Diffusivity influence of an elastic semiconductor medium under the effect of photo-thermoelasticity theory. Accurately, random processes are applied at the boundary of the semiconductor medium. The governing equations are expressed in the one-dimensional form (1D). The boundary conditions are considered random; the additional noise is regarded as white noise. The problem is set up to investigate the interaction between moisture diffusivity, thermo-elastic waves, and plasma waves. The investigation is carried out during a photothermal transport procedure while taking moisture diffusivity into consideration. The Laplace transform is used to solve the problem. The numerical solution for field distribution is obtained using the short-time approximation while performing inverse transformations of Laplace. The Wiener process notion has been used to arrive at the solutions for the stochastic case. Silicon (Si) material is used along several sample paths in a numerical study based on stochastic simulation. Additionally, a comparison of the stochastic and deterministic field variable distributions is provided. The effects of thermoelectric, thermoelastic, and reference moisture parameters of the applied force on all physical distributions are discussed graphically.

Linacs for high-energy physics, as well as for industry and medicine, require accelerating structures which are compact, robust, and cost-effective. Small foot-print linacs require high-accelerating gradients. Currently, stable-operating gradients, exceeding100MV/m, have been demonstrated at SLAC National Accelerator Laboratory, CERN, and KEK at X-band frequencies. Recent experiments show that accelerating cavities made out of hard copper alloys achieve better high-gradient performance as compared with soft copper cavities. In the scope of a decade-long collaboration between SLAC, INFN-Frascati, and KEK on the development of innovative high-gradient structures, this particular study focuses on the technological developments directed to show the viability of novel welding techniques. Two novel X-band accelerating structures, made out of hard copper, were fabricated at INFN-Frascati by means of clamping and welding. One cavity was welded with the electron beam and the other one with the tungsten inert gas welding process. In the technological development of the construction methods of high-gradient accelerating structures, high-power testing is a critical step for the verification of their viability. Here, we present the outcome of this step—the results of the high-power rf tests of these two structures. These tests include the measurements of the breakdown rate probability used to characterize the behavior of vacuum rf breakdowns, one of the major factors limiting the operating accelerating gradients. The electron beam welded structure demonstrated accelerating gradients of90MV/mat a breakdown rate of10−3/(pulsemeter)using a shaped pulse with a 150 ns flat part. Nevertheless, it did not achieve its ultimate performance because of arcing in the mode launcher power coupler. On the other hand, the tungsten inert gas welded structure reached its ultimate performance and operated at about a150MV/mgradient at a breakdown rate of10−3/(pulsemeter)using a shaped pulse with a 150 ns flat part. The results of both experiments show that welding, a robust, and low-cost alternative to brazing or diffusion bonding, is viable for high-gradient operation. This approach enables the construction of multicell standing and traveling-wave accelerating structures.