Hasil untuk "physics.acc-ph"

A. Clairembaud, M. Turner, M. Bergamaschi et al.

The self-modulation (SM) instability transforms a long charged particle bunch traveling in plasma into a train of microbunches that resonantly drives large-amplitude wakefields. We present the first determination of the saturation length of SM using experimental and numerical results. The saturation length is the distance over which wakefields reach their maximum amplitude along the plasma. By varying the plasma length and measuring the radius of the transverse distribution of the bunch, we find that the saturation length of SM decreases with plasma density and initial field amplitude, e.g., when seeding. The saturation length is a fundamental parameter of the instability, and these results are key for understanding SM and designing plasma wakefield accelerators driven by long bunches, such as AWAKE, or by long laser pulses for radiation production.

E. M. Starodubtseva, I. N. Tsymbalov, D. A. Gorlova et al.

We demonstrate a novel, direct method for measuring the phase velocity $v_φ$ of an intense laser pulse within a plasma channel - the crucial parameter that controls the resonance condition in direct laser acceleration (DLA). The technique exploits the second harmonic (SH) radiation generated at the channel sheath - a phenomenon previously observed in laser-wakefield acceleration experiments. The SH emission angle is governed by a phase-matching condition that directly depends on $v_φ$. Experimental measurements performed using a 1 TW, 10 Hz Ti:Sa laser system yield phase velocities in the range $v_φ=(1.010-1.030)c$ for plasma electron densities in the range $n_e=(0.01-0.06)n_{cr}$. The diagnostic is validated through quasi-3D particle-in-cell (PIC) simulations that reproduce the experimental conditions. This work provides a way to optimize DLA schemes by enabling in-situ measurement of the laser pulse phase velocity in plasma channels.

Roger J Smith

Fiber optic pulsed polarimetry is a LIDAR-like fiber sensing technique that uses a backscatter enhanced single mode backscatter-tailored optical fiber(BTOF) to measure the distributed B fields on all Magnetic Fusion Energy devices. The BTOF has a series of wavelength resonant reflection fiber Bragg gratings written uniformly along its length. The fiber's Verdet constant determines the strength of the Faraday effect which effectuates the measurement of local B along the fiber placed intimately next to or within a magnetized plasma volume. A robust measurement of the field distribution along the fiber is obtained at high rep rates, 5 MHz, high spatial resolution(1-10cm), high B field accuracy(<1%) and temporal response (ns). Multipathing in the BTOF produces 3rd order reflections that contaminate the LIDAR signal. Algorithms are given for calculating the level of contamination for uniform and flat reflection designs, in particular and any reflection series in general. The contamination is bracketed giving confidence in designing and implementing a BTOF. Applications include magnetic fusion devices, rail guns, high temperature superconducting magnets and magnetized target fusion research.

Ruohong Li, Yuan Liu, Dan W. Stracener et al.

High-lying Rydberg and autoionizing (AI) states of thorium (Th) have been studied via resonance laser ionization spectroscopy at both TRIUMF Canada's particle accelerator centre and Oak Ridge National Lab (ORNL). Multiple Rydberg series converging to the ionization potential (IP) were observed via different stepwise laser excitation schemes and were assigned to be the $6d^27s (^4F_{3/2})$ $np$, $nd$, and $nf$ series. Analysis of these series enabled the determination of the IP to be 50868.735(54) cm$^{-1}$, which improved the precision by two orders of magnitude over the current adopted NIST value of 50867(2) cm$^{-1}$. Additionally, four AI Rydberg series were identified and assigned to $nf$ and $nd$ series converging to the $6d^27s$ $^4F_{5/2}$ and $6d^27s$ $^2D_{3/2}$ metastable states of Th$^+$. The measured energies of the Rydberg and AI Rydberg states are reported, and observed perturbations within the series are discussed.

Driss Oumbarek Espinos, Alexei Zhidkov, Alexandre Rondepierre et al.

Particle in cell simulations are widely used in most fields of physics to investigate known and new phenomena which cannot be directly observed or measured yet. However, the computational and time resources needed for PICs make them impractical when high resolution and long time/distance simulations are required. In this work, we present a new PIC simulation code that takes advantage of the use of a relativistic reference frame and consequent time dilation and length contraction. These properties make a simulation capable of long (meter length) and high resolution simulations without the need of supercomputers. This new code is a step forward with regards to the previous tries enabling complex multiple body situations without additional filtering and smoothing of fields and currents. The usefulness of the relativistic frame PIC code is displayed by simulating electron beam bunching obtained in long undulator propagation and also the potential as a beam "buncher" of 10s of cm long low density plasmas.

Zhen-Ke Dou, Chong Lv, Yousef I. Salamin et al.

Compact spin-polarized positron accelerators play a major role in promoting significant positron application research, which typically require high acceleration gradients and polarization degree, both of which, however, are still great challenging. Here, we put forward a novel spin-polarized positron acceleration method which employs an ultrarelativistic high-density electron beam passing through any hole of multi-layer microhole array films to excite strong electrostatic and transition radiation fields. Positrons in the polarized electron-positron pair plasma, filled in the front of the multi-layer films, can be captured, accelerated, and focused by the electrostatic and transition radiation fields, while maintaining high polarization of above 90% and high acceleration gradient of about TeV/m. Multi-layer design allows for capturing more positrons and achieving cascade acceleration. Our method offers a promising solution for accelerator miniaturization, positron injection, and polarization maintaining, and also can be used to accelerate other charged particles.

Imran Khan, Vikrant Saxena

The interaction of a high-intensity ultrashort laser pulse with a few microns-thick hydrocarbon target is known to accelerate protons/ions to multi-MeV, on the rear side of the target, via the mechanism of target normal sheath acceleration. Micro-structuring the target front is one of the promising approaches to enhance the cut-off energy as well as to reduce the divergence of accelerated protons/ions. In this paper, the interaction of a normally incident intense laser pulse with targets having single micron-sized grooves, at their front side, of semi-circular, triangular, and rectangular shapes has been studied by using two-dimensional Particle-In-Cell (PIC) simulations. It is observed that as compared to a flat target for targets with a rectangular groove at the front side the focused hot electron beam at the rear side results in an approximately four-fold increase in the cut-off energy of accelerated protons. For triangular and semi-circular groove targets, the cut-off energy remains comparatively lower (higher than the flat target though). The angular divergence of the accelerated protons/ions is also found to be relatively much lower in the case of a rectangular groove.

J. E. Shrock, E. Rockafellow, B. Miao et al.

We show that laser wakefield electron accelerators in meter-scale, low density hydrodynamic plasma waveguides operate in a new nonlinear propagation regime where sustained beating of lowest order modes of the ponderomotively modified channel plays a significant role, whether or not the injected pulse is linearly matched to the guide. For a continuously doped gas jet, this mode beating effect leads to ionization injection and a striated multi-GeV energy spectrum of multiple quasi-monoenergetic peaks; the same process in a locally doped jet produces single multi-GeV peaks with <10% energy spread. A 3-stage model of drive laser pulse evolution and ionization injection characterizes the beating effect and explains our experimental results.

Xuesong Geng, Liangliang Ji, Baifei Shen

The compactness of laser wakefield acceleration (LWFA) is limited by its long focal length for high power lasers, e.g., more than 10 meters for 1-peatawatt (PW) laser pulse and up to hundreds of meters for 10-100 PW lasers. The long focal length originates from the low damage threshold of the optical off-axial parabolic (OAP) mirror and consequent large spot size. We propose implementing an OAP plasma mirror (PM) to form a telescope geometry, reducing the beam size and hence constraining the focal length to meter-range for LWFA driven by lasers beyond 1PW. Three-dimensional particle-in-cell simulations are performed to characterize the reflection of a 1-PW laser by the plasma OAP and find that optimal condition is achieved within only 1-m optical length. The new method successfully generates 9GeV electron bunch in the subsequent LWFA stage with consistent acceleration gradients to that of the 1-PW laser via ordinary focusing. The proposed geometry provides a solution of compact LWFAs available for even 100-PW laser systems.

Javier Carrasco, Daniel Valenzuela, Claudio Falcón et al.

Superconducting traveling-wave parametric amplifiers are a promising amplification technology suitable for applications in submillimeter astronomy. Their implementation relies on the use of Floquet transmission lines in order to create strong stopbands to suppress undesired harmonics. In the design process, amplitude equations are used to predict their gain, operation frequency, and bandwidth. However, usual amplitude equations do not take into account the real and imaginary parts of the dispersion and characteristic impedance that results from the use of Floquet lines, hindering reliable design. In order to overcome this limitation, we have used the multiple-scales method to include those effects. We demonstrate that complex dispersion and characteristic impedance have a stark effect on the transmission line's gain, even suppressing it completely in certain cases. The equations presented here can, thus, guide to a better design and understanding of the properties of this kind of amplifiers.

H. Backe

Monte-Carlo simulation calculation have been performed for 855 MeV electrons channeling in (110) planes of a diamond single crystal. The continuum potential picture has been utilized. Both, the transverse potential and the angular distributions of the scattered electrons at screened atoms are based on the Doyle-Turner scattering factors which were extrapolated with the functional dependence of the Molière representation to large momentum transfers. Scattering cross-sections at bound electrons have been derived for energies less than 30 keV from the double differential cross-section as function of both, energy and momentum transfer, taking into account also longitudinal and transverse excitations. For energies above 30 keV the Møller cross-section is used. The dynamics of the particle in the continuum transverse potential has been described classically. Results of the channeling process are presented in terms of instantaneous transition rates as function of the penetration depth, indicating that channeling can be described by a single exponential function only after the equilibration phase has been reached after about 15 $μ$m. As a byproduct, improved drift and diffusion coefficients entering the Fokker-Planck equation have been derived with which its predictive power can be improved.

Yi-Kai Kan, Franz X. Kärtner, Sabine Le Borne et al.

Space-charge effects are of great importance in particle accelerator physics. In the computational modeling, tree-based methods are increasingly used because of their effectiveness in handling non-uniform particle distributions and/or complex geometries. However, they are often formulated using an electrostatic force which is only a good approximation for low energy particle beams. For high energy, i.e., relativistic particle beams, the relativistic interaction kernel may need to be considered and the conventional treecode fails in this scenario. In this work, we formulate a treecode based on Lagrangian interpolation for computing the relativistic space-charge field. Two approaches are introduced to control the interpolation error. In the first approach, a modified admissibility condition is proposed for which the treecode can be used directly in the lab-frame. The second approach is based on the transformation of the particle beam to the rest-frame where the conventional admissibility condition can be used. Numerical simulation results using both methods will be compared and discussed.

K. Christensen, Jesse T. Myers, J. Swanson

E. Gillies, J. Fréchet

J. Whalen, Chi Chang, G. Clayton et al.

Z. Meng, A. Wolberg, D. Monroe et al.

Sang-Eun Oh, Steven W. van Ginkel, B. Logan

Mingchun Zhao, M. Liu, G. Song et al.

Xinlu Xu, Jorge Vieira, Mark Hogan et al.

Laser-triggered ionization injection is a promising way of generating controllable high-quality electrons in plasma-based acceleration. We show that ionization injection of electrons into a fully nonlinear plasma wave wake using a laser pulse comprising of one or more Laguerre-Gaussian modes with combinations of spin and orbital angular momentum can generate exotic three-dimensional (3D) spatial distributions of high-quality relativistic electrons. The phase dependent residual momenta and initial positions of the ionized electrons are encoded into their final phase space distributions, leading to complex spatiotemporal structures. The structures are formed as a result of the transverse (betatron) and longitudinal (phase slippage and energy gain) dynamics of the electrons in the wake immediately after the electrons are injected. Theoretical analysis and 3D simulations verify this mapping process leads to the generation of these complex topological beams. These beams may trigger novel beam-plasma interactions as well as produce coherent radiation with orbital angular momentum when sent through a resonant undulator.

J. Hachem, D. Crumrine, J. Fluhr et al.

Both exposure of stratum corneum to neutral pH buffers and blockade of acidification mechanisms disturb cutaneous permeability barrier homeostasis and stratum corneum integrity/cohesion, but these approaches all introduce potentially confounding variables. To study the consequences of stratum corneum neutralization, independent of hydration, we applied two chemically unrelated superbases, 1,1,3,3-tetramethylguanidine or 1,8-diazabicyclo [5,4,0] undec-7-ene, in propylene glycol:ethanol (7:3) to hairless mouse skin and assessed whether discrete pH changes alone regulate cutaneous permeability barrier function and stratum corneum integrity/cohesion, as well as the responsible mechanisms. Both 1,1,3,3-tetramethylguanidine and 1,8-diazabicyclo [5,4,0] undec-7-ene applications increased skin surface pH in parallel with abnormalities in both barrier homeostasis and stratum corneum integrity/cohesion. The latter was attributable to rapid activation (<20 min) of serine proteases, assessed by in situ zymography, followed by serine-protease-mediated degradation of corneodesmosomes. Western blotting revealed degradation of desmoglein 1, a key corneodesmosome structural protein, in parallel with loss of corneodesmosomes. Coapplication of serine protease inhibitors with the superbase normalized stratum corneum integrity/cohesion. The superbases also delayed permeability barrier recovery, attributable to decreased beta-glucocerebrosidase activity, assessed zymographically, resulting in a lipid-processing defect on electron microscopy. These studies demonstrate unequivocally that stratum corneum neutralization alone provokes stratum corneum functional abnormalities, including aberrant permeability barrier homeostasis and decreased stratum corneum integrity/cohesion, as well as the mechanisms responsible for these abnormalities.