Hasil untuk "physics.acc-ph"

G. Arnison, A. Astbury, G. Grayer et al.

C. Orchard, J. Kentish

Ting Sun, Zhen-Ke Dou, Ya-Qing Huang et al.

Plasma-based acceleration of positrons attracts extensive interests due to the ultrahigh accelerating gradient and ultrashort duration, while generating wakefield positron beam by the inherent injection is still a great challenge. Here, we put forward a superponderomotive injection method of positrons in the blowout regime of laser wakefield acceleration. In this method, transverse laser fields facilitate the trapping of positrons into the laser-modulated longitudinal wakefield which dominates the subsequent energy gaining, that is distinct from the electrostatic induced superponderomotive electrons. Particle-in-cell simulations demonstrate that this method can be implemented by the collision between a donut wake and a pair jet, resulting in the multi-cycle positron beam with hundreds of pC charge and brightness of $10^{13}~\rm{Am}^{-2}$. Such a collisional setup is available in the current laser-plasma experiments, and the generated positron beam is favorable for developing next generation of electron-positron collider, modeling the laboratory astrophysics and performing the ultrafast material diagnostics research.

He Zhang

In scientific computation, it is often necessary to calculate higher-order derivatives of a function. Currently, two primary methods for higher-order automatic differentiation exist: symbolic differentiation and algorithmic automatic differentiation (AD). Differential Algebra (DA) is a mathematical technique widely used in beam dynamics analysis and simulations of particle accelerators, and it also functions as an algorithmic automatic differentiation method. DA automatically computes the Taylor expansion of a function at a specific point up to a predetermined order and the derivatives can be easily extracted from the coefficients of the expansion. We have developed a Symbolic Differential Algebra (SDA) package that integrates algorithmic differentiation with symbolic computation to produce explicit expressions for higher-order derivatives using the computational techniques of algorithmic differentiation. Our code has been validated against existing DA and AD libraries. Moreover, we demonstrate that SDA not only facilitates the simplification of explicit expressions but also significantly accelerates the calculation of higher-order derivatives, compared to directly using AD.

A. A. Molavi Choobini, F. M. Aghamir

The present study explores radiation in THz spectrum region through the interaction of the wakefield of two-color laser pulses with magnetized plasma. The interaction of the two-color laser with plasma electrons induces transverse nonlinear current in two dimensions, resulting in generation of a wakefield and a forward wave. The investigation revealed that during the non-relativistic regime of laser-plasma interaction, interdependence exists between the electric fields of the forward wave and the wake. Conversely, in the relativistic regime, the dynamic of interaction changes, and plasma electrons are influenced not only by the electric field of the laser pulse but also by relativistic effects like Lorentz contraction, responding to both the electric and magnetic field components. This leads to generation of wake and forward wave radiations. The interplay between various laser and plasma parameters is analyzed, shedding light on the conditions leading to radiation angular distribution patterns in the forward and backward directions. The impact of spatial laser profile, a DC external magnetic field, polarization states, and plasma interaction length on the generated wake and forward wave patterns has been investigated. Through systematic variation of these parameters, the objective is to elucidate the controlled directional features of the resulting fields and radiation patterns.

Kenichi Nagaoka, Haruhisa Nakano, Taiga Hamajima et al.

Beam-focusing characteristics of negative ion beams have been experimentally investigated with a superimposition of a controlled perturbation of RF field in a filament-arc discharge negative ion source. Oscillations of a negative-ion beamlet width and axis responding to the RF perturbation were observed, which may be a cause of the larger beam divergence angle of the RF negative ion source for ITER. It is pointed out that the oscillation of the beamlet width depends on the perveance and on the RF frequency such that the oscillation is suppressed at perveance-matched conditions and at low RF frequency.

M. Kühl, Y. Cohen, T. Dalsgaard et al.

Jia Wang, Ming Zeng, Dazhang Li et al.

A new injection scheme using the interference of two coaxial laser pulses is proposed for generating high quality beams in laser wakefield accelerators. In this scheme, a relatively loosely focused laser pulse drives the plasma wakefield, and a tightly focused laser pulse with similar intensity triggers interference ring pattern which creates onion-like multi sheaths in the plasma wakefield. Due to the wavefront curvature change after the focal position of the tightly focused laser, the innermost sheath of the wakefield expands, which slows down the effective phase velocity of the wakefield and triggers injection of plasma electrons. Particle-in-cell simulations show that high quality electron beams with low energy spread (a few per mill), high charge (hundred picocoulomb) and small emittance (sub millimeter milliradian) at the same time can be generated using moderate laser parameters for properly chosen phase differences between the two lasers.

Kangtaek Lee, S. Asher

T. Silva, L. D. Amorim, M. C. Downer et al.

Hollow plasma channels are attractive for lepton acceleration because they provide intrinsic emittance preservation regimes. However, beam breakup instabilities dominate the dynamics. Here, we show that thin, warm hollow channels can sustain large-amplitude plasma waves ready for high-quality positron acceleration. We verify that the combination of warm electrons and thin hollow channel enables positron focusing structures. Such focusing wakefields unlock beam breakup damping mechanisms. We demonstrate that such channels emerge self-consistently during the long-term plasma dynamics in the blowout's regime aftermath, allowing for experimental demonstration.

Andrey V. Tyukhtin, Sergey N. Galyamin, Viktor V. Vorobev

Cherenkov radiation (CR) generated by a charge moving along one of the faces of a dielectric prism is analyzed. Unlike our previous papers, here we suppose that the charge moves from the the prism ``nose'' to its base. For CR analysis, we use the technique described in our previous papers and called the ``aperture method''. However, here we develop a new version of this technique which is suitable for objects with plane faces: it utilizes field expansion only over plane waves inside the object. This approach is especially convenient for objects having two or more plane interfaces on which the waves are reflected and/or refracted. Using this technique, we obtain the electromagnetic field distribution over the aperture and then apply Stratton-Chu formulas (aperture integrals). Further, the main attention is paid to the calculation of the radiation field in the Fraunhofer (far-field) area. It is notable that we obtain expressions for corresponding Fourier transforms in the form of single integrals. Using them, the series of typical angular diagrams are computed and physical conclusions are made.

D. Nordstrom, C. Alpers

A. Degen, M. Kosec

K. Straub, M. Benz, B. Schink

H. Ding, A. Döpp, M. Gilljohann et al.

Laser wakefield acceleration relies on the excitation of a plasma wave due to the ponderomotive force of an intense laser pulse. However, plasma wave trains in the wake of the laser have scarcely been studied directly in experiments. Here we use few-cycle shadowgraphy in conjunction with interferometry to quantify plasma waves excited by the laser within the density range of GeV-scale accelerators, i.e. a few 1e18 cm-3. While analytical models suggest a clear dependency between the non-linear plasma wavelength and the peak potential a_0, our study shows that the analytical models are only accurate for driver strength a_0<=1. Experimental data and systematic particle-in-cell simulations reveal that nonlinear lengthening of plasma wave train depends not solely on the laser peak intensity but also on the waist of the focal spot.

L. Feder, B. Miao, J. E. Shrock et al.

We demonstrate that an ultrashort high intensity laser pulse can propagate for hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by generating its own plasma waveguide as it propagates; the front of the pulse generates a waveguide that confines the rest of the pulse. A wide range of suitable initial index structures will support this "self-waveguiding" process; the necessary feature is that the gas density on axis is a minimum. Here, we demonstrate self-waveguiding of pulses of at least $1.5\times10^{17} W/cm^2$ (normalized vector potential $a_0\sim0.3)$ over 10 cm, or $\sim100$ Rayleigh ranges, limited only by our laser energy and length of our gas jet. We predict and observe characteristic oscillations corresponding to mode-beating during self-waveguiding. The self-waveguiding pulse leaves in its wake a fully ionized low density plasma waveguide which can guide another pulse injected immediately following; we demonstrate optical guiding of such a follow-on probe pulse

P. Snee, R. C. Somers, Gautham P Nair et al.

K. Kostikas, G. Papatheodorou, K. Ganas et al.

R. Buck, S. Rondinini, A. Covington et al.

M. Kosmulski