Evolution Of Beam Research Articles

Physics of beam self-modulation in plasma wakefield accelerators

The self-modulation instability is a key effect that makes possible the usage of nowadays proton beams as drivers for plasma wakefield acceleration. Development of the instability in uniform plasmas and in plasmas with a small density up-step is numerically studied with the focus at nonlinear stages of beam evolution. The step parameters providing the strongest established wakefield are found, and the mechanism of stable bunch train formation is identified.

Structure of the runaway electron loss during induced disruptions in TEXTOR

The loss of runaway electrons during an induced disruption is recorded by a synchrotron imaging technique using a fast infrared CCD camera. The loss is predominantly diffuse. During the “spiky-loss phase”, when the runaway beam moves close to the wall, a narrow channel between the runaway column and a scintillator probe is formed and lasts until the runaway beam is terminated. In some cases, the processed images show a stripe pattern at the plasma edge. A comparison between the MHD dominated disruptions and the MHD-free disruption is performed. A new mechanism of plasma disruptions with the runaway electron generation and a novel model which reproduces many characteristic features of the plasma beam evolution during a disruption is briefly described.

Potential performance for Pb-Pb,p-Pb, andp−pcollisions in a future circular collider

The hadron collider studied in the future circular collider (FCC) project could operate with protons and lead ions in similar operation modes as the LHC. In this paper the potential performances in lead-lead, proton-lead, and proton-proton collisions are investigated. Based on average lattice parameters, the strengths of intrabeam scattering and radiation damping are evaluated and their effect on the beam and luminosity evolution is presented. Estimates for the integrated luminosity per fill and per run are given, depending on the turnaround time. Moreover, the beam-beam tune shift and bound free pair production losses in heavy-ion operation are addressed.

Nonautonomous vector matter waves in two-component Bose-Einstein condensates with combined time-dependent harmonic-lattice potential

We construct exact self-similar soliton solutions of three-dimensional coupled Gross–Pitaevskii equations for two-species Bose–Einstein condensates (BECs) in a combined time-dependent harmonic-lattice potential. Based on these solutions, we investigate the control and manipulation of solitary waves for three kinds of BECs with changing diffraction and nonlinearity coefficients; the solutions include Ma breathers and Peregrine and Akhmediev soliton solutions. Our results indicate that matter waves readily propagate in this system. It is shown that diffraction and lattice potential factors play important roles in the beam evolution characteristics, such as the peak, the phase offset, the linear phase, and the chirp.

Coherence evolution of finite-energy Airy beam during its generation and diffraction

This work presents the analysis on the spatial coherence of the finite-energy Airy beam (FEAB) from the generation by the source Gaussian beam (SGB) to its propagation. It is shown that the coherence diameter of FEAB grows with the increasing of the value of truncation coefficient. But it reverses when it comes to that of SGB. And it reveals that the linear propagation is a positive element to the spatial coherence of FEAB and SGB with using the theoretical model of the mutual intensity. The coherence gets better with the increasing of propagation distance.

GEANT4 simulations for beam emittance in a linear collider based on plasma wakefield acceleration

Alternative acceleration technologies are currently under development for cost-effective, robust, compact, and efficient solutions. One such technology is plasma wakefield acceleration, driven by either a charged particle or laser beam. However, the potential issues must be studied in detail. In this paper, the emittance evolution of a witness beam through elastic scattering from gaseous media and under transverse focusing wakefields is studied.

Localization of light in a parity-time-symmetric quasi-periodic lattice.

We address the propagation of light in a parity-time (PT) symmetric quasi-periodic optical lattice. We show that the PT-symmetry breaking threshold of the system depends on the relative strength of two simple lattices forming quasi-periodic structure. The increase of the imaginary part of such structure leads to a dynamical phase transition from delocalization to re-localization for all eigenmodes. The nontrivial interplay between PT-symmetry breaking and the delocalization-localization transition in the system opens new prospects for the manipulation of dynamical evolution of light beams.

Enhanced patient care through collaborative team play: An orthodontist and an OMF.Radiologist’s collective perspective

The overarching theme of patient-centric care should serve as the fundamental basis for our treatment plans. Treatment plans that best utilize knowledge; training and technology must be developed on current evidence that rests on soundly researched principles. In orthodontics, the evolution of cone beam computed tomography as a new three-dimensional imaging modality has brought a paradigm shift in the radiographic evaluation of the maxillofacial structures. However, the coming of new technology does not warrant its use in routine practice and tight imaging protocols must be developed to include and promote the evidence-based patient-centric approach. This principle is best approached if the effort is a collaborative one. Collaboration between all the disciplines that contribute to patient care must act as a unified force to deliver superior care. In this editorial, we present a successful collaborative effort between orthodontics and oral and maxillofacial radiology that have engaged in collaborative efforts to deliver superior patient care, better predoctoral and postdoctoral training programs and how this effort has yielded excellent contribution to research.

Evolution of rotated anomalous hollow Gaussian beams in nonlinear media with a strongly spatial nonlocality

In this paper, using the rotating coordinates, we take the anomalous hollow Gaussian beam as an example to investigate the evolution of an ellipse-symmetric beam in strongly nonlocal nonlinear media. A set of analytical expressions are obtained and some numerical simulations are carried out to illustrate the relation between the evolution characteristics and the rotation angle. It is found that the evolution properties, such as the critical powers, beam widths, and intensity distributions in x and y directions, are all variational with the rotation angle.

Propagation of anomalous vortex beams in strongly nonlocal nonlinear media

The propagation properties of anomalous vortex beams in strongly nonlocal nonlinear media are investigated. Two equivalent analytical expressions for the evolution of anomalous vortex beams are obtained. It is found that the input power plays a key role in the beam evolutions. Selecting a proper input power, the beam width can be broadened or be compressed periodically, even it can keep invariant during propagation. The beam order and the topological charge mainly influence the intensity evolution and the phase evolution, respectively. The evolution period, the beam width, the phase distribution and the intensity distribution are discussed in detail. The results can also be generalized to other equivalent physical systems, such as an optical fractional Fourier transform system or a medium with a quadratic graded refractive index distribution.

Elliptical optical solitary waves in a finite nematic liquid crystal cell

The addition of orbital angular momentum has been previously shown to stabilise beams of elliptic cross-section. In this article the evolution of such elliptical beams is explored through the use of an approximate methodology based on modulation theory. An approximate method is used as the equations that govern the optical system have no known exact solitary wave solution. This study brings to light two distinct phases in the evolution of a beam carrying orbital angular momentum. The two phases are determined by the shedding of radiation in the form of mass loss and angular momentum loss. The first phase is dominated by the shedding of angular momentum loss through spiral waves. The second phase is dominated by diffractive radiation loss which drives the elliptical solitary wave to a steady state. In addition to modulation theory, the “chirp” variational method is also used to study this evolution. Due to the significant role radiation loss plays in the evolution of an elliptical solitary wave, an attempt is made to couple radiation loss to the chirp variational method. This attempt furthers understanding as to why radiation loss cannot be coupled to the chirp method. The basic reason for this is that there is no consistent manner to match the chirp trial function to the generated radiating waves which is uniformly valid in time. Finally, full numerical solutions of the governing equations are compared with solutions obtained using the various variational approximations, with the best agreement achieved with modulation theory due to its ability to include both mass and angular momentum losses to shed diffractive radiation.

Numerical simulation to study the transient self focusing of laser beam in plasma

In this paper, we present the numerical simulation for the coupled system of equations governing the dynamics of laser and Ion Acoustic Wave (IAW) in a collisionless plasma, when the coupling between the waves is through ponderomotive non-linearity. The nonlinear evolution of the laser beam is studied when the pump laser is perturbed by a periodic perturbation. By changing the perturbation wave number, we have studied its effect on the nonlinear evolution pattern of laser beam. In order to have a physical insight into the nonlinear dynamics of laser beam evolution in time and space, we have studied the laser and IAW spectra containing spatial harmonics. The magnitude of these harmonics changes with time and leads to time dependent localization of laser beam in spatial domain. The nonlinear dynamics of this localization is investigated in detail by using simulation and a semi-analytical model.

Blind search for radio-quiet and radio-loud gamma-ray pulsars with Fermi-LAT data

The Fermi Large Area Telescope (LAT) has observed more than a hundred of gamma-ray pulsars, about one third of which are radio-quiet, i.e., not detected at radio frequencies. The most of radio-loud pulsars are detected by Fermi LAT by using the radio timing models, while the radio-quiet ones are discovered in a blind search. The difference in the techniques introduces an observational selection bias and, consequently, the direct comparison of populations is complicated. In order to produce an unbiased sample, we perform a blind search of gamma-ray pulsations using Fermi-LAT data alone. No radio data or observations at optical or X-ray frequencies are involved in the search process. We produce a gamma-ray selected catalog of 25 non-recycled gamma-ray pulsars found in a blind search, including 16 radio-quiet and 9 radio-loud pulsars. This results in the direct measurement of the fraction of radio-quiet pulsars ɛRQ = 64 ± 10%, which is in agreement with the existing estimates from the population modeling in the outer magnetosphere model. The Polar cap models are disfavored due to a lower expected fraction and the prediction of age dependence. The age, gamma-ray energy flux, spin-down luminosity and sky location distributions of the radio-loud and radio-quiet pulsars from the catalog do not demonstrate any statistically significant difference. The results indicate that the radio-quiet and radio-loud pulsars belong to one and the same population. The catalog shows no evidence for the radio beam evolution.

Space charge induced beam instability in periodic focusing channel

The transverse evolution of the envelope of an intense, unbunched ion beam in a linear periodic transport channel can be modeled for the approximation of linear self-fields by the Kapchinskij-Vladimirskij envelope equation. The envelope mismatched modes, or the second order even mode [I. Hofmann, Phys. Rew. E 57, 4713 (1998)], are the lowest order of resonance leading to collective instability that the designer should avoid, which suggests that an accelerator system should be established in the parameter region where the zero beam current phase advance σ0 less than 90°. In this paper, we systemically studied the resonance mechanisms which result in confluent resonance in quadrupole Focusing-Defocusing (FD) channel and parametric resonance in solenoid channel. We propose that the mismatch modes cannot be exactly separated in FD channel; if one mode is excited, there is always some contribution of the other. To verify the influence of the confluent resonance and parametric resonance, the 2D Poissons solver in the self-consistent particle-in-cell simulation code TOPOPIC is adopted to study the beam evolution in both channels. Our simulations results show that the emittance show significant growth both in the confluent resonance stop band and parametric resonance stop band. The influences of the higher order of resonance are also discussed.

An intuitive model for on-axis pulse evolution of ultrashort pulsed Gaussian beams diffracted from a circular aperture

The diffraction of ultrashort pulsed Gaussian beams from a circular aperture is studied by means of Fresnel diffraction integral and Fourier transform method. A uniform analytical expression is derived for temporal pulse form of ultrashort pulsed Gaussian beams in two cases, i.e. with constant beam waist and with constant diffraction length. It is shown that the on-axis pulse can be formulated as a superposition of an unapertured pulse and an aperture-induced pulse. The superposition of these two pulses leads to an enhanced pulse intensity for small truncation parameters at certain distances in the near field. Our results may find applications in high-intensity laser waveform control.

Numerical Simulations of Elliptical Hollow Gaussian Beams Propagating in Strongly Nonlocal Media

The evolution of elliptical hollow Gaussian beams propagating in strongly nonlocal nonlinear media is investigated. As examples, this paper mainly focuses on the evolutions of the transverse intensity and the beam width of elliptical hollow Gaussian beams with the beam order being 1 and 3. The results show that the evolutions of the transverse intensity and the beam width are both periodical and the size of the evolution period is determined by the input power. There exists the transverse reverse transform for the beam width when selecting a proper input power, which is quiet different from the circularly symmetric hollow Gaussian beams.

Propagation evolution of the off-axis ellipse vector beam

The propagation characteristics of the off-axis ellipse vector beam (OEVB) are studied in this paper. The analytic expressions of the electric field and the intensity after OEVB propagating in free-space are derived. Numerical results indicate that the intensity distribution of OEVB is asymmetric and the intensity distribution after propagating is determined by propagation distance, dislocation displacement and ellipticity. In addition to expanding, the intensity distribution of OEVB tends to steady-state distribution finally and the dark core of vector beam disappears gradually during propagation. The major axis and minor axis of the ellipse intensity distribution in the steady-state are exchanged with each other as compared with in the initial plane. The results can help us to understand the dynamic propagation characteristics of the ellipse vector beam under the off-axis situation, and they can also guide the calibration of the ellipse vector beam in practice.

Envelope instability and the fourth order resonance

The well-known envelope instability or the second order even collective mode [I. Hofmann, Phys. Rev. E 57, 4 (1998)] and the fourth order resonance $4\ensuremath{\sigma}=360\ifmmode^\circ\else\textdegree\fi{}$ due to the nonlinear space charge effect in high intensity beams have been studied previously. A wide stop band around 15\ifmmode^\circ\else\textdegree\fi{} is found in a pure periodic focusing channel. In addition, it is illustrated that the fourth order resonance dominates over the envelope instability and practically replaces it in the stop band [D. Jeon et al., Phys. Rev. ST Accel. Beams 12, 054204 (2009)]. In this paper, for a continuous beam with remarkable space charge, our 2D self-consistent particle-in-cell simulation work with the code topopic shows these two kinds of effects respectively in a periodic focusing defocusing (FD) channel. For a fixed tune depression $\ensuremath{\eta}=0.8$, a stop band with a width of almost 15\ifmmode^\circ\else\textdegree\fi{} is also demonstrated. Moreover, it is confirmed that analytical results of the rms envelope instability diagram are a valid tool to interpret the width of the stop band. Emittance growth rates in stop band are also well explained. It is found that, for a nearly rms matched beam, the emittance growth in the stop band is almost proportional to the saturation time of the nonlinear instability of the envelope, which happens in a quick manner and takes only a few FD cells. In contrast, the fourth order resonance is independent of rms matching and will be accompanied by beam evolution as ``a long term effect'' once the related mechanism is excited.

Generation of quasiequally spaced ultrashort microbunches in a photocathode rf gun

A photocathode rf gun can generate trains of THz subpicosecond electron bunches by illuminating the cathode with trains of laser pulses, but it suffers from the increasing charge in the beam. The THz structure blurs and tends to disappear when the longitudinal space charge forces begin to play a significant role in the beam evolution. In this paper, we propose a scheme to restrain the space charge forces by expanding the transverse size of the laser pulses to reduce the charge density and adopting a multicell gun to increase the beam energy. Thus, quasiequally spaced ultrashort microbunches with relatively high charges can be generated according to our studies. Postacceleration can be used to freeze the longitudinal phase space dynamics. The proposed scheme is in principle able to generate intense multi-color narrow-band THz radiation and offers a promising way towards the tunable intense narrow-band THz sources.

Spatial profile and singularities of the edge-diffracted beam with a multicharged optical vortex

We analyze the edge diffraction of circular Laguerre–Gaussian beams LG02 and LG03 carrying multicharged optical vortices (OVs), with special attention to spatial properties and evolution of the diffracted beam. The problem is considered numerically within the frame of paraxial approximation and the Fresnel diffraction theory. The diffracted beam evolution is interpreted on the basis of the incident beam symmetry breakdown and decomposition of a higher-order OV into a set of single-charged secondary OVs. Upon propagation, the separate OVs describe specific trajectories within the diffracted beam cross section but in the far field they are localized on the symmetry axis parallel to the screen edge. The OVs' positions in each cross section reflect information of the screen edge position with respect to the incident beam axis. If the incident OV is stopped by the screen, the diffracted beam possesses no OV just behind the screen but in the course of its further propagation, single-order OVs appear consecutively so that in the far field the number of OVs equals to the absolute topological charge of the incident OV. Possible applications for the remote measurements of small linear displacements are discussed.