Wakefield Effects Research Articles

Analysis and comparison between electric and magnetic power couplers for accelerators in Free Electron Lasers (FEL)

Free-electron lasers represent a new and exciting class of coherent optical sources possessing broad wavelength tunability and excellent optical-beam quality. The FERMI seeded free-electron laser (FEL), located at the Elettra laboratory in Trieste, is driven by a 200m long, S-band linac: the high energy part of the linac is equipped with 6m long backward traveling wave (BTW) structures. The structures have small iris radius and a nose cone geometry which allows for high gradient operation. Development of new high-gradient, S-band accelerating structures for the replacement of the existing BTWs is under consideration. This paper investigates two possible solutions for the RF power couplers suitable for a linac driven FEL which require reduced wakefields effects, high operating gradient and very high reliability. The first part of the manuscript focuses on the reduction of residual field asymmetries, while in the second analyzes RF performances, the peak surface fields and the expected breakdown rate. In the conclusion, two solutions are compared and pros and cons are highlighted.

Terahertz radiation driven by two-color laser pulses at near-relativistic intensities: Competition between photoionization and wakefield effects.

We numerically investigate terahertz (THz) pulse generation by linearly-polarized, two-color femtosecond laser pulses in highly-ionized argon. Major processes consist of tunneling photoionization and ponderomotive forces associated with transverse and longitudinal field excitations. By means of two-dimensional particle-in-cell (PIC) simulations, we reveal the importance of photocurrent mechanisms besides transverse and longitudinal plasma waves for laser intensities >1015 W/cm2. We demonstrate the following. (i) With two-color pulses, photoionization prevails in the generation of GV/m THz fields up to 1017 W/cm2 laser intensities and suddenly loses efficiency near the relativistic threshold, as the outermost electron shell of ionized Ar atoms has been fully depleted. (ii) PIC results can be explained by a one-dimensional Maxwell-fluid model and its semi-analytical solutions, offering the first unified description of the main THz sources created in plasmas. (iii) The THz power emitted outside the plasma channel mostly originates from the transverse currents.

Ion wake field effects on the dust-ion-acoustic surface mode in a semi-bounded Lorentzian dusty plasma

The dispersion relation for the dust ion-acoustic surface waves propagating at the interface of semi-bounded Lorentzian dusty plasma with supersonic ion flow has been kinetically derived to investigate the nonthermal property and the ion wake field effect. We found that the supersonic ion flow creates the upper and the lower modes. The increase in the nonthermal particles decreases the wave frequency for the upper mode whereas it increases the frequency for the lower mode. The increase in the supersonic ion flow velocity is found to enhance the wave frequency for both modes. We also found that the increase in nonthermal plasmas is found to enhance the group velocity of the upper mode. However, the nonthermal particles suppress the lower mode group velocity. The nonthermal effects on the group velocity will be reduced in the limit of small or large wavelength limit.

Numerical investigation of the scale effect of hydrodynamic performance of the hybrid CRP pod propulsion system

The scale effect of hydrodynamic performance of the hybrid CRP pod propulsion system was investigated numerically using the RANS method combined with SST k−ω turbulence model and moving mesh method. The pod resistance influence factor was introduced to represent the effect of wake field of CRP on the pod resistance. Results showed the pod resistance influence factor to be a function of the Reynolds number and revolution ratio. Representative function expression can be obtained by regression analysis using multiplication of multinomial polynomials and linear function. The standard ITTC 1978 extrapolation procedure can be utilized to predict hydrodynamic performance of forward propeller because of the slightness of the influence of the pod unit on the forward propeller. The thrust and torque coefficient influence factors of aft propeller were introduced, and they were found to represent the effect of wake field of forward propeller and blockage effect of the pod on the hydrodynamic performance of aft propeller. It shows that thrust and torque coefficient influence factors are independent of the Reynolds number and have a linear relationship with the revolution ratio. On this basis, a method of estimating the hydrodynamic performance was proposed for full scale propulsion system.

Numerical study on scale effect of nominal wake of single screw ship

A 4000TEU container ship was studied without considering free surface effect, and the viscous flow fields of ship at different scales were solved numerically by RANS method, numerical uncertainty analysis was employed according to factors of safety method for Richardson extrapolation, scale effect of axial nominal wake field was analyzed in details. It shows that the reciprocal of mean axial wake fraction of propeller disc exhibits a near linear dependence on Reynolds number in logarithmic scale. For the single screw ship without bilge vortex, linear function is fit perfectly for the relationship between the reciprocal of mean axial wake fraction at each radius, reciprocal of amplitude of wake peak right above propeller disc and Reynolds number in logarithmic scales. In inner area of propeller disc, the reciprocal of amplitude of wake valley and wake peak right down propeller disc reveal nearly linear dependence on Reynolds number in logarithmic scales. While in outer area, the amplitude of wake peak and valley decline rapidly to potential wake fraction, and the wake width reveals negative exponent power function dependence on Reynolds number in logarithmic scales. On this basis, an extrapolated wake field scaling method is proposed.

Wakefields in SLAC linac collimators

When a beam travels near collimator jaws, it gets an energy loss and a transverse kick due to the backreaction of the beam field diffracted from the jaws. The effect becomes very important for an intense short bunch when a tight collimation of the background beam halo is required. In the Linac Coherent Light Source at SLAC a collimation system is used to protect the undulators from radiation due to particles in the beam halo. The halo is most likely formed from gun dark current or dark current in some of the accelerating sections. However, collimators are also responsible for the generation of wake fields. The wake field effect from the collimators not only brings an additional energy jitter and change in the trajectory of the beam, but it also rotates the beam on the phase plane, which consequently leads to a degradation of the performance of the Free Electron Laser at the Linac Coherent Light Source. In this paper, we describe a model of the wake field radiation in the SLAC linac collimators. We use the results of a numerical simulation to illustrate the model. Based on the model, we derive simple formulas for the bunch energy loss andmore » the average kick. In addition, we also present results from experimental measurements that confirm our model.« less

Wakefields in THz cylindrical dielectric lined waveguides driven by femtosecond electron bunches

This paper reports the wakefield effects driven by a high-intensity relativistic electron bunch in a dielectric lined waveguide (DLW). A state-of-the-art electron bunch is employed to serve as the drive bunch, which has an rms length of 10µm, i.e. 33fs, and a charge of 200pC. Such bunch parameters are comparable to those of DESY’s FLASH and SLAC’s LCLS and FACET facilities. It is demonstrated that coherent Cherenkov radiation (CCR) at the fundamental mode with frequency above 1THz and accelerating gradient as high as 2GV/m can be obtained in a single layer cylindrical diamond-DLW structure, as long as the geometrical parameters of the DLW are properly selected to match the drive bunch. Wakefield-induced energy modulations on the drive bunch itself are studied as well, which can be used to reduce its energy spread or to produce microbunches with much shorter length from it. The simulated results agree well with the theoretical predictions. Such wakefields can be used to accelerate or modulate electron bunches with ultra-high gradients, and produce high power THz radiations directly. These properties have potential applications in the fields of compact colliders and advanced radiation sources.

Numerical simulations of dust charging and wakefield effects

Charging of dust grains and related phenomena are fundamental problems in the physics of complex plasmas. The relative motion of grains and plasma breaks the symmetry in dust charging and gives rise to the wake in plasma density and potential, which can significantly influence the dynamics of other grains. This paper gives an overview of dust charging in two- and multi-component plasma flows and related wake effects, and presents recent results from self-consistent particle-in-cell (PIC) simulations. The role of wakefields is considered in the context of charging of downstream grains.

FEL simulation of the 0.1-nm Hard X-ray from the PAL-XFEL

The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) will provide X-ray FEL radiation in the range of 0.1 and 10 nm with five undulator beamlines. An undulator beamline for hard X-ray is designed for 0.1-nm self-amplified spontaneous emission FEL. When a 200-pC electron bunch with a 10-GeV energy and a 0.5-μm·rad normalized emittance passes the undulators, 1012 photons at 0.1 nm will be generated within 100 fs. The wakefield effect of the vacuum chamber may reduce the radiation power significantly, but a proper tapering in the undulator can recover the power. We present a study in which GENESIS 1.3 was used for the FEL simulation.

Wakefield effects on three-dimensional dust-lattice modes

AbstractDispersion relations of dust-lattice waves in three-dimensional body-centered cubic (bcc) and face-centered cubic (fcc) dusty plasma crystals are derived taking into account particle–wake interactions. Results show that resonance coupling between the longitudinal and out-of-plane transverse wave modes set in if the vertical confinement is below a certain critical value. The imaginary part of the wave frequency shows that the hybrid mode cannot propagate in the crystal.

The study of the wake field effects on the self-focusing and the compression of the laser pulse in plasma at the relativistic regime by Lagrangian method

The trial function in the variational-Lagrangian method is a fundamental method to explain the essential features of the laser pulse evolution in the plasma. Self-focusing behavior and compression of the laser pulse in plasma are analyzed, for both low and high intensity regimes. It is shown that the compression threshold depends on both pulse intensity and pulse length. In particular, in both regimes, the compression threshold is directly proportional to the pulse length. This is while this threshold is directly, in the linear regime, and inversely, in the nonlinear regime, proportional to pulse intensity. In the present work, the existence of oscillations is revealed, with a behavior akin to laser pulse width. Finally, the effects of pulse intensity, pulse length, and plasma density on compression are analyzed.

Analysis of the wake field effects in the PEP-II storage rings with extremely high currents

We present the history and analysis of different wake field effects throughout the operational life of the PEP-II SLAC B-factory. Although the impedance of the high and low energy rings is small, the intense high-current beams generated a lot of power. The effects from these wake fields are: heating and damage of vacuum beam chamber elements like RF seals, vacuum valves, shielded bellows, BPM buttons and ceramic tiles; vacuum spikes, vacuum instabilities and high detector background; and beam longitudinal and transverse instabilities. We also discuss the methods used to eliminate these effects. Results of this analysis and the PEP-II experience may be very useful in the design of new storage rings and light sources.

Effects of Higher-Order Relativistic Nonlinearity and Wakefield During a Moderately Intense Laser Pulse Propagation in a Plasma Channel

Using a variational approach, the propagation of a moderately intense laser pulse in a parabolic preformed plasma channel is investigated. The effects of higher-order relativistic nonlinearity (HRN) and wakefield are included. The effect of HRN serves as an additional defocusing mechanism and has the same order of magnitude in the spot size as that of the transverse wakefield (TWF). The effect of longitudinal wakefield is much larger than those of HRN and TWF for an intense laser pulse with the pulse length equaling the plasma wavelength. The catastrophic focusing of the laser spot size would be prevented in the present of HRN and then it varies with periodic focusing oscillations.

Beam heat load due to geometrical and resistive wall impedance in COLDDIAG

One of the still open issues for the development of superconductive insertion devices is the understanding of the heat intake from the electron beam. With the aim ofmeasuring the beam heat load to a cold bore and the hope to gain a deeper understanding in the underlying mechanisms, a cold vacuum chamber for diagnostics (COLDDIAG) was built. It is equipped with the following instrumentation: retarding field analyzers to measure the electron flux, temperature sensors to measure the beam heat load, pressure gauges, and mass spectrometers to measure the gas content. Possible beam heat load sources are: synchrotron radiation, wakefield effects due to geometrical and resistive wall impedance and electron/ion bombardment.The flexibility of the engineering design will allow the installation of the cryostat in different synchrotron light sources. COLDDIAG was first installed in the Diamond Light Source (DLS) in 2011. Due to a mechanical failure of the thermal transition of the cold liner, the cryostat had to be removed after one week of operation.After having implemented design changes in the thermal liner transition, COLDDIAG has been reinstalled in the DLS at the end of August 2012.In order to understand the beam heat load mechanism it is important to compare the measured COLDDIAG parameters with theoretical expectations. In this paper we report on the analytical and numerical computation of the COLDDIAG beam heat load due to coupling impedances deriving from unavoidable step transitions, ports used for pumping and diagnostics, surface roughness, and resistive wall. The results might have an important impact on future technological solutions to be applied to cold bore devices.

Wake Formation and Wake Field Effects in Complex Plasmas

AbstractThe formation of wake fields downstream of an objects in flowing plasmas has strong implications on structure, stability and dynamics of complex plasmas. This paper aims at putting recent experimental investigations and different theoretical approaches on charging of dust grains, stability of dust grain arrangements and the dynamical properties of complex plasmas into perspective. Further, the combination of wake fields and grain confinement is discussed, which is the generic situation in complex plasmas. It is shown that in spherical traps the resulting competition of trap geometry and vertical alignment leads to a sophisticated mixture of nested shell and string order (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the ion wake-field on the eikonal cross section for the electron–dust collision in dusty plasmas

AbstractThe ion wake-field effects on the elastic electron–dust collisions are investigated in complex dusty plasmas. The eikonal method is employed to investigate the behaviors of the scattering phase shift and scattering cross section due to the variation of the strength of the wake-field. It is shown that the eikonal phase shift decreases with an increase of the Mach number and increases with an increase of the impact parameter. It is also shown that the eikonal phase shift decreases with increasing Debye length. The eikonal cross section for the elastic electron–dust collision is found to be increased due to the influence of the wake-field. In addition, it is found that the wake-field effect on the eikonal cross section is almost independent of the Debye length.

Experimental demonstration of wakefield effects in a THz planar diamond accelerating structure

We have directly measured THz wakefields induced by a subpicosecond, intense relativistic electron bunch in a diamond loaded accelerating structure via the wakefield acceleration method. We present here the beam test results from the diamond based structure. Diamond has been chosen for its high breakdown threshold and unique thermoconductive properties. Fields produced by a leading (drive) beam were used to accelerate a trailing (witness) electron bunch, which followed the drive bunch at a variable distance. The energy gain of a witness bunch as a function of its separation from the drive bunch describes the time structure of the generated wakefield.

Influence of the ion wake-field on the collisional entanglement fidelity in complex dusty plasmas

The ion wake-field effects on the entanglement fidelity for the elastic electron-dust grain collisions are investigated in complex dusty plasmas. The screened effective potential and partial wave analysis are employed to obtain the entanglement fidelity in dusty plasmas as a function of the Mach number, Debye length, and collision energy. It is shown that the ion wake-field effect suppresses the entanglement fidelity in dusty plasmas. It is also found that the ion wake-field effect on the entanglement fidelity decreases with an increase of the collision energy.

Wakefield effects and solitary waves of an intense short laser pulse propagation in a plasma channel

In the presence of relativistic and channel-coupling nonlinearity and wakefield effects, the propagation characteristics and solitary waves of an intense short laser pulse in a preformed plasma channel are investigated. The evolution equation of the laser spot size is derived by using variational technique, the initial laser and plasma parameters for propagation with constant spot size, periodic defocusing and focusing oscillations, and solitary waves are identified. For illustration, some numerical results are also presented. It is found that the laser focusing is enhanced by the wakefield effects that result in a significant reduced focusing power.

CLIC crab cavity design optimisation for maximum luminosity

The bunch size and crossing angle planned for CERN's compact linear collider CLIC dictate that crab cavities on opposing linacs will be needed to rotate bunches of particles into alignment at the interaction point if the desired luminosity is to be achieved. Wakefield effects, RF phase errors between crab cavities on opposing linacs and unpredictable beam loading can each act to reduce luminosity below that anticipated for bunches colliding in perfect alignment. Unlike acceleration cavities, which are normally optimised for gradient, crab cavities must be optimised primarily for luminosity. Accepting the crab cavity technology choice of a 12 GHz, normal conducting, travelling wave structure as explained in the text, this paper develops an analytical approach to optimise cell number and iris diameter.