Transverse Instability Research Articles

Measurement and parametric analysis of transverse coupled bunch instabilities for electron synchrotron radiation source Indus-2

The performance of synchrotron radiation sources at high beam current gets often degraded due to the excitation of longitudinal and transverse coupled bunch instabilities (CBI). Hence, measurement of the coupled bunch instabilities may give insight into the performance of the machine, which in turn, may help supress these instabilities. Transverse coupled bunch instability is sensitive to the operating parameters of the accelerator such as betatron tune and sextupole magnet strength. In this paper, we report the measurement on transverse coupled bunch modes (CBM). The effect of betatron tune values and sextupole magnet strengths on transverse coupled bunch instability is discussed. For this study, betatron tune values and sextupole strengths are chosen in the vicinity of the routine operating points. With the help of this parametric study, an optimum operating point is identified in the beam operation with a minimum level of transverse coupled bunch instability. Further, an artificial neural network (ANN) is used to deduce a functional model between average transverse CBM levels and sextupole strengths.

Experimental study of transverse mode instability threshold dependence on seed wavelength in high power ytterbium-doped fiber amplifiers

As the output power increases, the transverse mode instability (TMI) has become a major limitation for power scaling of high power fiber lasers with nearly diffraction-limited beam quality. Many strategies in experimental or theoretical scopes have been proposed to mitigate the TMI. Theoretical studies have indicated that the TMI thresholds of the laser amplifiers are affected by the seed laser wavelength. In this work, the influences of seed wavelengths on the TMI threshold in high power large-mode-area ytterbium-doped fiber (YDF) amplifiers based on an all-fiber master oscillation power amplifier structure are investigated experimentally. The TMI characteristics and thresholds of the same fiber amplifier seeded by respective 1080 nm, 1070 nm, and 1065 nm fiber lasers are analyzed and compared. The experimental results show that the TMI threshold decrease with the seed wavelength in the region of 1065 nm–1080 nm. For every 5 nm decrease in seed wavelength, the TMI threshold power can be increased by about 290 W. Compared with the fiber amplifier of the 1080 nm seed wavelength, the TMI thresholds of the 1070 nm and 1065 nm wavelength fiber amplifiers are respectively increased by around 58.7% and 86.2%. Reducing the wavelength of seed can effectively improve the TMI threshold, but the amplified spontaneous emission (ASE) and nonlinear effects are also enhanced. The results have significant guidance for the mitigation of TMI in high-power YDF laser systems.

Dynamics of interacting dark soliton stripes

In the present work we examine the statics and dynamics of multiple parallel dark soliton stripes in a two-dimensional Bose-Einstein condensate. Our principal goal is to study the effect of the interaction between the stripes on the transverse instability of the individual stripes. We use a recently developed adiabatic invariant formulation to derive a quasi-analytical prediction for the stripe equilibrium position and for the Bogoliubov-de Gennes spectrum of excitations of stationary stripes. The cases of two-, three- and four-stripe states are studied in detail. We subsequently test our predictions against numerical simulations of the full two-dimensional Gross-Pitaevskii equation. We find that the number of unstable eigenmodes increases as the number of stripes increases due to (unstable) relative motions between the stripes. Their corresponding growth rates do not significantly change, although for large chemical potentials, the larger the stripe number, the larger the maximal instability growth rate. The instability induced dynamics of multiple stripe states and their decay into vortices are also investigated.

Experiments on the transverse aeroelastic instability of a single sphere in a swing configuration under a longitudinal wind

Overhead power lines are protected from electrical discharges (that occur during storms) by the so-called “guard cables”, which are placed above the power conductors. In some places, they are provided with beacon spheres. A new instability in these cables, under a longitudinal wind, has recently been reported. The motion of the cable can damage their supports, and causing interference with the power lines, leading to cable breakage, and reducing energy availability supplied by the transmission line. The reported instabilities include oscillation and autorotation motions. In previous papers, the vortex-induced vibration has been suggested as the mechanism that can explain the system oscillations at low speed, and presented the lateral quasi-steady aerodynamic force that can justify both the autorotation motion of a sphere and the oscillations at large speed.In this paper, results of wind tunnel tests concerning a simplified system, a single sphere in a swing configuration, are reported, considering the influence of the main parameters involved: cable span and sag, beacon mass, surface roughness, and reduced wind speed.The limits of the regions corresponding to different motion behaviours (oscillation, autorotation, transitions) are presented in the wind velocity-sag map. Autorotation results have been compared with available theoretical models.

Determination of a focusing system parameters in generation of THz radiation by bunch train

It is present electron optics optimization for terahertz radiation generation by a sequence of short electron bunches passing through the dielectric loaded waveguide. In this method the superposition of individual bunch wakefields is used to selectively excite high frequency TM0n modes in a slow-wave structure. The main problem of this scheme is the transverse beam instability that limits the length of THz radiation wave packet. The main goal of this paper is a new parameter optimization algorithm for the FODO focusing system. The numerical modelling of the beam dynamics was performed in the original code “Dielectric waveguide” which utilizes analytic expression of the Green’s function for the dielectric loaded waveguides with cylindrical and rectangular geometries.

Experimental Study of 5-kW High-Stability Monolithic Fiber Laser Oscillator With or Without External Feedback

In this paper, we construct a high power monolithic fiber laser oscillator based on a commercial ytterbium-doped fiber with core/inner cladding diameter of 25/400 μ m. The output performances of the fiber oscillator are experimentally investigated in the conditions of either with or without an external feedback. In the presence of external feedback, a very strong stimulated Raman scattering (SRS) (∼9 dB below the signal light) emerges at the output power of ∼3.56 kW, where the onset of the transverse mode instability (TMI) limits further power scaling. By large angle cleaving the signal arm of the forward combiner to avoid external feedback, the maximum output power of 5.07 kW is achieved with no sign of TMI. The Raman stokes light is ∼35 dB below the signal light and the M2 factor is measured to be ∼1.6. To directly characterize its power stability and engineering capability, the fiber oscillator is tested for continuous 5-h operation at ∼5.07 kW with power fluctuation less than 0.42%. During the test process, the output power, spectrum, beam quality, and temporal signal perform excellent stability. The experimental results reveal that thresholds of SRS and TMI strongly depend on the external feedback in high power fiber oscillators.

Intensity modulated terahertz vortex wave generation in air plasma by two-color femtosecond laser pulses.

We investigate the generation of broadband terahertz (THz) pulses with phase singularity from air plasmas created by fundamental and second harmonic laser pulses. We show that when the second harmonic beam carries a vortex charge, the THz beam acquires a vortex structure as well. A generic feature of this THz vortex is that the intensity is modulated along the azimuthal angle, which can be attributed to the spatially varying relative phase difference between the two pump harmonics. Fully space- and time-resolved numerical simulations reveal that transverse instabilities of the pump further affect the emitted THz field along nonlinear propagation, which may produce additional singularities resulting in a rich vortex structure. The predicted intensity modulation is experimentally demonstrated with a thermal camera, in excellent agreement with simulation results. The presence of phase singularities in the experiment is revealed by astigmatic transformation of the beam using a cylindrical mirror.

Transverse instability and disintegration of a domain wall of a relative phase in coherently coupled two-component Bose-Einstein condensates

We study transverse instability and disintegration dynamics of a domain wall of a relative phase in two-component Bose-Einstein condensates with a coherent Rabi coupling. We obtain analytically the stability phase diagram of the stationary solution of the domain wall for the one-dimensional coupled Gross-Pitaevskii equations in the plane of the Rabi frequency and the intercomponent coupling constant. Outside the stable region, the domain wall is dynamically unstable for the transverse modulation along the direction perpendicular to the phase kink. The nonlinear evolution associated with the instability is demonstrated through numerical simulations for both the domain wall without edges and that with edges formed by the quantized vortices.

Effect of dispersion on the nanoscale patterns produced by ion sputtering.

Our simulations show that dispersion can have a crucial effect on the patterns produced by oblique-incidence ion sputtering. It can lead to the formation of raised and depressed triangular regions traversed by parallel-mode ripples, and these bear a strong resemblance to nanostructures that are commonly observed in experiments. In addition, if dispersion and transverse smoothing are sufficiently strong, highly ordered ripples form. Finally, dispersion can cause the formation of protrusions and depressions that are elongated along the projected beam direction even when there is no transverse instability. This may explain why topographies of this kind form for high angles of ion incidence in cases in which ion-induced mass redistribution is believed to dominate curvature-dependent sputtering.

Instability transverse mode phase transition of fiber oscillator for extreme power lasers.

High-power fiber lasers have been widely explored in engineering and science, and improving the beam quality restricted by transverse mode instability (TMI) is waiting to be solved for extra high-power applications. Here, we theoretically propose a phase transition model for understanding TMI in a fiber oscillator. A general dynamics model describing TMI is established by modifying the heat equation, and then a special dynamics model of a two-mode fiber (TMF) oscillator is obtained by applying this general dynamics model to TMF oscillator case. Theoretical analysis shows that there is a reversible phase transition point in this TMF oscillator model, which can well explain the sudden and reversible change of TMI. Based on linear stability analysis near the phase transition point, an analytical threshold formula of TMI is given to calculate the TMI threshold in the TMF oscillator. The calculated results are consistent with the reported experimental results. Furthermore, the relationship between the TMI threshold and several parameters was also discussed in detail such as laser wavelength, pump wavelength, core radius, cladding radius, etc. This theoretical model will be useful to understand and suppress the TMI in fiber oscillators.

Physics of relativistic collisionless shocks: The scattering-center frame.

In this first paper of a series dedicated to the microphysics of unmagnetized, relativistic collisionless pair shocks, we discuss the physics of the Weibel-type transverse current filamentation instability that develops in the shock precursor, through the interaction of an ultrarelativistic suprathermal particle beam with the background plasma. We introduce in particular the notion of the "Weibel frame," or scattering center frame, in which the microturbulence is of mostly magnetic nature. We calculate the properties of this frame, using first a kinetic formulation of the linear phase of the instability, relying on Maxwell-Jüttner distribution functions, then using a quasistatic model of the nonlinear stage of the instability. Both methods show that (i) the Weibel frame moves at subrelativistic velocities relative to the background plasma, therefore at relativistic velocities relative to the shock front; (ii) the velocity of the Weibel frame relative to the background plasma scales with ξ_{b}, i.e., the pressure of the suprathermal particle beam in units of the momentum flux density incoming into the shock; and (iii) the Weibel frame moves slightly less fast than the background plasma relative to the shock front. Our theoretical results are found to be in satisfactory agreement with the measurements carried out in dedicated large-scale 2D3V particle-in-cell simulations.

Comparative study on transverse mode instability of fiber amplifiers based on long tapered fiber and conventional uniform fiber

Compared with conventional uniform fibers, long tapered fibers provide an effective way to maintain beam quality and suppress the generation of nonlinear effects in fiber lasers. In this work, the transverse mode instability (TMI) of two amplifiers based on ytterbium-doped fiber (YDF) with uniform core diameter and tapered core diameter has been investigated experimentally. For a reasonable comparison of TMI in these two fiber amplifiers, the same effective core diameters and design parameters are applied to both. In the uniform YDF amplifier, the TMI threshold is around 1046 W, and the beam quality M2 factor increases with the power and reaches 3.2 when the power is around 1500 W. In a tapered YDF (T-YDF) amplifier, the maximum output power is up to 2170 W with a beam quality of M2 ~ 2.2 and no sign of TMI is observed. To the best of our knowledge, this is the first reported all-fiber tapered fiber laser with a 2 kW-level average output power. The experimental results and theoretical analysis indicate that the TMI threshold of the T-YDF amplifier is higher than the uniform YDF amplifier in spite of sharing the same effective core diameter.

Studies of transverse single-bunch instabilities in booster synchrotrons

Abstract The capability of providing full-charge bunches has become an important requirement for quite a few boosters used in ultralow-emittance storage rings based synchrotron light sources. The transverse single-bunch instabilities may limit the single-bunch charge in these type of boosters. The relatively weak synchrotron radiation damping makes the bunch more unstable at the injection energy than at the extraction energy. Therefore, studies of the transverse single-bunch instabilities are carried out first at the injection energy, with the chromaticity varied, to understand how the threshold changes accordingly. Our studies show that the single-bunch threshold current tends to drop when the chromaticity becomes nonzero at the injection energy of the used booster lattice. The corresponding simulation results are presented and analyzed. Furthermore, the transverse single-bunch instabilities during the energy ramping process are also studied.

Analytical and experimental studies on dynamic instability of simply supported rectangular plates with arbitrary concentrated masses

This paper presents innovative analytical and experimental investigations of transverse dynamic instability of a simply supported rectangular plate attached with arbitrary concentrated masses owing to parametric resonance excited by an in-plane uniformly distributed periodic load along two opposite edges, which has not been reported in the literature. Based on the von-Kármán large deflection theory of thin plate, differential equations of transverse motion are established by using the Galerkin method. The analytical results of dynamic instability regions and nonlinear response curves of the plate having arbitrary concentrated masses for various instability modes are obtained. Tests investigating the influence of concentrated mass on the out-of-plane dynamic instability of various modes of a plate under the in-plane periodical loading presented in the paper are first time reported in the literature. Independent swept frequency tests are carried out to investigate instability frequency-amplitude regions for transverse dynamic instability of the plate and the test results agree well with the theoretical counterparts. It is shown that concentrated masses significantly influence out-of-plane dynamic instability of the plate. The investigation leads to the following novel findings: (1) When the concentrated masses are located at positions corresponding to non-zero modal displacements, the masses increase the rate of energy dissipation and damping ratio of the plate, leading to an increase of the critical excitation amplitude for dynamic instability of the plate, but to a decrease of the natural frequencies of the plate and the critical excitation frequencies for dynamic instability of the plate. (2) Under the same excitation amplitude, the excitation frequencies and the widths of excitation frequency region for dynamic instability of the plate as well as excitation frequency intervals for nonlinear dynamic instability of the plate decrease with an increase of the concentrated masses, which are located at positions corresponding to non-zero modal displacements. (3) When the concentrated masses are located at positions corresponding to zero modal displacements, the concentrated masses almost do not affect the critical excitation frequency and the widths of excitation frequency region for dynamic instability of the plate as well as excitation frequency intervals for nonlinear dynamic instability of the plate. (4) The widths of excitation frequency regions for dynamic instability of the plate as well as excitation frequency intervals for nonlinear dynamic instability of the plate increases when masses move away from the position of the largest modal displacement of the plate.

Transverse mode instability, thermal lensing and power scaling in Yb3+-doped high-power fiber amplifiers.

Transverse mode instability (TMI) is compared to thermal lensing (TL) power threshold and used to derive power scaling limits in high-power fiber amplifiers. The TMI power threshold is shown to be ~65% of the TL one and dominates power scaling. In addition to commonly used limiting effects, we introduce a bend-induced mechanical reliability criterion, which limits the maximum allowable cladding diameter to ~600μm. This also results in the introduction of a critical pump brightness, the minimum required pump brightness at which the maximum signal power is achieved. The maximum achievable power depends primarily on the choice of pumping wavelength, amplifier gain and heat coefficient. Maximum signal powers of ~28kW to ~38kW, for diode pumping (λp = 976nm), and ~35kW to ~52kW, for tandem pumping (λp = 1018nm), are predicted for single-mode fiber amplifiers operating at signal wavelength λs = 1070nm, when the amplifier gain is increased from 10dB to 20dB. For an amplifier gain of 10dB, the maximum achievable signal power varies from 85kW to 25kW for tandem pumping and 35kW to 20kW for diode pumping, when the heat coefficient varies from 1% to 15% and 5.5% to 20%, respectively. The corresponding critical pump brightness varies from ~0.50 W/(μm2 sr) to ~0.14 W/(μm2 sr) for tandem pumping and ~0.25 W/(μm2 sr) to ~0.13 W/(μm2 sr) for diode pumping.

Comment on “Instability studies at the CERN Proton Synchrotron during transition crossing”

In the article of Migliorati, Aumon, Koukovini-Platia, Huschauer, Metral, Sterbini and Wang [Phys. Rev. Accel. Beams 21, 120101 (2018)], hereafter the Article and the Authors, the long-standing qualification of the PS instability as the transverse mode-coupling instability, or TMCI, is not mentioned, while a model of the beam breakup (BBU) is discussed. Do the observations and modeling of the Article refute the long-standing idea that the PS instability is the TMCI?

Transverse instability of gravity–capillary solitary waves on deep water in the presence of constant vorticity

The bifurcation of two-dimensional gravity–capillary waves into solitary waves when the phase velocity and group velocity are nearly equal is investigated in the presence of constant vorticity. We found that gravity–capillary solitary waves with decaying oscillatory tails exist in deep water in the presence of vorticity. Furthermore we found that the presence of vorticity influences strongly (i) the solitary wave properties and (ii) the growth rate of unstable transverse perturbations. The growth rate and bandwidth instability are given numerically and analytically as a function of the vorticity.

Investigating ride quality of the machine-tractor unit with a variable mass during transport work

The disclosure of issues related to solving problems of transport work has always been relevant. Today, in the study of types of transport works used in agriculture, there is a problem in the study of the movement of machine-tractor unit with trailed or semi-trailer tanks, which are filled with liquid. Since the transportation of liquid cargo causes significant internal oscillations, the impact on the operation of the wheeled tractor and tank is significant. This process causes longitudinal and transverse instability, which contributes to the increase of both energy costs and emergency situations (there is an effect on vibrations of motion, controllability and stability of the vehicle, increasing the dynamic loading of the running system, etc.). The goal of the work is to study the smoothness of the machine-transport unit with variable weight (semitrailer-tank and tank-trailer) by simulating the vibrational system of the vehicle. Namely, to study the effect of forced oscillations on the movement of a semi-trailer tank and a trailer tank, as well as when varying the rigidity of the tires of the wheeled tractor. In solving the set goal, a methodology was used which included the analysis of the results of mathematical modeling of the vibration of the wheeled tractor and the tank with a liquid containing a free surface in the dynamic solution of the problem. The model used takes into account the redistribution of liquid in the tank, which is caused by fluctuations of the shell, using the characteristics of the Rayleigh surface waves. As a result, the data of theoretical research, which take into account the variable weight of the semi-trailer and trailer unit during the performance of the machine-tractor unit of transport work with varying rigidity of tires, is obtained. The originality is in determining the dependence of the rigidity index of the wheels of the wheeled tractor on the smoothness of the machine-tractor unit with a semi-trailer and trailer tanker. The practical value of the work is in the recommendations on the use of one or another pressure in the tires of the wheeled tractor during operation.Key words: wheeled tractor; transport work; tank, vibrations; mass redistribution.

Improvement of ion acceleration in radiation pressure acceleration regime by using an external strong magnetic field

AbstractTwo-dimensional particle-in-cell (PIC) simulations have been used to investigate the interaction between a laser pulse and a foil exposed to an external strong longitudinal magnetic field. Compared with that in the absence of the external magnetic field, the divergence of proton with the magnetic field in radiation pressure acceleration (RPA) regimes has improved remarkably due to the restriction of the electron transverse expansion. During the RPA process, the foil develops into a typical bubble-like shape resulting from the combined action of transversal ponderomotive force and instabilities. However, the foil prefers to be in a cone-like shape by using the magnetic field. The dependence of proton divergence on the strength of magnetic field has been studied, and an optimal magnetic field of nearly 60 kT is achieved in these simulations.

Transverse instability magnetic field thresholds of electron phase-space holes.

A detailed comparison is presented of analytical and particle-in-cell simulation investigation of the transverse instability, in two dimensions, of initially one-dimensional electron phase-space hole equilibria. Good quantitative agreement is found between the shift-mode analysis and the simulations for the magnetic field (B) threshold at which the instability becomes overstable (time oscillatory) and for the real and imaginary parts of the frequency. The simulation B threshold for full stabilization exceeds the predictions of shift-mode analysis by 20-30%, because the mode becomes substantially narrower in spatial extent than a pure shift. This threshold shift is qualitatively explained by the kinematic mechanism of instability.