Transverse Density Gradients Research Articles

Single-Shot Diagnosis of Electron Energy Evolution via Streaked Betatron X Rays in a Curved Laser Wakefield Accelerator.

We report on an experimental observation of the streaking of betatron x rays in a curved laser wakefield accelerator. The streaking of the betatron x rays was realized by launching a laser pulse into a plasma with a transverse density gradient. By controlling the plasma density and the density gradient, we realized the steering of the laser driver, electron beam, and betatron x rays simultaneously. Moreover, we observed an energy-angle correlation of the streaked betatron x rays and utilized it in diagnosing the electron acceleration process in a single-shot mode. Our work could also find applications in advanced control of laser beam and particle propagation. More importantly, the angular streaked betatron x ray has an intrinsic spatiotemporal correlation, which makes it a promising tool for single-shot pump-probe applications.

Particle acceleration by sub-proton cyclotron frequency spectrum of dispersive Alfven waves in inhomogeneous solar coronal plasmas

ABSTRACT The problem of explaining observed soft X-ray fluxes during solar flares, which invokes acceleration of large fraction of electrons, if the acceleration takes places at the solar coronal loop-top, can potentially be solved by postulating that flare at loop-top creates dispersive Alfven waves (DAWs) which propagate towards the foot-points. As DAWs move in progressively denser parts of the loop (due to gravitational stratification) the large fraction of electrons is no longer needed. Here, we extend our previous results by considering f−1 frequency spectrum of DAWs and add He++ ions using fully kinetic particle-in-cell (PIC) simulations. We consider cases when transverse density gradient is in the range 4–40c/ωpe and DAW driving frequency is 0.3–0.6ωcp. We find that (i) The frequency spectrum case does not affect electron acceleration fraction in the like-to-like cases, but few times larger percentage of He++ heating is seen due to ion cyclotron resonance; (ii) In cases when counter propagating DAWs collide multiple-times, much larger electron and ion acceleration fractions are found, but the process is intermittent in time. This is because intensive heating (temperature increase) makes the-above-thermal-fraction smaller; Also more isotropic velocity distributions are seen; (iii) Development of kink oscillations occurs when DAWs collide; (iv) Scaling of the magnetic fluctuations power spectrum steepening in the higher-density regions is seen, due to wave refraction. Our PIC runs produce much steeper slopes than the orginal spectrum, indicating that the electron-scale physics has a notable effect on DAW spectrum evolution.

Underdense plasma lens with a transverse density gradient

We explore the implications of a transverse density gradient on the performance of an underdense plasma lens and nonlinear plasma-based accelerator. Transverse density gradients are unavoidable in plasma sources formed in the outflow of standard gas jets, which are used heavily in plasma accelerator communities. These density gradients lead to longitudinal variations in the transverse wakefields, which can transversely deflect an electron beam within the blowout wake. We present a theoretical model of the fields within the plasma blowout cavity based on empirical analysis of 3D particle-in-cell (PIC) simulations. Using this model, the transverse beam dynamics may be studied analytically, allowing for an estimation of the net kick of a witness electron bunch from an underdense plasma lens and for density uniformity tolerance studies in plasma accelerators and plasma lenses. This model is compared to PIC simulations with a single electron bunch and constant density profile, and to PIC simulations with two bunches and a thin, underdense plasma lens density profile with density ramps.

On a Guiding of Whistler‐Mode Waves by Density Gradients

AbstractObservations from satellites demonstrate that in the magnetosphere, VLF whistler‐mode waves are frequently detected in the narrow transition regions, where the plasma density changes its magnitude over a short distance across the ambient magnetic field. These observations suggest that the small‐scale, isolated density gradients can guide the VLF whistler‐mode waves along the field. We investigate the guiding of the whistler‐mode waves by the transverse density gradients with a size much less than the characteristic perpendicular size of the wave. We found analytical solutions describing these waves in the plasma with a sharp density discontinuity between two homogeneous regions, and confirm with time‐dependent, two‐dimensional simulations that these waves are indeed guided by the discontinuity. Simulations also reveal that the parameters of the guided waves (the frequency and parallel wavelength) relate to the parameters of the plasma.

Influence of self-focused Laguerre-Gaussian laser beam on second harmonic generation in collisionless plasma having density transition

The influence of self-focused Laguerre-Gaussian beam (L-G beam) on second harmonic generation (SHG) in collisionless plasma with density transition is explored in present work. In presence of collisionless nonlinearity, the ponderomotve force causes redistribution of electrons thereby establishing transverse density gradients inside plasma. These density gradients causes production of electron plasma wave(EPW) at main beam frequency. The nonlinear interaction of EPW with main beam produces 2nd harmonics. The authors have employed the moment theory in order to obtain differential equation controlling the growth of beam waist and second harmonic yield (SHY). It is perceived from results that L-G beam modes and density transition play a major role in increasing beam’s focusing behavior and SHY.

Generation of straight and curved hollow plasma channels by laser-generated nonlinear wakefields and studies of ultra-intense laser pulse guiding

We report on a study of highly controllable, quasi-static hollow plasma channels generated by ion motion following a laser–plasma interaction via numerical simulations. These channels are generated by ion motion in a nonlinear wakefield, which has an asymmetry between focusing and defocusing periods. Such plasma channels, driven by a fs duration, 100 TW level laser pulse, have the capability to guide more powerful laser pulses. Moreover, we show that curved plasma channels can be generated in a transverse density gradient plasma and all-optical guiding of a second, significantly more powerful laser pulse in such a curved plasma channel. This mechanism may be useful for creating plasma optics for multi-stage TeV laser plasma accelerators and compact synchrotron radiation sources.

Effect of plasma density gradients on generation of terahertz radiation in magnetized plasma column during relaxation of kiloampere REB inside it

Generation of terahertz radiation in the magnetized plasma column due to the two-stream instability developing in the relativistic electron beam (REB) with a current of 10 kA is studied at the GOL-PET facility. In the first series of the experiments, at the regular transverse plasma density gradients, power of the generated terahertz radiation increased thirty times. At a frequency of the upper-hybrid oscillations (0.2–0.3 THz) and pulse duration of few microseconds, it reached 4 MWs. In the next experiments, when the additional longitudinal density gradients were created in plasma, the radiation power exceeded 10 MWs. These results are described in this paper.

Chromospheric evaporation and phase mixing of Alfvén waves in coronal loops

Context. Phase mixing of Alfvén waves has been studied extensively as a possible coronal heating mechanism but without the full thermodynamic consequences considered self-consistently. It has been argued that in some cases, the thermodynamic feedback of the heating could substantially affect the transverse density gradient and even inhibit the phase mixing process. Aims. In this paper, for the first time, we use magnetohydrodynamic (MHD) simulations with the appropriate thermodynamical terms included to quantify the evaporation following heating by phase mixing of Alfvén waves in a coronal loop and the effect of this evaporation on the transverse density profile. Methods. The numerical simulations were performed using the Lagrangian Remap code Lare2D. We set up a 2D loop model consisting of a field-aligned thermodynamic equilibrium and a cross-field (background) heating profile. A continuous, sinusoidal, high-frequency Alfvén wave driver was implemented. As the Alfvén waves propagate along the field, they undergo phase mixing due to the cross-field density gradient in the coronal part of the loop. We investigated the presence of field-aligned flows, heating from the dissipation of the phase-mixed Alfvén waves, and the subsequent evaporation from the lower atmosphere. Results. We find that phase mixing of Alfvén waves leads to modest heating in the shell regions of the loop and evaporation of chromospheric material into the corona with upflows of the order of only 5–20 m s−1. Although the evaporation leads to a mass increase in the shell regions of the loop, the effect on the density gradient and, hence, on the phase mixing process, is insignificant. Conclusions. This paper self-consistently investigates the effect of chromospheric evaporation on the cross-field density gradient and the phase mixing process in a coronal loop. We found that the effects in our particular setup (small amplitude, high frequency waves) are too small to significantly change the density gradient.

Single-cycle THz signal accompanying laser wake in photoionized plasmas

Photoionization of an ambient gas by a weakly relativistic, femtosecond laser pulse leaves behind the pulse a flat-top plasma column. The uniform-density core of the column is surrounded by a micron-thin shell, within which the density of plasma species falls down to zero. As the laser pulse ponderomotive force drives the wave of charge separation (the laser wake), electron fluid velocity oscillates along this boundary layer. Coupling velocity oscillations to the transverse density gradient forms an azimuthally polarized rotational current confined within the boundary layer. At each radial offset, this current oscillates with a local (THz-scale) Langmuir frequency, transforming the cylindrical electrostatic plasma wave into a radially evanescent Fourier component of an electromagnetic signal. Summing up these Fourier components yields the time-domain THz signal which is (a) non-local (i.e. not phase-locked to the Langmuir oscillation at any given location), (b) contains the entire frequency band from zero to the Langmuir frequency of the density plateau, and (c) is detectable at a distance orders of magnitude larger than the plasma column radius. A few millimeters away from the column, rapid evanescence of the higher-frequency components transforms the signal into a radially polarized, single-cycle picosecond pulse with a kV m−1-scale electric field. Detecting this pulse with existing (e.g. electro-optical) techniques may bring information on the efficiency of laser coupling to the plasma, the amplitude of wake fields, and the structure of the column boundary.

Sulfur hexafluoride bubble evolution in shock accelerated flow with a transverse density gradient

Sulfur hexafluoride (SF6) is a colorless, odorless, non-toxic, non-flammable stable gas, which has been widely adopted as the heavy gas in the Richtmyer–Meshkov instability study. In this paper, a computational analysis of SF6 bubble evolution in shock-accelerated flow with a transverse density gradient is presented. The influences of different incident shock Mach numbers on various interactions were clarified using high-resolution computation schemes. The results showed that the incident shock wave becomes curved during propagation because of the transverse density gradient. Based on this, two separate shock-focusing processes were identified when Ma = 1.21 and three separate shock-focusing processes were identified when Ma = 2.0. However, the shock-focusing intensity was weaker than previously observed in a flow field with a uniform density distribution. High- and relevant-pressure impingement played vital roles in the formation of three jets near the downstream pole of the SF6 bubble in both cases. In addition, impingement by incident and reflected shocks could induce additional vorticities in the bubble region and promote increased bubble volumes, but these increased bubble volumes could weaken the average vorticity. Upon increasing the incident shock Mach number, the effective bubble volume decreased with the enhanced shock intensity, but the vorticities were strengthened. Furthermore, analyzing the factors that affected vorticity evolution allowed us to find that the compression term had a stronger influence on vorticity evolution than the baroclinic term or the viscosity term. All of these studies complement the Richtmyer–Meshkov instability study.

Анализ устойчивости прямоточных струй с градиентом плотности

Using Chebyshev spectral collocation method, the temporal and spatial stability analysis of a round jet with tangential velocity profile and transverse density gradient is carried out. The characteristic frequencies, wave numbers and eigen modes of the most unstable normal perturbations are analyzed. It is concluded that considering transverse density gradient in most cases leads to decrease in the growth rates and phase velocities of all unstable modes. It is shown that the relations between the phase and group velocities obtained from the temporal and spatial stability analysis of round jet turn out to be valid only for large shear layer thicknesses.

Laser and electron deflection from transverse asymmetries in laser-plasma accelerators.

We report on the deflection of laser pulses and accelerated electrons in a laser-plasma accelerator (LPA) by the effects of laser pulse front tilt and transverse density gradients. Asymmetry in the plasma index of refraction leads to laser steering, which can be due to a density gradient or spatiotemporal coupling of the laser pulse. The transverse forces from the skewed plasma wave can also lead to electron deflection relative to the laser. Quantitative models are proposed for both the laser and electron steering, which are confirmed by particle-in-cell simulations. Experiments with the BELLA Petawatt Laser are presented which show controllable 0.1-1 mrad laser and electron beam deflection from laser pulse front tilt. This has potential applications for electron beam pointing control, which is of paramount importance for LPA applications.

Generation of second harmonics by a self-focused Hermite-Gaussian laser beam in collisionless plasma

In this work, the generation of second harmonics of a Hermite–Gaussian laser beam in collisionless plasma has been presented. On incidence of the Hermite–Gaussian laser beam in plasma, the charge carriers shift from the high field region to the low field region on account of the ponderomotive force which results in the generation of a transverse density gradient in the background plasma which in turn generates plasma waves at incident beam frequency ω0. Interaction of this plasma wave with the pump beam generates the second harmonics of the incident laser beam with frequency 2ω0. The moment theory approach has been used to derive the coupled differential equations for the beam widths of the laser beam in the transverse x- and y-directions which are further solved numerically. The effect of different modes and initial beam widths of the Hermite–Gaussian beam in the x- and y-directions has been investigated for the self-focusing and second harmonic yield (SHY) of the laser beam in plasma. Also, the effect of increasing plasma density is visualized on the self-focusing and SHY of the beam. It has been observed that the SHY significantly depends on different modes and initial widths of the Hermite–Gaussian laser beam as well as on plasma density.

Численное моделирование электромагнитной эмиссии при инжекции электронного пучка в плазму с сильными поперечными градиентами плотности

Recent experiments on the injection of kiloampere electron beams into a magnetized plasma at the GOL-PET facility have shown that the power of sub-terahertz radiation escaping from the plasma along the magnetic field increases by more than an order of magnitude if strong transverse density gradients are preliminarily created in the plasma. In this paper, the influence of transverse in homogeneities of plasma density on the efficiency of electromagnetic radiation generation near the harmonics of the plasma frequency is studied using particle-in-cell simulations. Simulations performed for the real relative density of the beam and the real spatial scales of the in homogeneity show that the beam instability develops only in the density wells, and the small transverse size of its localization comparable with the wavelength contributes to a more efficient conversion of unstable oscillations into electromagnetic ones. Despite the fact that radiation at the plasma frequency is blocked across the leading magnetic field, it can leave the generation region with the decrease of the plasma density in the longitudinal direction.

Angular streaking of betatron X-rays in a transverse density gradient laser-wakefield accelerator

In a plasma with a transverse density gradient, laser wavefront tilt develops gradually due to phase velocity differences in different plasma densities. The wavefront tilt leads to a parabolic trajectory of the plasma wakefield and hence the accelerated electron beam, which leads to an angular streaking of the emitted betatron radiation. In this way, the temporal evolution of the betatron X-ray spectra will be converted into angular “streak,” i.e., having a critical energy-angle correlation. An analytical model for the curved trajectory of a laser pulse in a transverse density gradient is presented. This gives the deflection angle of the electron beam and the betatron X-rays as a function of the plasma and laser parameters, and it was verified by particle-in-cell simulations. This angular streaking could be used as a single-shot diagnostic technique to reveal the temporal evolution of betatron X-ray spectra and hence the electron acceleration itself.

Enhancement of betatron x rays through asymmetric laser wakefield generated in transverse density gradients

Laser wakefield acceleration of electrons usually offers an axisymmetry around the laser propagation axis. Thus, the accelerating electrons that are focused on axis often execute small transverse oscillations. In this Article, we propose a simple scheme to break this symmetry, which enhances the transverse wiggling of electrons and boosts the betatron radiation emission. Through 3D particle-in-cell simulations, we show that sending the laser with a small angle of incidence on a transverse plasma density gradient generates an asymmetric wakefield. It first provokes injection and then increases the wiggling of the electrons through the transverse shifting of the wakefield axis which occurs when the laser pulse leaves the gradient. Consequently, we show that the radiated energy per unit of charge can increase by a factor $>20$ when using this scheme, and that the critical energy of the radiation quintuples compared with a reference case without the transverse density gradient.

Generation of a wakefield undulator in plasma with transverse density gradient

We show that a short relativistic electron beam propagating in a plasma with a density gradient perpendicular to the direction of motion generates a wakefield in which a witness bunch experiences a transverse force. A density gradient oscillating along the beam path would create a periodically varying force—an undulator, with an estimated strength of the equivalent magnetic field more than 10 T. This opens an avenue for the creation of a high-strength, short-period undulator, which eventually may lead to all-plasma, free electron lasers where a plasma wakefield acceleration is naturally combined with a plasma undulator in a unifying, compact setup.

Unthrottled Flows with Complex Background Waves in Curved Isolators

The unthrottled flows in curved isolators are studied experimentally and numerically. Wind-tunnel tests are conducted at a freestream Mach number of 4.92. The internal flowfield is visualized and measured. Large-scale separation bubbles in the inlet are observed, and the downstream duct curvature is found to be able to decrease the separation bubble size via lowering the local pressure downstream of the separation bubble. Complex background waves appear in these isolators, and the increase of duct curvature tends to intensify the reflected shocks. Particularly, the strength distributions along the shock direction of the background shocks behave differently under the influence of duct curvature. For the right-running shocks, the strength distribution changes from a descending trend to an ascending trend with the increase of duct curvature, whereas the strength of left-running shocks always descends with an increasing speed. The strength of background shocks is particularly related to the left-running expansion waves, which get intensified and become the dominant expansion waves in the isolators as the duct curvature increases. The schlieren images indicate a significant transverse density gradient and pressure gradient in the curved isolators. The wall pressure distributions further show that the radial pressure gradient in small curvature isolators switches between centripetal and centrifugal repeatedly; whereas it remains centripetal consistently in large curvature isolators, which is also determined by the left-running expansion waves. As a result, the secondary flow gets stronger as the duct curvature increases, resulting in a larger low-momentum flow region near the bottom wall. In general, the compression capability of the isolator gets attenuated with the increase of the duct curvature.

Effect of self-focusing of elliptical laser beam on second harmonic generation in collisionless plasma

In the present paper, effect of self-focusing of elliptical laser beam on second harmonic generation is investigated in collisionless plasma by making use of paraxial ray approximation. There is redistribution of carriers from high field region to low field region by ponderomotive force and hence transverse density gradient gets established in plasma. This transverse density gradient in turn generates the electron plasma wave at pump wave frequency. The electron plasma wave so generated interacts with pump wave there by producing second harmonic. Effect of variation in laser beam intensity and plasma density on second harmonic yield is also analyzed.

Second harmonic generation by self focused q-Gaussian laser beam in preformed collisional parabolic plasma channel

Abstract This paper presents an investigation of self-focusing of a q-Gaussian laser beam and its effect on second generation in a preformed collisional parabolic plasma channel. In the presence of a q-Gaussian laser beam redistribution of carriers takes place on account of nonuniform heating produced due to collisional nonlinearity, as a result of which, a transverse density gradient is established in the channel which in turn generates plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and generate second harmonics of the incident laser beam. Moment theory has been used to derive differential equation governing the evolution of spot size of laser beam propagating through the channel. The differential equation so obtained has been solved numerically. The effect of the intensity of laser beam, deviation of intensity distribution of laser beam along its wavefront from Gaussian distribution, plasma density and depth of channel on beam width of laser beam as well as second harmonic yield has been investigated.