Shock Wave Acceleration Research Articles

Generation of quasi-monoenergetic heavy ion beams via staged shock wave acceleration driven by intense laser pulses in near-critical plasmas

Laser-driven ion acceleration potentially offers a compact, cost-effective alternative to conventional accelerators for scientific, technological, and health-care applications. A novel scheme for heavy ion acceleration in near-critical plasmas via staged shock waves driven by intense laser pulses is proposed, where, in front of the heavy ion target, a light ion layer is used for launching a high-speed electrostatic shock wave. This shock is enhanced at the interface before it is transmitted into the heavy ion plasmas. Monoenergetic heavy ion beam with much higher energy can be generated by the transmitted shock, comparing to the shock wave acceleration in pure heavy ion target. Two-dimensional particle-in-cell simulations show that quasi-monoenergetic ion beams with peak energy 168 MeV and considerable particle number are obtained by laser pulses at intensity of in such staged shock wave acceleration scheme. Similarly a high-quality ion beam with a well-defined peak with energy 250 MeV and spread can also be obtained in this scheme.

Diaphragm opening effects on shock wave formation and acceleration in a rectangular cross section channel

Shock wave formation and acceleration in a high-aspect ratio cross section shock tube were studied experimentally and numerically. The relative importance of geometric effects and diaphragm opening time on shock formation are assessed. The diaphragm opening time was controlled through the use of slit-type (fast opening time) and petal-type (slow opening time) diaphragms. A novel method of fabricating the petal-type diaphragms, which results in a consistent burst pressure and symmetric opening without fragmentation, is presented. High-speed schlieren photography was used to visualize the unsteady propagation of the lead shock wave and trailing gas dynamic structures. Surface-mounted pressure sensors were used to capture the spatial and temporal development of the pressure field. Unsteady Reynolds-Averaged Navier–Stokes simulation predictions using the shear-stress-transport turbulence model are compared to the experimental data. Simulation results are used to explain the presence of high-frequency pressure oscillations observed experimentally in the driver section as well as the cause of the initial acceleration and subsequent rapid decay of shock velocity measured along the top and bottom channel surfaces. A one-dimensional theoretical model predicting the effect of the finite opening time of the diaphragm on the rate of driver depressurization and shock acceleration is proposed. The model removes the large amount of empiricism that accompanies existing models published in the literature. Model accuracy is assessed through comparisons with experiments and simulations. Limitations of and potential improvements in the model are discussed.

Interaction of an acceleration wave with a characteristic shock in a non-ideal relaxing gas

The amplitude of an acceleration wave propagating along the characteristic associated with the largest eigenvalue in a non-ideal relaxing gas is evaluated. The evolution of a characteristic shock and its interaction with the acceleration wave is studied. The amplitudes of the reflected and transmitted waves and the jump in the shock wave acceleration after interaction are computed. The effects of relaxation and non-ideality on the amplitude of acceleration wave are discussed.

Optical shaping of gas targets for laser–plasma ion sources

We report on the experimental demonstration of a technique to generate steep density gradients in gas-jet targets of interest to laser–plasma ion acceleration. By using an intentional low-energy prepulse, we generated a hydrodynamic blast wave in the gas to shape the target prior to the arrival of an intense CO$_{2}$ (${\it\lambda}\approx 10~{\rm\mu}\text{m}$) drive pulse. This technique has been recently shown to facilitate the generation of ion beams by shockwave acceleration (Tresca et al., Phys. Rev. Lett., vol. 115 (9), 2015, 094802). Here, we discuss and introduce a model to understand the generation of these blast waves and discuss in depth the experimental realisation of the technique, supported by hydrodynamics simulations. With appropriate prepulse energy and timing, this blast wave can generate steepened density gradients as short as $l\approx 20~{\rm\mu}\text{m}$ ($1/e$), opening up new possibilities for laser–plasma studies with near-critical gaseous targets.

Shock wave acceleration in a gravitational field. A special case

A one-dimensional problem of shock wave acceleration in a uniform gravitational field is exactly solved. In front of the shock wave, the medium state is initially in equilibrium and its density decreases according to a power law. The shock wave is generated using a piston moving freely in the gravitational field. The adiabatic index is assumed to be equal to 3. The obtained solution is represented in terms of elementary functions.

Publisher's Note: “Shock wave acceleration of protons in inhomogeneous plasma interacting with ultrashort intense laser pulses” [Phys. Plasmas 22, 043103 (2015)

Publisher's Note: “Shock wave acceleration of protons in inhomogeneous plasma interacting with ultrashort intense laser pulses” [Phys. Plasmas <b>22</b>, 043103 (2015)

Acceleration of weak shock waves

Exact solutions can demonstrate possible ways of the behavior of gas dynamics solutions when the density of the medium, being initially in equilibrium, decreases. In the present study a method of solving the wave equation with variable speed of sound is designed within the framework of the acoustic approximation using an expansion in series in terms of the characteristic variable. It is shown that in each step there exists an integral of the equations of motion which makes it possible to express the solution in finite form. The motion is initiated by the impact of a piston which generates a weak accelerated shock wave. The presence of a homogeneous gravity field is taken into account.

Laser-driven flier impact experiments at the SG-III prototype laser facility

Laser-driven flier impact experiments have been designed and performed at the SG-III prototype laser facility. The continuum phase plate (CPP) technique is used for the 3 ns quadrate laser pulse to produce a relatively uniform irradiated spot of 2 mm. The peak laser intensity is 2.7×1013 W/cm2 and it accelerates the aluminum flier with a density gradient configuration to a high average speed of 21.3 km/s, as determined by the flight-of-time method with line VISAR. The flier decelerates on impact with a transparent silica window, providing a measure of the flatness of the flier after one hundred microns of flight. The subsequent shock wave acceleration, pursuing, and decay in the silica window are interpreted by hydrodynamic simulation. This method provides a promising method to create unique conditions for the study of a material’s properties.

Cosmic rays, clouds and climate

I much enjoyed Henr ik Svensmark’s contribution Cosmic Rays, Clouds and Climate in number 2 of volume 46. Indeed, we desperately try to quantify manmade climate changes, without understanding much larger climate changes in our past, not manmade, with 'ice ages', and possibly also with 'desert ages'. In other words, we do not yet fully understand all the different heating and cooling mechanisms of our planet. Svensmark mentions heating by infalling 'cosmic rays' and 'gamma-rays', from both our Sun and our Galaxy. (The cosmic-ray energy spectrum extends from thermal energies, of order eV/particle, up to extreme values of 1020.5 eV, with its dominant share below 1GeV). He also mentions observed fluctuations thereof, on timescales between hours, and hundreds of Megayears. Ideally, we should like to know the power spectra of their fluctuations, between years and Megayears, and the precise screening mechanisms of our atmosphere against them, via aerosols and clouds. Svensmark mentions 'Forbush decreases', during hours, and oceans as calorimeters, on timescales of kyr, via varying isotopic-rate distributions, and measured (varying) supernova rates. So which of all the aforementioned inputs into our terrestrial temperature balance are the dominant ones? Starlight, supernova explosions, and/or active galactic nuclei and their twin-jets? I have never seen a curve like Svensmark’s Fig.2 drawn for all the above inputs, on timescales of yrs to kyrs. It could be useful to try and draw such curves. A problem for this may be our as yet poor understanding. For instance, mainstream knowledge talks of Galactic rays and cosmic rays, meaning that the highest-energy CRs came from beyond our Galaxy – an energetically unfeasible task, among others because of propagation losses – or of their shock-wave acceleration in SN remnants – a violation of the Second Law (cf. Physikalische Mythen auf dem Pruefstand by W. Kundt & O. Marggraf, Springer, 2014). Many of our astrophysical insights are as yet preliminary, because they are untested. So there remains a lot to be explored by the next generation. n

Shock wave acceleration of protons in inhomogeneous plasma interacting with ultrashort intense laser pulses

The acceleration of protons, triggered by solitary waves in expanded solid targets is investigated using particle-in-cell simulations. The near-critical density plasma is irradiated by ultrashort high power laser pulses, which generate the solitary wave. The transformation of this soliton into a shock wave during propagation in plasma with exponentially decreasing density profile is described analytically, which allows to obtain a scaling law for the proton energy. The high quality proton bunch with small energy spread is produced by reflection from the shock-front. According to the 2D simulations, the mechanism is stable only if the laser pulse duration is shorter than the characteristic development time of the parasitic Weibel instability.

Shock wave amplification by shock wave self-generated magnetic field driven by laser and the external magnetic field

Shock wave is a common phenomenon in astrophysics. Shock wave acceleration has been regarded as a source of high-energy cosmic rays. Very strong magnetic field exists in the surrounding of the shock wave at the edge of the supernova remnants. But the mechanisms of generation and amplification of such a strong magnetic field are not clear yet. In this paper, the properties of shock wave driven by the laser irradiating on un-magnetized and magnetized plasmas are investigated using two-dimensional particle-in-cell (PIC) simulations. It is found that very strong spontaneous magnetic field can be generated around the laser-driven shock front in the un-magnetized plasma. The spontaneous magnetic field can store energy and accelerate electrons further. When an external magnetic field is introduced, the electrons and ions are accelerated more efficiently by the shock wave than in the un-magnetized plasma. The external magnetic field can transfer its energy to electrons and ions, and strengthen the shock wave. In simulations, the introduced external magnetic field has three different strengths: 1072 MG, 107.2 MG and 10.72 MG, which determine the shock structures through the driven currents. There are two single-polar magnetic arcs that constitute the shock structure when the external magnetic field is 1072 MG, i.e., one is the shock itself and the other is actually the reverse shock, whereas only one magnetic arc is produced but with a bipolar structure in the direction perpendicular to the shock propagation when the externally added magnetic fields are much lower (107.2 MG and 10.72 MG). The two bipolar magnetic structures will evolve into a single-polar arc when the externally added magnetic field is 107.2 MG, but they are kept for all the time when the external magnetic field is 10.72 MG. It can be explained by taking the Larmor radius into the consideration. That the amplification ratio of the magnetic field decreases as the introduced external magnetic field increases implies that the magnetic amplification in the space is possibly due to the local field generation rather than the field compression. An amplification ratio of tens of the external magnetic field is achieved due to the pseudo Rayleigh-Taylor instability, but still much smaller than that around the astrophysical shock front, indicating that other efficient mechanisms are responsible for the observed magnetic amplification around shocks in the supernova remnants.

Shock wave acceleration in a magnetic field

An exact solution of MHD equations with plane waves describing the solid-body motion of an ideally conducting gas in a given uniform gravitational field is derived. The motion is due to a piston producing a shock wave propagating throughout the initial equilibrium state with a decreasing density. The solution involves an arbitrary function of the Lagrangian variable, whose choice influences the flow pattern.

Ion acceleration in the ‘dragging field’ of a light-pressure-driven piston

We propose a new acceleration scheme by employing the enormous electric potential behind a light-pressure-driven piston, namely ‘dragging field acceleration’. When a thin foil driven by light pressure of an ultra-intense laser pulse propagates in underdense background plasma, it serves as a shock-like piston, trapping and reflecting background protons to ultra-high energies. Unlike in shock wave acceleration, the piston velocity is not limited by the Mach number and can be highly relativistic. Background protons can be trapped and reflected forward by the ‘dragging field’ potential attached to a piston, which is not employed in light-pressure acceleration. Our one-dimensional particle-in-cell simulations and analytical model both show that proton energies of several tens to hundreds of GeV can be obtained. The injection conditions are investigated. Generation of mono-energetic tens-of-GeV proton bunch in two-dimensional geometry are discussed. Finally, the scheme is demonstrated with a three-dimensional simulation, where the effect of radiation reaction is included.

Self-similar gas motions in a gravity field

The decelerating effect of a homogeneous gravity field on the plane shock wave acceleration near the outside surface of a gas layer, initially in equilibrium, is analyzed within the framework of the self-similar formulation. A qualitative investigation is performed, the cases in which the first integrals exist are noted, and certain exact solutions of the problem are obtained at different power laws of the initial density variation.

Effects of density profile and multi-species target on laser-heated thermal-pressure-driven shock wave acceleration

The shock wave acceleration of ions driven by laser-heated thermal pressure is studied through one-dimensional particle-in-cell simulation and analysis. The generation of high-energy mono-energetic protons in recent experiments (D. Haberberger et al., 2012 Nat. Phys. 8 95) is attributed to the use of exponentially decaying density profile of the plasma target. It does not only keep the shock velocity stable but also suppresses the normal target normal sheath acceleration. The effects of target composition are also examined, where a similar collective velocity of all ion species is demonstrated. The results also give some reference to future experiments of producing energetic heavy ions.

On the acceleration of shock waves and concentration of energy

By a series of simple examples related to exact solutions of problems in gas dynamics and magnetohydrodynamics, possible mechanisms of acceleration of shock waves and concentration of energy are elucidated. The acceleration of a shock wave is investigated in the problem of motion of a plane piston at a constant velocity in the case when the initial density of the medium drops in the presence of constant counterpressure. It is shown that in this situation a “blow-up” regime is induced by a shock wave going to infinity in finite time even for limited work of the piston. A simple spherically symmetric solution with a converging shock wave is constructed and shown to lead to the concentration of energy. A general method for solving one-dimensional non-self-similar problems related to matching the equilibrium state to a motion with homogeneous deformation on a shock wave is discussed; this method leads to a solution in quadratures.

ELECTRON AND PROTON ACCELERATION DURING THE FIRST GROUND LEVEL ENHANCEMENT EVENT OF SOLAR CYCLE 24

High-energy particles were recorded by near-Earth spacecraft and ground-based neutron monitors (NMs) on 2012 May 17. This event was the first ground level enhancement (GLE) of solar cycle 24. In this study, we try to identify the acceleration source(s) of solar energetic particles by combining in situ particle measurements from the WIND/3DP, GOES 13, and solar cosmic rays registered by several NMs, as well as remote-sensing solar observations from SDO/AIA, SOHO/LASCO, and RHESSI. We derive the interplanetary magnetic field (IMF) path length (1.25 +/- 0.05 AU) and solar particle release time (01:29 +/- 00:01 UT) of the first arriving electrons by using their velocity dispersion and taking into account contamination effects. We found that the electron impulsive injection phase, indicated by the dramatic change in the spectral index, is consistent with flare non-thermal emission and type III radio bursts. Based on the potential field source surface concept, modeling of the open-field lines rooted in the active region has been performed to provide escape channels for flare-accelerated electrons.Meanwhile, relativistic protons are found to be released 10 minutes later than the electrons, assuming their scatter-free travel along the same IMF path length. Combining multi-wavelength imaging data of the prominence eruption and coronal mass ejection (CME), we obtain evidence that GLE protons, with an estimated kinetic energy of 1.12 GeV, are probably accelerated by the CME-driven shock when it travels to 3.07 solar radii. The time-of-maximum spectrum of protons is typical for shock wave acceleration.

Existence and interaction of acceleration wave with a characteristic shock in transient pinched plasma

In this paper, the evolution of an acceleration wave and a characteristic shock for the system of partial equations describing one dimensional, unsteady, axisymmetric motion of transient pinched plasma has been considered. The amplitude of the acceleration wave propagating along the characteristic associated with the largest eigenvalue has been evaluated. The interaction of the acceleration wave with the characteristic shock has been investigated. The amplitudes of the reflected and transmitted waves and the jump in the shockwave acceleration after interaction are evaluated.

Numerical study of shock wave entry and propagation in a microchannel

The entry of a shock wave with the Mach number Mis = 2.03 into a microchannel and its further propagation is numerically studied with the use of kinetic and continuum approaches. Numerical simulations on the basis of the Navier — Stokes equations and the Direct Simulation Monte Carlo method are performed for different Knudsen numbers Kn = 8·10−3 and 8·10−2 based on the microchannel half-height. At the Knudsen number Kn = 8·10−3, amplification of the shock wave after its entry into the microchannel is observed. Further downstream, the shock wave is attenuated, which is in qualitative agreement with experimental data. It is demonstrated that results predicted by a quasi-one-dimensional model (which ignores viscosity and heat conduction) of shock wave propagation over a channel with an abrupt change in the area agrees with results of numerical simulations on the basis of the Euler equations. In both cases, shock wave acceleration (amplification) after its entry into the microchannel is observed. At the Knudsen number Kn = 8·10−2, the influence of the entrance shape on shock wave propagation over the microchannel is examined. Intense attenuation of the shock wave is observed in three cases: channel with sudden contraction, junction of two channels with an additional thin separating plate, and rounded junction in the form of a sector with an angle of 90° (quarter of a circumference). It is shown that the microchannel entrance shape can affect further propagation of the shock wave. The wave has the highest velocity in the case with a rounded entrance.

Visualization of Obstacle-Induced Shock-Compression Ignition

Experimental and numerical investigations of shock-compression ignition induced by a disturbance have been carried out. Experiments were performed in a square cross-section shock tube behind reflected shock waves at temperature between 1401 and 1681 K and the total densities were kept constant at around 30 mol/m3. The mixture used was a stoichiometric methane/oxygen/argon with 89.5 percent argon dilution. A thin plate or a fine wire was installed apart from the tube-end of the shock tube to generate density disturbance. The shape of shock-compression ignition flame under different disturbance was observed by direct and schlieren high-speed photography. The disturbance in the shape of a vortex appeared after the reflected shock wave passes through the obstacles. Ignition appears in the vicinity downstream of the obstacle in the tube-end side. The ignition flame is the same shape of the density disturbance. These disturbances made with obstacles promote ignition delay and fixes the position of ignition. The temperature in the disturbed region was evaluated using the temperature-dependent molar extinction coefficients of CO2 at 220 nm. An infrared absorption method with an infrared He-Ne laser (3.39 μm) was used for measuring the density in the disturbed region. The second order explicit MacCormack-TVD scheme is used to solve two-dimension Navier-Stokes equations. The density of the downstream side of the obstacle is lower than that of the upstream side. Temperature in the disturbed region was higher than others; this high temperature was caused by reflected shock wave acceleration in a rarefaction wave region at the rear of the obstacle.