A plasma wiggler beamline for 100 TW to 10 PW lasers

We introduce the theoretical framework of laser wakefield acceleration and plasma wiggler radiation generation in the matched regime, give scaling laws and apply the scheme to laser systems planned for the near future. We compare the anticipated electron and x-ray beam parameters for a 100 TW, 1 PW and 10 PW short pulse Ti:Sapphire laser with previous experimental results. Depending on the chosen laser configuration, x-rays from a plasma wiggler beamline (PWB) can be several orders of magnitude brighter than current betatron sources, and comparable to or better than 3rd generation synchrotron facilities. Furthermore, increasing the laser power from 0.1 to 10 PW, the spectral peak of the betatron radiation shifts into the hard x-ray and γ-ray regime. We also discuss a basic layout of a PWB and motivate 100 TW, 1 PW and 10 PW beamlines with a wide range of uses, experiments and applications. The ability to couple the PWBs with various optical laser drivers has the potential to facilitate uses across the spectrum of light source applications.     

Effect of initial deformations on wave front propagation in anisotropic plates

The effects of large amplitudes and initial deformations on shock waves and acceleration waves propagating in fiber-reinforced laminated plates are investigated. Three cases are discussed, namely the large amplitude shock under initial in-plane deformations, small amplitude waves under in-plane deformations, and small amplitude waves propagating in a plate with large deflection. It is found that the in-plane force has a substantial effect on the transverse shear mode but little effects on other modes. The large initial deflection, however, is found to have considerable effects on all modes. A general procedure for constructing the wave surfaces is also presented.     

Propagation of Quasiacoustic Pulses in an Elastoplastic Medium

Expressions for the velocity of a plastic shock wave and phase velocity of longitudinal waves in an elastoplastic medium with hardening are obtained in a quasiacoustic approximation. An analytical solution of the problem of shockpulse attenuation is constructed. A special feature of the amplitude of the attenuating plastic shock wave is that it reaches the amplitude of the elastic precursor in a finite time, whereas in hydrodynamics, the amplitude of a quasiacoustic shock pulse tends to zero asymptotically.     

Evolution of weak shocks in one dimensional planar and non-planar gasdynamic flows

Asymptotic decay laws for planar and non-planar shock waves and the first order associated discontinuities that catch up with the shock from behind are obtained using four different approximation methods. The singular surface theory is used to derive a pair of transport equations for the shock strength and the associated first order discontinuity, which represents the effect of precursor disturbances that overtake the shock from behind. The asymptotic behaviour of both the discontinuities is completely analysed. It is noticed that the decay of a first order discontinuity is much faster than the decay of the shock; indeed, if the amplitude of the accompanying discontinuity is small then the shock decays faster as compared to the case when the amplitude of the first order discontinuity is finite (not necessarily small). It is shown that for a weak shock, the precursor disturbance evolves like an acceleration wave at the leading order. We show that the asymptotic decay laws for weak shocks and the accompanying first order discontinuity are exactly the ones obtained by using the theory of non-linear geometrical optics, the theory of simple waves using Riemann invariants, and the theory of relatively undistorted waves. It follows that the relatively undistorted wave approximation is a consequence of the simple wave formalism using Riemann invariants.     

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.     

Measurement and simulations of hollow atom X-ray spectra of solid-density relativistic plasma created by high-contrast PW optical laser pulses

K-shell spectra of solid Al excited by petawatt picosecond laser pulses have been investigated at the Vulcan PW facility. Laser pulses of ultrahigh contrast with an energy of 160 J on the target allow studies of interactions between the laser field and solid state matter at 10²⁰ W/cm². Intense X-ray emission of KK hollow atoms (atoms without n = 1 electrons) from thin aluminum foils is observed from optical laser plasma for the first time. Specifically for 1.5 μm thin foil targets the hollow atom yield dominates the resonance line emission. It is suggested that the hollow atoms are predominantly excited by the impact of X-ray photons generated by radiation friction to fast electron currents in solid-density plasma due to Thomson scattering and bremsstrahlung in the transverse plasma fields. Numerical simulations of Al hollow atom spectra using the ATOMIC code confirm that the impact of keV photons dominates the atom ionization. Our estimates demonstrate that solid-density plasma generated by relativistic optical laser pulses provide the source of a polychromatic keV range X-ray field of 10¹⁸ W/cm² intensity, and allows the study of excited matter in the radiation-dominated regime. High-resolution X-ray spectroscopy of hollow atom radiation is found to be a powerful tool to study the properties of high-energy density plasma created by intense X-ray radiation.     

Limit currents in a plasma betatron

The radial motion of a plasma column is considered for electron acceleration in a plasma betatron. The limit value of the relativistic currents which can be obtained in devices of this type is computed.G. I. Budker [1] proposed using the runaway effect in a plasma with a strong electric field for converting a cold ring plasma into an intense compensated beam of relativistic electrons. To confine such a beam within an annular vacuum chamber one can use either a betatron-type magnetic field or the field of the image currents produced in the metal shell enclosing the vacuum chamber with the electron beam. In the latter case, as estimates show, the number of accelerated electrons must already be considerable; this leads to an increase in the difficulties which impede the successful acceleration of all plasma electrons.Accordingly, most of the experiments on accelerating plasma electrons have employed betatron fields in devices called plasma betatrons [2–4], A feature of these accelerators is total compensation of the space charge of the accelerated electrons and hence an increased possibility of obtaining high accelerated currents. In this article, we compute the magnitude of the limit currents which may be obtained in a plasma betatron as a function of its parameters and operating conditions.The first results in this direction, published in 1949 [5], were rough estimates. Subsequently, other more accurate calculations were published [6], but these, in our opinion, did not give sufficient information on the characteristic quantities.The author thanks A. E. Bazhanov for his help in interpreting Eq. (16).     

Studying astrophysical collisionless shocks with counterstreaming plasmas from high power lasers

Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless”, since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10¹⁶ W/cm² laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide “head-on”. Our data showed that the flow velocity and electron density were 10⁸ cm/s and 10¹⁹ cm⁻³, respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments.     

Evolution of a laser-generated shock wave in iron and its interaction with martensitic transformation and twinning

Effects of shock waves (generated by a nanosecond laser pulse in plates of Armco-iron) on structural changes are analysed. Localisation of processes of martensitic transformation and twinning – for various values of laser pulse duration – is studied both experimentally and numerically. A proposed model accounts for interaction of shock wave propagation and structure changes. Realisation of martensitic transformation and twin formation influences wave front modification. A stress amplitude decrease with increasing distance from a microcrater determines, together with the pulse duration, a character of spatial localisation of structural changes. Numerical results are compared with experimental data and serve as a basis for additional interpretation of phenomena. Received 9 August 1994 / Accepted 30 June 1997     

Production of periodically modulated laser-driven blast waves in a clustering gas

To study hydrodynamic behavior on thin shell high Mach number blast waves, experiments have been performed in which spatially tailored shock waves have been launched in a gas of clusters using an intense 35 fs laser pulse. The target medium was first modified by destroying clusters in specific locations using a spatially modulated laser focus. Under subsequent intense laser irradiation, the efficient absorption properties of the remaining clustered regions compared to those regions with no clusters led to a pattern of hot and cold plasma resulting in a cylindrical blast wave with a periodic modulation imprinted on the shock front. This technique may provide a method for studying thin shell instabilities in strongly radiative blast waves.     

Collimated quasi-monoenergetic electron beam generation from intense laser solid interaction

A quasi-monoenergetic electron beam with divergence of 3° and energy peak of 1 MeV is observed along the target surface from interaction of a bulk Cu target and an intense relativistic laser pulse of 1 TW and 70 fs at a grazing incident angle. A preplasma formed by high-contrast picosecond prepulse plays a crucial role. Particle-in-cell simulations broadly reproduce the result and show that a preplasma with the proper density and a large angle of incidence is required. The preplasma sets up a static electric field along the surface can accelerate electrons. The static electric field is formed just after the passage of the laser. This approach can be extended to higher intensities to generate higher energy beams.     

Behaviour of the acceleration and shock waves in materials with internal state variables

The explicit expressions for the change in the amplitudes of one-dimensional acceleration and shock waves propagating through arbitrary homogeneous materials described by the strain and internal state variables/parameters/are derived. The existence of a critical amplitude β for the acceleration wave and a critical strain gradient λ for the shock wave is established. For an infinitesimal shock wave the general form of the solution of the governing differential equation is furnished. The differential equations for the amplitudes of these two kind of waves are applied to an elastic-viscoplastic material.     

MEASUREMENT AND ANALYSIS OF LASER GENERATED RAYLEIGH AND LAMB WAVES CONSIDERING ITS PULSE DURATION

In this article, laser generated Rayleigh and Lamb waves are studied by taking into account its pulse duration. The physical model and theoretical solution are presented to predict the corresponding waveforms for aluminum samples under the ablation generation regime.The waveforms of the excited Rayleigh and Lamb waves by laser with selected pulse duration were measured by laser interferometer and analyzed theoretically, and the agreement between measurement and analysis is demonstrated for the validation of the theoretical model and solution.The broadening of the Rayleigh wave and the disappearing of high order Lamb wave modes can be found with the increase of the pulse duration by the laser ultrasonic technique.     

Thermo-acceleration waves and shock formation in Extended Thermodynamics of gravitational gases

The possibility of shock formation as degeneration of acceleration waves in a thermoviscous gravitational ideal gas is studied by exploiting the hyperbolic system of Extended Thermodynamics. The mathematical aspects of this problem are discussed by considering the different contributions of gravity and dissipative effects. In particular, we evaluate the critical time (i.e. the instant in which a shock wave starts) proving that it exists, in the usual physical situations, only for a sufficiently large critical initial amplitude of the acceleration jump. We show that an acceleration wave can never degenerate into a shock wave except in some limiting cases and so, since gravity force is overcome by dissipative effects, our results do not differ, qualitatively, from the case without gravity: this result implies the asymptotic stability (in the sense of [1]) of the static isothermal solutions.     

Theory of nonlinear waves in a plasma

Stationary nonlinear waves propagating in a cold rarefied plasma composed of electrons and two types of ions are considered. The structure of isolated waves and shock waves is found. In recent years an intensive study has been made of finite-amplitude waves and collisionless shock waves in a rarefied plasma, in connection with laboratory experiments [1] and astrophysical applications (the problem of the interaction of the solar wind with the Earth's magnetosphere [2]). When allowance is made for dispersion effects associated with the departure of the dispersion law =(k) from the linear, and for the compensating nonlinear twisting of the wave profile, we are able to obtain the profile of stationary nonlinear waves of finite amplitude, and when allowance is made for damping we can also obtain the structure of a collisionless shock wave [3]. Such waves have been studied fairly fully for the case of a two-component plasma. The present paper examines stationary nonlinear waves propagating across a magnetic field in a cold rarefied quasi-neutral plasma composed of electrons and two types of ions.     

Decay of longitudinal plasma oscillations to ion-sound oscillations

Recently, the question of the nonlinear relation between various plasma oscillations has been the subject of much attention as a result of a series of circumstances. The most important of these is the fact that in the majority of experiments on beam instabilities [1, 2] the intensity of the oscillations excited is very large, so that nonlinear effects in the interaction of oscillations must be significant. It should be noted that beam instability is not the only method of exciting highfrequency plasma oscillations. As was shown in [3], very intense oscillations may also be excited by beams of transverse waves of various frequency ranges, among which are powerful light beams [4]. Finally, excitation is possible by means of shock waves [5] and large-amplitude waves propagating through a plasma.Nonlinear coupling of plasma and low-frequency ion-sound oscillations leads, in particular, to the generation of the latter [6]. On the one hand, this is of interest as regards the problem of turbulent heating of a plasma, since the absorption of ion-sound oscillations in a plasma is usually stronger than the absorption of plasma oscillations. On the other hand, ion-sound oscillations may bring about the acceleration of low-energy ions due to the effects of induced erenkov absorption and radiation of waves by ions, as considered in the work of one of the authors [7], Although plasma oscillations accelerate particles more effectively [8], the injection conditions in the configuration for acceleration by plasma oscillations are very stringent v > v_e. The number of ions with such velocity for small ion temperatures T_i is small. Thus, the acceleration of ions will arise in this case as a result of the interaction of ion-sound oscillations until such time as their velocity reaches values of the order v_e. This question is of interest not only for the acceleration of ions (heating) in the presence of high-frequency turbulence created by beams of charged particles or as a result of the action of powerful radiation on a plasma, but also for the problem of neutron radiation from powerful impulse discharges in a plasma and for a series of astrophysical problems.In what follows we consider a number of one-dimensional self-consistent problems regarding the interaction (decay and fusion) of plasma and ion-sound oscillations resulting from the induced Raman scattering of the former by the latter. It is shown that the development of instability in a turbulent plasma with a high level of excited plasma oscillations leads both to the excitation of ion-sound oscillations, and also to the appearance in the plasma oscillation spectrum of satellites differing from the basic frequency _0e by a frequency of the order _0i and with greater intensities for the lower frequencies. The qualitative change of the plasma oscillation spectrum may serve as an immediate indication of the excitation of ion-sound oscillations in the system. The results obtained allow one to trace the process of development of instabilities. It is shown that in a plasma with a high level of ion-sound oscillations violet satellites are excited in the plasma oscillation spectrum, while the intensities of the violet satellites have a tendency to level out and form a satellite plateau if the level of ion-sound waves is high enough.     

Waves of finite amplitude in a hot plasma

A study is made of a plane shock wave of arbitrary strength propagating in a hot rarefied plasma across the magnetic field. The question of the propagation of nonstationary waves of finite but small amplitude under these conditions is examined.Fairly detailed studies have been made of waves of finite amplitude in a cold rarefied plasma. The profile of such waves is formed as the result of nonlinear and dispersion effects, the dispersion effects being caused by electron inertia and plasma anisotropy. If the gas-kinetic pressure of the plasma is taken into account, then dispersion effects appear which are associated with the fact that the Larmor radius of the ions is finite. Stationary waves of small but finite amplitude propagating across the magnetic field in a hot plasma (when the gas-kinetic pressure p is comparable with the magnetic pressure H²/87) have been treated in [1, 2]. In [1] an isolated rarefaction wave was found in a hot plasma, instead of the compression wave characteristic of a cold plasma, and a qualitative picture of the shock wave structure was given. In [2] a study was made of a small-amplitude shock wave with the finite size of the ion Larmor radius taken into account. The present paper investigates the structure of shock waves of arbitrary strength which propagate across the magnetic field in a fairly hot rarefied plasma, and also examines nonstationary waves of finite but small amplitude excited in a plasma by a magnetic piston acting over a limited time interval.Notation p gas-kinetic pressure - H magnetic field - u, v macroscopic velocities along the x and y axes - density - m_e(m_i) mass of electron (ion) - plasma conductivity - _H ion-cyclotron frequency - V_A Alfvèn velocity - c velocity of light - adiabatic exponent - V specific volume - _0e(_0i) electron (ion) plasma frequency - S₀ velocity of sound. In conclusion the author thanks R. Z. Sagdeev and N. N. Yanenko for discussing the paper, and also R. N, Makarov for helping with the numerical computations.     

Shock and acceleration waves with large amplitudes in laminated composite plates

Propagation of shock and acceleration waves with large amplitudes is studied. The geometrical nonlinearity in the von Karman sense is included in deriving the plate equations. The dynamical conditions on the wave fronts are derived from the three-dimensional conditions in a way consistent with the derivation of the plate equations. General equations governing the propagation velocities are obtained. Solutions are presented for the case where the plates are initially at rest. It is found that, in this case, the large amplitude has a substantial effect only on the transverse shear shock wave. Finally, stability of the wave front is discussed.     

脉冲激光等离子体与正激波相互作用的PIV实验研究

脉冲激光等离子体与超声速流场相互作用在飞行器减阻隔热、点火助燃等方面具有重要的应用价值.纹影实验方法只能定性或半定量地反映流动状态.为定量研究速度分布和旋涡结构,针对激光等离子体及其与正激波相互作用过程开展粒子图像测速PIV实验研究.在激波管实验平台上建立了纳秒脉冲激光能量沉积系统和PIV测量系统,通过定量测量,探明了激光等离子体引致的激光空气泡以及热核的流动特性,揭示了激光等离子体在正激波冲击下的流动特性与演化规律,并给出了激光能量大小和位置对相互作用过程的影响.结果表明:激光空气泡内的速度分布在激光入射方向上并不关于击穿点对称,而是在靠近激光入射方向一侧的流速略大于远离激光入射方向一侧;斜压导致热核在演化初期产生涡环,后期则由剪切主导;正激波与激光空气泡界面、热核界面相互作用时,产生斜压涡量,当激光能量为87.8 mJ、正激波马赫数1.4时,热核在正激波作用下产生的涡量比在静止空气中演化时大1个数量级;激光与正激波相互作用的关键过程是热核在正激波冲击下演化成涡环,在激波波前注入激光能量能够获得更加显著的涡环.     

Generation of higher electron cyclotron frequency harmonics in plasma with a beam

We examine nonlinear excitation of the higher electron-cyclotron frequency harmonics for waves propagating perpendicular to an external uniform magnetic field in a Maxwell plasma for the case of low-density electron beam passage through the plasma. It is shown that the nonlinear excitation mechanism leads to the possibility of generating cyclotron harmonics for plasma parameters for which generation does not occur from the linear theory viewpoint. The nonlinear cyclotron harmonic generation increments are calculated for nonlinear scattering by the beam and plasma electrons of the high frequency longitudinal waves excited in the plasma by the beam.Translated from Zhurnal Prikladnoi Mekhaniki Tekhnicheskoi Fiziki, Vol. 10, No. 6, pp. 40–51, November–December, 1969.The author wishes to thank V. N. Tsytovich for posing the problem and for many discussions of the questions touched upon in the article.