Bright multi-keV harmonic generation from relativistically oscillating plasma surfaces

The first evidence of x-ray harmonic radiation extending to 3.3 A, 3.8 keV (order n>3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (eta approximately n{-2.5}-n{-3}) at multi-keV energies. This scaling holds up to a maximum order, n{RO} approximately 8{1/2}gamma;{3}, where gamma is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy hnu>1 keV is found to be into a cone angle approximately 4 degrees , significantly less than that of the incident laser cone (20 degrees ).     

Ultrashort relativistically strong solitons in plasma

An equation that describes a reflectionless propagation of non-one-dimensional relativistically strong wave packets in plasma has been studied. A family of three-dimensional solitons is found and their stability is analytically justified. The dynamics of soliton formation and their interactions have been numerically simulated.     

Generation of high order optical harmonics from solid surfaces

Received: 26 November 1998     

Localization of intense electromagnetic waves in a relativistically hot plasma

We consider nonlinear interactions between intense short electromagnetic waves (EMWs) and a relativistically hot electron plasma that supports relativistic electron holes (REHs). It is shown that such EMW-REH interactions are governed by a coupled nonlinear system of equations composed of a nonlinear Schro dinger equation describing the dynamics of the EMWs and the Poisson-relativistic Vlasov system describing the dynamics of driven REHs. The present nonlinear system of equations admits both a linearly trapped discrete number of eigenmodes of the EMWs in a quasistationary REH and a modification of the REH by large-amplitude trapped EMWs. Computer simulations of the relativistic Vlasov and Maxwell-Poisson system of equations show complex interactions between REHs loaded with localized EMWs.     

Scanning tunneling microscopy of RF oscillating surfaces

Amplitude and phase of high frequency surface acoustic wave (SAW) fields are investigated by a novel scanning tunneling microscopy technique. The gap voltage is modulated at a slightly detuned high frequency. Due to the nonlinearity of the tunneling process a frequency mixing appears. For scanned areas with dimensions much smaller than the wavelength of the SAW a remarkable local variation of amplitude and phase of the tunneling current at the difference frequency is observed. Depending on the local morphology different components of the particle displacement vector are detected. Model calculations of amplitude and phase images are presented for a real topography.     

Nonlinear evolution of a relativistically strong electromagnetic wave in self-created electron-positron plasma

The nonlinear interaction between the electron-positron pairs produced by an electromagnetic wave in plasma and the wave leads to damping of the wave, frequency upshift, change of polarization, and particle acceleration. The case of a circularly polarized wave is investigated in the framework of the relativistic Vlasov equation with a source term based on the Schwinger formula for the pair creation rate.     

Interaction of relativistically strong electromagnetic waves with a layer of overdense plasma

Plasma-field structures that arise under the interaction between a relativistically strong electromagnetic wave and a layer of overdense plasma are considered within a quasistationary approximation. It is shown that, together with known solutions, which are nonlinear generalizations of skin-layer solutions, multilayer structures containing cavitation regions with completely removed electrons (ion layers) can be excited when the amplitude of the incident field exceeds a certain threshold value. Under symmetric irradiation, these cavitation regions, which play the role of self-consistent resonators, may amplify the field and accumulate electromagnetic energy.     

Nonlinear propagation of relativistically intense electromagnetic waves in a collisionless plasma

We discuss the nonlinear propagation of relativistically intense electromagnetic waves into collisionless plasmas with special emphasis on one dimensional plane wave solutions of the propagating, standing and modulated types. These solutions exhibit a rich variety of phenomena associated with relativistic electron mass variation and coupling between transverse electromagnetic and longitudinal fields. They have important applications to problems of laser propagation, self-focusing in overdense plasmas, particle and photon acceleration and to electromagnetic radiation around pulsars.     

The output of a quartz transducer oscillating in its fundamental and higher harmonics

The output from a quartz crystal oscillating in three liquids, at its fundamental, 3rd harmonic and 5th harmonic have been measured by the calorimetric method keeping the crystal amplitude the same. The output is found to be much less than that one can expect from theoretical considerations.     

Temporal coherence of high-order harmonics generated at solid surfaces

We present interferometric measurements of the temporal coherence of high-order harmonics generated by reflection of a titanium sapphire laser off a solid surface. It is found that the coherence length of the harmonic emission is significantly reduced compared with the bandwidth limited case. To identify the responsible mechanism, the acquired data were analyzed by means of particle-in-cell simulations, whose results show good agreement between the calculated spectra and the measured coherence times. We show that the observed broadening can be understood consistently by the occurrence of a Doppler shift induced by the moving plasma surface, which is dented by the radiation pressure of the laser pulse. In this case, this Doppler effect would also lead to positive chirp of the emitted radiation.     

Motions of particles in an oscillating plasma

An alternative consideration of oscillations and waves in plasma is suggested based on equations for small deflections of particles from their equilibrium trajectories instead of equations for perturbations of distribution functions and fields, so that average values are calculated with equilibrium distribution functions. Instead of the Maxwell equations for an electromagnetic field their solutions in the Lienard-Wiechert form are used. All the known results of the linear kinetic theory of plasma oscillations are shown to be derivable in this way, and small correlations for dispersion relations of the first order in the plasma parameter due to the Debye screening have been obtained. A simplified consideration of a rarefield plasma in a strong magnetic field is given showing a non-cyclotron character of the motion of the particles. Such a combination of statistical and dynamical approaches may be useful in many problems of plasma physics.     

Generation of harmonics and chaos in the dynamics of anharmonic surfaces

Effective equations of motion of a surface atom placed in an anharmonic surface potential are derived for three kinds of substrates (i) continuous dispersionless, (ii) discrete with nearest neighbours harmonic interactions, and (iii) a substrate showing a strong spatial dispersion. The resulting equations of motion are differential equations in the case (i) and Volterra Integro-Differential Equations (VIDE) in the cases (ii) and (iii). Examples of non-linear surface response to phonons coming from the bulk are given for the dispersionless substrate.     

New regime of anomalous penetration of relativistically strong laser radiation into an overdense plasma

It is shown that penetration of relativistically intense laser light into an overdense plasma, accessible by self-induced transparency, occurs over a finite length only. The penetration length depends crucially on the overdense plasma parameter and increases with increasing incident intensity after exceeding the threshold for self-induced transparency. Exact analytical solutions describing the plasma-field distributions are presented.     

Calculation of free-free Gaunt factors for 1 keV electrons in a 1 keV normal density Ce plasma

We compare numerical results for free-free Gaunt factors in a hot dense cesium plasma, obtained in the average atom approximation either with a simple approximate analytic potential or with the finite temperature Thomas-Fermi model. We obtain results (for the spectrum of radiation from a 1 keV electron incident on a cesium ion in a 1 keV plasma) from an exact numerical calculation in partial waves of the relativistic electron Bremsstrahlung cross section treated as a single electron transition within these potentials. Comparison shows that Born approximation results in the same potentials fail by large factors, especially at the soft photon end of the spectrum.     

A radiotracer technique for measuring the reflection of heavy ions (of keV energies) from solid surfaces

A radiotracer technique for measuring the reflected fraction of heavy ions impinging at keV energies on solid surfaces is described. This technique consists essentially in measuring the distribution of radioactivity between the target and a cylindrical collector surrounding it. A series of experiments has been performed to establish the reliability and the limits of the technique.     

Charge states of 25–150 keV H and 4He backscattered from solid surfaces

Measurements have been made of the ion-fractions of H and ⁴He backscattered with energies of 25–160 keV from Cu, Au, and Si surfaces which were etched and washed but not atomically clean. The ion-fractions for H range from 0.37 at 25keV to 0.92 at 160 keV, and for ⁴He from 0.10 at 30keV to 0.58 at 150 keV, depending to a small extent on the target material. Where comparisons can be made the data agree rather closely with results of others for particles traversing thin foils. The data are useful for calibration of an electrostatic analyzer in surface analysis. Plots of ion-fraction against particle velocity show a primary dependence on velocity, as expected, but there is a small difference in slope between the H and He curves. Charge states of particles scattered from surface impurities did not deviate significantly from those of particles scattered from the substrate at the same energy.     

激光等离子体相互作用的局域振荡电子加热机制

 用2.5维粒子模拟程序模拟了超强激光与等离子体的相互作用过程，发现超强激光可以通过J×B加热机制加速电子并引起电荷分离，从而产生很强的静电场并形成电场势阱，电子在静电场势阱中振荡，被多次加速，使得高速电子被甩出势阱，进而增强电荷分离，然后静电场结构被破坏，静电势能传给电子。在此过程中，电子在此阱中作局域振荡，并且被J×B机制多次加速，激光的能量会有效地传给电子，使电子能量高达10MeV。这是一种新的电子加热机制，称之为局域振荡电子加热机制。