Su-Ming Weng

Ultra-intense laser pulse propagation in plasmas: from classic hole-boring to incomplete hole-boring with relativistic transparency

Relativistic laser pulse propagation into homogeneous plasmas has been investigated as a function of plasma density. At first, the propagation features are compared systematically between relativistic transparency (RT) and hole-boring (HB). Paramountly, a considerably broad intermediate regime, namely the incomplete HB regime, has been found between the RT regime and the HB regime for an extremely intense circularly polarized (CP) pulse. In this regime HB proceeds in collaboration with RT, resulting in a much faster propagation speed and a higher cut-off energy of fast ions than in the classic HB regime. Similarly to the classic HB regime, formulae are presented to model the laser propagation and the ion acceleration according to the modified momentum flux balance in this incomplete HB regime. The simulations give the density boundary between this incomplete HB regime and the classic HB regime for CP pulses, which is crucial for estimating the maximum mean ion energy and the maximum conversion efficiency that can be achieved by the classic HB acceleration at a given laser intensity. For linear polarization (LP) the propagation mechanism apparently undergoes a transition in time between these two regimes. A detailed comparison between LP and circular polarization is made for these phenomena.

Ion acceleration by colliding electrostatic shock waves in laser-solid interaction

Based on particle-in-cell simulations, ion acceleration by collisionless electrostatic shock waves in the interaction of intense laser pulses with solid targets containing two ion species of different charge-to-mass ratios R is studied. The acceleration processes of the ions with different R and located in different regions of the target have been considered. Simulations also show that ions with larger R can be reflected and accelerated dramatically between two oppositely propagating shocks generated in plasma dominated by ions with smaller R. The studies suggest a new way to generate high energy ions. The involved phenomena may also occur in space and astrophysical plasmas.

Quasi-monoenergetic ion generation by hole-boring radiation pressure acceleration in inhomogeneous plasmas using tailored laser pulses

It is proposed that laser hole-boring at a steady speed in inhomogeneous overdense plasma can be realized by the use of temporally tailored intense laser pulses, producing high-fluence quasi-monoenergetic ion beams. A general temporal profile of such laser pulses is formulated for arbitrary plasma density distribution. As an example, for a precompressed deuterium-tritium fusion target with an exponentially increasing density profile, its matched laser profile for steady hole-boring is given theoretically and verified numerically by particle-in-cell simulations. Furthermore, we propose to achieve fast ignition by the in-situ hole-boring accelerated ions using a tailored laser pulse. Simulations show that the effective energy fluence, conversion efficiency, energy spread, and collimation of the resulting ion beam can be significantly improved as compared to those found with un-tailored laser profiles. For the fusion fuel with an areal density of 1.5 g cm–2, simulation indicates that it is promising to reali...

Relativistic critical density increase and relaxation and high‐power pulse propagation

High‐power laser pulse propagation in an overdense plasma due to the relativistic critical density increase has been investigated in one dimension. In a first step the conditions for the existence of a relativistic critical density are delimited and supported by particle‐in‐cell simulations. Its accurate determination is made possible by the installation of a new numerical diagnostics. Guided by this we show that the critical density increase strongly depends on both laser polarization and plasma density profile. Further, we find a new relaxation time ranging from several to many laser cycles, which sets a limit for short laser pulse manipulation and tailoring. Paramountly, it is proved that in the power optics domain the pulse propagation velocity is inhibited by the relativistic energy density in the medium and by the efficient reflection, in contrast to the group velocity from standard dispersion optics.

Analysis of the Brunel model and resulting hot electron spectra

Among the various attempts to model collisionless absorption of intense and superintense ultrashort laser pulses, the so-called Brunel mechanism plays an eminent role. A detailed analysis reveals essential aspects of collisionless absorption: Splitting of the electron energy spectrum into two groups under p-polarization, prompt generation of fast electrons during one laser cycle or a fraction of it, insensitivity of absorption with respect to target density well above nc, robustness, simplicity, and logical coherence. Such positive aspects contrast with a non-Maxwellian tail of the hot electrons, too low energy cut off, excessively high fraction of fast electrons, and inefficient absorption at moderate angles of single beam incidence and intensities. Brunel’s pioneering idea has been the recognition of the role of the space charges induced by the electron motion perpendicular to the target surface that make irreversibility possible. By setting the electrostatic fields inside the overdense target equal to ...

A compact tunable polarized X-ray source based on laser-plasma helical undulators.

Laser wakefield accelerators have great potential as the basis for next generation compact radiation sources because of their extremely high accelerating gradients. However, X-ray radiation from such devices still lacks tunability, especially of the intensity and polarization distributions. Here we propose a tunable polarized radiation source based on a helical plasma undulator in a plasma channel guided wakefield accelerator. When a laser pulse is initially incident with a skew angle relative to the channel axis, the laser and accelerated electrons experience collective spiral motions, which leads to elliptically polarized synchrotron-like radiation with flexible tunability on radiation intensity, spectra and polarization. We demonstrate that a radiation source with millimeter size and peak brilliance of 2 × 1019 photons/s/mm2/mrad2/0.1% bandwidth can be made with moderate laser and electron beam parameters. This brilliance is comparable with third generation synchrotron radiation facilities running at similar photon energies, suggesting that laser plasma based radiation sources are promising for advanced applications.

Inverse bremsstrahlung absorption and the evolution of electron distributions accounting for electron-electron collisions

Two-dimensional Fokker-Planck simulations have been conducted to investigate the inverse bremsstrahlung absorption and the evolution of the electron distribution function (EDF), where the electron-electron (e-e) collisions are taken into account, allowing for highly anisotropic electron distributions. The numerical results show that the anisotropic part of the EDF is comparable to the isotropic part even for a moderate laser field. The resulting EDF is no longer a simple super-Gaussian, but a hybrid of the super-Gaussian and Maxwellian distributions. Furthermore, the e-e collisions tend to enhance the inverse bremsstrahlung absorption; the contribution ratio of e-e collisions to the inverse bremsstrahlung absorption rate increases with increasing ion charge state Zi in the high laser frequency regime, while it decreases in the low laser frequency regime. It indicates that one cannot simply neglect the e-e collisions in high Zi cases.

Generation of quasi-monoenergetic carbon ions accelerated parallel to the plane of a sandwich target

A new ion acceleration scheme, namely, target parallel Coulomb acceleration, is proposed in which a carbon plate sandwiched between gold layers is irradiated with intense linearly polarized laser pulses. The high electrostatic field generated by the gold ions efficiently accelerates the embedded carbon ions parallel to the plane of the target. The ion beam is found to be collimated by the concave-shaped Coulomb potential. As a result, a quasi-monoenergetic and collimated C6+-ion beam with an energy exceeding 10 MeV/nucleon is produced at a laser intensity of 5 × 1019 W/cm2.

Nonlocal heat transport in laser-produced aluminum plasmas

The spatial and temporal evolutions of nonlocal heat transport in laser-produced aluminum plasmas are studied with the improvements of the Thomson scattering experiments and the kinetic Fokker–Planck simulations. The results are compared with the hydrodynamic simulations with the classical Spitzer–Harm theory. When another heater beam is used, the electron temperature decreases slowly and the temperature gradient becomes steep in the conduction zone. The nonlocal heat flux can be sustained at a high value with slow decrease for long time. The Fokker–Planck simulations considering electron-electron collisions can well describe the nonlocal heat transport process in laser-produced plasmas.

Formation and evolution of a pair of collisionless shocks in counter-streaming flows

A pair of collisionless shocks that propagate in the opposite directions are firstly observed in the interactions of laser-produced counter-streaming flows. The flows are generated by irradiating a pair of opposing copper foils with eight laser beams at the Shenguang-II (SG-II) laser facility. The experimental results indicate that the excited shocks are collisionless and electrostatic, in good agreement with the theoretical model of electrostatic shock. The particle-in-cell (PIC) simulations verify that a strong electrostatic field growing from the interaction region contributes to the shocks formation. The evolution is driven by the thermal pressure gradient between the upstream and the downstream. Theoretical analysis indicates that the strength of the shocks is enhanced with the decreasing density ratio during both flows interpenetration. The positive feedback can offset the shock decay process. This is probable the main reason why the electrostatic shocks can keep stable for a longer time in our experiment.