Guiding of laser pulses in plasma waveguides created by linearly-polarized femtosecond laser pulses

CNN-based neural network model for amplified laser pulse temporal shape prediction with dynamic requirement in high-power laser facility

The temporal shape of laser pulses is one of the essential performances in the inertial confinement fusion (ICF) facility. Due to the complexity and instability of the laser propagation system, it is hard to predict the pulse shapes precisely by pure analytic methods based on the physical model [Frantz-Nodvik (F-N) equation]. Here, we present a data-driven model based on a convolutional neural network (CNN) for precise prediction. The neural network model introduces sixteen parameters neglected in the F-N equation based models to expand the representation dimension. The sensitivity analysis of the experimental results confirms that these parameters have different degrees of influence on the temporal output shapes and cannot be ignored. The network characterizes the whole physical process with commonality and specificity features to improve the description ability. The prediction accuracy evaluated by a root mean square of the proposed model is 7.93%, which is better compared to three optimized physical models. This study explores a nonanalytic methodology of combining prior physical knowledge with data-driven models to map the complex physical process by numerical models, which has strong representation capability and great potential to model other measurable processes in physical science.

Observation of transverse injection and enhanced beam quality in laser wakefield acceleration of isolated electron bunches using an optimized plasma waveguide

The laser wakefield acceleration of monoenergetic multi-GeV electron beams in the bubble regime is investigated via particle-in-cell simulations considering laser guiding of sub-petawatt pulses by an optimized plasma waveguide. The density profile of the plasma has a transverse transition from a low value for the laser guiding central channel to an optimal higher value for the surrounding plasma. Multidimensional particle-in-cell simulations in the nonlinear bubble regime show that when the spot size of the Gaussian laser pulse is matched to the diameter of the low-density laser-guiding plasma channel, electron self-injection can be transversely provided from the surrounding high-density plasma mitigating the need for a minimum electron density of the low-density channel to trigger the self-injection. Accordingly, the pump depletion and electron dephasing lengths can be increased by reducing the electron density of the axial channel, and the electron bunch can be accelerated to considerably longer distances. As a result, the energy gain of the trapped electrons, injected from the surrounding high-density region, can be efficiently enhanced. Under such conditions, a completely localized electron bunch with considerably decreased energy spread (<2%) and enhanced peak energy (∼2.5GeV) is accelerated over a length of ∼6mm by a sub-petawatt laser pulse (∼86TW).

Analysis of light-wave nonstaticity in the coherent state

The characteristics of nonstatic quantum light waves in the coherent state in a static environment is investigated. It is shown that the shape of the wave varies periodically as a manifestation of its peculiar properties of nonstaticity like the case of the Fock-state analysis for a nonstatic wave. A belly occurs in the graphic of wave evolution whenever the wave is maximally displaced in the quadrature space, whereas a node takes place every time the wave passes the equilibrium point during its oscillation. In this way, a belly and a node appear in turn successively. Whereas this change of wave profile is accompanied by the periodic variation of electric and magnetic energies, the total energy is conserved. The fluctuations of quadratures also vary in a regular manner according to the wave transformation in time. While the resultant time-varying uncertainty product is always larger than (or, at least, equal to) its quantum-mechanically allowed minimal value ([Formula: see text]), it is smallest whenever the wave constitutes a belly or a node. The mechanism underlying the abnormal features of nonstatic light waves demonstrated here can be interpreted by the rotation of the squeezed-shape contour of the Wigner distribution function in phase space.

Millisecond-long suppression of spectroscopic optical signals using laser filamentation

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Optical Guiding in Meter-Scale Plasma Waveguides

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable laser-driven electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J_{0} beam, generates the core of the waveguide, while the delayed second pulse, here a J_{8} or J_{16} beam, generates the waveguide cladding, enabling wide control of the guide's density, depth, and mode confinement. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on-axis plasma densities as low as N_{e0}∼5×10^{16}  cm^{-3}.

Direct refractive index retrieval from interferometry measurements

We present an alternative numerical method to the Abel inversion technique, which can be applied to complex non-symmetrical systems. A comparison with the Abel inversion algorithm was conducted. For benchmarking, the method was applied to a synthetic trace representing a plasma waveguide characterized by a constant parabolic density profile. Furthermore, the temperature and refractive index of a plume of hot air surrounding a non-cylindrical soldering iron were retrieved. Temperatures between 50 °C and 200 °C were successfully retrieved within the instrument precision. The proposed method allows robust and fast data retrieval while maintaining the accuracy and resolution of well-known methods, as Abel inversion.

Hydrodynamic optical-field-ionized plasma channels

We present experiments and numerical simulations which demonstrate that fully ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical field ionization. Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of 200 mm long plasma channels with axial densities of order n_{e}(0)=1×10^{17}cm^{-3} and lowest-order modes of spot size W_{M}≈40μm. These simulations show that the laser energy required to generate the channels is modest: of order 1 mJ per centimeter of channel. The simulations are confirmed by experiments with a spherical lens which show the formation of short plasma channels with 1.5×10^{17}cm^{-3}≲n_{e}(0)≲1×10^{18}cm^{-3} and 61μm≳W_{M}≳33μm. Low-density plasma channels of this type would appear to be well suited as multi-GeV laser-plasma accelerator stages capable of long-term operation at high pulse repetition rates.