Na Xie

Strong magnetic fields generated with a simple open-ended coil irradiated by high power laser pulses

A simple scheme to produce strong magnetic fields due to cold electron flow in an open-ended coil heated by high power laser pulses is proposed. It differs from previous generation of magnetic fields driven by fast electron current in a capacitor-coil target [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. The fields in our experiments are measured by B-dot detectors and proton radiography, respectively. A 205u2009T strong magnetic field at the center of the coil target is generated in the free space at Iλ2 of 6.85u2009×u20091014u2009Wu2009cm−2u2009μm2, where I is the laser intensity, and λ is the laser wavelength. The magnetic field strength is proportional to Iλ2. Compared with the capacitor-coil target, the generation mechanism of the magnetic field is straightforward and the coil is easy to be fabricated.

Demonstration of resonant backward Raman amplification in high-density gas-jet plasma

Backward Raman amplification was observed in a 0.7 mm-long high-density gas jet plasma. The 800 nm 30 fs seed pulse was amplified by a factor ~28, with an output energy of 2.8 mJ. The output spectra showed that the waveband around 800 nm was significantly amplified. The experimental result demonstrated that the resonant Raman amplification can be realized in high-density plasma against strong plasma instability.

Progress on the XG-III high-intensity laser facility with three synchronized beams

The paper presents the technical design and progress on a special high-power laser facility, i.e. XG-III, which is being used for high-field physics research and fast ignition research. The laser facility outputs synchronized nanosecond, picosecond and femtosecond beams with three wavelengths, i.e. 527 nm, 1053 nm and 800 nm respectively, and multiple combinations of the beams can be used for physics experiments. The commissioning of the laser facility was completed by the end of 2013. The measurement results show that the main parameters of the three beams are equal to or greater than the designed ones.

An angular-resolved multi-channel Thomson parabola spectrometer for laser-driven ion measurement

A multi-channel Thomson parabola spectrometer was designed and employed to diagnose ion beams driven by intense laser pulses. Angular-resolved energy spectra for different ion species can be measured in a single shot. It contains parallel dipole magnets and wedged electrodes to fit ion dispersion of different charge-to-mass ratios. The diameter and separation of the entrance pinhole channels were designed properly to provide sufficient resolution and avoid overlapping of dispersed ion beams. To obtain a precise energy spectral resolving, three-dimensional distributions of the electric and magnetic fields were simulated. Experimental measurement of energy-dependent angular distributions of target normal sheath accelerated protons and deuterons was demonstrated. This novel compact design provides a comprehensive characterization for ion beams.

Efficient production of strong magnetic fields from ultraintense ultrashort laser pulse with capacitor-coil target

The ion beam bunching in a cascaded target normal sheath acceleration is investigated by theoretical analysis and particle-in-cell simulations. It is found that a proton beam can be accelerated and bunched simultaneously by injecting it into the rising sheath field at the rear side of a laser-irradiated foil target. In the rising sheath field, the ion phase rotation may take place since the back-end protons of the beam feels a stronger field than the front-end protons. Consequently, the injected proton beam can be compressed in the longitudinal direction. At last, the vital role of the ion beam bunching is illustrated by the integrated simulations of two successive stages in a cascaded acceleration.An ultraintense femtosecond laser pulse was used, for the first time, to produce a strong magnetic field with controlled shapes by interactions with a capacitor-coil target with high efficiency. The temporal evolution of the strong magnetic field was obtained by the time-gated proton radiography method. A comparison of high-resolution radiographic images of proton deflection and particle-track simulations indicates a peak magnetic field of ∼20u2009T. The energy conversion efficiency from the ultraintense laser pulse to the magnetic field is as high as ∼10%. A simple model of the ultraintense laser-driven capacitor-coil target gives a relationship between the magnetic field strength and the electron temperature produced by the laser. Our results indicate that magnetic fields of tens of tesla could be stably produced by most of the existing ultraintense laser facilities. It potentially opens new frontiers in basic physics which require strong magnetic field environments.An ultraintense femtosecond laser pulse was used, for the first time, to produce a strong magnetic field with controlled shapes by interactions with a capacitor-coil target with high efficiency. The temporal evolution of the strong magnetic field was obtained by the time-gated proton radiography method. A comparison of high-resolution radiographic images of proton deflection and particle-track simulations indicates a peak magnetic field of ∼20u2009T. The energy conversion efficiency from the ultraintense laser pulse to the magnetic field is as high as ∼10%. A simple model of the ultraintense laser-driven capacitor-coil target gives a relationship between the magnetic field strength and the electron temperature produced by the laser. Our results indicate that magnetic fields of tens of tesla could be stably produced by most of the existing ultraintense laser facilities. It potentially opens new frontiers in basic physics which require strong magnetic field environments.

Proton beam shaped by “particle lens” formed by laser-driven hot electrons

Two-dimensional tailoring of a proton beam is realized by a “particle lens” in our experiment. A large quantity of electrons, generated by an intense femtosecond laser irradiating a polymer target, produces an electric field strong enough to change the trajectory and distribution of energetic protons flying through the electron area. The experiment shows that a strip pattern of the proton beam appears when hot electrons initially converge inside the plastic plate. Then the shape of the proton beam changes to a “fountain-like” pattern when these hot electrons diffuse after propagating a distance.

Novel front end design for synchronized output pulses with zero timing jitter in XG-III laser facility

XG-III laser facility is a petawatt laser which has a unique feature of three synchronized pulses output for various pump-probe experiments. To realize the synchronization with zero timing jitter, we have designed and implemented a novel front-end system based on super-continuum injected femtosecond optical parametric amplification (fs:OPA). Critical parameters of fs:OPA were optimized for the best conversion efficiency. Experimental results verified that major design specifications such as pulse energy, central wavelength and spectral width were fully accomplished and a high pulse contrast ratio was also achieved by the fs:OPA process.

Improvement of temporal contrast by 2 order of magnitude based on the deformable mirrors

Chirp pulse amplification (CPA) has been promoted as an effective way to explore the intensity frontier. High order dispersion induced by the stretcher and materials in the CPA system, which deteriorates both the pulse duration and temporal contrast, however, can not be absolutely compensated by the compressor. Placed at the Fourier plane of a 4f zero-dispersion stretcher consisting of a grating, the deformable mirror (DM) has been demonstrated as the modulator to compensate high order dispersion. Using the method of ray tracing, the relation between spectrum and position on DM has been obtained. It shows that the resolution of the deformable mirror can be controlled by adjusting the focal length and incident angle. We have simulated a typical Ti:sappire CPA system to revise the spectral phase by the DM. The result illustrates that if the spectral phase can be compensated, the temporal contrast will be improved by 2 order of magnitude.