Spectrally peaked electron beams produced via surface guiding and acceleration in femtosecond laser-solid interactions

Kα X-ray Emission from Nanowire Cu Targets Driven by Femtosecond Laser Pulses for X-ray Conversion and Backlight Imaging

A high-quality X-ray source was proposed by modifying the target material structure characteristics driven by ultrahigh laser energy. The experiments were performed on the Ti:sapphire femtosecond laser beam device (4.3-6 J, 30 fs), one of the three XG-III lasers in Laser Fusion Research Center of China Academy of Engineering Physics. The femtosecond laser beam drove the nanowire copper material with an average length of 18-50 μm and a diameter of about 260 nm. A single-photon counting charge-coupled device was employed to measure the copper Kα X-ray emission of the nanowire and foil targets. A clear maximum photon yield of the nanowire target was calculated to be 3.6 × 10⁸ photons sr^-1 s^-1, the conversion efficiency was up to 0.0087%, and the average yield was 2.5 times that of the copper foil targets. In addition, by using a pinhole imaging method of φ10 μm, the minimum full width at half maximum spot size of the X-ray source was calculated in the range of 85-240 μm, which was similar to that of the copper foil material with a long radius of 170 μm and a short radius of 63 μm. The experimental data illustrate that the nanowire has the potential to enhance the energy absorption of femtosecond laser for X-ray conversion and backlight imaging.

Ultrahigh-charge electron beams from laser-irradiated solid surface

Compact acceleration of a tightly collimated relativistic electron beam with high charge from a laser-plasma interaction has many unique applications. However, currently the well-known schemes, including laser wakefield acceleration from gases and vacuum laser acceleration from solids, often produce electron beams either with low charge or with large divergence angles. In this work, we report the generation of highly collimated electron beams with a divergence angle of a few degrees, nonthermal spectra peaked at the megaelectronvolt level, and extremely high charge (∼100 nC) via a powerful subpicosecond laser pulse interacting with a solid target in grazing incidence. Particle-in-cell simulations illustrate a direct laser acceleration scenario, in which the self-filamentation is triggered in a large-scale near-critical-density plasma and electron bunches are accelerated periodically and collimated by the ultraintense electromagnetic field. The energy density of such electron beams in high-Z materials reaches to [Formula: see text], making it a promising tool to drive warm or even hot dense matter states.

Fast electrons generated by quasistatic electric fields of a fs-laser-pulse-induced plasma

We present a new acceleration mechanism for electrons taking place during the interaction of an ultrashort, nonrelativistic laser pulse with a plasma generated at the surface of a solid density target. In our experiments, the plasma is created by a laser pulse with femtosecond duration and an energy of about 1 mJ focused to intensities of above 10^{17}W/cm^{2}. We observe that the electron energies acquired by this mechanism exceed the ponderomotive potential of the laser by an order of magnitude. This result was reproduced and quantitatively confirmed by particle-in-cell simulations, which further revealed that the observed electron acceleration is based on quasistatic electric fields caused by the space charges of ponderomotively preaccelerated electrons. This acceleration process is examined in more detail by a simplified numerical model, which allows a qualitative explanation of the final electron energies.

Relativistic electron acceleration by mJ-class kHz lasers normally incident on liquid targets

We report observation of kHz-pulsed-laser-accelerated electron energies up to 3 MeV in the -klaser (backward) direction from a 3 mJ laser interacting at normal incidence with a solid density, flowing-liquid target. The electrons/MeV/s.r. >1 MeV recorded here using a mJ-class laser exceeds or equals that of prior super-ponderomotive electron studies employing lasers at lower repetition-rates and oblique incidence. Focal intensity of the 40-fs-duration laser is 1.5 · 10¹⁸ W cm^-2, corresponding to only ∼80 keV electron ponderomotive energy. Varying laser intensity confirms electron energies in the laser-reflection direction well above what might be expected from ponderomotive scaling in normal-incidence laser-target geometry. This direct, normal-incidence energy spectrum measurement is made possible by modifying the final focusing off-axis-paraboloid (OAP) mirror with a central hole that allows electrons to pass, and restoring laser intensity through adaptive optics. A Lanex-based, optics-free high-acquisition rate (>100 Hz) magnetic electron-spectrometer was developed for this study to enable shot-to-shot statistical analysis and real-time feedback, which was leveraged in finding optimal pre-plasma conditions. 3D Particle-in-cell simulations of the interaction show qualitative super-ponderomotive spectral agreement with experiment. The demonstration of a high-repetition-rate, high-flux source containing >MeV electrons from a few-mJ, 40 fs laser and a simple liquid target encourages development of future ≥kHz-repetition, fs-duration electron-beam applications.

Divergence-free transport of laser-produced fast electrons along a meter-long wire target

We demonstrate that, from a 10-μm metal wire irradiated by a 10(19)  W/cm2 laser pulse, fast electrons form a nearly perfect circular beam around the wire and propagate along it. The total charge and diameter of the electron beam are maintained over a propagation distance of 1 m. Moreover, the electron beam can be guided along a slightly bent wire. Numerical simulations suggest that a relatively weak steady electric field, which does not decay for several nanoseconds, is generated around the wire and plays a key role in the long-distance guidance.