Yifei Li

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.

Intense high repetition rate Mo Kα x-ray source generated from laser solid interaction for imaging application

We report an efficient Mo Kα x-ray source produced by interaction of femtosecond Ti: sapphire laser pulses with a solid Molybdenum target working at 1 kHz repetition rate. The generated Mo Kα x-ray intensity reaches to 4.7 × 10(10) photonsu2009sr(-1)u2009s(-1), corresponding to an average power of 0.8 mW into 2π solid angle. The spatial resolution of this x-ray source is measured to be 26 lp/mm. With the high flux and high spatial resolution characteristics, high resolving in-line x-ray radiography was realized on test objects and large size biological samples within merely half a minute. This experiment shows the possibility of laser plasma hard x-ray source as a new low cost and high resolution system for radiography and its ability of ultrafast x-ray pump-probe study of matter.

Multiple quasi-monoenergetic electron beams from laser-wakefield acceleration with spatially structured laser pulse

By adjusting the focus geometry of a spatially structured laser pulse, single, double, and treble quasi-monoenergetic electron beams were generated, respectively, in laser-wakefield acceleration. Single electron beam was produced as focusing the laser pulse to a single spot. While focusing the laser pulse to two spots that are approximately equal in energy and size and intense enough to form their own filaments, two electron beams were produced. Moreover, with a proper distance between those two focal spots, three electron beams emerged with a certain probability owing to the superposition of the diffractions of those two spots. The energy spectra of the multiple electron beams are quasi-monoenergetic, which are different from that of the large energy spread beams produced due to the longitudinal multiple-injection in the single bubble.

Plasma optical shutter in ultraintense laser-foil interaction

We report on a plasma optical shutter to reduce the intensity level of a nanosecond-duration pedestal of amplified spontaneous emission (ASE) using an ultrathin foil. The foil is ionized by the ASE prepulse and forms an expanding underdense preplasma, which enables the main laser pulse transmission, leading to an enhancement in temporal contrast. When such a plasma shutter is placed in front of a main target of interest, the preplasma profiles observed are similar to that produced from a single-layer reference target irradiated by a high-contrast laser, and can be finely tuned by varying the shutter thickness. Proton beams with significantly reduced divergence and higher flux density were measured experimentally using the double-foil design. The reduction in beam divergence is a characteristic signature of higher contrast laser production as a combined consequence of less target deformation and flatter sheath-acceleration field, as supported by the two-dimensional (2D) hydrodynamic and particle-in-cell si...

Demonstration of laser-produced neutron diagnostic by radiative capture gamma-rays

We report a new scenario of the time-of-flight technique in which fast neutrons and delayed gamma-ray signals were both recorded in a millisecond time window in harsh environments induced by high-intensity lasers. The delayed gamma signals, arriving far later than the original fast neutron and often being ignored previously, were identified to be the results of radiative captures of thermalized neutrons. The linear correlation between the gamma photon number and the fast neutron yield shows that these delayed gamma events can be employed for neutron diagnosis. This method can reduce the detecting efficiency dropping problem caused by prompt high-flux gamma radiation and provides a new way for neutron diagnosing in high-intensity laser-target interaction experiments.

Ultrahigh-charge electron beams from laser-irradiated solid surface

Significance In the last three decades, the laser–plasma accelerator (LPA) has shown a rapid development owing to its super–high-accelerate gradients, which makes it a very promising compact accelerator and light source. Acceleration of a high-quality electron beam with divergence angle as small as possible and beam charge as high as possible has been a long-term goal ever since the inception of the LPA concept. However, until now the most popular acceleration scenario has failed to achieve both goals. We solved this problem and obtained tightly collimated electron beams with small divergence angle and extremely high beam charge (∼100 nC) via the powerful ps laser pulse interacting with a solid target. 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 ∼1012u2009J/m3, making it a promising tool to drive warm or even hot dense matter states.

Proton acceleration from vacuum-gapped double-foil target with low-contrast picosecond intense laser

Proton emissions from vacuum-gapped cascaded-ultrathin-foil targets irradiated with low-contrast intense picosecond laser pulses were measured. The maximum energy of the proton beam and the laser-to-proton energy conversion efficiency were both increased in comparison with those from the single-layer reference targets. A transition from plateau to exponential profile in proton energy spectral shape was found for the target with a front-foil thickness of above 500u2009nm. The measured annular x-ray emissions from both target front and rear sides indicate that the proton enhancement could be attributed to the modified preplasma distribution. A simple model and hydrodynamic simulations further show that the optimal acceleration occurs when the front shutter foil is right swelled onto the front surface of the rear source foil by the prepulses at the arrival of the main laser pulse. This cascaded thin-foil target design can be popularized in improving laser-driven proton beams for wide applications.Proton emissions from vacuum-gapped cascaded-ultrathin-foil targets irradiated with low-contrast intense picosecond laser pulses were measured. The maximum energy of the proton beam and the laser-to-proton energy conversion efficiency were both increased in comparison with those from the single-layer reference targets. A transition from plateau to exponential profile in proton energy spectral shape was found for the target with a front-foil thickness of above 500u2009nm. The measured annular x-ray emissions from both target front and rear sides indicate that the proton enhancement could be attributed to the modified preplasma distribution. A simple model and hydrodynamic simulations further show that the optimal acceleration occurs when the front shutter foil is right swelled onto the front surface of the rear source foil by the prepulses at the arrival of the main laser pulse. This cascaded thin-foil target design can be popularized in improving laser-driven proton beams for wide applications.

Ultrafast pulsed magnetic fields generated by a femtosecond laser

An ultrafast pulsed magnetic field from a two-loop solenoid is generated by a femtosecond (fs) laser. High temporal resolution is needed to measure the magnetic field. We describe an improved Faraday-rotation measurement to evaluate the evolution of the magnetic field with a resolution of ∼3.3 picoseconds (ps) in a single shot, with an uncompressed chirped pulse from a Ti:sapphire laser as the optical probe. A magnetic field of 0.52u2009T with a rise time of 20.8 ps has been measured with this chirped Faraday probe. In addition, we demonstrate the magnetic field strength driven by the femtosecond laser can be modified by adjusting the focal spot size.An ultrafast pulsed magnetic field from a two-loop solenoid is generated by a femtosecond (fs) laser. High temporal resolution is needed to measure the magnetic field. We describe an improved Faraday-rotation measurement to evaluate the evolution of the magnetic field with a resolution of ∼3.3 picoseconds (ps) in a single shot, with an uncompressed chirped pulse from a Ti:sapphire laser as the optical probe. A magnetic field of 0.52u2009T with a rise time of 20.8 ps has been measured with this chirped Faraday probe. In addition, we demonstrate the magnetic field strength driven by the femtosecond laser can be modified by adjusting the focal spot size.