Yaoxiang Liu

Direct observation of laser guided corona discharges

Laser based lightning control holds a promising way to solve the problem of the long standing disaster of lightning strikes. But it is a challenging project due to insufficient understanding of the interaction between laser plasma channel and high voltage electric filed. In this work, a direct observation of laser guided corona discharge is reported. Laser filament guided streamer and leader types of corona discharges were observed. An enhanced ionization took place in the leader (filament) through the interaction with the high voltage discharging field. The fluorescence lifetime of laser filament guided corona discharge was measured to be several microseconds, which is 3 orders of magnitude longer than the fluorescence lifetime of laser filaments. This work could be advantageous towards a better understanding of laser assisted leader development in the atmosphere.

Collisionless shocks driven by 800 nm laser pulses generate high-energy carbon ions

We present experimental studies on ion acceleration from diamond-like carbon (DLC) foils irradiated by 800 nm, linearly polarized laser pulses with peak intensity of 1.7 × 1019 W/cm2 to 3.5 × 1019 W/cm2 at oblique incidence. Diamond-like carbon foils are heated by the prepulse of a high-contrast laser pulse and expand to form plasmas of near-critical density caused by thermal effect before the arrival of the main pulse. It is demonstrated that carbon ions are accelerated by a collisionless shock wave in slightly overdense plasma excited by forward-moving hot electrons generated by the main pulse.

Effects of nanosecond-scale prepulse on generation of high-energy protons in target normal sheath acceleration

A pulse cleaner based on noncollinear optical-parametric amplification and second-harmonic generation processes is used to improve the contrast of a laser of peak intensity ∼2 × 1019 W/cm2 to ∼1011 at 100 ps before the peak of the main pulse. A 7 MeV proton beam is observed when a 2.5 μm-thick Al foil is irradiated by this high-contrast laser. The maximum proton energy decreases to 2.9 MeV when a low-contrast (∼108) laser is used. Two-dimensional particle-in-cell simulations combined with MULTI simulations show that the maximum proton energy sensitively relies on the detecting direction. The ns-time-scale prepulse can bend a thin target before the main pulse arrives, which reduces maximum proton energy in the target normal sheath acceleration.

Large-scale proton radiography with micrometer spatial resolution using femtosecond petawatt laser system

An image of dragonfly with many details is obtained by the fundamental property of the high-energy proton source on a femtosecond petawatt laser system. Equal imaging of the dragonfly and high spatial resolution on the micrometer scale are simultaneously obtained. The head, wing, leg, tail, and even the internal tissue structures are clearly mapped in detail by the proton beam. Experiments show that image blurring caused by multiple Coulomb scattering can be reduced to a certain extent and the spatial resolution can be increased by attaching the dragonfly to the RCFs, which is consistent with theoretical assumptions.

Corona discharge induced snow formation in a cloud chamber

Artificial rainmaking is in strong demand especially in arid regions. Traditional methods of seeding various Cloud Condensation Nuclei (CCN) into the clouds are costly and not environment friendly. Possible solutions based on ionization were proposed more than 100 years ago but there is still a lack of convincing verification or evidence. In this report, we demonstrated for the first time the condensation and precipitation (or snowfall) induced by a corona discharge inside a cloud chamber. Ionic wind was found to have played a more significant role than ions as extra CCN. In comparison with another newly emerging femtosecond laser filamentation ionization method, the snow precipitation induced by the corona discharge has about 4 orders of magnitude higher wall-plug efficiency under similar conditions.

Probing the effective length of plasma inside a filament.

We present a novel method based on plasma-guided corona discharges to probe the plasma density longitudinal distribution, which is particularly good for the weakly ionized plasmas (~1014 cm-3). With this method, plasma density longitudinal distribution inside both a weakly ionized plasma and a filament were characterized. When a high voltage electric field was applied onto a plasma channel, the original ionization created by a laser pulse would be enhanced and streamer coronas formed along the channel. By measuring the fluorescence of enhanced ionization, in particular, on both ends of a filament, the weak otherwise invisible plasma regions created by the laser pulse were identified. The observed plasma guided coronas were qualitatively understood by solving a 3D Maxwell equation through finite element analysis. The technique paves a new way to probe low density plasma and to precisely measure the effective length of plasma inside a filament.

Waveform control of enhanced THz radiation from femtosecond laser filament in air

We report on a waveform control of enhanced THz radiation along a femtosecond laser filament in air with a high voltage technique. By applying a DC high-voltage electric field from two sharp electrodes in a direction parallel to the laser filament and scanning it along the filament, the longitudinal evolution of amplified THz emission was demonstrated. By changing the position of the pair of electrodes along the laser filament, different waveforms of THz radiation were obtained. Due to the change of the plasma density distribution at the leading and trailing ends of a laser filament, the enhanced THz waveforms could have a phase shift of ∼π. The technique is very simple. It could help to understand the THz generation process through external electric field assisted laser filamentation.

Focal spot effects on the generation of proton beams during target normal sheath acceleration

Focal spot effects on the generation of proton beams are investigated by a high-intensity high-contrast laser irradiating on solid foil in target normal sheath acceleration experiments. Different spot size, transverse shape, and intensity of the laser are obtained by appropriately using deformable mirrors and parabolic mirrors. Experiments show that the maximum proton energy is mainly determined by the laser intensity if the focal spot size is not seriously changed. Compared with the previous experimental results, the optimum foil thickness d o is scaled by the laser intensity I as d o ~ I 0.33. The corresponding theoretical estimation is carried out as d o ~ I 0.25 for ultra-high intensity laser systems with similar contrast. MULTI and particle-in-cell simulations are used to interpret the experimental results.

Laser guided ionic wind

We report on a method to experimentally generate ionic wind by coupling an external large electric field with an intense femtosecond laser induced air plasma channel. The measured ionic wind velocity could be as strong as >4 m/s. It could be optimized by increasing the strength of the applied electric field and the volume of the laser induced plasma channel. The experimental observation was qualitatively confirmed by a numerical simulation of spatial distribution of the electric field. The ionic wind can be generated outside a high-voltage geometry, even at remote distances.

Laser guided coronas and its application on probing plasma density distribution

Applying high voltage electric field on an intense femtosecond laser ionized plasma channel, enhanced ionization and lifetime extension up to few microsecond of plasma channel was observed. Streamer and leader types of coronas along laser filament were generated. The technique has been used to precisely probe plasma density distribution.