Q. L. Dong

Plasmoid Ejection and Secondary Current Sheet Generation from Magnetic Reconnection in Laser-Plasma Interaction

Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al. [Phys. Rev. Lett. 97, 255001 (2006)] by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (>1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS.

Hot electron generation via vacuum heating process in femtosecond laser–solid interactions

Hot electron generation by the vacuum heating process has been studied in the interaction of 150 fs, 5 mJ, 800 nm P-polarized laser pulses with solid targets. The measurements have suggested that the “vacuum heating” is the main heating process for the hot electrons with high energies. The energy of the vacuum-heated hot electrons has been found to be higher than the prediction from the scaling law of resonance absorption. Particle-in-cell simulations have confirmed that the hot electrons are mainly generated by the vacuum heating process under certain experimental conditions.

Collisionless shockwaves formed by counter-streaming laser-produced plasmas

The interaction between two counter-streaming laser-produced plasmas is investigated using the high-power Shenguang II laser facility. The shockwaves observed in our experiment are believed to be excited by collisionless mechanisms. The dimensionless parameters calculated with the results suggest that it is possible to scale the observation to the supernova remnants using transformation and similarity criteria.

Enhanced K α output of Ar and Kr using size optimized cluster target irradiated by high-contrast laser pulses

We observed that increasing the clusters size and laser pulse contrast can enhance the X-ray flux emitted by femtosecond-laser-driven-cluster plasma. By focusing a high contrast laser (10(-10)) on large argon clusters, high flux Kα-like X-rays (around 2.96 keV) is generated with a total flux of 2.5 × 10(11) photons/J in 4π and a conversion efficiency of 1.2 × 10-4. In the case of large Kr clusters, the best total flux for L-shell X-rays is 5.3 × 1011 photons/J with a conversion efficiency of 1.3 × 10-4 and, for the Kα X-ray (12.7 keV), it is 8 × 10(8) photons/J with a conversion efficiency of 1.6 × 10-6. Using this X-ray source, a single-shot high-performance X-ray imaging is demonstrated.

Z-dependence of hot electron generation in femtosecond laser interaction with solid targets

Hot electron emission is studied using 798 nm, 150 fs, p-polarized laser pulses at 1 × 1016 W cm−2, irradiating on plastic (CH), aluminium (Al), titanium (Ti) and copper (Cu) planar targets. The dependence of hot electron generation on atomic number Z is observed experimentally.

Directional transport of fast electrons at the front target surface irradiated by intense femtosecond laser pulses with preformed plasma

The effects of laser incidence angle on lateral fast electron transport at front target surface, when a plasma is preformed, irradiated by intense (>10(18) W/cm(2)) laser pulses, are studied by K-alpha imaging technique and electron spectrometer. A horizontally asymmetric K-alpha halo, resulting from directional lateral electron transport and energy deposition, is observed for a large incidence angle (70 degrees). Moreover, a group of MeV high energy electrons is emitted along target surface. It is believed that the deformed preplasma and the asymmetrical distribution of self-generated magnetic field, at large incidence angle, play an important role in the directional lateral electron transport.

Laboratory spectroscopy of silicon plasmas photoionized by mimic astrophysical compact objects

Photoionized plasmas are encountered in astrophysics wherever low-temperature gas/plasma is bathed in a strong radiation field. X-ray line emissions in the several kiloelectronvolts spectral range were observed from accreting clouds of binary systems, such as CYGNUS X-3 and VELA X-1, in which high-intensity x-ray continua from compact objects (neutron stars, black holes or white dwarfs) irradiate the cold and rarefied clouds. X-ray continuum- induced line emission accurately describes the accreting clouds, but experimental verification of this photoionized plasma model is scarce. Here we report the generation of photoionized plasmas in the laboratory under well-characterized conditions using a high-power laser. A blackbody radiator at a temperature of 500 eV, corresponding to a compact object, was created by means of a laser-driven implosion. The emerging x-rays irradiate a low-density (n(e) < 10(20) cm(-3)) and low- temperature (T(e) < 30 eV) silicon plasma. Line emissions from lithium- and helium-like silicon ions were observed from a thermally cold silicon plasma in the 1.8-1.9 keV spectral region, far from equilibrium conditions. This result reveals the laboratory generation of a photoionizing plasma. Atomic kinetic calculations imply the importance of direct K-shell photoionization by incoming hard x-rays.

Electron acceleration by static and oscillating electric fields produced in the interaction between femtosecond laser pulses and solid targets

The interaction of modest, femtosecond (fs) laser pulses with solid targets is studied with particle-in-cell (PIC) simulations. A bi-temperature distribution of hot electrons is found. The first hot electron temperature can be attributed to the resonance absorption of the laser field, whereas the second hot electron temperature is identified to be due to the combined acceleration by the static electric field in front of the target and by the laser induced oscillating electric field in the thin plasma layer between the vacuum and the target.

X-ray lasers from Inner-shell transitions pumped by the Free-electron laser

We present a approach of generating femtosecond coherent x-ray pulses by using the self-amplified free-electron laser (SASE FEL) to pump the inner-shell x-ray lasers (ISXRLs). Theoretical simulations are performed. The gain characteristics are analyzed for the two representative schemes of inner-shell x-ray transitions, ie. the self-terminated x-ray lasing (1s)(-1)-->(2p)(-1) (lambda = 4.5nm) in carbon (Z = 6) and the quasi-stationary x-ray lasing (2p)(-1 )-->(3s)(-1) (lambda = 4.1nm) in calcium (Z = 20). When the 10fs x-ray FEL pulses are available at 284eV and 360eV with the pumping intensities of 1.2x10(15)W/cm(2) and 2x10(17)W/cm(2) for C and Ca, respectively, a net gain of 140cm(-1) can be predicted. Using a one-dimensional model, the properties of output ISXRLs are studied. By the Carbon ISXRL scheme, the multi-spiky SASE FEL x-ray pulse with chaotic temporal structure is smoothed to a temporally continuous x-ray pulse with a comparable duration but at a different wavelength. The Calcium scheme, can be used to create one single x-ray laser pulse with a duration as short as 2fs. The spectral bandwidth of the output ISXRLs is an order of 10 (-3), which is one order narrower than that of the SASE FELs.

Nonlocal heat transport in laser-produced aluminum plasmas

The spatial and temporal evolutions of nonlocal heat transport in laser-produced aluminum plasmas are studied with the improvements of the Thomson scattering experiments and the kinetic Fokker–Planck simulations. The results are compared with the hydrodynamic simulations with the classical Spitzer–Harm theory. When another heater beam is used, the electron temperature decreases slowly and the temperature gradient becomes steep in the conduction zone. The nonlocal heat flux can be sustained at a high value with slow decrease for long time. The Fokker–Planck simulations considering electron-electron collisions can well describe the nonlocal heat transport process in laser-produced plasmas.