Yanjun Gu

Fast magnetic-field annihilation in the relativistic collisionless regime driven by two ultrashort high-intensity laser pulses.

The magnetic quadrupole structure formation during the interaction of two ultrashort high power laser pulses with a collisionless plasma is demonstrated with 2.5-dimensional particle-in-cell simulations. The subsequent expansion of the quadrupole is accompanied by magnetic-field annihilation in the ultrarelativistic regime, when the magnetic field cannot be sustained by the plasma current. This results in a dominant contribution of the displacement current exciting a strong large scale electric field. This field leads to the conversion of magnetic energy into kinetic energy of accelerated electrons inside the thin current sheet.

Fast magnetic field annihilation driven by two laser pulses in underdense plasma

Fast magnetic annihilation is investigated by using 2.5-dimensional particle-in-cell simulations of two parallel ultra-short petawatt laser pulses co-propagating in underdense plasma. The magnetic field generated by the laser pulses annihilates in a current sheet formed between the pulses. Magnetic field energy is converted to an inductive longitudinal electric field, which efficiently accelerates the electrons of the current sheet. This new regime of collisionless relativistic magnetic field annihilation with a timescale of tens of femtoseconds can be extended to near-critical and overdense plasma with the ultra-high intensity femtosecond laser pulses.

Explosion of relativistic electron vortices in laser plasmas

The interaction of high intensity laser radiation with underdense plasma may lead to the formation of electron vortices. Though being quasistationary on an electron timescales, these structures tend to expand on a proton timescale due to Coloumb repulsion of ions. Using a simple analytical model of a stationary vortex as initial condition, 2D PIC simulations are performed. A number of effects are observed such as vortex boundary field intensification, multistream instabilities at the vortex boundary, and bending of the vortex boundary with the subsequent transformation into smaller electron vortices.

Evolution of relativistic solitons in underdense plasmas

Relativistic solitons arising from the interaction of an intense laser pulse with underdense plasmas are investigated. We show the formation and evolution of the relativistic solitons in a collisionless cold plasma with two dimensional particle-in-cell simulations. Such a kind of solitons will evolve into postsolitons if the time scale is longer than the ion response time. Generally, a substantial part of the pulse energy is transformed into solitons during the soliton formation. This fairly high efficiency of electromagnetic energy transformation can play an important role in the interaction between the laser pulse and the plasma. The energy exchange between the electromagnetic field and the kinetic energy of the soliton is discussed. In homogeneous plasmas, the solitons tend to stay close to the region where they are generated and dissipate due to the interaction with surrounding particles eventually. While the laser pulse propagates through inhomogeneous plasmas, the solitons are accelerated along the plasma density gradient towards lower density.

Magnetic reconnection research with petawatt-class lasers

Magnetic reconnection is regarded as a fundamental phenomenon in space and laboratory plasmas. It converts magnetic energy to kinetic energy of plasma particles through the topological rearrangements of the magnetic field lines. Magnetic reconnection is believed to play an important role in the solar systems, such as solar flares and coronal mass ejections. Observations of rapid energy release in solar flare and the global convection pattern within the magnetosphere are strongly suggestive that reconnection must be occurring. With the development of laser technology, high power laser facilities have made great progress in recent decades. Ultra powerful pulse with TW and PW are available now. As a result, the laser-matter interaction enters regimes of interest for laboratory astrophysics such as magnetic reconnection. J. Y. Zhong et al.1 reported an experiment about Xray source emission by reconnection outflows. Two intense lasers with long pulse duration are focused on the solid Aluminum target to generate hot electrons. In this paper, we employ a hydrogen foam target with near critical density to investigate the reconnection. Two parallel ultra intense pulses are injected into the target. By the effect of laser wakefield acceleration, two strong electron beam are generated and both of them induce a magnetic dipole structure. With the expansion of the dipole, magnetic field annihilation occurs in the center part of the target. The induced electric field and particle acceleration are detected in the simulations as evidence for magnetic reconnection. The effects of separation distance between two laser pulses and laser intensity on magnetic reconnection are also discussed.

Evolution of laser induced electromagnetic postsolitons in multi-species plasma

The evolution of an s-polarized relativistic electromagnetic soliton created in multi-species plasma by an intense short laser pulse is investigated using two-dimensional particle-in-cell simulations. The multi-component plasma consists of electrons and high-Z ions with a small addition of protons. By comparison, the evolution of postsoliton is very different from that in hydrogen plasma. A halo-like structure is found in spatial patterns of both electromagnetic fields and electron densities. The process of energy depletion is much slower due to the smaller charge-to-mass ratio of ions, which implies a better way of detecting postsolitons in simulations and experiments. In addition, it is found that the Coulomb explosion of high-Z ions in the postsoliton stage facilitates low-Z ion acceleration, and the maximum energy of low-Z ions increases with the component ratio of high-Z to low-Z ions.