Cavity-Defect Plasma Bragg Gratings

Plasma Bragg grating （PBG） is composed of periodic variations of plasma and dielectric or vacuum. The defect mode characteristic of the PBG with a cavity-defect is studied by one-dimensional particle-in-cell （1D PIC） simulation. It is shown that the laser pulse with the defect frequency can be localized around the defect partly and at the same time leak out of both sides of the grating slowly because of the few number of the grating period. This results in local high laser field intensity and high plasma density produced at the defect area, from which the third harmonic is enhanced by one order of magnitude. With the enhancement of the light coupled to the defect and the decrease of the light leaking out of the defect, the conversion efficiency of the third harmonic from the incident laser can be increased.     

Relativistic Electron Acceleration in a Wake Field Generated by the Intense Femtosecond Laser Interaction with a Mixture Jet of Deuterium Clusters and Molecules

High energy electron acceleration in a wake field generated in the intense ultrashort （30fs） laser pulse cluster gas jet interaction is experimentally demonstrated. Relativistic electrons with energy of 60 MeV were observed. These high energy electrons split into two beams due to the relativistic self-focusing of the laser.     

Self-Injection and Acceleration of Monoenergetic Electron Beams from Laser Wakefield Accelerators in a Highly Relativistic Regime

Self-injection and acceleration of monoenergetic electron beams from laser wakefield accelerators are first investigated in the highly relativistic regime, using 100 TW class, 27 fs laser pulses. Quasi-monoenergetic multi- bunched beams with energies as high as multi-hundredMeV are observed with simultaneous measurements of side-scattering emissions that indicate the formation of self-channelfing and self-injection of electrons into a plasma wake, referred to as a ＇bubble＇. The three-dimensional particle-in-cell simulations confirmed multiple self-injection of electron bunches into the bubble and their beam acceleration with gradient of 1.5 GeV/cm.     

Neutron Generation and Kinetic Energy of Expanding Laser Plasmas

We investigate the kinetic energy of expanding plasma of a solid target heated by a ultra-short and ultra-intense laser pulse and the efllciency of energy coupling between the ultra-intense laser pulse and the solid target, in order to increase the utilization ratio of laser energy and to raise the neutron generation farther. Some new ideas about improving the energy utilization by head-on collision~, between the expanding plasmas are proposed. The significance is the raise of generation of shorter duration neutron, of the order of picoseconds, which allows for an increase of energy resolution in time-of-flight experiments and also for the investigation of the dynamics of nuclear processes with high temporal resolution.     

Excitation of multiple wakefields by short laser pulses in quantum plasmas

We present a theoretical investigation of the excitation of multiple electrostatic wakefields by the ponderomotive force of a short electromagnetic pulse propagating through a dense plasma. It is found that the inclusion of the quantum statistical pressure and quantum electron tunneling effects can qualitatively change the classical behavior of the wakefield. In addition to the well-known plasma oscillation wakefield, with a wavelength of the order of the electron skin depth (λe=c/ωpe, which in a dense plasma is of the order of several nanometers, where c is the speed of light in vacuum and ωpe is the electron plasma frequency), wakefields in dense plasmas with a shorter wavelength (in comparison with λe) are also excited. The wakefields can trap electrons and accelerate them to extremely high energies over nanoscales.     

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×10¹⁶ W/cm², which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm² energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.This revised version was published online in August 2005 with a corrected cover date.     

A DC probe diagnostics for fast electron temperature measurements in tokamak edge plasmas

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. It provides simultaneous measurements of electron temperature and parallel ion current density with high frequency at the same point in space. We describe ongoing work to characterize the ion flows within the probe, and to calibrate the diagnostics using 2D kinetic simulations. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreus, Seitzerland, June 23–24, 2002.     

Zonal flows, GAM, and radial electric field in the H-1 heliac

Experimental results on the role of zonal flows and geodesic acoustic modes in formation of transport barriers in the H-1 heliac are reviewed. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Roma, Italy, June 26–27, 2006.     

Fluctuation measurements with emissive probes in tokamaks

A new probe head, composed of two electro-emissive probes and two Langmuir probes has been designed to measure edge fluctuations in the CASTOR tokamak. In this contribution, only one Langmuir and one electron-emissive probe are used to measure the temperature and potential fluctuations and the phase angle between them.     

Dynamics of fully nonlinear drift wave-zonal flow turbulence system in plasmas

We present numerical simulations of fully nonlinear drift wave-zonal flow (DW-ZF) turbulence systems in a nonuniform magnetoplasma. In our model, the drift wave (DW) dynamics is pseudo-three-dimensional (pseudo-3D) and accounts for self-interactions among finite amplitude DWs and their coupling to the two-dimensional (2D) large amplitude zonal flows (ZFs). The dynamics of the 2D ZFs in the presence of the Reynolds stress of the pseudo-3D DWs is governed by the driven Euler equation. Numerical simulations of the fully nonlinear coupled DW-ZF equations reveal that short scale DW turbulence leads to nonlinear saturated dipolar vortices, whereas the ZF sets in spontaneously and is dominated by a monopolar vortex structure. The ZFs are found to suppress the cross-field turbulent particle transport. The present results provide a better model for understanding the coexistence of short and large scale coherent structures, as well as associated subdued cross-field particle transport in magnetically confined fusion plasmas.     

Diagnosis of Multiple Gases Separated from Transformer Oil Using Cavity-Enhanced Raman Spectroscopy

The Raman signal of gas molecules is very weak due to its small scattering cross section. Here, a near-confocal cavity-enhanced Raman detection system is demonstrated. In the cavity, a high power light of 9W is achieved by using a cw 200mW 532nm laser, which greatly enhances the detection sensitivity of gas species. A photomultiplier tube connected to a spectrometer is used as the detection system. The Raman spectra of the mixed gases separated from transformer oil has been observed. The relationship of absolute Raman intensity and gas pressure is also obtained. To our knowledge, this is the first Raman system to detect the gases separated from transformer oil.     

Propagation of Ion-Acoustic Wave in an Inhomogeneous Dusty Plasma with. Dust Charge Fluctuation

The propagation of dust ion-acoustic wave in an inhomogeneous dusty plasma is studied by taking the dust charge fluctuation and collisions into account. It is shown that the dust charge fluctuation brings a phase shift to the wave. Furthermore, because of the presence of dust charge fluctuation, a new damping term rises, which makes the damping more sharply.     

Generation of Multiphoton Entangled States with Linear Optical Elements

We propose a linear optical protocol to generate three-photon and four-photon entangled states without resorting to entangled sources. The setup in this protocol is composed of three beam splitters and two half-wave plates. We can obtain three-photon and four-photon entangled states with postselection, as with other protocols. This protocol has the advantage of high efficiency and is more feasible than others.     

Laser-Induced Particle Jet and Its Ignition Application in Premixed Combustible Gases

A hot particle jet is induced as a laser pulse from a free oscillated Nd：YAG laser focused on a coal target. The particle jet successfully initiates combustion in a premixed combustible gas consisting of hydrogen, oxygen, and air. The experiment reveals that the ionization of the particle jet is enhanced during the laser pulse. This characteristic is attributed to the electron cascade process and the ionization of the particles or molecules of the target. The initial free electrons, which are ablated from the coal target, are accelerated by the laser pulse through the inverse Bremsstrahfung process and then collide with the neutrals in the jet, causing the latter to be ionized.     

Generation of Anti-Stokes Line in Fundamental Mode of Photonic Crystal Fibre

A photonic crystal fibre （PCF） with zero-dispersion wavelength around 800 nm is designed and fabricated. Simulated results show that the zero-dispersion wavelength of fundamental mode for this PCF is at 826nm, and phase-matched four-wave mixing can be achieved in fundamental mode. Using 20ors Ti：sapphire laser with central wavelength at 810nm as pump, the anti-Stokes line around 610hm ＆ generated efficiently. The output signal has a Gaussian-like profile, which indicates that the anti-Stokes signal is in the fundamental mode of the PCF. The energy of anti-Stokes signal is higher than that of residual pump laser and the maximum ratio of the anti-Stokes signal to the pump component in the output spectrum is estimated to be 1.2.     

Guiding of an electromagnetic pulse in a plasma immersed in combined wiggler and axial magnetic fields

We present a new plasma-based method of guiding an electromagnetic pulse. The scheme consists of an inhomogeneous magnetic field and a uniform density plasma, in contrast to existing schemes that rely on transverse plasma density gradients but need not be magnetized. The refractive index of a magnetized plasma depends on the strength and direction of the magnetic field as well as the plasma density. A guiding channel is formed by using field inhomogeneity to generate the desired transverse profile of the index of refraction. The concept is analyzed with an envelope equation and, for the specific example of a wiggler magnetic field, with a two-dimension particle-in-cell simulation. A simplified model of this scheme as producing a magnetic wall in analogy to metallic waveguides is presented, for which corresponding approximate relations for the guided mode axial wavelength and radius are derived as functions of the plasma and magnetic field parameter. These are seen to be in good agreement with particle-in-cell simulation results. Since the desired inhomogeneity of the refractive index can be made easily when the electromagnetic wave frequency is close to the cyclotron frequency, this guiding scheme is most readily applied in the microwave regime.     

Effect of Discharge Voltage on an Ion Sheath Formed at a Grid in a Multi-Dipole Discharge Plasma

It is experimentally demonstrated that a relatively strong ion-rich sheath formed at a fixed negative bias of the grid can be changed to a rather weak ion sheath （sheath potential weakly retards electrons） only by increasing the discharge voltage in the system. At sufficiently high negative grid bias, an increase of discharge voltage enhances the ion collection current at the grid. An explanation is put forward in support of this experimental observation. A slight density enhancement with a fall in plasma electron temperature is also observed with the increasing negative grid bias.     

Statistical properties of turbulence in a toroidal magnetized ECR plasma

The statistical analyses of fluctuation data measured by electrostatic-probe arrays clearly show that the self-organized criticality (SOC) avalanches are not the dominant behaviors in a toroidal ECR plasma in the SMT (Simple Magnetic Torus) mode of KT-5D device. The f⁻¹ index region in the auto-correlation spectra of the floating potential Vf and the ion saturation current Is, which is a fingerprint of a SOC system, ranges only in a narrow frequency band. By investigating the Hurst exponents at increasingly coarse grained time series, we find that at a time scale of τ>100 μs, there exists no or a very weak long-range correlation over two decades in τ. The difference between the PDFs of Is and Vf clearly shows a more global nature of the latter. The transport flux induced by the turbulence suggests that the natural intermittency of turbulent transport maybe independent of the avalanche induced by near criticality. The drift instability is dominant in a SMT plasma generated by means of ECR discharges.     

Two-temperature relaxation and melting after absorption of femtosecond laser pulse

The theory and experiments concerned with the electron-ion thermal relaxation and melting of overheated crystal lattice constitute the subject of this paper. The physical model includes two-temperature (2T) equation of state, many-body interatomic potential, the electron-ion energy exchange, electron thermal conductivity, and optical properties of solid, liquid, and two phase solid-liquid mixture. Two-temperature hydrodynamics and molecular dynamics codes are used. An experimental setup with pump-probe technique is used to follow evolution of an irradiated target with a short time step 100 fs between the probe femtosecond laser pulses. Accuracy of measurements of reflection coefficient and phase of reflected probe light are 1% and ∼1 nm, respectively. It is found that, firstly, the electron-electron collisions make a minor contribution to a light absorption in solid Al at moderate intensities; secondly, the phase shift of a reflected probe results from heating of ion subsystem and kinetics of melting of Al crystal during  ps, where t is time delay between the pump and probe pulses measured from the maximum of the pump; thirdly, the optical response of Au to a pump shows a marked contrast to that of Al on account of excitation of d-electrons.     

Generation and propagation of hot electrons in laser-plasmas

Hot electrons are generated in the interaction between intense ultrashort laser pulses with targets. The process depends on the laser intensity, polarization, incident angle, scale length of plasmas and target materials. In this paper, the recent progress on generation and propagation of hot electrons in non-relativistic and relativistic laser-plasma interactions at the Institute of Physics, Chinese Academy of Sciences, are reviewed.