P. X. Wang

Vacuum electron acceleration by an intense laser

Using 3D test particle simulations, the characteristics and essential conditions under which an electron, in a vacuum laser beam, can undergo a capture and acceleration scenario (CAS). When a{sub 0} {approx}> 100 the electron can be captured and violently accelerated to energies {approx}> 1 GeV, with an acceleration gradient {approx}> 10 GeV/cm, where a{sub 0} = eE{sub 0}/m{sub e}{omega}c is the normalized laser field amplitude. The physical mechanism behind the CAS is that diffraction of the focused laser beam leads to a slowing down of the effective wave phase velocity along the captured electron trajectory, such that the electron can be trapped in the acceleration phase of the wave for a longer time and thus gain significant energy from the field.

Scheme for a quantum random number generator

A scheme for a random number generator based on the intrinsic randomness of quantum mechanics is proposed. A Fresnel multiple prism which can act as a perfect 50∕50 beam splitter is used to realize the random events by choosing single photons from a polarized laser beam. A procedure to get rid of the bias of the raw sequences is discussed in detail together with the random number generation efficiency per light pulse.

Accurate description of Gaussian laser beams and electron dynamics

In this paper, the higher order corrections to the description of a Gaussian laser field are derived and expressed as power functions of the parameter s ¼ 1=kw0, where k is the laser wave number and w0 the beam width at the focus center. Using the test particle simulation programs, the electron dynamics obtained using the paraxial approximation, the fifthorder correction, and the seventh-order correction are compared. Special attention is given to electron acceleration in vacuum by intense laser beams. The results reveal that, when kw0 J 50, the paraxial approximation field is good enough to reproduce all the electron dynamic characteristics. In the range of 40 K kw0 < 50, the fifth-order corrected field should be used. For very tightly focused laser beams kw0 K 30, one has to utilize seventh-order or higher order corrections to describe more accurately the field of a Gaussian beam. 2002 Published by Elsevier Science B.V.

Mechanism of electron acceleration by chirped laser pulse

We studied the mechanism of electron acceleration by a chirped laser pulse. We found that, because of the chirp effect, a region exists where the laser wave phase experienced by the electron varies slowly, so that the electron can be accelerated for a long time. The mechanism of chirped laser acceleration is different to that of the capture and acceleration scenario, although both of them have a main acceleration stage in which the electrons are trapped for long periods.

Polarization effect of fields on vacuum laser acceleration

Concerning laser-driven electron acceleration in vacuum, a comparison was made between using circularly polarized (CP) laser field and linearly polarized (LP) field. It has been found that the main advantage for using CP field is that its acceleration channel occupies relatively larger phase space, which can give rise to greater acceleration efficiency. This feature chiefly comes from the difference in the distribution of the longitudinal electric components of these two kinds of fields. One of the disadvantages with CP field is the “energy saturation” phenomenon as the laser intensity is sufficiently high, resulting from the enhanced Lorentz force component in CP field. Physical explanations of these characteristics are addressed as well.

Efficient energy conversion from laser to proton beam in a laser-foil interaction

Demonstrated is a remarkable improvement on the energy conversion efficiency from laser to protons in a laser-foil interaction by particle simulations. The total laser-proton energy conversion efficiency becomes 16.7%, although a conventional plane foil target serves a rather low efficiency. In our previous study we found that Al multihole thin-foil target was efficient for the energy conversion from laser to protons [Y. Nodera and S. Kawata, Phys. Rev. E 78, 046401 (2008)], and the energy conversion efficiency was 9.3%. In our 2.5-dimensional particle-in-cell simulations the Al multihole structure is also employed, and the parameters of the Al multihole wing width and length are optimized in the paper. The present results clarify the roles of the target Al hole width and depth in the laser-proton energy conversion. The main physical reason for the enhancement of the conversion efficiency is a reduction of the laser reflection at the target surface area. The optimized multihole foil target provides a rema...

Output features of vacuum laser acceleration

Electrons acceleration by the vacuum laser acceleration scheme CAS [see, e.g., P. X. Wang et al., Appl. Phys. Lett. 78, 2253 (2001)] (capture and acceleration scenario) is simulated. The general features of the outgoing electrons are examined at different laser intensities. Explanations based on the mechanism behind the CAS scheme of those output characteristics are presented. The results show it is hopeful that CAS becomes a useful scheme for laser accelerators.

A formula on phase velocity of waves and application

Phase velocity plays a key role in wave-matter interactions where phase matching is essential. Laser acceleration of electrons is a good example. We have derived an exact formula with two deductions of the phase velocity for a monochromatic wave field in homogeneous medium from the fundamental wave equation. The core of these formulae is that the phase velocity is sufficiently determined in terms of the wave amplitude with no explicit reference to the phase. We applied these formulae to make out the phase velocity distribution of a Gaussian laser beam and compared with the traditional method.

Field structure and electron acceleration in a laser beam of a high-order Hermite-Gaussian mode

We analyze the axial electric field intensity distribution and the phase velocity distribution of high-order Hermite-Gaussian (HG) mode laser beams. Using a three-dimensional test particle simulation, the numerical results of electrons accelerated by Hermite-Gaussian (0,0), and (3,0) mode laser beams are presented. It is established that electrons can be more favorably captured and accelerated in an odd high-order Hermite-Gaussian mode laser beam.

Properties of electron acceleration by a circularly polarized laser in vacuum

The dynamic characteristics of an electron accelerated by ultraintense circularly polarized laser pulses in vacuum following the capture and acceleration scenario were studied. Comparing them with that from the use of linearly polarized laser pulses, we found (i) that the acceleration channel is wider, leading to greater acceleration efficiency, and (ii) that the maximum energy gains rise much more slowly as the laser intensity increases. This slow rise is caused by the magnetic-field force, which weakens the longitudinal acceleration force at higher laser intensity.