Laser-plasma accelerator group

Abstract:

Isolated photon production in deep inelastic ep scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 320 pb - 1. Measurements were made in the isolated-photon transverse-energy and pseudorapidity ranges 4 < E Tγ < 15 GeV and - 0.7 < η γ < 0.9 for exchanged photon virtualities, Q 2, in the range 10 < Q 2 < 350 GeV 2 and for invariant masses of the hadronic system W X > 5 GeV. Differential cross sections are presented for inclusive isolated photon production as functions of Q 2, x, E Tγ and η γ. Leading-logarithm parton-shower Monte Carlo simulations and perturbative QCD predictions give a reasonable description of the data over most of the kinematic range. © 2010 Elsevier B.V.

Abstract:

Two techniques are demonstrated to produce ultrashort pulse trains capable of quasi-phase-matching highharmonic generation. The first technique makes use of an array of birefringent crystals and is shown to generate high-contrast pulse trains with constant pulse spacing. The second technique employs a grating-pair stretcher, a multiple-order wave plate, and a linear polarizer. Trains of up to 100 pulses are demonstrated with this technique, with almost constant inter-pulse separation. It is shown that arbitrary pulse separation can be achieved by introducing the appropriate dispersion. This principle is demonstrated by using an acousto-optic programmable dispersive filter to introduce third- and fourth-order dispersions leading to a linear and quadratic variation of the separation of pulses through the train. Chirped-pulse trains of this type may be used to quasi-phase-match high-harmonic generation in situations where the coherence length varies through the medium. © 2010 Optical Society of America.

Abstract:

The generation of quasimonoenergetic electron beams, with energies greater than 500 MeV, in a laser-plasma accelerator driven by 2.5 J, 80 fs laser pulses guided in a low density plasma channel, is investigated. The laser energy required to achieve electron injection is found to depend strongly on the quality of the input laser focal spot. Simulations show that, although the matched spot size of the plasma channel is greater than the self-focused spot size, the channel assists relativistic self-focusing and enables electron injection to occur at lower plasma densities and laser powers than would be possible without a waveguide. © 2010 The American Physical Society.

Abstract:

Differential cross sections for dijet photoproduction and this process in association with a leading neutron, e ++p→e ++jet+jet+X(+n), have been measured with the ZEUS detector at HERA using an integrated luminosity of 40 pb -1. The fraction of dijet events with a leading neutron was studied as a function of different jet and event variables. Single- and double-differential cross sections are presented as a function of the longitudinal fraction of the proton momentum carried by the leading neutron, x L, and of its transverse momentum squared, pT2. The dijet data are compared to inclusive DIS and photoproduction results; they are all consistent with a simple pion-exchange model. The neutron yield as a function of x L was found to depend only on the fraction of the proton beam energy going into the forward region, independent of the hard process. No firm conclusion can be drawn on the presence of rescattering effects. © 2009 Elsevier B.V.

Abstract:

A method for generating non-uniformly spaced (chirped) trains of high-energy, high-contrast, femtosecond pulses is described and demonstrated. In this method a temporally stretched laser pulse is passed through an acousto-optic programmable dispersive filter (AOPDF), a birefringent plate, and a linear polarizer. It is demonstrated that linear and nonlinear variation of the pulse separation within the train may be controlled by changing respectively the third-and fourth-order dispersion introduced by the AOPDF. Programmable, non-uniform pulse trains of this type may find applications in quasi-phase matching high-harmonic generation. © 2010 IOP Publishing Ltd.