Laser-plasma accelerator group

Abstract:

We report the operation of an optical parametric amplifier (OPA) capable of producing gigawatt peak-power laser pulses with tunable wavelength in either the visible or near-infrared spectrum. The OPA has two distinct operation modes (i) generation of > 350 μJ, sub 100 fs pulses, tunable between 1250 - 1550 nm; (ii) generation of > 170 μJ, sub 150 fs pulses tunable between 490 - 530 nm. We have recorded high-order harmonic spectra over a wide range of driving wavelengths. This flexible source of femtosecond pulses presents a useful tool for exploring the wavelength-dependence of strong-field phenomena, in both the multi-photon and tunnel ionization regimes.

Abstract:

We describe in detail a low power Compact Marx Bank (CMB) circuit that can provide 20 kV, 500A pulses of approximately 100–200 ns duration. One application is the generation of capillary discharge plasmas of density ≈ 1018 cm􀀀3 used in laser plasma accelerators. The CMB is tiggered with a high speed solid state switch and gives a HV output pulse with a ns scale rise time into a 50Ω load (coaxial cable) with < 4 ns voltage jitter. Its small size (10 cm × 25 cm × 5 cm) means that it can be placed right next to the capillary discharge in the target chamber so avoiding the need to impedance match. The electrical energy required per discharge is < 1 J and the CMB can be run at shot repetition rates of >∼ 1 Hz. This low power requirement means the circuit can easily be powered by a small lead acid battery and so therefore can be floated relative to laboratory earth. The CMB is readily scalable and pulses > 45 kV are demonstrated in air discharges.

Abstract:

The exclusive deep inelastic electroproduction of ψ(2S) and J/ψ(1S) at an ep centre-of-mass energy of 317 GeV has been studied with the ZEUS detector at HERA in the kinematic range 2 < Q2 < 80 GeV2, 30 <W< 210 GeV and |t| < 1 GeV2, where Q2 is the photon virtuality, W is the photon–proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The data for 2 < Q2 < 5 GeV2 were taken in the HERA I running period and correspond to an integrated luminosity of 114 pb−1. The data for 5 < Q2 < 80 GeV2 are from both HERA I and HERA II periods and correspond to an integrated luminosity of 468 pb−1. The decay modes analysed were μ+μ− and J/ψ(1S)π+π− for the ψ(2S) and μ+μ− for the J/ψ(1S). The cross-section ratio σψ(2S)/σJ/ψ(1S) has been measured as a function of Q2, W and t. The results are compared to predictions of QCD-inspired models of exclusive vector-meson production.

Abstract:

A method for single-shot, non-destructive characterisation of broadband X-ray beams, based on energy-resolved powder diffraction, is described. Monte-Carlo simulations are used to simulate data for X-ray beams in the keV range with parameters similar to those generated by betatron oscillations in a laser-driven plasma accelerator. The retrieved X-ray spectra are found to be in excellent agreement with those of the input beams for realistic numbers of incident photons. It is demonstrated that the angular divergence of the X-rays can be deduced from the deviation of the detected photons from the Debye-Scherrer rings which would be produced by a parallel beam. It is shown that the angular divergence can be measured as a function of the photon energy, yielding the angularly-resolved spectrum of the input X-ray beam.