Laser-plasma accelerator group

Abstract:

To drive a laser-plasma wakefield, high peak-power laser pulses are required. For useful accelerator applications, it is also necessary to have driving lasers with high efficiency, repetition rates, and average power. The coherent combination of Ytterbium-doped fibre laser amplifiers is a promising potential solution, and previous work has demonstrated the successful combination of near-identical ultrafast fibre lasers. We report here the combination of significantly mismatched Ytterbium-doped photonic crystal fibre amplifiers with a combined efficiency of 96%, while the locked power output remained stable for 6 hours. The combined output of the system had a total gain of 12 dB, with no detrimental effect on the compressed pulse width observed.

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.