arXiv subjects

C. J. Hooker

Publications and source records attributed to C. J. Hooker.

6 recordsLinked to original sources

arXiv2020

Bright X-ray radiation from plasma bubbles in an evolving laser wakefield accelerator

We show that the properties of the electron beam and bright x-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion length. With a 200~TW laser pulse this results in an x-ray beam with median photon energy of \unit[20]{keV}, $> 6\times 10^{8}$ photons above \unit[1]{keV} per shot and a peak brightness of $\unit[3 \times 10^{22}]{photons~s^{-1}mrad^{-2}mm^{-2} (0.1\% BW)^{-1}}$.

physics.plasm-ph↗

arXiv2018

Temporal Feedback Control of High-Intensity Laser Pulses to Optimize Ultrafast Heating of Atomic Clusters

We describe how active feedback routines can be applied at limited repetition rate (5 Hz) to optimize high-power $>10$ TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

physics.plasm-ph↗

arXiv2018

Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

We report on the depletion and power amplification of the driving laser pulse in a strongly-driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including localized redshift and subsequent group delay dispersion at the laser pulse front.

physics.plasm-ph↗

arXiv2010

Observation of a Long-Wavelength Hosing Modulation of a High-Intensity Laser Pulse in Underdense Plasma

We report the first experimental observation of a long-wavelength hosing modulation of a high-intensity laser pulse. Side-view images of the scattered optical radiation at the fundamental wave-length of the laser reveal a transverse oscillation of the laser pulse during its propagation through underdense plasma. The wavelength of the oscillation λ_hosing depends on the background plasma density n_e and scales as λ_hosing~n_e^-3/2. Comparisons with an analytical model and 2-dimensional particle-in-cell simulations reveal that this laser hosing can be induced by a spatio-temporal asymmetry of the intensity distribution in the laser focus which can be caused by a misalignment of the parabolic focussing mirror or of the diffraction gratings in the pulse compressor.

physics.plasm-ph↗

arXiv2010

Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-polarized radiation. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage `shake-up' reaction. Here we report a unique combination of experimental techniques that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry, equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond XUV radiation sources. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in-vivo cells and nanoscale XUV lithography.

physics.atom-ph↗

arXiv2005

Geometry- and diffraction-independent ionization probabilities in intense laser fields: probing atomic ionization mechanisms with effective intensity matching

We report a novel experimental technique for the comparison of ionization processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser pulses is observed over the intensity range 10 TW/cm^2 to 10 PW/cm^2 using Effective Intensity Matching (EIM), which is coupled with Intensity Selective Scanning (ISS) to recover the geometry-independent probability of ionization. Such measurements, made possible by quantifying diffraction effects in the laser focus, are compared directly to theoretical predictions of multiphoton, tunnel and field ionization, and a remarkable agreement demonstrated. EIM-ISS allows the straightforward quantification of the probability of recollision ionization in a linearly polarized laser pulse. Furthermore, probability of ionization is discussed in terms of the Keldysh adiabaticity parameter, gamma, and the influence of the precursor ionic states present in recollision ionization is observed for the first time.

physics.atom-ph↗