Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Many astrophysical systems are effectively “collisionless,” that is, the mean free path for collisions between particles is much longer than the size of the system. The absence of particle collisions does not preclude shock formation, however, as shocks can be the result of plasma instabilities that generate and amplify electromagnetic fields. The magnetic fields required for shock formation may either be initially present, for example, in supernova remnants or young galaxies, or they may be self-generated in systems such as gamma-ray bursts (GRBs). In the case of GRB outflows, the Weibel instability is a candidate mechanism for the generation of sufficiently strong magnetic fields to produce shocks. In experiments on the OMEGA Laser, we have demonstrated a quasi-collisionless system that is optimized for the study of the non-linear phase of Weibel instability growth. Using a proton probe to directly image electromagnetic fields, we measure Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows. The collisionality of the system is determined from coherent Thomson scattering measurements, and the data are compared to similar measurements of a fully collisionless system. The strong, persistent Weibel growth observed here serves as a diagnostic for exploring large-scale magnetic field amplification and the microphysics present in the collisional-collisionless transition.

Abstract:

A blind variant of digital holographic microscopy is presented that removes the requirement for a well-characterized, highly divergent reference beam. This is achieved by adopting an off-axis recording geometry where a sequence of holograms is recorded as the reference is tilted, and an iter ative algorithm that estimates the amplitudes and phases of both beams while simultaneously enhancing the numerical aperture. Numerical simulations have demonstrated the accuracy and robustness of this approach when applied to the coherent imaging problem.

Abstract:

A forwardly-peaked bright neutron source was produced using a laser-driven, deuteron-rich ion beam in a pitcher-catcher scenario. A proton-free ion source was produced via target normal sheath acceleration from Au foils having a thin layer of D2O ice at the rear side, irradiated by sub-petawatt laser pulses (∼200 J, ∼750 fs) at peak intensity . The neutrons were preferentially produced in a beam of ∼70 FWHM cone along the ion beam forward direction, with maximum energy up to ∼40 MeV and a peak flux along the axis for neutron energy above 2.5 MeV. The experimental data is in good agreement with the simulations carried out for the d(d,n)3He reaction using the deuteron beam produced by the ice-layered target.

Abstract:

Proton radiography is a technique extensively used to resolve magnetic field structures in high-energy-density plasmas, revealing a whole variety of interesting phenomena such as magnetic reconnection and collisionless shocks found in astrophysical systems. Existing methods of analyzing proton radiographs give mostly qualitative results or specific quantitative parameters, such as magnetic field strength, and recent work showed that the line-integrated transverse magnetic field can be reconstructed in specific regimes where many simplifying assumptions were needed. Using artificial neural networks, we demonstrate for the first time 3D reconstruction of magnetic fields in the nonlinear regime, an improvement over existing methods, which reconstruct only in 2D and in the linear regime. A proof of concept is presented here, with mean reconstruction errors of less than 5% even after introducing noise. We demonstrate that over the long term, this approach is more computationally efficient compared to other techniques. We also highlight the need for proton tomography because (i) certain field structures cannot be reconstructed from a single radiograph and (ii) errors can be further reduced when reconstruction is performed on radiographs generated by proton beams fired in different directions.

Abstract:

The direct laser-acceleration mechanism, nonlinear parametric resonance, of relativistic electrons in a linearly polarized laser-produced plasma channel is examined by a self-consistent model including the relativistic laser dispersion in plasmas. Nonlinear parametric resonance can be excited, and the oscillation amplitude of electrons grows exponentially when the betatron frequency of electron motion varies roughly twice the natural frequency of the oscillator. It is shown analytically that the region of parametric resonance is defined by the self-similar parameter ne/nca0. The width of this region decreases with ne/nca0, but the energy gain and oscillation amplitude increases. In this regime, the electron transverse momentum grows faster than that in the linear classical resonance regime.