Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The remarkable discovery by the Chandra X-ray observatory that the Crab nebula's jet periodically changes direction provides a challenge to our understanding of astrophysical jet dynamics. It has been suggested that this phenomenon may be the consequence of magnetic fields and magnetohydrodynamic instabilities, but experimental demonstration in a controlled laboratory environment has remained elusive. Here we report experiments that use high-power lasers to create a plasma jet that can be directly compared with the Crab jet through well-defined physical scaling laws. The jet generates its own embedded toroidal magnetic fields; as it moves, plasma instabilities result in multiple deflections of the propagation direction, mimicking the kink behaviour of the Crab jet. The experiment is modelled with three-dimensional numerical simulations that show exactly how the instability develops and results in changes of direction of the jet.

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 most established route to create a laser-based neutron source is by employing laser accelerated, low atomic-number ions in fusion reactions. In addition to the high reaction cross-sections at moderate energies of the projectile ions, the anisotropy in neutron emission is another important feature of beam-fusion reactions. Using a simple numerical model based on neutron generation in a pitcher–catcher scenario, anisotropy in neutron emission was studied for the deuterium–deuterium fusion reaction. Simulation results are consistent with the narrow-divergence (∼70° full width at half maximum) neutron beam recently served in an experiment employing multi-MeV deuteron beams of narrow divergence (up to 30° FWHM, depending on the ion energy) accelerated by a sub-petawatt laser pulse from thin deuterated plastic foils via the Target Normal Sheath Acceleration mechanism. By varying the input ion beam parameters, simulations show that a further improvement in the neutron beam directionality (i.e. reduction in the beam divergence) can be obtained by increasing the projectile ion beam temperature and cut-off energy, as expected from interactions employing higher power lasers at upcoming facilities.