Laboratory astroparticle physics

Abstract:

We present a mechanism for the generation of magnetic field in the early universe prior to the QCD crossover, assuming that the dark matter is made of pseudoscalar axions. Thermoelectric fields arise due to pressure gradients in the primordial plasma resulting from the difference in charge, energy density and equation of state of the quark and lepton components. The axion field is coupled to the EM field and when its spatial gradient is misaligned with the thermoelectric field an electric current is driven. Due to the finite resistivity of the plasma a generally rotational electric field appears. For a QCD axion mass consistent with observational constraints, a magnetic field is thus generated with strength $B \sim 10^{-11 }$ G and characteristic scale $L_B \sim $ 1 pc at present, and viable values for the combination $BL_B^{1/2}$, which is probed in cosmic voids through $\gamma$-ray observations of distant blazars. The amplitude and spatial/temporal scales of the pressure gradients may in principle be inferred through the detection of the concomitant emission of gravitational waves, while experiments are underway to confirm or rule out the existence of axions, with direct consequences for our predictions.

Abstract:

In this paper, we present a model characterizing the interaction of a radiative shock (RS) with a solid material, as described in a recent paper (Koenig et al., Phys. Plasmas, 24, 082707 (2017)), the new model is then related to recent experiments performed on the GEKKO XII laser facility. The RS generated in a xenon gas cell propagates towards a solid obstacle that is ablated by radiation coming from the shock front and the radiative precursor, mimicking processes occurring in astrophysical phenomena. The model presented here calculates the dynamics of the obstacle expansion, which depends on several parameters, notably the geometry and the temperature of the shock. All parameters required for the model have been obtained from experiments. Good agreement between experimental data and the model is found when spherical geometry is taken into account. As a consequence, this model is a useful and easy tool to infer parameters from experimental data (such as the shock temperature), and also to design future experiments.

Abstract:

We present the first experimental measurement of temperature and density of a warm dense plasma produced by a pulsed power driver at the Nevada Terawatt Facility (NTF). In the early phases of discharge, most of the mass remains in the core, and it has been challenging to diagnose with traditional methods, e.g. optical probing, because of the high density and low temperature. Accurate knowledge of the transport coefficients as well as the thermodynamic state of the plasma is important to precisely test or develop theoretical models. Here, we have used spectrally resolved non-collective X-ray Thomson scattering to characterize the dense core region. We used a graphite load driven by the Zebra current generator (0.6 MA in 200 ns rise time) and the Ti He-α line produced by irradiating a Ti target with the Leopard laser (30 J, 0.8 ns) as an X-ray probing source. Using this configuration, we obtained a signal-to-noise ratio ~2.5 for the scattered signal. By fitting the experimental data with predicted spectra, we measured T=2±1.9 eV, ρ=0.6±0.5 gr/cc, 70 ns into the current pulse. The complexity of the dense core is revealed by the electrons in the dense core that are found to be degenerate and weakly coupled, while the ions remain highly coupled.