Experimental studies of high-energy-density magnetized plasmas : in route to magneto-inertial fusion

Overview & Synopsis

This thesis is devoted to the exploration of magnetized high-energy-density (HED) plasmas driven by laser along with the understanding of the physics involved in the magnetic field (B-field) generation using laser-driven coil targets (LDCs). The latter is one of the possible strategies to magnetize HED plasmas in facilities without external pulsed power.One of the main axes was the dimensioning and the deployment of laser-driven cylindrical implosion experiments under seed B-fields to study magnetization effects on the implosions and, more specifically, on the transport of energy and magnetic flux in HED plasmas. The thesis presents K-shell emission X-ray spectra obtained from the radial implosion of Ar-doped D2-filled cylinders at the OMEGA laser, with 15 kJ of UV laser drive, that exhibit noticeable changes when a B-field is present, as compared to when it is not. This work was supported, through collaborations, by extended magnetohydrodynamics (MHD) and atomic physics simulations modeling the implosions, namely the transport of energy, magnetic flux and radiation.A multi-emission zone analysis of the experimental spectra allows for quantitative measurements of the plasma conditions in the core, indicating a 50% increase in temperature at half the mass density, when a 30 T seed field is applied. Besides, the predictions from atomic physics post-processing of extended-MHD simulations closely match the experimental spectra, providing compelling evidence that the core conditions at peak compression are in line with the impact of a 10 kT compressed field. The findings provide a foundation for verifying magnetized transport models in highly compressed plasmas – namely to test the microphysics involved in the transport coefficients used in the MHD macroscopic models – and to a more robust design of bigger scale experiments at LMJ and NIF.The other main axis of the thesis was the further understanding of seed B-field generation from LDCs. The thesis presents new measurements of such B-fields driven by high-energy laser pulses of sub-nanosecond or multiple nanosecond durations, either at IR or UV wavelengths, with typical intensities of the order of 1015 W/cm2.At the LULI2000 facility, the B-fields were characterized by proton deflectometry in two perpendicular probing axes. We succeeded to identify effects of both magnetic and electric fields on the probing protons, therefore quantifying both coil discharge current (of well identified polarity) and static charge at different times. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped RL circuit model, supporting the quasi-steady assessment of the discharges. Peak B-fields of 50 T at the center of 0.5 mm diameter coils were obtained at the explored, rather moderate, laser intensity.At the PALS facility, we obtained a more complete physical picture of the active current-driving region in LDCs, that is the laser interaction region. We simultaneously measured the B-field near the coil (by optical polarimetry) and the self-generated B-field in the laser-produced plasma plume (by complex interferometry), along with a thorough characterization of the plasma particle sources and ground discharges through the target holder. We discuss the correlation between the coil current and the plasma plume current across the plates, inferred respectively from the above B-field measurements. This platform allowed to optimize peak B-fields for different target geometrical parameters.Finally, we also discuss B-field measurements from LDCs driven by UV laser light at the OMEGA and LMJ facilities, as a prelude of their potential use for magnetizing large-scale HED plasma experiments.Different theoretical models for the evolution of LDC-generated B-fields are confronted with the experimental results of the various experiments.