Speaker
Description
The use of seed magnetic-fields (B-fields) in laser-driven target-compression experiments may lead to > 10 kT B-fields across the compressed core due to advection of the in-flow plasma. B-fields exceeding 10 kT are promising for magneto-inertial fusion since they reduce electron thermal conduction perpendicular to the field lines and may even increase alpha-particle energy deposition in the hot spot. Studying the formation of these compressed B-fields may also improve our understanding of extreme plasma magnetization phenomena relevant to astrophysics or extended magnetohydrodynamics.
In order to reach compressed B-fields exceeding 10 kT, one important challenge is to generate strong seed B-fields on major laser facilities. Where external pulsed power hardware is not available, we can use laser-driven coil (LDC) targets to supply a multi-tesla quasi-static field. These targets allow easy access for diagnostics and do not produce a significant quantity of debris.
We have tested LDCs on several different nanosecond laser facilities under laser drive conditions relevant to the Laser MegaJoule (LMJ). The goal was to predict the B-fields that might be achieved on LMJ by benchmarking a laser-driven diode model of B-field generation [1]. At the LULI2000 and OMEGA facilities we used comparable laser intensities, ~1015 -1016 W/cm2, at 1.06µm and 0.35µm wavelengths respectively. We generated discharge currents of ~20 kA and ~8 kA yielding B-fields of ~50 T and ~6 T respectively, with targets of different size (and inductance).
Where possible, magnetic fields were measured using proton deflectometry directed along two axes of the target. Comparing our experimental deflectograms with proton tracking simulations enables us to identify various deflection features that can be linked to the looping current or static charging of the coil’s wire surface. Measured discharge currents are broadly consistent with predictions from our model for all the experimentally tested conditions, which give grounds for the successful use of LDCs on large-scale facilities like LMJ [2].