Andrew McKelvey

Measurements of the energy spectrum of electrons emanating from solid materials irradiated by a picosecond laser

In this work, we present the results of experiments observing the properties of the electron stream generated laterally when a laser irradiates a metal. We find that the directionality of the electrons is dependent upon their energies, with the higher-energy tail of the spectrum (∼1 MeV and higher) being more narrowly focused. This behavior is likely due to the coupling of the electrons to the electric field of the laser. The experiments are performed by using the Titan laser to irradiate a metal wire, creating the electron stream of interest. These electrons propagate to nearby spectator wires of differing metals, causing them to fluoresce at their characteristic K-shell energies. This fluorescence is recorded by a crystal spectrometer. By varying the distances between the wires, we are able to probe the divergence of the electron stream, while by varying the medium through which the electrons propagate (and hence the energy-dependence of electron attenuation), we are able to probe the energy spectrum of the stream.

Target surface area effects on hot electron dynamics from high intensity laser–plasma interactions

Reduced surface area targets were studied using an ultra-high intensity femtosecond laser in order to determine the effect of electron sheath field confinement on electron dynamics. X-ray emission due to energetic electrons was imaged using a Ka imaging crystal. Electrons were observed to travel along the surface of wire targets, and were slowed mainly by the induced fields. Targets with reduced surface areas were correlated with increased hot electron densities and proton energies. Hybrid Vlasov-Fokker-Planck simulations demonstrated increased electric sheath field strength in reduced surface area targets.

X-Ray imaging of ultrafast magnetic reconnection driven by relativistic electrons

Evidence of magnetic reconnection (MR) events driven by relativistic electrons is observed between two high-intensity laser/plasma interaction sites. The two laser foci were on average 20um FWHM containing 50TW of power each, delivered with a split f/3 paraboloid onto copper foil targets at a focused intensity of 1019 W/cm2 with the HERCULES laser system. Cu K-alpha emissions from the interactions were imaged with a spherically bent Quartz crystal, and by motorizing one half of the paraboloid vertically the focal separation was varied between 0- 400um. Splitting the beam halves revealed an enhanced region between the foci with the highest a maximized K-alpha signal intensity at one inter-beam separation, evidencing inflow from relativistic electron driven MR. A filtered LANEX screen was imaged to search for outflow/jet electrons along the plane of the target surface and normal to the axis defined by the two spots, to calculate the electron temperature and to search for spatial profile nonuniformities potentially directly originating from reconnection events. Ongoing 2D and 3D PIC simulations are being conducted to better understand and model the measured electron outflow dynamics.

Counter-HPM window experiments and theory

Summary form only given. Microwave windows that protect sensitive electronics from high power microwaves are important to military and civilian applications. We are testing a microwave window with crosspolarized, inter-digitized conducting strips that are biased at moderate (100s V) to high voltage (kVs) DC. The goal is to determine whether the microwave breakdown threshold can be controlled by argon gas pressure and DC bias voltage. DC Paschen curves have been measured in argon for these window-structures. Experiments are underway to expose the window-structures to high power (10-100 MW) microwaves in a single output waveguide of the UM relativistic magnetron facility. Theory and simulations will utilize previous multipactor susceptibility curves for perpendicular and parallel DC fields to evaluate the effect of DC fields on multipactor breakdown of windows.