Byoung-ick Cho

Hot electron generation from intense laser irradiation of microtipped cone and wedge targets

X-ray production from the interaction of femtosecond laser pulses focused to relativistic intensity into re-entrant targets etched into silicon has been investigated. Kα and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though Kα is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell simulations.

Controlled reproducible alignment of cone targets and mitigation of preplasma in high intensity laser interactions

The use of cone targets in high intensity laser-plasma experiments has been of recent interest because of their potential use in integrated fast ignition experiments. Simpler experiments provide a good avenue for understanding the underlying physics, however precise control of the alignment along with good pointing accuracy is of crucial importance. While on big laser facilities target alignment is done precisely with several microscopes, it is not always the case on smaller facilities. This can have a detrimental effect on the quality of the results. We have developed and characterized a method for accurate alignment of intense laser pulses into a cone target. This, along with optimal positioning of the focus compared to the tip, efficiently uses the shape of the target to microfocus the laser light and concentrates the hot electrons in the tip, and can mitigate preplasma issues.

Study of hot electron transport in foil, wedge, and cone targets irradiated with ultraintense laser pulses

We investigate hot electron transport in foil, wedge, and cone targets irradiated with ultraintense femtosecond laser pulses by observing the transition radiation emitted from the targets rear side. Two-dimensional images and spectra of coherent transition radiation along with x-ray detection have revealed the spatial, temporal, and temperature characteristics of hot electron micropulses. Various patterns from different target and laser configurations suggest that hot electrons were guided by the strong static electromagnetic fields at the target boundary.

Pyramidal Targets as an Advanced Radiation Source in Laser-Solid Interactions

A novel target with hollow pyramidal shape is studied as a radiation source of electrons, protons, and Kα x-rays. Experiments compare this source to flat foil and cone targets under similar laser conditions.

WE‐E‐330D‐01: The Production of Ultrafast Bright K‐Alpha X‐Rays From Laser Produced Plasmas for Medical Imaging

Purpose: To show the potential of improving image quality with a cleaner, brighter, quasi‐monochromatic X‐ray micro‐source via laser produced plasmas(LPP).Method and Materials: First generation targets consisting of ten micron thick gold formed into free‐standing pyramids have been built. PIC (Particle‐In‐Cell) simulations have been performed in order to validate this target geometry. Preliminary experiments with a Ti‐Sapphire CPA laser have been achieved with these targets. Second generation parabolic cone targets with an optimal angle for electron transport have also been built. This new nano‐fabricated target could optimize X‐ray source characteristics. Results:PIC (Particle‐In‐Cell) simulations show that conical targets optically guide laser light resulting in a higher density of hot electrons at the apex These simulations show a possible ten times augmentation in hot electron density and a three times increase in electron temperature with a conical verses flat target. This increase in collimated suprathermal electrons boosts total photon yield as well as possibly enhancing line emission verses the bremsstrahlung continuum. Preliminary experiments demonstrate a three‐fold higher X‐ray yield and a two‐fold reduction in focal spot with the pyramidal verses the flat target. Furthermore, the geometry of the conical targets not only reduces focal spot size to a few microns and pulse duration to a couple picoseconds, but allows the particles to escape the target perpendicular to the surface resulting in a particle‐free, ultra‐short X‐ray micro‐beam. Conclusion: Comparing LPP X‐ray source parameters to that of a standard X‐ray tube shows substantial improvements in focal spot size, photon flux, spectral range and emission duration. Focusing on target design can provide a cleaner, brighter, quasi‐monochromatic X‐ray source that could improve image quality in any medicaldiagnostic regime. Such advancements show promising applications in mammography and angiography. Conflict of Interest: Research sponsored by DOE/NNSA under University of Nevada Reno grant #DE‐FC52‐01NV14050.

Single-shot measurements of ultrafast dynamics in a dense plasma

The ultrafast dynamics in a dense plasma were studied with a single-shot measurement of the time-dependent reflectivity and phase shift at the rear surface of a free standing aluminum foil following femtosecond irradiation.

Observation of hot electron beams in intense laser-solid interactions characterized with coherent transition radiation

Coherent transition radiation was observed from aluminum foils irradiated by an ultra-intense laser. Comparison of experiments and theoretical calculations indicated two streams of hot electron micro-pulses were generated by resonance absorption and jtimesB heating.

Development of third harmonic generation as a short pulse probe of shock heated material

We are studying high‐pressure laser produced shock waves in silicon (100). To examine the material dynamics, we are performing pump‐probe style experiments utilizing 600 ps and 40 fs laser pulses from a Ti:sapphire laser. Two‐dimensional interferometry reveals information about the shock breakout, while third harmonic light generated at the rear surface is used to infer the crystalline state of the material as a function of time. Sustained third harmonic generation (THG) during a ∼100 kbar shock breakout indicate that the rear surface remains crystalline for at least 3 ns. However, a decrease in THG during a ∼300 kbar shock breakout suggests a different behavior, which could include a change in crystalline structure.

Study of hot electron transportation in foils and wedge targets irradiated with ultrashort laser pulses

We investigated the hot electron transport in foil and wedge shaped targets irradiated with ultra-intense laser pulses. The results suggest that the electrons are guided by the strong quasi-static electromagnetic fields at the wedge boundary.

Equation of state measurements on solid density matter isochorically heated by a laser-accelerated proton beam

A laser-accelerated beam of protons is used to isochorically heat a silicon foil to multi-eV temperatures. Proton dosimetry, self-emission, and synchronized reflective interferometry are used simultaneously to infer internal energy, temperature, and pressure.