EMP control and characterization on high-power laser systems

Giant electromagnetic pulses(EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers(e.g. the Extreme Light Infrastructure).We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3 D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP,providing an opportunity for comparison with existing charge separation models.

Role of magnetic field evolution on filamentary structure formation in intense laser–foil interactions

Filamentary structures can form within the beam of protons accelerated during the interaction of an intense laser pulse with an ultrathin foil target. Such behaviour is shown to be dependent upon the formation time of quasi-static magnetic field structures throughout the target volume and the extent of the rear surface proton expansion over the same period.This is observed via both numerical and experimental investigations. By controlling the intensity profile of the laser drive,via the use of two temporally separated pulses, both the initial rear surface proton expansion and magnetic field formation time can be varied, resulting in modification to the degree of filamentary structure present within the laser-driven proton beam.

Reflection of intense laser light from microstructured targets as a potential diagnostic of laser focus and plasma temperature

The spatial-intensity profile of light reflected during the interaction of an intense laser pulse with a microstructured target is investigated experimentally and the potential to apply this as a diagnostic of the interaction physics is explored numerically. Diffraction and speckle patterns are measured in the specularly reflected light in the cases of targets with regular groove and needle-like structures, respectively, highlighting the potential to use this as a diagnostic of the evolving plasma surface. It is shown, via ray-tracing and numerical modelling, that for a laser focal spot diameter smaller than the periodicity of the target structure, the reflected light patterns can potentially be used to diagnose the degree of plasma expansion, and by extension the local plasma temperature, at the focus of the intense laser light. The reflected patterns could also be used to diagnose the size of the laser focal spot during a high-intensity interaction when using a regular structure with known spacing.

Bremsstrahlung emission from high power laser interactions with constrained targets for industrial radiography

Laser-solid interactions are highly suited as a potential source of high energy X-rays for nondestructive imaging.A bright,energetic X-ray pulse can be driven from a small source,making it ideal for high resolution X-ray radiography.By limiting the lateral dimensions of the target we are able to confine the region over which X-rays are produced,enabling imaging with enhanced resolution and contrast.Using constrained targets we demonstrate experimentally a(20±3)μm X-ray source,improving the image quality compared to unconstrained foil targets.Modelling demonstrates that a larger sheath field envelope around the perimeter of the constrained targets increases the proportion of electron current that recirculates through the target,driving a brighter source of X-rays.

Effect of rear surface fields on hot, refluxing and escaping electron populations via numerical simulations

After a population of laser-driven hot electrons traverses a limited thickness solid target,these electrons will encounter the rear surface,creating TV/m fields that heavily influence the subsequent hot-electron propagation.Electrons that fail to overcome the electrostatic potential reflux back into the target.Those electrons that do overcome the field will escape the target.Here,using the particle-in-cell(PIC)code EPOCH and particle tracking of a large population of macro-particles,we investigate the refluxing and escaping electron populations,as well as the magnitude,spatial and temporal evolution of the rear surface electrostatic fields.The temperature of both the escaping and refluxing electrons is reduced by 30%–50%when compared to the initial hot-electron temperature as a function of intensity between 1019 and 1021 W/cm^2.Using particle tracking we conclude that the highest energy internal hot electrons are guaranteed to escape up to a threshold energy,below which only a small fraction are able to escape the target.We also examine the temporal characteristic of energy changes of the refluxing and escaping electrons and show that the majority of the energy change is as a result of the temporally evolving electric field that forms on the rear surface.

Effects of target pre-heating and expansion on terahertz radiation production from intense laser-solid interactions

The first experimental measurements of intense(～7 × 1019 W cm-2) laser-driven terahertz(THz) radiation from a solid target which is preheated by an intense pulse of laser-accelerated protons is reported. The total energy of the THz radiation is found to decrease by approximately a factor of 2 compared to a cold target reference. This is attributed to an increase in the scale length of the preformed plasma, driven by proton heating, at the front surface of the target,where the THz radiation is generated. The results show the importance of controlling the preplasma scale length for THz production.