Session JO7: Short Pulse Laser Plasma Interactions

Electron heating mechanisms for a relativistic intensity laser pulse interacting with a near-critical plasma

Two and three dimensional particle-in-cell simulations have been performed to understand the electron heating mechanisms taking place when a picosecond duration, relativistically intense laser pulse interacts with a near-critical density plasma. Results from experiments using the Titan laser (LLNL) interacting with very low-density foam targets indicated that from very overdense targets the electron temperature, $T_{e}$, was close to the expected ponderomotive potential energy, but an increase in $T_{e}$ was observed as the plasma density was reduced towards the critical density. Numerical modeling will be presented to show that the differences in the electron heating for different target densities can be understood in terms of the complex interplay between the laser fields and evolving plasma fields as the interaction progresses.   

Study of spatio-temporal dynamics of laser-hole boring in near critical plasma

At high-intensities of light, radiation pressure becomes one of the dominant mechanisms in laser-plasma interaction. The radiation pressure of an intense laser pulse can steepen and push the critical density region of an overdense plasma creating a cavity or a hole. This hole boring phenomenon is of importance in fast-ignition fusion, high-gradient laser-plasma ion acceleration, and formation of collisionless shocks. Here multi-frame picosecond optical interferometry is used for the first direct measurements of space and time dynamics of the density cavity as it is pushed forward by a train of CO$_{\mathrm{2}}$ laser pulses in a helium plasma. The measured values of the hole boring velocity into an overdense plasma as a function of laser intensity are consistent with a theory based on energy and momentum balance between the heated plasma and the laser and with two-dimensional numerical simulations. We show possibility to extract a relative plasma electron temperature within the laser pulse by applying an analytical theory to the measured hole boring velocities.   

Enhanced ion acceleration in the transition regime from opaque to transparent plasmas

Using particle in cell (PIC) simulations, we investigate ion acceleration in high-intensity laser-plasma interactions in targets that become transparent during the interaction process. A theoretical model is developed to derive an optimal target electron areal density `n.L' as a function of laser normalized intensity and the pulse duration in the laser transparent regime. A large schematic parametric scan for a wide range of target electron density (n) and thickness (L) is performed and shown to be consistent with analytical theory. Ion acceleration in optimal conditions relies on the re-heating of the expanding sheath electrons by the laser and enhancing the Target Normal Sheath Acceleration (TNSA) electric field after the plasma becomes transparent to the laser light. This enhanced TNSA field decays slower compared to conventional TNSA resulting in significantly higher proton energies.   

Shock-Wave Acceleration of Protons on OMEGA EP

Recent experimental results using shock-wave acceleration (SWA) driven by a CO$_{2}$ laser in a H$_{2}$ gas-jet plasma have shown the possibility of producing proton beams with energy spreads \textless 10{\%} and with energies of up to 20 MeV using a modest peak laser power of 4 TW.\footnote{D. Haberberger \textit{et al.}, Nature Phys., \textbf{8}, 95 (2012).} Here we propose the investigation of the scaling of the SWA mechanism to higher laser powers using the 1-$\mu $m OMEGA EP Laser System at the Laboratory for Laser Energetics. The required tailored plasma profile is created by expanding a CH target using the thermal x-ray emission from a UV ablated material. The desired characteristics optimal for SWA are met: (a) peak plasma density is overcritical for the 1-$\mu $m main pulse and (b) the plasma profile exponentially decays over a long scale length on the rear side. Results will be shown using a 4$\omega $ probe to experimentally characterize the plasma density profile. Scaling from simulations of the SWA mechanism\footnote{F. Fiuza \textit{et al}., Phys. Rev. Lett. \textbf{109}, 215001 (2012).} shows that ion energies in the range of 100 MeV/amu are achievable with a focused $a_{0}$ of 5 from the OMEGA~EP Laser System. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Proton shock acceleration using a high contrast high intensity laser

Laser-driven proton acceleration is a field of intense research due to the interesting characteristics of this novel particle source including high brightness, high maximum energy, high laminarity, and short duration. Although the ion beam characteristics are promising for many future applications, such as in the medical field or hybrid accelerators, the ion beam generated using TNSA, the acceleration mechanism commonly achieved, still need to be significantly improved. Several new alternative mechanisms have been proposed such as collisionless shock acceleration (CSA) in order to produce a mono-energetic ion beam favorable for those applications. We report the first results of an experiment performed with the TITAN laser system (JLF, LLNL) dedicated to the study of CSA using a high intensity (5x10$^{19}$W/cm$^{2})$ high contrast ps laser pulse focused on 55 $\mu$m thick CH and CD targets. We show that the proton spectrum generated during the interaction exhibits high-energy mono-energetic features along the laser axis, characteristic of a shock mechanism.   

Late-time plasma dynamics in the relativistic transparency regime via self-generated fields

The interaction of a high intensity laser with ultra-thin foils and low density foams often results in relativistic transparency. In this regime, the volumetric laser-plasma interaction, upon the onset of the relativistic transparency, converts the laser energy into electrons' thermal and directional motion with increased efficiency. The thermal motion leads to further target expansion while the directional motion is responsible for a longitudinal electron current that emerges at the backside of the target. The electron sheath from the expanded target and the current on the order of the Alfv\'{e}n limit set up large self-generated quasi-static plasma electric and magnetic fields. When the laser exists the plasma, the accelerated bulk ions can further interact with the electron flow sustained by the front sheath field and regulated by the self-generated azimuthal magnetic and longitudinal electric fields. We present detailed Particle-In-Cell simulations that reveal such electron-ion coupling assisted by the self-generated plasma fields in this second step, leading to high energy ions with smaller energy spread that may be useful for various applications.   

Efficient quasi-monoenergetic ion beams up to 18 MeV/nucleon via self-generated plasma fields in relativistic laser plasmas

Table-top laser-plasma ion accelerators seldom achieve narrow energy spreads, and never without serious compromises in efficiency, particle yield, etc. Using massive computer simulations, we identify a self-organizing scheme that exploits persisting self-generated plasma electric ($\sim$ TV/m) and magnetic ($\sim$ 10$^{4}$ Tesla) fields to reduce the ion energy spread after the laser exits the plasma -- separating the ion acceleration from the energy spread reduction. Consistent with the scheme, we experimentally demonstrate aluminum and carbon ion beams with narrow spectral peaks at energies up to 310 MeV (11.5 MeV/nucleon) and 220 MeV (18.3 MeV/nucleon), respectively, with high conversion efficiency ($\sim$ 5{\%}, i.e., 4J out of 80J laser). This is achieved with 0.12 PW high-contrast Gaussian laser pulses irradiating planar foils with optimal thicknesses of up to 250 nm that scale with laser intensity. When increasing the focused laser intensity fourfold (by reducing the focusing optic f/number twofold), the spectral-peak energy increases twofold. These results pave the way for next generation compact accelerators suitable for applications. For example, 400 MeV (33.3 MeV/nucleon) carbon-ion beam with narrow energy spread required for ion fast ignition could be generated using PW-class lasers.   

Proton acceleration enhanced by a plasma jet in expanding foils undergoing relativistic transparency

The interaction of a sufficiently intense laser pulse with an ultrathin target can induce it to rapidly transition from an overdense to a relativistically underdense plasma. In recent years many insights have been made into aspects of this regime - from the onset of transparency itself, to the spatial profile of electrons accelerated and ion acceleration. We present an experimental study of laser-ion acceleration in this regime that demonstrates the complex interplay between mechanisms including sheath fields, radiation pressure and transparency-driven field enhancements. This is experimentally demonstrated by separating signature components within the proton beam. Using PIC simulations, it is shown that a plasma jet is formed during the transition to transparency resulting in higher laser energy absorption to electrons and enhanced ion acceleration. The final ion energy is demonstrated to be highly sensitive to the picosecond rising edge profile of the laser pulse.   

Collisional and collision-less surface heating in intense laser matter interaction

We explore the interaction of high-contrast intense sub-100 fs laser pulses with solid density tar- gets, using numerically converged collisional particle-in-cell simulations in one two and three dimen- sions. We observe a competition between two mechanisms that can lead to plasma heating. Inverse bremsstrahlung at solid density on one hand, and electrons scattering off plasma waves on the other, can both heat the skin layer to keV temperatures on a femtosecond time scale, facilitating a heat wave and a source of MeV electrons that penetrate and heat the bulk target. Collision-less effects heat the surface effectively starting at the relativistic intensity threshold, independent of plasma density. Our numerical results show that a high-contrast 1J/100fs laser can drive a solid target into the warm dense matter regime. This system is suitable to ab-initio modeling and experimental probing.   

Measurements of Fast Magnetic Reconnection Driven by Relativistic Electrons

Magnetic reconnection is a process whereby opposing magnetic field lines are forced together and topologically rearrange, resulting in lower magnetic potential energy and in corresponding plasma heating. Such occurrences are ubiquitous in astrophysics as well as appearing in laboratory plasmas such as in ICF in the form of instabilities. We report measurements in the domain of ultra-fast, ultra-intense lasers, in which the mechanism responsible follows from radially expanding surface electrons with $v \approx c$. Results are compared from two laser facilities (HERCULES and Omega EP), both of which produced two relativistic intensity pulses focused within close proximity onto copper foils. A spherical X-ray crystal was used to image the $K_{\alpha}$ radiation induced by electron currents, revealing the midplane diffusion region wherein electrons are accelerated into the target by the electric field generated during reconnection. The characteristics of this signal are studied as a function of the focal spot separation, laser energy, and pulse duration. The results are then compared to 3D PIC simulations.   

Controlling the dynamics of a femtosecond laser-driven shock in hot dense plasma

We present the dependence of the dynamics of a plasma super-critical layer on the laser intensity contrast in the regime of intense femtosecond laser-solid interaction. Time-resolved pump-probe diagnostics reveal the interplay of inward shock strength and laser contrast of a femtosecond laser at an intensity of $10^{18}$ W cm$^{-2}$. The measurements show that the pulse with 2 orders of magnitude higher intensity contrast than that with a usual lower contrast one ($\sim 10^{-5}$) launches the shock-like disturbance (into the target) having 10 times more speed. This observation is further supplemented by employing an external prepulse (for manipulating the preplasma scale length) which helps to control the inward motion of the critical surface. This opens up the possibility of controlling the inward moving shock disturbance and leads to medical, science and engineering applications.   

Supersonic Propagation of a K-Shell Ionization Front in Metal Targets

The supersonic propagation of a K-shell ionization front has been measured in high-energy-density metal targets using 1-D monochromatic streaked x-ray imaging. The ionization front was driven by hot electrons generated by a 10-ps laser pulse focused to an intensity of $10^{18}$ W/cm$^{2}$. The data show the ionization front travelling at $0.11c \pm 0.02c$. The measurements are in good agreement with implicit-hybrid particle-in-cell and collisional-radiative code calculations that predict the hot-electron transport and the K-shell ionization front dynamics. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

X-ray backlit Talbot-Lau Moir\'{e} deflectometry

Talbot-Lau interferometer is a refraction-based diagnostic capable of characterizing highly localized electron density gradients in High Energy Density (HED) plasma through phase shift measurements. Talbot-Lau X-ray Deflectometry (TXD) allows for simultaneous acquisition of attenuation, refraction, elemental composition, and scatter information from a single x-ray image. The TXD diagnostic, previously benchmarked in the laboratory using continuous x-ray illumination [M. P. Valdivia, et al., Rev. Sci. Instrum. 85, 073702 (2014)], was tested using a pulsed x-ray source at the Multi-TeraWatt (MTW) facility at LLE for future applications as a HED diagnostic. A Talbot-Lau interferometer composed of free standing and ultrathin gratings, used the copper K-shell emission at 8 keV from a 30 J, 8 ps laser pulse focused on a 500 x 500 $\mu$m$^{2}$ Cu target of 20 $\mu$m thickness. The spatial resolution achieved was limited by the x-ray source size. Future experiments will use smaller x-ray sources (1-10 $\mu$m). These experiments demonstrated that the source grating can survive and high contrast images can be recorded. TXD images from test objects, as well as the electron density profiles retrieved from the interferograms with a 10{\%} accuracy, will be shown.   

Monochromatic x-ray radiography of laser-driven spherical targets using high-energy, picoseconds LFEX laser

Formation of a high density fusion fuel is essential in both conventional and advanced Inertial Confinement Fusion (ICF) schemes for the self-sustaining fusion process. In cone-guided Fast Ignition (FI), a metal cone is attached to a spherical target to maintain the path for the injection of an intense short-pulse ignition laser from blow-off plasma created when nanoseconds compression lasers drive the target. We have measured a temporal evolution of a compressed deuterated carbon (CD) sphere using 4.5 keV K-alpha radiography with the Kilo-Joule, picosecond LFEX laser at the Institute of Laser Engineering. A 200 $\mu$m CD sphere attached to the tip of a Au cone was directly driven by 9 Gekko XII beams with 300 J/beam in a 1.3 ns Gaussian pulse. The LFEX laser irradiated on a Ti foil to generate 4.51 Ti K-alpha x-ray. By varying the delay between the compression and backlighter lasers, the measured radiograph images show an increase of the areal density of the imploded target. The detail of the quantitative analyses to infer the areal density and comparisons to hydrodynamics simulations will be presented. This work was performed with the support and under the auspices of the NIFS Collaboration Research program (NIFS13KUGK072). H.S. was supported by the UNR's International Activities Grant program.