Session GO7: Laser Plasma Ion Acceleration

Towards 100 MeV maximum energy for laser-accelerated proton beams

Since the discovery of the target normal sheath acceleration mechanism, laser-driven ion acceleration has been a field of very active research. Despite numerous investigations to optimize laser and target conditions the initially observed maximum proton energy of about 60 MeV can neither be reproduced routinely with up to date laser systems nor has this limit been exceeded significantly. At the same time simulations show promising results for alternative mechanisms based on ultrathin targets and high temporal contrast. We report on results of an experimental campaign at the PHELIX laser. Using micrometer thick plastic targets and laser intensities on the order of 10$^{20}$W/cm$^{2}$ we achieved monotonically decreasing proton spectra with cutoff energies in excess of 85 MeV and particle numbers of 10$^{9}$ in an energy bin of 1 MeV around this maximum. For a certain match of laser and target conditions we could also observe angular separation of proton beams accelerated via different mechanisms. In this contribution we define the experimental conditions that enable such high energy maxima and particle numbers and we discuss the limitations that prevent maximum energies of hundreds of MeV as predicted by simulations.   

Energy gain and spectral tailoring of ion beams using ultra-high intensity laser beams

The field of laser driven ion acceleration over the past decade has produced a huge amount of research. Nowadays, several multi-beam facilities with high rep rate system, e.g. ELI, are being developed across the world for different kinds of experiments. The study of interaction dynamics of multiple beams possessing ultra-high intensity and ultra-short pulse duration is of vital importance. Here, we present the first experimental results on ion acceleration using two ultra-high intensity beams. Thanks to the unique capability of Arcturus laser at HHU D\"{u}sseldorf, two almost identical, independent beams in laser parameters such as intensity (\textgreater 10$^{20}$ W/cm$^{2}$), pulse duration (30\textit{fs}) and contrast (\textgreater 10$^{10}$), could be accessed. Both beams are focused onto a 5 $\mu$m thin Ti target. While ensuring spatial overlap of the two beams, at relative temporal delay of $\sim$ 50\textit{ps} (optimum delay), the proton and carbon ion energies were enhanced by factor of 1.5. Moreover, strong modulation in C$^{4+}$ions near the high energy cut-off is observed later than the optimum delay for the proton enhancement. This offers controlled tailoring of the spectral content of heavy ions.   

Acceleration of energetic ions using 40 femtosecond laser pulses and ultrathin targets

We report on experiments conducted using the 40 fs HERCULES laser at intensity 3x10$^{20}$ W/cm$^{2}$ with sub-micron Si$_{3}$N$_{4}$ and pure metal foils. The target thickness scan shows that the ion species distribution transitions from low ionization states of protons and carbon to high ionization states of carbon and substrate ions such as Si$^{12+}$ when the thickness is reduced incrementally from 1300 nm to 50 nm. The change in thickness also results in dramatic increase in maximum energy and particle number. The ion beam generation characteristics were improved for thicknesses 50-150 nm. Targets with thicknesses 35 nm and below yielded similar high charge state ions and high maximum energy yet reduced particle number. The results are consistent with 2D PIC modeling where realistic laser and target parameters were used [1]. This work was performed with the support of the Air Force Office of Scientific Research under grant FA9550-14-1-0282. \\[4pt] [1] G. Petrov, ``Proton acceleration from short pulse lasers interacting with ultrathin foils,'' this meeting.   

Demonstrated Efficient Quasi-Monoenergetic Carbon-Ion Beams Approaching Fast Ignition (FI) Requirements

Using massive computer simulations of relativistic laser-plasma interactions, we have identified 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 of intense laser-driven ion beams after the laser exits the plasma [1]. Consistent with the scheme, we have demonstrated on the LANL Trident laser carbon-ion beams with narrow spectral peaks at 220 MeV, with high conversion efficiency ($\approx $ 5{\%}) [1]. These parameters are within a factor of 2 of FI requirements [2]. The remaining gap may be bridged by increasing the laser intensity by a factor of 4, according to our data [1]. We also discuss how this beam may be focused, to address the remaining requirement for FI, besides the total laser energy. \\[4pt] [1] S. Palaniyappan, et al$.$, \textit{Efficient quasi-monoenergetic ion beams up to 18 MeV/nucleon via self-generated plasma fields in relativistic laser plasmas, arXiv}:1506.07548v1; S. Palaniyappan, et al., this conference\\[0pt] [2] J.C. Fern\'{a}ndez, et al., \textit{Fast ignition with laser-driven proton and ion beams.} Nuclear Fusion, \textbf{54}(5), 054006 (2014)   

Computational study of transport and energy deposition of intense laser-accelerated proton beams in solid density matter

With intense proton beams accelerated by high power short pulse lasers, solid targets are isochorically heated to become partially-ionized warm or hot dense matter. In this regime, the thermodynamic state of the matter significantly changes, varying the proton stopping power where both bound and free electrons contribute. Additionally, collective beam-matter interaction becomes important to the beam transport. We present self-consistent hybrid particle-in-cell (PIC) simulation results of proton beam transport and energy deposition in solid-density matter, where the individual proton stopping and the collective effects are taken into account simultaneously with updates of stopping power in the varying target conditions and kinetic motions of the beam in the driven fields. Broadening of propagation range and self-focusing of the beam led to unexpected target heating by the intense proton beams, with dependence on the beam profiles and target conditions. The behavior is specifically studied for the case of an experimentally measured proton beam from the 1.25 kJ, 10 ps OMEGA EP laser transporting through metal foils. This work was supported by the U.S. DOE under Contracts No. DE-NA0002034 and No. DE-AC52-07NA27344 and by the U.S. AFOSR under Contract FA9550-14-1-0346.   

Spectral Features in Laser Driven Proton Acceleration from Cylindrical Solid-density Hydrogen Jets

The generation of monoenergetic proton beams by ultrashort high-intensity laser-plasma interactions is of great interest for applications such as stopping power measurements, fast ignition laser confinement fusion, and ion beam therapy. In general, the commonly used mechanism of target normal sheath acceleration (TNSA) does not provide the required energy spread or maximum proton energy. Here we study alternative acceleration mechanisms, which have been identified in particle in cell (PIC) simulations, to overcome the limitations of TNSA. Using the Titan laser system at the Lawrence Livermore National Laboratory, we investigate proton acceleration from wire targets and a cryogenic solid-density hydrogen jet. Due to the cylindrical geometry, TNSA is suppressed allowing other accelerations mechanisms to become observable. Quasi-monoenergetic features in laser-forward direction are observed in the proton spectrum indicating radiation-pressure-driven acceleration mechanisms. Our experimental results are accompanied by supporting PIC simulations.   

Laser-driven proton and deuteron acceleration from a pure solid-density H2/D2 cryogenic jet

Laser-driven proton acceleration has become of tremendous interest for the fundamental science and the potential applications in tumor therapy and proton radiography. We have developed a cryogenic liquid hydrogen jet, which can deliver a self-replenishing target of pure solid-density hydrogen or deuterium. This allows for a target compatible with high-repetition-rate experiments and results in a pure hydrogen plasma, facilitating comparison with simulations. A new modification has allowed for the formation of jets with rectangular profiles, facilitating comparison with foil targets. This jet was installed at the Titan laser and driven by laser pulses of 40-60 J of 527 nm laser light in 1 ps. The resulting proton and deuteron spectra were measured in multiple directions with Thomson parabola spectrometers and RCF stacks. The spectral and angular information suggest contribution from both the TNSA and RPA acceleration mechanisms.   

Effect of the Rayleigh-Taylor-instability on radiation-pressure-accelerated protons from solid-density hydrogen jets

Proton beams generated by relativistic laser-plasma interactions are of great interest in warm dense matter research due to applications such as isochoric heating and stopping power measurements. Radiation pressure acceleration (RPA) from pure hydrogen targets is a promising approach towards developing low emittance beams with high particle flux, one of the key requirements for above studies. We developed a novel target utilizing cryogenic hydrogen jets at solid densities for ion acceleration experiments. Using the 150 TW laser system DRACO at HZDR we measured pure proton spectra exceeding 10 MeV for peak intensities of $~5x10^{20}$ W/cm$^2$ at a repetition rate of 1 Hz. The proton beam shows a net-like structure. The experimental results will be discussed with the support of particle-in-cell simulations to assess the impact of the Rayleigh-Taylor-instability on radiation-pressure-accelerated protons   

Self-aligning concave relativistic plasma mirror with ultrafast adjustable focus

Plasma mirrors (PMs) excited at sub-relativistic intensity (\textless 10$^{18}$W/cm$^{2}$) are widely used to improve the temporal contrast of ultrashort laser pulses that are subsequently focused to ultra-relativistic intensity. However, new applications demand PMs that reflects efficiently with high beam quality when excited directly at relativistic intensity. We report a quantitative laboratory study of space-/time-integrated and space-/time- resolved reflectivity of PMs excited by high-contrast, 30 fs, 800 nm relativistically intense laser pulses. We observe high reflectivity (\textgreater 0.8) for intensities up to 5x10$^{18}$W/cm$^{2}$, provided laser contrast exceeds 10$^{4}$ at 1 ps and angle of incidence is less than 5$^{\circ}$. Particle-in-cell simulations suggest that sharp drops observed outside these limits are caused by refocusing of reflected light outside the collection optics due to depression of the reflecting surface by light pressure (deformation, usually a concave curvature) and self-induced relativistic transparency. Furthermore, the reflected relativistic intensity can be enhanced multiple times and the second focus position can be adjusted in the range of few tens of micron away from PM surface by controlling the contrast at 1 ps.   

Optimized ion acceleration using high repetition rate, variable thickness liquid crystal targets

Laser-based ion acceleration is a widely studied plasma physics topic for its applications to secondary radiation sources, advanced imaging, and cancer therapy. Recent work has centered on investigating new acceleration mechanisms that promise improved ion energy and spectrum. While the physics of these mechanisms is not yet fully understood, it has been observed to dominate for certain ranges of target thickness, where the optimum thickness depends on laser conditions including energy, pulse width, and contrast. The study of these phenomena is uniquely facilitated by the use of variable-thickness liquid crystal films, first introduced in P. L. Poole \textit{et al. PoP} \textbf{21}, 063109 (2014). Control of the formation parameters of these freely suspended films such as volume, temperature, and draw speed allows on-demand thickness variability between 10 nanometers and several 10s of microns, fully encompassing the currently studied thickness regimes with a single target material. The low vapor pressure of liquid crystal enables in-situ film formation and unlimited vacuum use of these targets. Details on the selection and optimization of ion acceleration mechanism with target thickness will be presented, including recent experiments on the Scarlet laser facility and others.   

Liquid crystal film development for plasma mirrors and waveplates

Many laser-plasma phenomena currently under study depend critically on the quality of the pulse contrast. Costly sacrificial plasma mirrors are now commonly used to improve the temporal laser contrast before target interaction, especially for ion acceleration where high contrast is necessary to achieve interesting new mechanisms. Liquid crystal films were originally developed as variable thickness thin-film targets, and were demonstrated for this purpose in P. L. Poole \textit{et al.} \textit{PoP} \textbf{21}, 063109 (2014). Varying film formation parameters such as volume, temperature, and draw speed allows thickness control between 10 nm and several 10s of microns, in-situ and under vacuum. Development since that initial work has allowed large area films to be formed, several cm$^{\mathrm{2}}$ in extent, with the same thickness range. The molecular flatness of a freely suspended film renders these films excellent low-cost plasma mirrors, given appropriate formation control. Additionally, the birefringence of the liquid crystal used here permits these films to be used as large area zero-order waveplates at the appropriate thickness. Details on the current state of liquid crystal film application development, including a \textgreater 1 Hz small area film formation device, will be presented. This work was performed with support from the DARPA PULSE program through a grant from AMRDEC and by the NNSA under contract DE-NA0001976.   

Laser acceleration of monoenergetic protons with a near-critical, optically-shaped gas target

Laser-based ion acceleration is studied using the intense terawatt CO2 laser pulse with a near-critical hydrogen gas target. The gas density profile is tailored by a hydrodynamic shock, which is launched by ablation of solid with a moderate-energy, nanosecond Nd:YAG laser pulse in the vicinity of the gas jet. A sharp density gradient is thus created near the edge of the gas column, resulting to $\sim$ 6X local density enhancement up to several times of critical density within $\alt$ 100 micrometers before CO2 laser pulse arrives. With such density profile, we have observed quasi-monoenergetic proton beams with energies \textgreater 1 MeV and good shot-to-shot reproducibility. In contrast, no protons were observed when the hydrodynamic shock is absent. Results from experiments and simulations will be presented. This work is supported by U.S. Department of Energy.   

Enhanced Ion Acceleration from Micro-tube Structured Targets

We present an enhanced ion acceleration method that leverages recent advancements in 3D printing for target fabrication. Using the three-dimensional Particle-in-Cell simulation code Virtual Laser-Plasma Lab (VLPL), we model the interaction of a short pulse, high intensity laser with a micro-tube plasma (MTP) structured target. When compared to flat foils, the MTP target enhances the maximum proton energy by a factor of about 4. The ion enhancement is attributed to two main factors: high energy electrons extracted from the tube structure enhancing the accelerating field and light intensification within the MTP target increasing the laser intensity at the location of the foil. We also present results on ion energy scaling with micro-tube diameter and incident laser pulse intensity.   

Proton Accelerated from a Laser Driven Z-Pinch

We will present experimental and numerical results of intense laser acceleration of protons from a sharp near critical density plasma-vacuum interface. Protons were accelerated from a hydrodynamically tailored gaseous hydrogen target using the 10TW TFL laser at NRL. At sufficiently high plasma densities (\textgreater 7x10$^{21}$ cm$^{-3}$), the observed proton beam characteristics were consistent with the Target Normal Sheath Acceleration mechanism. At lower densities (\textless 7x10$^{21}$ cm$^{-3}$), the protons were characterized by a \textless 700keV axial beam with a high-energy halo and energies approaching 2MeV. 3D PIC simulations indicate that these energetic protons result from a laser driven Z-pinch that collapses at the plasma-vacuum interface. Further experimental results and laser-plasma scaling will be discussed.   

A novel parametric instability of propagating critical layer affecting the laser longitudinal envelope

A parametric instability that affects the longitudinal envelope of a laser pulse interacting with a propagating critical layer is presented [Sahai,PoP 21,056707, 2014; Sahai, arXiv:1411.2401, 2014]. It is shown that non-linear mixing between the incident and reflected laser pulse from a propagating critical layer electron compression results in a beat-wave with a complete modulation of the incident wave envelope. This beat-wave modulates the velocity of the propagating critical layer, resulting in a new Doppler frequency which creates a second beat-wave, further modulating the laser envelope. The frequency spread of the laser envelope grows in time resulting in a large spectral spread of the laser pulse envelope. The velocity of the propagating critical layer acceleration structure is correspondingly modulated as is the space-charge potential. Thus, the ions that are accelerated off the potential have a large energy spread. Since, the growth rate of this instability depends upon the acceleration structure velocity, longer pulses are unfavorable for accelerating ions to higher energies with a narrow energy spread. This instability is also relevant to laser-driven fusion and laser hole-boring based fast-ignition but due to much smaller velocities, its effect is mitigated.