Session PO7: Intense Laser Interactions

Observation of spectral modulations in laser ion acceleration from underdense plasmas

Ions have been accelerated radially by irradiating a supersonic helium gas jet ($400\mu m$ diameter) with ultrashort, relativistic laser pulses ($I\lambda^{2}>10^{18}\mathrm{Wcm^{-2}\mu m^{2}}$). Accelerated ions have been measured over a wide range of densities ($0.01 n_{c} < n_{e} < 0.1 n_{c}$). Strong modulations in the ion spectra have been observed, depending on the interaction conditions. Whereas Monte-Carlo particle tracking indicates recombination of ions accelerated by Coulomb explosion, due to charge exchange with neutral gas, detailed 2D1/2 PIC simulations suggest the trapping and acceleration of ions from plasma edges by radially propagating waves. These results evidence new insights on the laser gas interaction in this density range.   

Narrow energy spread, 25MeV protons from the interaction of a time-structured CO$_{2}$ laser pulse with a gas target

Experimental results and 2D OSIRIS simulations of laser-driven proton acceleration from the interaction of a time-structured 10$\mu $m CO$_{2}$ laser pulse train and a gaseous target are presented. The wavelength of a CO$_{2}$ laser provides a unique opportunity to change the target density from 0.5 to 5n$_{cr}$ in a controlled manner by changing the H$_{2}$ gas jet pressure. The CO$_{2}$ laser pulses consist of a train of 3ps pulses separated by 18ps with a peak power of $\sim $4TW and a total energy of $\sim $50J. The initial results show the production of proton energies of up to 25MeV, which far exceeds that predicted by ponderomotive force scaling for an a$_{o}\sim $2. Furthermore, in the density range around 2n$_{cr}$, these high energy protons are contained within a narrow energy spread of $\Delta $E/E $\sim $ 10{\%}. These results are attributed to the unique time structure of the CO$_{2}$ laser pulses, underdense LPI's such as self-focusing due to large P/P$_{cr}$ values, and profile steepening/hole boring.   

Energetic ion production using a 5 TW Ti:sapphire Laser interacting with an underdense plasma

From laser-plasma interactions it is now possible to generate energetic particles such as MeV ions and GeV energy electrons using the short-pulse high-intensity lasers available. In this work we present an experimental study where energetic ions were produced in an underdense $\sim $1x10$^{19}$ cm$^{-3}$ plasma created by a 50 fs Ti:Sapphire laser with 5 TW's of power. The acceleration mechanism is mainly based on the generation of a longitudinal electric field at the plasma-vacuum boundary created by the charge separation and the current produced by accelerated electrons. We are exploring if the higher energy electrons produced by laser induced wakes can in turn produce higher energy ions than the usual scaling found using solid targets. The physics of the interaction is studied with 2D and 3D particle-in-cell simulations. Work supported by DOE grants DE-FG02-92ER40727, NSF grants PHY-0936266 and FCT grant SFRH/BD/37838/2007.   

Laser-driven Ion-, electron- and photon-beams from relativistically overdense plasmas

As one of the main tools for the experimental investigation of relativistic plasmas in the laboratory, ultrahigh intensity lasers have seen rapid growth with ever extremer parameters of energy and pulse duration. At peak powers, already exceeding 10$^{22}$ W/cm$^2$, in virtually every experiment in relativistic laser physics, the laser pulse interacts with a more or less extended and heated plasma, due to prepulses and ASE. By drastically improving this contrast, we initiated a paradigm shift in relativistic laser-matter interactions, allowing us to interact ultrarelativistic pulses volumetrically with overdense targets, that will turn relativistically transparent during the few 10s -- 100s fs of the interaction. Specifically, we increased the contrast of the 200TW Trident laser to better than 2x10$^{-12}$ at 500ps and better than 1$^{-7}$ at 5ps enabling an interaction with overdense targets between 3 to 300nm. This volumetric overdense interaction enables new particle acceleration mechanisms for both electrons and ions, as well as forward directed relativistic surface harmonics. In first experiments we were able to experimentally demonstrate a new ion acceleration mechanisms, the Break-Out Afterburner, reaching carbon energies of $>$0.5GeV and proton energies $>$65MeV. This work was supported by the DOE OFES and by the DFG through LMUexcellent.   

The Shaped Critical Surface in High Intensity Laser-Plasma Interactions

The interaction between an intense laser and under-dense plasma involves relativistic effects and instabilities that can drastically alter pulse properties. Characterization of the plasma, particularly the density profile, is paramount in understanding what effects dominate. Simulations of the reflection of light off of plasma show that the formation of the relativistically shaped critical surface, dependent on both the laser intensity and plasma density profiles, plays an important role in the divergence of the specularly reflected light. We have found that these effects can be modeled as a simple problem in Gaussian optics, with the plasma acting as a combination of lenses and mirrors. This work suggests that by measuring the divergence of the specularly reflected light, the plasma density profile can be determined in regions where shadographic and interferometric techniques are limited.   

Single-cycle relativistic pulse generation by laser foil interaction

A scheme producing nearly single-cycle relativistic laser pulses of wavelength 800 nm is proposed. When a laser pulse interacts with an overdense thin foil, the latter will be more transparent to the more-intense part of the laser, so that a transmitted pulse can be much shorter than the incident pulse. It is found that a transmitted pulse of duration 4 fs and peak intensity 3x10$^{20}$ W/cm$^{2}$ can be generated [1] When two counter-propagating circularly polarized (CP) pulses interact with an overdense foil, the driving pulse (with larger laser field amplitude) will accelerate the whole foil, and the scattered pulse (with smaller laser field amplitude) is reflected by this flying-layer. Due to the Doppler Effect and varying velocity of the layer, the reflected pulse is up-shifted for frequency and highly chirped, thus could be compressed to nearly single-cycled relativistic laser pulse with short wavelength. Simulations show that a sub-femtosecond nearly single-cycled relativistic pulse can be generated with wavelength of 0.2$\mu $m after dispersion compensation. [2] \\[4pt] [1] L. Ji, B. Shen et al., Phys. Rev. Lett. 103, 215005 (2009) \\[0pt] [2] L. Ji, B. Shen et al., Phys. Rev. Lett. 105, 025001 (2010)   

Simulations of efficient Raman amplification into the multi-Petawatt regime

The laser architectures being considered for the Extreme Light Infrastructure (ELI) facility are based upon solid state lasers which are very successful in providing petawatt peak powers to target. The breakdown threshold for optical components in these systems, however, demands meter-scale beams. For a number of years, Raman amplification, an approach mostly free of breakdown problems, has promised a breakthrough by the use of much smaller amplifying media, i.e. (millimetre diameter wide) plasmas, but to date, only 60 GW peak powers have been obtained in the laboratory, far short of the desired multi-petawatt regime. Here we show, through the first large scale multi-dimensional particle-in-cell simulations of this process, that multi-petawatt peak powers can be reached only in a narrow parameter window dictated by the growth of plasma instabilities. The control of these instabilities promises greatly reduced costs and complexity of intense lasers, allowing much greater access to higher intensity regimes for fundamental science and industrial applications. Furthermore, it is shown that this process scales to short wavelengths allowing compression of free electron laser pulses to attosecond duration.   

Radiation reaction effects in the laser-charged particle interaction

During the interaction with ultra-high intensity laser pulses, an electron experiences a strong influence from the self-emitted radiation. This process may strongly affect the experiments being planned with next generation lasers (e.g., mulitple laser pulses with peak powers $>$100 PW and with a combined focused intensity $>$ 10$^{25}$ W/cm$^{2})$, which will also lead to the investigation of new regimes of laser pulse interaction not available before. We study the dependence of the radiation reaction effects on the electromagnetic pulse polarization for different pulse configurations. This dependence will fully manifest itself at extremely high intensities. For circularly polarized colliding pulses, the EM avalanche of photons and electron-positron pairs will immediately follow the production of a single pair at focus, dominating both the charged particle motion and the pulse evolution. We show that, in contrast, for linearly polarized colliding pulses of the same total energy, the effects of radiation reaction are much weaker.   

Stimulation of Nonlinear Optical Effects in Vacuum by Increasing Lorentz Invariants

In a regime of nonlinear quantum electrodynamics, vacuum becomes a nonlinear optical medium in strong electromagnetic fields due to the polarization generated by virtual electron-positron pairs. We have investigated nonlinear optical effects in vacuum induced by ultra-intense laser beam theoretically. Nonlinear polarization in vacuum is calculated by the use of the effective Lagrangian density presented by Heisenberg, Euler and Schwinger, and the representation of polarization contains Lorentz Invariants. As a result of calculations, it is found that lower f-number focusing optics enhances nonlinear polarization and the photon number generated by the interaction between vacuum and laser field. This result shows that fast focusing optics dramatically decreases the laser intensity to detect the nonlinear optical properties of vacuum.   

Laser Compton Cooling of Relativistic Electron and Positron Beams and Pair Plasmas

With the advance of high energy intense lasers, it becomes conceivable to use photons to slow down relativistic electron and positron beams, or rapidly cool a relativistic electron-positron pair plasma. Here we present results from the Monte Carlo simulations of the Compton cooling of relativistic electrons and positrons using intense lasers of one micron wavelength. We find that several hundred kJ to a MJ of laser energy is sufficient to Compton cool multi-MeV electrons/positrons down to keV energies and below. We also explore the use of resonant Compton cooling in a strong magnetic field (100 MG and above). Preliminary results using Doppler shifted laser light look promising.   

Observation of radiation pressure driven Rayleigh-Taylor instability

Radiation pressure acceleration (RPA)$\left[1\right]$ uses the high pressures of a high intensity laser pulses to accelerate low mass targets to high energy with potentially low energy spreads. However, simulations have shown susceptibility to the radiation pressure driven Rayleigh-Taylor instability $\left[1,2\right]$. Here we report on experiments performed with the VULCAN Petawatt laser at the Rutherford-Appleton Laboratory using intensities up to $10^{21}$~Wcm$^{-2}$ in linear polarisation. Peaks within the C$^{6+}$ spectra indicate RPA acceleration of the foils. Most remarkably though, strong spatial modulation is observed in the recorded proton beams. The form and extent of these modulations suggest Rayleigh-Taylor break-up of the foil. Simulations provide further support for these conclusions.\\ $\left[ 1 \right]$ A.P.L. Robinson et al., NJP, \textbf{10}, 013021 (2008)\\ $\left[ 2 \right]$ F. Pegoraro et al., PRL, \textbf{99}, 065002 (2007)