Proton radiography of a shock-compressed target

In this paper we report on the radiography of a shock-compressed target using laser produced proton beams. A low-density carbon foam target was shock compressed by long pulse high-energy laser beams. The shock front was transversally probed with a proton beam produced in the interaction of a high intensity laser beam with a gold foil. We show that from radiography data, the density profile in the shocked target can be deduced using Monte Carlo simulations. By changing the delay between long and short pulse beams, we could probe different plasma conditions and structures, demonstrating that the details of the steep density gradient can be resolved. This technique is validated as a diagnostic for the investigation of warm dense plasmas, allowing an in situ characterization of high-density contrasted plasmas.

Electromagnetic pulse compression and energy localization in quantum plasmas

The evolution of the intensity of a relativistic laser beam propagating through a dense quantum plasma is investigated, by considering different plasma regimes. A cold quantum fluid plasma and then a thermal quantum description(s) is (are) adopted, in comparison with the classical case of reference. Considering a Gaussian beam cross-section, we investigate both the longitudinal compression and lateral/longitudinal localization of the intensity of a finite-radius electromagnetic pulse. By employing a quantum plasma fluid model in combination with Maxwell's equations, we rely on earlier results on the quantum dielectric response, to model beam-plasma interaction. We present an extensive parametric investigation of the dependence of the longitudinal pulse compression mechanism on the electron density in cold quantum plasmas, and also study the role of the Fermi temperature in thermal quantum plasmas. Our numerical results show pulse localization through a series of successive compression cycles, as the pulse propagates through the plasma. A pulse of 100 fs propagating through cold quantum plasma is compressed to a temporal size of approximate to 1.35 attosecond and a spatial size of approximate to 1.08 10(-3) cm. Incorporating Fermi pressure via a thermal quantum plasma model is shown to enhance localization effects. A 100 fs pulse propagating through quantum plasma with a Fermi temperature of 350 K is compressed to a temporal size of approximate to 0.6 attosecond and a spatial size of approximate to 2.4 10(-3) cm. (c) 2010 Elsevier B.V. All rights reserved.

The plasma mirror - A subpicosecond optical switch for ultrahigh power lasers

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers ~TW–PW!. The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csDt,lLaser must be met (cs : sound speed, Dt: time from plasma formation to the peak of the pulse!. The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.

Measurements of relativistic self-phase-modulation in plasma

We report the first systematic observations of relativistic self-phase-modulation (RSPM) due to the interaction of a high intensity laser pulse with plasma. The plasma was produced in front of a solid target by the prepulse of a 100 TW laser beam. RSPM was observed by monitoring the spectrum of the harmonics generated by the intense laser pulse during the interaction. The multipeaked broadened spectral structure produced by RSPM was studied in plasmas with different density scale lengths for laser interactions at intensities up to 3.0x1019 W cm(-2) (a=p(osc)/m(e)c=4.7). The results are compared with calculated spectra and agreement is obtained.

Measurements of negative ion densities by absorption spectroscopy

A novel diagnostic technique for the measurement of negative ions is presented. The absorption of a cw-laser beam with a wavelength close to the maximum of the photodetachment cross section is measured. The laser beam undergoes multiple reflections through the discharge volume using a multipass cell there a Herriott arrangement). Proof-of-principle measurements of negative hydrogen ions in a hybrid volume source are presented. The measured H- density is in agreement with results of conventional, Langmuir probe-based photodetachment measurements. (C) 1998 American Institute of Physics. [S0003-6951(98)02119-6].

Comparison of plasma transport theory with measurements in a H- discharge

This paper and its companion paper describe the comparison between a one-dimensional theoretical model of a hydrogen discharge in a magnetic multipole plasma source and experimental measurements of the plasma parameters. The discharge chamber, described here, has been designed to produce significant densities of H- ions by incorporating a weak transverse field through the discharge to obtain electron cooling so as to maximize H- production. Langmuir probes are used to monitor the plasma, determining the ion density, the electron density and temperature and the plasma potential. The negative density is measured by photo-detachment of the extra electron using an intense laser beam. The model, described in the companion paper, uses the presented source geometry to calculate these plasma quantities as a function of the major are parameters; namely the are current and voltage and gas pressure. Good agreement is obtained between theory and experiment as a function of position and arc parameters.

Optical Cooling of Atoms in Microtraps by Time-Delayed Reflection

We present a theoretical analysis of a novel scheme for optical cooling of particles that does not in principle require a closed optical transition. A tightly confined laser beam interacting with a trapped particle experiences a phase shift, which upon reflection from a mirror or resonant microstructure produces a time-delayed optical potential for the particle. This leads to a nonconservative force and friction. A quantum model of the system is presented and analyzed in the semiclassical limit.

IMAGING OF SURFACE-PLASMON LAUNCH AND PROPAGATION USING A PHOTON SCANNING TUNNELING MICROSCOPE

The spectroscopic capability of the photon scanning tunneling microscope is exploited to study directly the launch and propagation of surface plasmons on thin silver films. Two input beams, of different wavelength, are incident through the prism in a prism-Ag film-air-fibre tip system. Both excite surface plasmons at the Ag-air interface and light of both wavelengths is coupled into the fibre probe via the respective surface plasmon evanescent fields. One laser beam is used for instrument control. The second, or probe beam is tightly focused on the sample, within the area of the unfocused or control beam, giving a well-defined and symmetrical, confined surface plasmon launch site. However, the image at the probe wavelength is highly asymmetrical in section with an exponential tail extending beyond one side of the launch site. This demonstrates in a very direct fashion;the propagation of surface plasmons; a propagation length of similar to 11.7 mu m is measured at a probe wavelength of 543.5 nm. On rough Ag films the excitation of localised scattering centres is also observed in addition to the launch of delocalised surface plasmons.

INTERPLAY OF CAF2 SUBSTRATE ROUGHENING AND FILM DEPOSITION RATE ON RADIATIVE SCATTERING OF SURFACE-PLASMON POLARITONS FROM AG FILMS

The radiative decay of surface plasmon polaritons has been investigated in an attempt to characterize the surface roughness of Ag films prepared under different conditions. The polaritons were excited by the method of attenuated total reflection of light. The films were deposited on the face of a 60-degrees BK-7 glass prism at a rate that was deliberately fixed in two different ranges (centred on 0.1 and 10 nm s-1) and in some cases a CaF2 underlayer was used to roughen the film surfaces. The intensity of the scattered light emitted from the opposite face of the films was measured as a function of direction for each using the same sensitivity scale and was correlated with the preparation of the film. It was found that on nominally smooth substrates fast-deposited thinner films give out more light and are deduced to have greater short wavelength (300-600 nm) roughness amplitude. There is also evidence for long wavelenth (7 mum) periodic roughness due to the prism substrate itself. On CaF2 roughened surfaces the light output from the films is further increased and the peak intensity is backward directed with respect to the exciting laser beam direction. Here roughness on a lateral scale of 350 nm is responsible. Also, elastic scattering of surface plasmon polaritons at grain boundaries reduces the light output from fast deposited, small grain, films on CaF2 roughened surfaces. Overall, a consistent picture of roughness induced radiative polariton decay emerges for all cases studied.

Scaling of Ion Spectral Peaks in the Hybrid RPA-TNSA Region

The role of the radiation pressure of an intense laser beam in the formation of proton and carbon spectra from thin foils is discussed. The data presented suggests that, in competition with the Target Normal Sheath Acceleration mechanism, the onset of the Light Sail (LS) region of Radiation Pressure Acceleration can be obtained for suitably thin targets at currently available laser intensities,. The spectral features and their scaling with the laser and target parameters are consistent with the scenario of Light Sail (LS) acceleration.

XFEL resonant photo-pumping of dense plasmas and dynamic evolution of autoionizing core hole states

Similarly to the case of LIF (Laser-Induced Fluorescence), an equally revolutionary impact to science is expected from resonant X-ray photo-pumping. It will particularly contribute to a progress in high energy density science: pumped core hole states create X-ray transitions that can escape dense matter on a 10 fs-time scale without essential photoabsorption, thus providing a unique possibility to study matter under extreme conditions. In the first proof of principle experiment at the X-ray Free Electron Laser LCLS at SCLAC [Seely, J., Rosmej, F.B., Shepherd, R., Riley, D., Lee, R.W. Proposal to Perform the 1^st High Energy Density Plasma Spectroscopic Pump/Probe Experiment", approved LCLS proposal L332 (2010)] we have successfully pumped inner-shell X-ray transitions in dense plasmas. The plasma was generated with a YAG laser irradiating solid Al and Mg targets attached to a rotating cylinder. In parallel to the optical laser beam, the XFEL was focused into the plasma plume at different delay times and pump energies. Pumped X-ray transitions have been observed with a spherically bent crystal spectrometer coupled to a Princeton CCD. By using this experimental configuration, we have simultaneously achieved extremely high spectral (λ/δλ ≈ 5000) and spatial resolution (δx≈70 μm) while maintaining high luminosity and a large spectral range covered (6.90 - 8.35 Å). By precisely measuring the variations in spectra emitted from plasma under action of XFEL radiation, we have successfully demonstrated transient X- ray pumping in a dense plasma.

Fast-beam study of H2+ ions in an intense femtosecond laser field

A fast beam of H-2(+) ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused laser intensity and angular distributions of neutral dissociation products have been measured. Significant differences are observed between the present results and those obtained from experiments involving neutral H-2 molecules. These differences are indicative of the precursor H-2 molecule playing an important and hitherto neglected formative role in the laser-induced fragmentation processes.

Laser-Driven Proton Beams: Acceleration Mechanism, Beam Optimization, and Radiographic Applications

This paper reviews recent experimental activity in the area of optimization, control, and application of laser accelerated proton beams, carried out at the Rutherford Appleton Laboratory and the Laboratoire pour l’Utilisation des Lasers Intenses 100 TW facility in France. In particular, experiments have investigated the role of the scale length at the rear of the plasma in reducing target-normal-sheath-acceleration acceleration efficiency. Results match with recent theoretical predictions and provide information in view of the feasibility of proton fast-ignition applications. Experiments aiming to control the divergence of the proton beams have investigated the use of a laser-triggered microlens, which employs laser-driven transient electric fields in cylindrical geometry, enabling to focus the emitted
protons and select monochromatic beam lets out of the broad spectrum beam. This approach could be advantageous in view
of a variety of applications. The use of laser-driven protons as a particle probe for transient field detection has been developed and
applied to a number of experimental conditions. Recent work in this area has focused on the detection of large-scale self-generated magnetic fields in laser-produced plasmas and the investigation of fields associated to the propagation of relativistic electron both on the surface and in the bulk of targets irradiated by high-power laser pulses.

Stable GeV Ion Beam Acceleration from Thin Foils by Circularly Polarized Laser Pulses

A stable relativistic ion acceleration regime for thin foils irradiated by circularly polarized laser pulses is suggested. In this regime, the "light-sail" stage of radiation pressure acceleration for ions is smoothly connected with the initial relativistic "hole-boring" stage, and a defined relationship between laser intensity I(0), foil density n(0), and thickness l(0) should be satisfied. For foils with a wide range of n(0), the required I(0) and l(0) for the regime are theoretically estimated and verified with the particle-in-cell code ILLUMINATION. It is shown for the first time by 2D simulations that high-density monoenergetic ion beams with energy above GeV/u and divergence of 10 degrees are produced by circularly polarized lasers at intensities of 10(22) W/cm(2), which are within reach of current laser systems.