Ned Saleh

An optical trap for relativistic plasma

In this paper, we discuss an optical trap capable of confining extremely high density ~close to critical density! and hot ~relativistic! plasmas, of kinetic energy up to 350 keV, by means of the interference of two terawatt-class ~TW! femtosecond laser pulses. In the intersection region of laser beams, the modulated total laser intensity formed ponderomotive potential troughs of subwavelength width ~0.7 mm!, and very high ponderomotive potential gradients, up to 10 12 eV/m. The Thomson scattering, stimulated Raman scattering, analysis, and computer simulation all indicate that the electrons were bunched by the strong ponderomotive force into sheets of thickness two orders of magnitude less than the laser wavelength, and an electron density up to 10 times higher than that of the background n0 . Correspondingly, the stimulated Raman side scattering indicates strong electron density deletion ~0.4% of n0) between the density-bunched regions. An electrostatic field of 10 11 eV/m was produced by the bunched electrons. Unlike the electric field of an electron plasma wave, 9‐12 the electrostatic field in this optical trap was a localized direct-current field, with zero phase velocity and a fixed field direction during the laser beam interference.

Pulse radiolysis of liquid water using picosecond electron pulses produced by a table-top terawatt laser system

A laser based electron generator is shown, for the first time, to produce sufficient charge to conduct time resolved investigations of radiation induced chemical events. Electron pulses generated by focussing terawatt laser pulses into a supersonic helium gas jet are used to ionize liquid water. The decay of the hydrated electrons produced by the ionizing electron pulses is monitored with 0.3 μs time resolution. Hydrated electron concentrations as high as 22 μM were generated. The results show that terawatt lasers offer both an alternative to linear accelerators and a means to achieve subpicosecond time resolution for pulse radiolysis studies.

High contrast 150-terawatt laser for high field laser-plasma interaction studies

Summary form only given. High power chirped pulse amplification (CPA) lasers are capable of providing enough power to study relativistic regime-of laser-plasma interaction, including effects such as high-energy particle acceleration, relativistic self-focusing and nonlinear Thomson scattering. To achieve this regime focused laser intensity must be over 10/sup 18/ W/cm/sup 2/ (typically-10/sup 19/-10/sup 20/ W/cm/sup 2/). If solid metal target is irradiated the laser intensity contrast on the nanosecond timescale must be better than 12 orders of magnitude to ensure that the target does not disintegrate before the pulse arrives. To improve the pulse contrast in the 25 fs CPA laser system, we are currently building for high field laser-plasma studies at CUOS, we pursue an approach, originally developed at CUOS for longer pulses (/spl sim/100 fs). We pre-amplify the oscillator output pulse to submicrojoule energy level without stretching the pulse, improve the pulse contrast by using a saturable absorber and stretch the pulse thereafter for further amplification.

Observation of Relativistic Cross-Phase Modulation in High Intensity Laser-Plasma Interactions

We report a novel nonlinear optical phenomena, relativistic cross-phase modulation between Raman light and a high intensity laser pulse in an underdense plasma. The experimental spectra compared well with calculations from a proposed theoretical model

Evidence of Ionization Blue Shift Seeding of Forward Raman Scattering

We report on the results of spectroscopic experiments that were conducted by focusing an intense ultra‐short laser pulse onto a helium gas target. The scattered light from the interaction region was measured spectrally and spatially from various directions as a function of laser intensity and plasma density. The experimental data showed that forward Stimulated Raman Scattering (SRS) was sensitive to the focus position of laser relative to the nozzle. Together with the plasma channel that was imaged by a CCD camera, the measurements indicate that SRS is seeded by the ionization blue‐shifted light. The cross‐phase modulation between the SRS and laser beam was also observed in the experiment.

Near diffraction limited high-contrast 10 terawatt laser

Summary from only given. We report on further progress in developing a Ti:sapphire high contrast laser, namely reaching a 10 TW milestone. Large output energy of the regenerative amplifier allows us to do this by amplifying the regenerative amplifier output pulse in a single 4-pass amplifier.

Developments in Relativistic Nonlinear Optics

We report recent results of experiments and simulations in the regime of peak laser intensities above 1019 W/cm2, including the following topics: (1) electron and proton acceleration to energies in excess of 10 MeV in well collimated beams; (2) use of laser chirp to control the growth of plasma waves and acceleration of electrons by the Raman instability; (3) all optical injection and acceleration of electrons; (4) relativistic self-focusing by means of the mutual index of refraction of two overlapping laser pulses; (5) creation of a radioisotope by the reaction 10B(d,n)11C; (6) high-order harmonic generation from relativistic free electrons in an underdense plasma.

A Proof‐of‐Principle Experiment of Optical Injection of Electrons in Laser‐Driven Plasma Waves

We report on a proof‐of‐principle experiment that demonstrates, for the first time, the feasibility of optically injecting electrons into laser‐driven plasma waves, first proposed by [D. Umstadter, J.‐K. Kim, and E. Dodd, Phys. Rev. Lett. 76, 2073 (1996)]. Using a Table‐Top‐Tera Watt laser system (I ∼ 5×1018 W/cm2, λ = 1 μm, τ = 400 fs), whose output beam is split by 1:4 ratio into a pump and injection beams, respectively, spatial (within 10 μm) and temporal (within 400 fs) overlap of the two beams were achieved by intersecting them orthogonally in an under‐dense (∼ 4×1019 cm−3), supersonically produced He plasma jet. The interference of the two beams in the plasma, produces an intensity grid, directed along the bisector of the their propagation directions. This causes the plasma electrons to be trapped and periodically bunched in the intersection region, reaching modulation densities an order of magnitude higher than that of the cold relativistic plasma wave‐breaking limit reported so far, and theoretica...

Status of the LILAC Experiment

We present the status of the LILAC experiment, including results on the propagation of 30-fs duration laser pulses in plasmas of the requisite density, and measurements of the dark current. We also discuss the status of a laser upgrade, an electron beam line and plans for the future.

New developments in laser acceleration of beams

We report experimental results in which ultra-short duration (femtosecond) laser pulses from tabletop lasers are focused to intensities above 10/sup 19/ W/cm/sup 2/ onto either gas jets or thin solid-density films. At such extreme electromagnetic field strengths (10/sup 11/ V/cm), plasmas are formed in which the electrons oscillate relativistically, creating gigabar pressure. The displacement of electrons-but not the heavier ions-from the region of the laser focus drives large space-charge fields (exceeding 1 GeV/cm). For laser pulses that are short compared with a plasma period, this takes the form of a wakefield, which accelerates MeV energy beams of electrons. For pulses long compared with a plasma period, we show that a Coulomb explosion accelerates protons (or other ions) to energy in excess of 10 MeV in well-collimated beams. In both cases, not only is this acceleration gradient up to a thousand times greater than in radio-frequency accelerators, but we also found that their transverse geometrical emittances are at least comparable, e.g., up to 10/sup 10/ particles per pulse and divergence angles as low as 1 for electrons and 20 for protons. Additionally, the repetition rate of the electron gun is 10 Hz, a thousand-fold improvement over its past performance. In order to reduce the large electron energy spread, we show experimentally the injection of electrons into a laser-driven plasma wave by use of a separate synchronized laser pulse.