James Easter

Temperature scaling of hot electrons produced by a tightly focused relativistic-intensity laser at 0.5 kHz repetition rate

The energy spectrum of hot electrons emitted from the interaction of a relativistically intense laser with an Al plasma is measured at a repetition rate of 0.5 kHz by accumulating ∼103 highly reproducible laser shots. In the 1017–2×1018 W/cm2 range, the temperature of electrons escaping the plasma along the specular direction scales as (Iλ2)0.64±0.05 for p-polarized pulses incident at 45°. This scaling is in good agreement with three-dimensional particle-in-cell simulations and a simple model that estimates the hot-electron temperature by considering the balance between the deposited laser intensity and the energy carried away by those electrons.

Vacuum-free x-ray source based on ultrashort laser irradiation of solids

A vacuum-free ultrafast laser-based x-ray source is demonstrated. Hard x-rays up to 80KeV are generated from Cu, Mo, Ag, Sn, and Ge targets in a laminar helium flow surrounded by atmosphere using tightly focused 33fs, 3mJ laser pulses. X-ray spectra, conversion efficiencies, and source sizes are presented. Six-fold efficiency improvement is observed, over similar sources found in the literature [1]. Source sizes determined for Cu and Mo show distinct dependences on laser pulse energy. It is also shown that the Cu source size has no dependence on the presence of the spectral band around the 8KeV K-shell lines.

Laser-ion acceleration through controlled surface contamination

In laser-plasma ion accelerators, control of target contamination layers can lead to selection of accelerated ion species and enhancement of acceleration. To demonstrate this, deuterons up to 75 keV are accelerated from an intense laser interaction with a glass target simply by placing 1 ml of heavy water inside the experimental chamber prior to pumping to generate a deuterated contamination layer on the target. Using the same technique with a deuterated-polystyrene-coated target also enhances deuteron yield by a factor of 3 to 5, while increasing the maximum energy of the generated deuterons to 140 keV.

Directional properties of hard x-ray sources generated by tightly focused ultrafast laser pulses

Directional properties of ultrafast laser-based hard x-ray sources are experimentally studied using tightly focused approximately millijoule laser pulses incident on a bulk Mo target. Energy distributions of Kα and total x rays, as well as source-size distributions are directionally resolved in vacuum and in flowing helium, respectively. Directional distributions of x-ray emission is more isotropic for p-polarized pump than for s-polarized. Based on source-size measurements, a simple two-location model, with expanded plasma and bulk material, is employed to represent the x-ray source profile.

Generation of hard x rays by femtosecond laser pulse interaction with Cu in laminar helium flow in ambient air

Hard x rays are generated in laminar helium flow in atmosphere (without a vacuum vessel) through the interaction of tightly focused 35fs laser pulses of varying energy with a Cu target. The energy conversion efficiency from laser to Kα x rays is measured to be 5.4×10−6, giving a flux of 5.9×109photons∕s into 2π sr from a source measuring approximately 9×12μm2 in size (vertical×horizontal). Results are compared to those measured in vacuum and in static helium environments.

High-order harmonic generation from solid targets with 2 mJ pulses

Harmonics up to the 18th order are generated from solid targets by focusing 2 mJ, 50 fs pulses at 800 nm to a spot size of 1.7 μm (FWHM). To our knowledge, this is the first demonstration of high-harmonic generation with a very short focal length paraboloid (f/1.4) and kilohertz laser system. The harmonics have a low divergence (<4°) compared to the driving beam and conversion efficiencies (>10(-7) per harmonic) comparable to gas harmonics. No contrast enhancement techniques are employed, and the system is capable of operating at 500 Hz.

Angular emission and polarization dependence of harmonics from laser–solid interactions

Laser plasma interaction experiments have been performed to characterize high order harmonic emission up to the 18th order using high rep rate mJ level laser pulses at relativistic intensities. The experiments were compared to two- and three-dimensional particle-in-cell simulations. The harmonic divergence was found to be less than 4° (full-width at half-maximum) at highest intensity and increased as the laser was defocused (i.e. as the intensity was reduced). The polarization dependence on the harmonic generation efficiency and divergence was also measured. Circular polarization was found to cause a deflection in the angle of emission of the harmonics—an effect which may be beneficial in the use of such harmonics for efficient isolated attosecond pulse production.

Compressor optimization with compressor-based multiphoton intrapulse interference phase scan (MIIPS)

The multiphoton intrapulse interference phase scan (MIIPS) technique is modified to optimize the compressor settings of a chirped pulse amplification (CPA) laser system. Here, we use the compressor itself to perform the phase scan inherent in MIIPS measurement . A frequency-resolved optical gating measurement shows that the pulse duration of the compressor optimized using the modified MIIPS technique is 33.8 fs with a 2.24 rad temporal phase variation above 2% intensity. The measured time-bandwidth product is 0.60, which is close to that of transform-limited Gaussian pulse (0.44).

Divergence of high order harmonic emission from intense laser interactions with solid targets

Laser plasma interaction reveals harmonic divergence <;4 degrees and increasing with defocus. Circular polarization was found to cause a change in the angle of emission, which may be beneficial in efficient isolated attosecond pulse production.

High-repetition rate wakefield electron source driven by few-millijoule ultrashort laser pulses

10mJ ultrafast laser pulses acting on gas flow from capillary nozzles generate stable 100keV electron beams at 500Hz. Particle-in-cell simulations indicate that slow high amplitude plasma waves form and accelerate the electron beams.