Jonathan Grava

Multiply ionized carbon plasmas with index of refraction greater than one

Over the last decade, X-ray lasers in the wavelength range14 - 47 nm have been used to do interferometry of plasmas. Just as for optical interferometry of plasmas, the experimental analysis assumed that the index of refraction is due only to the free electrons. This makes the index of refraction less then one. Recent experiments in Al plasmas have observed fringe lines bend the wrong way as though the electron density is negative. We show how the bound electrons can dominate the index of refraction in many plasmas and make the index greater than one or enhance the index such that one would greatly overestimate the density of the plasma using interferometry.

Soft X-Ray Laser Interferometry of a Dense Plasma Jet

Soft X-ray laser interferograms were acquired to map the evolution of a dense plasma jet created by the laser irradiation of a solid copper triangular target. The plasma is observed to rapidly expand along the symmetry plane of the target, forming a narrow plasma plume with measured electron densities of up to 1.2times1020 cm-3.

Soft x-ray laser interferometry of colliding plasmas

We have used soft x-ray laser interferometry to study dense colliding plasmas produced by laser irradiation of semi-cylindrical targets. Results are reported on the evolution of 1 mm long plasmas created by heating 500 μm diameter half holhraum copper targets with an intensity of ~1.6 1012 W.cm-2 from 120 ps duration laser pulses of 800 nm wavelength. The setup combines a robust high throughput amplitude division interferometer based on diffraction gratings with a 46.9 nm table-top capillary discharge laser. Series of high contrast interferograms were obtained depicting the evolution of the copper plasmas into a localized plasma that reaches densities above 1×1020 cm-3 when the plasmas collide near the center of the cavity. The technique allows the generation of high resolution density maps of colliding plasma with various degree of collisionality for comparison with code simulations.

Soft x-ray laser interferometry study of dense plasma jet collimation

Soft x-ray laser interferometry and hydrodynamic simulations were used to study the increase in collimation of laboratory plasma jets created with low energy (⇐ 1 J) short laser pulses irradiating target of varying atomic number.

Elucidating the collimation of laboratory plasma jets using soft x-ray interferometry

The mechanisms responsible for an increase in collimation of laboratory plasma jets with higher atomic number was studied using soft x-ray laser interferometry and 2D model simulations. Dense plasma jets (Ne~ 1020 cm-3) were produced by irradiating V-shaped grooves of different materials (C, Al, and Cu) with 120 ps Ti:Sa laser pulses at peak intensities of 1 x 1012 W cm-2. High contrast soft x-ray interferograms of these plasmas were generated by combining a Mach-Zehnder interferometer that uses diffraction gratings as beam-splitters and a 46.9 nm table-top capillary discharge laser probe. A significant increase in jet collimation was observed for the higher Z materials. Simulations performed with the radiation hydrodynamic code HYDRA attribute differences in jet collimation to an increased radiation cooling of the higher Z jets.

Plasma interactions in laser irradiated semi-cylindrical cavities studied with soft x-ray interferometry using a capillary discharge laser

Soft x-ray interferometry was used to measure the evolution of dense converging plasmas created by laser irradiation of 500 μm diameter semi-cylindrical carbon targets. Optical laser pulses with an intensity of ~1×1012W cm-2 and 120 ps duration were used to heat the surface of the cavities. The dense plasma formed expands from the walls converging slightly off the semi-cylinders axis, giving rise to a bright localized high density plasma region. A sequence of electron density maps were measure using a 46.9 nm wavelength tabletop capillary discharge soft x-ray laser probe and an amplitude division interferometer based on diffraction gratings. The measured density profiles are compared with simulations conducted using the multi-diminensional hydrodynamic code HYDRA. The benchmarked model was then used to simulate particle trajectories which reveal that the increase in electron density near the axis is mainly the result of the convergence of plasma that originated at the bottom of the groove during laser irradiation.

Soft X-Ray Laser Interferometry of Colliding Al Plasmas in a Semi-Cylindrical Cavity

Soft x-ray laser interferometry was used to study the evolution of dense colliding plasmas produced by laser irradiation of semi-cylindrical targets. We present a series of interferograms that map the evolution of 1 mm long Aluminum plasmas created by irradiating 500 μm diameter semi-cylindrical targets with an intensity of ~ 1.1× 1012 Wcm-2 from 120 ps duration laser pulses of 800 nm wavelength. The interferograms were obtained combining a 46.9 nm tabletop capillary discharge soft x-ray laser with a high throughput amplitude division interferometer based on diffraction gratings. The interferograms show that the plasma expands converging on the axis of the semi-cylindrical cavity where it reaches electron densities above 1× 1020 cm-3. The code HYDRA was used to simulate the plasmas expansion and give insight on the plasma dynamics.

Soft x-ray laser interferometry unveils plasmas with index of refraction greater than one

We report clear evidence of the existence of multiply ionized plasmas with index of refraction greater than one at soft x-ray wavelengths. Moreover, it is shown to be a general phenomenon affecting broad spectral regions in numerous highly ionized plasmas. The experimental evidence consists of the observation of anomalous fringe shifts in soft x-ray laser interferograms of laser-created Al plasmas probed at 14.7 nm and of Ag and Sn laser-created plasmas probed at 46.9 nm. The comparison of measured and simulated interferograms shows that these anomalous fringe shifts result from the dominant contribution of low charge ions to the index of refraction. This usually neglected bound electron contribution can affect the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements and at different wavelengths.