Showing papers by "Christina Back published in 2004"

Efficient multi-keV X-ray sources from Ti-doped aerogel targets.

Abstract: We have measured the production of hnu approximately 4.7 keV x rays from low-density Ti-doped aerogel (rho approximately 3 mg/cc) targets at the OMEGA laser facility (University of Rochester), with the goal of maximizing x-ray output. Forty OMEGA beams (lambda(L)=0.351 microm) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser fully ionizes the target (n(e)/n(crit)

Effects of plasma composition on backscatter, hot electron production, and propagation in underdense plasmas

Abstract: A series of underdense laser plasma interaction experiments performed on the Helen laser [M. J. Norman et al., Appl. Opt. 41, 3497 (2002)] at the Atomic Weapons Establishment (AWE), U.K., using 2ω light have uncovered a strong dependence of laser backscatter and hot electron production on plasma composition. Using low-Z materials, we find a behavior familiar from previous 3ω work, the interchange of stimulated Raman scattering for Brillouin scattering as we change from gases that have high ion wave damping (e.g., C5H12) to gases with low ion wave damping (e.g., CO2). However, as Z is increased, we find that Brillouin scattering drops while Raman scattering remains low. For gases with Z greater than 18, it is possible to have long scalelength, underdense plasmas with both low Brillouin and Raman backscatter losses. Complementary measurements of hot electron production show efficient production of hot electrons in C5H12 plasmas approaching 0.25ncr, but changing the plasma composition can greatly suppress th...

Multi-keV x-ray conversion from prepulsed foil experiments

Abstract: Starting from FCI2 simulations showing good multi-keV conversion efficiencies of a preformed plasm from thin foils heated by two laser pulses, experiments have been performed with titanium and copper on the Omega laser facility at University of Rochester. The advantages of using this method are efficiencies close to gas targets due to the under-dense plasma created by the pre-pulse and X-ray emissions available at high photon energies that cannot be reached with gas targets. Optimum parameters (laser intensities, delay between the two pulses and thickness of the foil) for titanium and copper foils were estimated from simulations. An increase in the multi-keV conversion efficiency (above 4 keV) by a factor of 2, compared to the case without pre-pulse, is clearly shown on titanium targets. X-ray emission was measured by different diagnostics in good agreement and close to simulations results.

Efficient multi-keV x-ray sources from Ti-doped aerogel targets

Abstract: We have measured the production of hv ≈ 4.7 keV x‐rays from low‐density Ti‐doped aerogel (ρ ≈ 3 mg/cc) targets at the OMEGA laser facility (University of Rochester), with the goal of maximizing x‐ray output. Forty OMEGA beams (λL = 0.351 μm) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser fully ionizes the target (ne/ncrit ⩽ 0.1), and a laser‐bleaching wave excites, supersonically, the high‐Z emitter ions in the sample. Heating in the target was imaged with gated x‐ray framing cameras and an x‐ray streak camera. Ti K‐shell x‐ray emission was spectrally resolved with a two‐channel crystal spectrometer and also with a set of filtered aluminum x‐ray diodes, both instruments provide absolute measurement of the multi‐keV x‐ray emission. We find between 40 – 260 J of output with 4.67 ⩽ hv ⩽ 5.0 keV. Radiation‐hydrodynamic calculations predict late time enhancement of the x‐ray power due first to axial stagnation of the heating waves, then, ablatively...

Efficient multi-keV x-ray sources from Ti-doped aerogel targets

Abstract: We have measured the production of hv {approx} 4.7 keV x-rays from low-density Ti-doped aerogel ({rho} {approx} 3 mg/cc) targets at the OMEGA laser facility (University of Rochester), with the goal of maximizing x-ray output. Forty OMEGA beams ({lambda}{sub L} = 0.351 {micro}m) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser fully ionizes the target (n{sub e}/n{sub crit} {le} 0.1), and a laser-bleaching wave excites, supersonically, the high-Z emitter ions in the sample. Heating in the target was imaged with gated x-ray framing cameras and an x-ray streak camera. Ti K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and also with a set of filtered aluminum x-ray diodes, both instruments provide absolute measurement of the multi-keV x-ray emission. We find between 40 - 260 J of output with 4.67 {le} hv {le} 5.0 keV. Radiation-hydrodynamic calculations predict late time enhancement of the x-ray power due first to axial stagnation of the heating waves, then, ablatively-driven radial compression from the target walls.

Shielding a streak camera from hard x rays

Abstract: The targets used in the hot halfraum campaign at OMEGA create many hot electrons, which result in a large flux of hard x rays. The hard x rays produce a high background in the streak camera. The background was significantly reduced by wrapping the streak camera with a high-Z material; in this case, 1/8 in. of Pb. The large hard x-ray flux also adds noise to images from framing cameras which use charge-coupled devices.

Supersonic propagation of ionization waves in an under-dense, laser-produced plasma

Abstract: We observe a laser-driven supersonic ionization wave heating a mm-scale plasma of sub-critical density up to 2-3 keV electron temperatures. Propagation velocities initially 10 times the sound speed were measured by means of time-resolved x-ray imaging diagnostics. The measured ionization wave trajectory is modeled analytically and by a 2D radiation-hydrodynamics code. The comparison to the modeling suggests that nonlocal heat transport effects may contribute to the attenuation of the heat wave propagation.