LLE Review 119

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Highlights

This volume of the LLE Review, covering April–June 2009, features shock-ignition experiments on OMEGA that have been performed in spherical geometry. Shock ignition is a two-step inertial confinement fusion (ICF) concept in which a strong shock wave is launched at the end of the laser drive pulse to ignite the compressed core of a low-velocity implosion. Fuel assembly and ignition are separated by the two steps, relaxing the driver requirements and promising high gains. In the experiments described in this article, room-temperature plastic shells were compressed on a low adiabat by 40 beams of the 60-beam OMEGA Laser System. The remaining 20 beams were delayed and tightly focused onto the target to drive a strong shock into the compressed core. Near the inner shell surface this ignitor shock collides with the return shock driven by the hot-spot pressure and propagating outward through the shell. After the ignitor and return-shock collision, a third shock wave, resulting from the collision, propagates inward, leading to further compression of the hot spot. Good coupling of the shock-beam energy was observed in these experiments, leading up to an ~20× increase in neutron yield. The authors observed significant stimulated Raman backscattering of laser energy and resultant fast electrons that are actually beneficial for shock ignition as they augment the strong hydrodynamic shock.

Additional highlights of research presented in this issue include the following:

• Laser-driven magnetic-flux compression in high-energy-density plasma experiments are described that demonstrated, for the first time, magnetic-field compression to many tens of megagauss (MG) in cylindrical implosions of inertial confinement fusion targets. The very high magnetic-flux compression was achieved using the ablative pressure of the OMEGA laser to drive a cylindrical shell at high-implosion velocity, trapping and compressing an embedded external field to tens of MG, high enough to magnetize the hot-spot plasma. Line-averaged magnetic fields between 30 and 40 MG were observed.
• It is shown that monoenergetic proton radiography combined with Lorentz mapping can be used to uniquely detect and discriminate magnetic and electric fields. This unique detection of electromagnetic fields and identification of field type and strength as a function of position was used to determine the nature of self-generated fields in a novel experiment with laser-generated plasma bubbles on two sides of a plastic foil. The results provided absolute identification and measurement of a toroidal magnetic field around each bubble and determined that any electric field component parallel to the foil was below measurement uncertainties.
• A spatially resolved spectral interferometry technique, known as S² imaging, is used to measure higher-order mode content of a large-mode-area amplifier at full power for the first time. The technique was adapted for the short-fiber amplifier at full power and revealed a small amount of a co-polarized LP11 mode. This mode’s power relative to the fundamental LP01 mode depends on the alignment of the input signal.
• Optical differentiation in a regenerative amplifier with temperature-tuned volume Bragg grating (VBG) as an intracavity spectral filter is demonstrated for the first time. A simple, reliable laser system that produces multimillijoule ~150-ps pulses without mode-locking using an RA with VBG as an optical differentiator is described.
• Crack growth in brittle glass plates is examined using finite element modeling. Fracture is analyzed in terms of strength, fracture toughness, or slow crack growth and this article outlines a procedure that estimates the deepest-allowable surface flaw.
• Finite element analysis is applied to ultrafast photoconductive switches of the metal–semiconductor–metal (MSM) type to explain why MSM devices with alloyed electrodes show improved photoresponse efficiency compared to devices with surface contact electrodes. The model is also used to predict improved responsivity, based on electrode spacing and antireflective coating.