DT-filled glass microspheres have been imploded at power levels exceeding 4 TW using the Lawrence Livermore Laboratory 1.06-/um Argus laser. Thermonuclear neutron yields in excess of 1.5 x lo^ have been observed implying a DT burn efficiency of 1.6 x lo" ^ Neutron and a time-of-flight measurements indicate DT burn temperatures of 4-8 keV, implying that a DT gain of approximately 10"^ and a nr of 10^^ were obtained, As part of the effort to understand the physics of laser imploded targets, a series of experiments examining both basic laser-plasma interaction phenomena and the parameter space of exploding-pusher experiments have been performed over the last three years, both at Lawrence Livermore Laboratory and other laboratories.^"^ A laser irradiated target is said to operate in the exploding-pusher^ mode when the pusher significantly decompresses in the process of compressing the fueL This is characteristic of a high rate of energy addition to the pusher. Laser absorption mainly by collective processes producing super thermal electrons, early energy deposition in the hsell by these super thermal electrons, a near supersonic electron thermal wave driven by electron thermal conduction from the laser-absorption region, and significant shock compression of the fuel causing a large entropy change are other characteristics of the exploding-pusher mode. This results in a limited density increase, but significant heating of the fuel. Early exploding-pusher experiments with DT-filled glass microshells indicated that for fixed DT fill, the target performance, measured in terms of neutron yield, should increase^° as ^^10/3^2, HcJv)T -1/2 Here t*o is the target radius, w the shell wall thickness, T the time-and spaceaveraged final DT fuel temperature, and {uv) the Maxwell averaged DT cross section. It is assumed thatr is proportional to the useful specific absorbed energy, SQ, The useful fraction is essentially the absorbed energy corrected for any temporal mismatch between the input laser pulse length and the characteristic target-implosion time scale, and is thus the time in the laser pulse beyond which further absorption can no longer influence the final implosion phase. This time is found empirically to be roughly determined by the amount of energy absorbed until the pusher has traversed '^ 30% of the initial target radius, with the instantaneous pusher velocity assumed proportional to the energy absorbed up to that time. Since pusher velocities are -(2.5 -^.5)x 10^ cm/sec,^'^^ a 90-/im-diam target would find nearly all the energy ^'useful" for laser pulses with a full width at half-maximum (FWHM) ^ 40 ps. Experiments performed at KMS Fusion, Inc. also gave results consistent with this description.^^ Targets irradiated at a fixed peak power, with pulse lengths increasing in steps of 40 ps up to 240 ps, showed no increase in neutron yield for pulse lengths beyond 80 ps. Both to confirm the "useful energy'* hypothesis and to extend our data 1570

Measurements have been made of the energy spectrum of neutrons emitted as a result of laser compression of D-T-filled glass microshells using a time-of-flight spectrometer. From the width of unfolded energy spectra, plasma ion temperatures of approximately 6 keV have been determined for 90-μm-diam targets irradiated with 2 TW of 1.06-μm light. The measured mean neutron energy of 14.00±0.10 MeV agrees with the expected value of 14.05 MeV.

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This letter reports experiments that used a mode-locked TEA CO2 laser to modulate a cw He–Ne laser. This technique uses the optical Kerr effect in CS2. The optical Kerr coefficient, n2, at 10.6 μm is estimated from our data to be approximately 10−20 m2/V2.