Howard T. Powell

Petawatt laser pulses

We have developed a hybrid Ti:sapphire-Nd:glass laser system that produces more than 1500 TW (1.5 PW) of peak power. The system produces 660 J of power in a compressed 440+/-20 fs pulse by use of 94-cm master diffraction gratings. Focusing to an irradiance of >7x10(20) W/cm (2) is achieved by use of a Cassegrainian focusing system employing a plasma mirror.

Direct-Drive Laser Fusion; Status and Prospects

Techniques have been developed to improve the uniformity of the laser focal profile, to reduce the ablative Rayleigh–Taylor instability, and to suppress the various laser–plasma instabilities. There are now three direct-drive ignition target designs that utilize these techniques. An evaluation of these designs is still ongoing. Some of them may achieve the gains above 100 that are necessary for a fusion reactor. Two laser systems have been proposed that may meet all of the requirements for a fusion reactor.

Kinoform phase plates for focal plane irradiance profile control

A versatile, rapidly convergent, iterative algorithm is presented for the construction of kinoform phase plates for tailoring the far-field intensity distribution of laser beams. The method consists of repeated Fourier transforming between the near-field and the far-field planes with constraints imposed in each plane. For application to inertial confinement fusion, the converged far-field pattern contains more than 95% of the incident energy inside a desired region and is relatively insensitive to beam aberrations.

Random phase plates for beam smoothing on the Nova laser

We discuss the design and fabrication of 80-cm-diameter random phase plates for target-plane beam smoothing on the Nova laser. Random phase plates have been used in a variety of inertial confinement fusion target experiments, such as studying direct-drive hydrodynamic stability and producing spatially smooth x-ray backlighting sources. These phase plates were produced by using a novel sol-gel dip-coating technique developed by us. The sol-gel phase plates have a high optical damage threshold at the second- and third-harmonic wavelengths of the Nd:glass laser and have excellent optical performance.

Designing fully continuous phase screens for tailoring focal-plane irradiance profiles

An iterative algorithm for constructing fully continuous phase screens for tailoring far-field intensity profiles is presented. The algorithm is robust, stable, and, if run properly, maintains the continuous nature of the phase throughout the iterative process. The iterative procedure is applied to generate continuous phase screens to produce a 12th-power super-Gaussian far-field intensity profile.

Laser oscillation on the green bands of XeO and KrO

Laser oscillation has been observed on the green bands of the XeO and KrO excimers using direct electron beam excitation of high‐pressure Xe or Kr containing small concentrations of O2. These bands are transitions between molecular levels correlating to the 1S0 and 1D2 metastable levels of atomic oxygen plus ground‐state Xe or Kr. The XeO laser emission is at a number of wavelengths between 5300 and 5550 A, while the KrO emission is at a single wavelength near the free atom line at 5577 A. Laser pulse energies of 10 mJ and peak powers of 100 kW are seen for both excimers.

1.047-/spl mu/m Yb:Sr/sub 5/(PO/sub 4/)/sub 3/F energy storage optical amplifier

The pumping and gain properties of Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm/sup 2/ based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF/sub 4/ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0/spl times/10/sup -20/ and 1.5/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3/spl times/10/sup -20/ and 1.4/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25/spl times/25/spl times/175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics ( >

KrCl laser oscillation at 222 nm

Laser oscillation has been observed on the 2Σ+ 1/2‐2Σ+ 1/2 band of KrCl at 222 nm in an electron‐beam‐excited mixture of argon, krypton, and chlorine. The laser performance and spectral features of KrCl and KrF are compared.

Photodissociation lasers using forbidden transitions of selenium atoms

Weak laser oscillation has been demonstrated on the 489‐ and 777‐nm electric‐dipole‐forbidden 1S0–3P1 and 1S0–1D2 transitions of the selenium atom. Carbonyl selenide has been photolyzed at 172 nm by Xe*2 radiation to produce the Se(1S0) excited state.

Sulfur 1S0‐1D2 laser by OCS photodissociation

Strong laser oscillation has been obtained on the electric‐dipole‐forbidden 1S0‐1D2 transition of atomic sulfur using 146‐nm photodissociation of OCS. A photolytic mixture of OCS with N2 and SF6 produced optimum laser performance. Based on the laser output energy of 5 mJ at 772.5 nm, we infer that an energy density of 2.5 J/liter was stored in the laser transition immediately following photolysis. The one‐half quenching time of S(1S) atoms was approximately 1 μs for these conditions.