Thin film optical coatings are susceptible to damage by high intensity x rays. Time-resolved measurements of this damage are required to better understand the mechanism, so that more rugged coatings can be developed. In the present experiment, dark-field shadowgraphy was used to temporally map the x-ray damage across the surface of certain anti-reflecting (AR) coatings. Two beams from the NRL PHAROS III high power Nd:glass laser system were utilized to generate a point source of plasma x rays, which in turn was used to irradiate and damage the optical coatings. Thin, opaque filters, coupled with permanent magnets and pinholes, were used to shield the optical samples from ultraviolet and charged-particle damage, respectively. The absolute, time-integrated x-ray fluence was measured with a crystal spectrograph, and also was temporally resolved with an x-ray diode. The surface morphology of the damaged optical samples was examined after each shot visually, and later with a profilometer as well as with both scanning electron- and atomic-force microscopes. A measured threshold fluence for damage of 0.049±30% cal/cm2 agrees very well with a radiation-damage code prediction of 0.046 cal/cm2.

We present results of experiments on very-high Mach number (> 100) shocks and very-high Mach number and Reynolds number (> 100 × 106) turbulence, which are initiated with a powerful laser driver. In one experiment we examined laser-produced-shock solid-surface interactions and Mach stems and triple points, and also measured a new phenomenon termed a blast wave decursor. In a second experiment we found that shocks become unstable if they propagate through a gas which has a low adiabatic index and we measured the growth rate of the instability. In a third experiment we have shown that a high Mach number shock dramatically enhances the structure of a turbulent field through which it passes and that the shock itself is badly distorted. This result is unexpected since common wisdom has it that high Mach number shocks self-heal as they pass through a turbulent field.

Thin film coatings are susceptible to high intensity x-ray damage. The PHAROS III laser was utilized to generate a point source of x-ray emission used to determine the damage threshold of AR-coated space optics. Thin filters coupled with magnets were used to shield the specimens from thermal radiation and plasma debris. Grids supporting the thin filters could be patterned into the coatings. The surface morphology of damaged specimens has been examined with SEM and AFM microscopes to determine the nature of the damage in multilayer AR coatings. Microscopic techniques were used to measure the depths of coating damage and edge sharpness in the patterned region.

X-rays in the 0.5–1.6 keV photon energy range, useful for debris-free damage testing of optical components, are produced efficiently by a laser-produced plasma. Energies exceeding 1 kJ in three beams from the NRL Pharos III laser irradiate a copper target to produce L-shell x-rays from plasma ions. A near-isotropic x-ray fluence of typically 5 cal/cm2 at 1 cm from the source is deduced from measurements with filtered Si-PIN and biplanar vacuum diodes. The corresponding conversion efficiency of laser to x-ray energy is typically 11%.

Visible spectral lines from n = 3, DELTAn = 0 transitions in N+ and N2+ ions are used for measuring the plasma electron density and temperature in a region of two colliding blast waves, propagating through a 1.5-10-Torr nitrogen atmosphere. The blast waves originate at the tips of two aluminum rods irradiated with two beams of the Naval Research Laboratory (NRL) Pharos-III 1.054-mum-wavelength Nd:glass laser operated at an energy of 200-430 J for each beam in 5-ns pulses. An electron density in the colliding-blast-wave region of N(e) almost-equal-to 10(18) cm-3 was deduced from Stark broadening of spectral lines from N+ ions. An electron temperature of T(e) almost-equal-to 4 eV was measured in this region from a spectral-line intensity ratio between N2+ and N+ ions. Near one target, an electron density of N(e) almost-equal-to 8 X 10(20) cm-3 was determined from series-limit x-ray spectral-line merging; a mean electron temperature of kT(e) almost-equal-to 225 eV was determined from x-ray fine-intensity ratios. Some evidence was found for enhanced velocities for blast waves propagating through a plasma formed by a preceding blast wave.

A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub-Alfvénically expanding into a magnetic field. Results from an experimental study of this instability are reported and compared with theory, notably a magnetohydrodynamic (MHD) treatment that includes the Hall term, a generalized kinetic lower-hybrid drift theory, and with computer simulations. Many theoretical predictions are confirmed while several features remain unexplained. New and unusual features appear in the development of this instability. In the linear stage there is an onset criterion insensitive to the magnetic field, initial density clumping (versus interchange), linear growth rate much higher than in the ‘‘classic’’ MHD regime, and dominant instability wavelength of order of the plasma density scale length. In the nonlinear limit free-streaming flutes, apparent splitting (bifurcation) of flutes, curling of flutes in the electron cyclotron sense, and a highly asymmetric expansion are found. Also examined is the effect on the instability of the following: an ambient background plasma (that adds collisionality and raises the expansion speed/Alfvén speed ratio), magnetic-field line tying, and expansion asymmetries (that promotes plasma cross-field jetting).

The Panel finds that the steady-state/advanced-to-kamak mission is a critical element in the U.S. fusion strategy as established by FPAC. An attractive SS/AT device can be constructed for about the $400M FY'92 guideline proposed by the SEAB-TF. The design and construction of such a facility should proceed on a schedule to enter operation in 1990–00. Adequate funding for peak construction years should become available following the D-T operation of TFTR. In all its phases, the new device should be managed as a national facility.

We present the first measurements of an instability in Taylor-Sedov blast waves propagating through a uniform gas. The instability occurred in a gas whose adiabatic index was low. Amplitude perturbations grew as a power of time. Our observations are compared to theory.

We observe, for the first time in a laboratory, the formation of decursors at intersections of planar surfaces with powerful blast waves. The blast waves, which have hundreds-kilobar overpressures, are created by rapid ablation of material heated with an intense laser beam.

X-ray data were acquired from intermediate- and high-Z element targets with the Pharos III laser system. Targets, mounted at the tip of thin glass stalks, were microscopic fragments or foils of single or multiple high-Z element composition. The laser irradiance was 4 x 10(14) W/cm2 with about 300 J of a focused, 1.05-mu-m wavelength laser beam.A convex KAP crystal spectrograph was used to collect high-resolution spectra in the 4-20 angstrom soft X-ray region. The recorded spectral film densities were microdensitometered and converted by computer-processing to absolute continuum intensities. The spectral features were identified with the aid of ab initio atomic structure calculations. The continuum background was evaluated as a source of pseudo-continua for absorption studies of soft X-rays.

Laser-produced plasmas have many properties similar to, or which can be scaled to, those encountered in space, magnetospheric, ionospheric, and astrophysical situations. We describe several such experiments performed with the PHAROS III Nd-laser facility at NRL.

The output energy of present x‐ray lasers is ≲10−6 of that of the driver laser, for laser‐produced plasma media. A significant improvement is essential for more compact, useful, and less costly devices. Some plasma/atomic processes that may bear on this goal and that deserve attention are described. Some experiments already indicate promise for improved efficiency. One recent one from the RIKEN laboratory (Japan) is described, along with initial attempts at the Naval Research Laboratory (NRL) to duplicate the results.

Interest in x-ray absorption derives from soft x-ray transport studies to characterize laser-solid-interaction effects. The object of this work was to determine whether x-ray absorption features were measurable in laser-produced plasmas from solid targets. X-ray emission was made using high-atomic-number microsphere targets irradiated with a focused beam of the Pharos laser while the laser-solid-interaction plasmas were generated by a second beam in Al foil targets. High-resolution, spatially resolved x-ray spectra were collected with instrumentation and geometry similar to previous tracer-dot spectroscopy measurements of plasma profile parameters. Soft x-ray absorption lines, observed at different distances from the aluminum target surface, may be related to density gradients in the laser-generated plasmas.

: X-ray data were acquired from mass-limited targets with the Pharos III laser system. Targets, mounted at the tip of thin glass stalks, were microscopic pieces of single or multiple high-Z element composition. The laser irradiance was 4 x 10 to the 14th power w/sg cm with about 300J of focused 1.05 microns wavelength laser beam. A convex KAP crystal spectrograph was used for its capability to collect high resolution spectra in the 4-20 A soft x-ray region. This recorded spectral film densities were microdensitometered and converted by computer-processing to absolute continuum and emission line intensities. The spectral features were identified with the aid of ab-initio atomic structure calculations. The continuum background was evaluated as a source of pseudo-continua for absorption studies of soft x-rays. Keywords: X-ray spectroscopy; Laser heated plasmas; Continuum emission.