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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 s...

X‐ray damage to optical surfaces consisting of melting, fracturing, cracking, and cratering is produced, and then analyzed using interference contrast microscopy and stylus profilometery. The test samples are irradiated by 0.5–1.6 keV x rays at fluence levels up to 5.5 cal/cm2. The x rays originate from L‐shell transitions in copper ions, produced when 1.25‐μm‐thick targets are irradiated at 1.3 kJ energy by a 1.054 μm wavelength laser. The x‐ray emission is found to be nearly isotropic over 2π sr, while the plasma mass‐flow distribution is peaked along the laser axis. Hence, contamination of the test sample by target debris or plasma is greatly reduced by placing the samples off axis from the laser beam, in addition to the use of beryllium shields.

A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub‐Alfvenically 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/Alfven speed ratio), magnetic‐field line tying, and expansion asymmetries (that promotes plasma cross‐field jetting).

High‐intensity laser irradiation of hollow glass cylinders immersed in a magnetic field results in plasma expansions strongly collimated in the direction transverse to both the initial flow and the magnetic field, but jetlike in the direction parallel to the initial flow. Magnetic fields from B=0 kG to B=10 kG produced plasmas with markedly different geometrical features. Fast framing camera photographs show the plasmas propagating across magnetic field lines and undergoing structuring indicative of transverse velocity shear‐driven instabilities. Comparison is made between the observed instability characteristics and predictions of Rayleigh–Taylor, classical Kelvin–Helmholtz, and the electron–ion hybrid instabilities. Only the electron–ion hybrid instability is consistent with the experimental results.

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 × 1014W/cm2 with about 300J of a focused, 1.05 μm wavelenght laser beam. A convex KAP crystal spectrograph was used to collect high-resolution spectra in the 4–20 A 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.

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.

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.

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.

The successful compression of laser‐driven pellets to thermonuclear ignition depends on the stability and uniformity of the motion with which dense shells can be imploded. The motion of planar foils accelerated by the Pharos II laser has been studied by two‐dimensional, flash x radiography employing pinhole imaging and slitted crystal imaging. The acceleration was driven by a 3–5‐ns duration, 1.05‐μm laser focused to 3–6×1012 W/cm2 in a millimeter diameter spot, while a second laser beam of shorter duration produced the x‐ray flash for imaging purposes. The x‐ray images obtained clearly show that the planar foil targets are ablatively accelerated to velocities of 3×106 cm/s while maintaining a density above 3% of solid. The axial extent of the accelerated, high‐density material has been observed to be as small as 25% of the distance traveled. The sides of accelerated portions of the foil connect smoothly to the stationary regions removed from the laser illumination. This connection apparently isolates the...

We describe a new efficient absorption mechanism for intense laser light in plasma resulting from an ion acoustic instability generated by a return current. Comparisons with experimental results are presented.

A simple electrostatic‐deflection multichannel time‐of‐flight ion analyzer optimized to study high energy ion species from laser‐produced plasmas is described. For this application moderate species and energy resolution over a wide dynamic energy range is desirable. The analyzer was designed to achieve a dynamic energy range from 1 to 1000 keV with a 15 kV power supply. Design considerations are presented together with some typical experimental results.

We describe the use of a ballistic pendulum array to measure the momentum of a laser‐produced plasma. An in situ calibration method is described and the pendulum results are compared to measurements made with other diagnostics.

We present results of a laser Doppler‐shift technique which gives the velocity profiles of ablatively accelerated targets. Measurements of the effects of laser beam nonuniformity on the target acceleration are presented and interpreted in the context of laser pellet fusion.

Electron plasma waves driven unstable by the electron beam-plasma instability are observed to decay into convectively and absolutely growing ion waves and electron waves propagating in opposite directions. Ion waves excited to large amplitudes further couple with the high-frequency unstable waves to produce nonresonant short-wavelength electron modes. The nonresonant modes are found effective in accelerating electrons and thermalizing the beam.

A steady state, dc discharge, electrostatically focused ion beam source with permanent magnet multipole confinement is described. The ion beam is low density, nb [inverted lazy s] 108 cm−3, low divergence, θ [inverted lazy s] 1°, energetic, Vb≤5 kV, and is suitable for investigating ion beam‐plasma interactions. Ion beam speed and species are determined either by modulating the beam and analyzing the resulting free‐streaming perturbations or by a pulse and time‐of‐flight technique. The amplitude of the free‐streaming signal is enhanced in the electron cyclotron harmonic passbands due to the decreased dielectric function of the background plasma. Two beam‐plasma instabilities, the electron cyclotron drift instability, and an instability at the lower hybrid frequency are observed using this ion source.

The growth of an ion acoustic instability caused by a slow electron drift is studied experimentally. A small-amplitude test wave in this unstable system develops into a large-amplitude steady state which is found to be a Bernstein-Greene-Kruskal---like ion mode with trapped electrons.

The electron cyclotron drift instability has been observed in a controlled, steady-state experiment. Enhanced noise bands are seen in the first two Bernstein wave dispersion branches. Dispersion, noise frequency dependence upon magnetic field and ion beam velocity, and saturation are demonstrated.

The dependence of stimulated Brillouin backscatter upon pulse amplitudes, timing, target material, and geometry from a prepulse‐formed plasma is found. The applicability of these results to backscatter from a structured continuous pulse is discussed.

Resonant laser probing diagnostics are used to observe laser‐produced barium plasmas (n≤1015 cm−3, T≂1 eV) streaming (v≂106 cm/s) across a strong transverse magnetic field (10 kG). The interaction of the plasma with the magnetic field produces structure in the plasma. The observation of this structure requires resonant diagnostics because of the very low plasma density in this experiment. A YAG‐pumped dye laser is tuned near the Ba ii 4554‐A transition for resonant absorption, shadowgraphy, and scattering measurements. Resonant absorption and shadowgraphy images reveal that the plasma expands across the field with virtually no inhibition from the field, while narrowing in the plane perpendicular to the field. In the plane of the magnetic field the expansion is highly structured, taking the form of very narrow jets aligned along the field lines. This structure appears to be a manifestation of a beam‐plasma‐type instability. For a comparison with theoretical predictions the plasma density and scale lengths ...

Measurements of the asymptotic energy distribution of the different high‐energy ion species at Nd‐laser irradiance 1015–1016 W/cm2 on a CH2 planar target are presented and the acceleration mechanism of these high‐energy ions is discussed. Multipeak structure between 10 and 100 keV is observed on the H+ ion distribution. A disproportionate number of H+ ions with E/Z higher than the C+z ions is also observed.

The momentum, energy, and velocity characteristics of plasma ablating from planar targets irradiated by long Nd‐laser pulses (4 ns

The fundamental and second through the fifth harmonic spectral lines have been observed from the plasma produced when a 75‐psec Nd : glass laser (∼1016 W/cm2) is focused onto a thick planar polystyrene target. Both line profiles and relative intensities of these harmonic are given.

Angular distributions of scattered laser light from Nd‐laser‐produced plasma at irradiances of 1015–1016 W/cm2 were measured. Total reflectivities in the range 20–50% are found. The total reflectivity remains approximately constant as the target is rotated from normal incidence up to 50°–60°, implying the existence of a steep density gradient near the critical density and/or absorption of laser light in the underdense region of the plasma.

The transport of energy from a 1.06‐μm, 95‐psec laser pulse at an irradiance of 1015 W/cm2 through a thin layer of polystyrene into an Al substrate was studied by x‐ray, ion, and scattered‐light measurements. The intensities of the following quantities were measured as a function of polystyrene thickness: (1) x‐ray line radiation from the Al backing, (2) bremsstrahlung continuum from 3 to 88 keV, (3) ions of several keV energy, and (4) scattered laser light. The results indicate that a polystyrene thickness of no more than 0.5 μm is sufficient to inhibit substantial heating of the Al substrate.

An optical shutter is described which opens on a few picoseconds time scale. The shutter consists of a thin foil irradiated by a laser.

Laser plasma coupling under near reactor‐size target conditions is studied at Iλ2≂1014 W/cm2 μm2 with 1‐mm size plasmas, 3‐kJ, 3‐ns irradiations on the 1.064‐μm Shiva laser. Evidence for the occurrence of parametric processes, such as stimulated Brillouin sidescatter and the two‐plasmon decay instability, is observed.

Using a time‐of‐flight spectroscopic technique, measurements were made of the ion energy distributions of very fast ions and thermal ions produced when a 7–15‐J 100‐psec Nd : glass laser pulse (1.06 μm) strikes a (CH2)n slab target. Ion energies greater than 0.5 MeV have been observed for the first time with this technique of measurement. A simultaneous comparison is made between the signal of an ion charge collector placed 30 cm from the target and the intensity of the C VI 3434‐A ion line at 1 cm from the target.

An experimental technique, especially well adapted to measure secondary‐electron coefficients for energetic ions incident on any charge‐collector surface, is described. The technique uses a laser‐produced plasma as an ion source, a high‐energy ion analyzer as an ion energy and species filter, and a tubular electrode arrangement to suppress or attract the secondary electrons at the charge collectors. The secondary‐electron coefficients for several ionization states of carbon and hydrogen ions incident on copper surfaces are given as a function of incident ion energy between 15 and 150 keV/Z.

We have used the SHIVA laser system to accelerate carbon disks to speeds in excess of 100 km/sec. The 3KJ/3 ns pulse, on a 1 mm diameter spot of a single disk produced a conventional shock of about 5 MB. The laser energy can, however, be stored in kinetic motion of this accelerated disk and delivered (reconverted to thermal energy) upon impact with another carbon disk. This collision occurs in a time much shorter than the 3 ns pulse, thus acting as a power amplifier. The shock pressures measured upon impact are estimated to be in the 20 MB range, thus demonstrating the amplification power of this colliding disk technique in creating ultra-high pressures. Theory and computer simulations of this process will be discussed, and compared with the experiment.

A novel method is described from which estimates of the internal pressure or temperature of highly accelerated material can be made. The technique utilizes the relationship of the initial free expansion properties of a fluid boundary to internal pressure. The technique is applied to Nd‐laser ablatively accelerated targets; the results compare well with target rear surface temperature measurements using time‐resolved optical pyrometry.

The interaction physics and hydrodynamic motion of thin‐foil targets irradiated by long, low‐flux Nd‐laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin‐foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

The width of ion‐velocity distributions from laser‐produced plasmas can be controlled experimentally by varying the size of the laser spot. This ion‐velocity width is determined primarily by whether the ions are mostly in the rarefaction or steady‐state regime. It is not dominated by the thermal distribution of the ions, as is sometimes assumed. A simple analytic theory shows that the distribution width is governed by the scaling parameter rs/cτ, where rs is the focal spot radius, c is the sound speed, and τ is the laser pulse length.

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double‐foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.

A laser‐produced plasma generates blast waves as it expands supersonically into a stationary photoionized background gas (N2) at a pressure of 1–5 Torr.1 Using a combination of spectroscopic and interferometric measurements, time‐ and space‐resolved values of the temperature and density are obtained. This study was performed on targets in the NRL Pharos III laser facility with laser energies of 20–120 J and pulse durations of ∼5 ns. A 1‐m spectrograph equipped with three photomultiplier channels, which are calibrated on an absolute scale, is used for the spectroscopic measurements. The interferometry is done with a folded‐wavefront interferometer. It uses an optical probe pulse at 5270 A that is split off of the main laser pulse, reduced in pulse duration (∼300 ps), and time delayed. Interferometric measurements can be made simultaneously with the spectroscopic measurements. Experimental data and the technique of analysis will be shown. This work was supported by the Defense Nuclear Agency.

When a short‐pulse laser beam is focused onto a solid target in a few torr ambient gas, the expanding target plasma couples to the resulting ambient plasma. Dark‐field shadowgrams show that a thin, nearly spherical blast front is produced. These shadowgrams also show that certain regions of the blast front (called aneurisms) may project well ahead of the neighboring, spherical part. Several mechanisms for aneurisms are discussed. One class of aneurisms is consistently produced along the laser axis when the laser energy is greater than about 10 J. Two‐dimensional, hydrodynamic computer simulations of these on‐axis aneurisms show that they can be accounted for by the laser heating of an on‐axis channel.

Absolute spectral measurements of 50–500‐keV electrons emitted from a Nd‐laser‐produced plasma are presented. For normally incident irradiation (1015–1016 W/cm2) onto polystyrene slab targets a total emitted energy of ∼1010 electrons is inferred with a total emitted energy of ∼0.2 mJ. The electron emission is observed to be almost the same both in and out of the plane of polarization. Properties of the emitted spectrum, such as spectral hardness and intensity, are observed to be dependent on the pulse shape and the target material.

The self‐generated magnetic field produced when a pulsed Nd:glass laser is focused onto a carbon foil target is measured for the first time using the Zeeman effect. Emission from the CV 1s2s3S1–1s2p3P2 (2270.9 A) was used for the measurement. At a laser irradiance of ∼5×1012 W/cm2, the magnetic field increased from 100 to 200 kG when the focal spot was changed from a flat‐topped distribution to a ring pattern, which caused higher density gradients. A Faraday rotation measurement at approximately 200 kG agreed with the Zeeman effect measurement. Opacity effects in the plasma enhanced the sensitivity, allowing the Zeeman measurement to be made in this magnetic field range.

Spatial electron plasma wave echo experiments are described in which the relation between the echo wavelength and frequency is given by ω3/=vc, where vc is approximately equal to the velocity at which the electron distribution function has maximum slope. It is concluded that the echo response is dominated by free‐streaming electrons. Other evidence is also presented which supports this conclusion. The experimental results corroborate many of the properties of free‐streaming disturbances derived from an expansion of the Vlasov‐Poisson system. Numerical integrations of the derived expressions for the second‐order ballistic echo electric field compare favorably with experiment. The mechanism by which electrons are perturbed by the externally applied fields (e.g., in velocity or density) is found to influence the detailed structure of the echo response.

Ballistic second‐order plasma wave echoes are observed in a Maxwellian plasma. In some cases a small resonant coupling is also seen.