Insulators are critical components in high-energy, pulsed power systems. It is known that the vacuum surface of the insulator will flashover when illuminated by ultraviolet (UV) radiation depending on the insulator material, insulator cone angle, applied voltage and insulator short-history. A testbed comprised of an excimer laser (KrF, 248 nm, ~2 MW/cm2, 30 ns FWHM,), a vacuum chamber (low 1.0E-6 torr), and dc high voltage power supply (<60 kV) was assembled for insulator testing to measure the UV dose during a flashover event. Five in-house developed and calibrated fast D-Dot probes (>12 GHz, bandwidth) were embedded in the anode electrode underneath the insulator to determine the time of flashover with respect to UV arrival. A commercial energy meter were used to measure the UV fluence for each pulse. Four insulator materials high density polyethylene, Rexolitereg 1400, Macortrade and Mycalex with side-angles of 0, plusmn30, and plusmn45 degrees, 1.0 cm thick samples, were tested with a maximum UV fluence of 75 mJ/cm2 and at varying electrode charge (10 kV to 60 kV). This information clarified/corrected earlier published studies. A new phenomenon was observed related to the UV power level on flashover that as the UV pulse intensity was increased, the UV fluence on the insulator prior to flashover was also increased. This effect would bias the data towards higher minimum flashover fluence.

Ultrashort pulsed lasers can accurately ablate materials which are refractory, transparent, or are otherwise difficult to machine by other methods. The typical method of machining surfaces with ultrashort laser pulses is by raster scanning, or the machining of sequentially overlapping linear trenches. Experiments in which linear trenches were machined in alumina at various pulse overlaps and incident fluences are presented, and the dependence of groove depth on these parameters established. A model for the machining of trenches based on experimental data in alumina is presented, which predicts and matches observed trench geometry. This model is then used to predict optimal process parameters for the machining of trenches for maximal material removal rate for a given laser.

Laser machining technology has been used to demonstrate the ability to rapidly perform jobs on all aspects of ICF targets. Lasers are able to rapidly perform modifications and repairs to the gold metal parts on hohlraums, make cuts in the delicate polymer parts of the hohlraum, and drill holes in the capsules to enable them to be filled with fuel. Lasers investigated in this work include 193 nm ArF and 248 nm KrF excimers and 810 nm chirped-pulse amplification Ti:Sapphire lasers. The excimer lasers showed a definite advantage in drilling and machining of polymeric materials and the ultrashort inftared pulses of the Ti:Sapphire laser were far better for the gold structures.

A dynamic transmission electron microscope (DTEM) has been designed and implemented to study structural dynamics in condensed matter systems. The DTEM is a conventional in situ transmission electron microscope (TEM) modified to drive material processes with a nanosecond laser, “pump” pulse and measure it shortly afterward with a 30-ns-long probe pulse of ∼107 electrons. An image with a resolution of <20nm may be obtained with a single pulse, largely eliminating the need to average multiple measurements and enabling the study of unique, irreversible events with nanosecond- and nanometer-scale resolution. Space charge effects, while unavoidable at such a high current, may be kept to reasonable levels by appropriate choices of operating parameters. Applications include the study of phase transformations and defect dynamics at length and time scales difficult to access with any other technique. This single-shot approach is complementary to stroboscopic TEM, which is capable of much higher temporal resolution but is restricted to the study of processes with a very high degree of repeatability.

Spectral emission from optical breakdown in the bulk of a transparent dielectric contains information about the nature of the breakdown medium. We have made time resolved measurements of the breakdown induced emission caused by nanosecond and femtosecond infrared laser pulses. We previously demonstrated that the emission due to ns pulses is blackbody in nature allowing determination of the fireball temperature and pressure during and after the damage event. The emission due to femtosecond pulse breakdown is not blackbody in nature; two different spectral distributions being noted. In one case, the peak spectral distribution occurs at the second harmonic of the incident radiation, in the other the distribution is broader and flatter and presumably due to continuum generation. The differences between ns and fs breakdown emission can be explained by the differing breakdown region geometries for the two pulse durations. The possibility to use spectral emission as a diagnostic of the emission region morphology will be discussed.

Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.

We report an experimental investigation of mitigating surface damage growth at. 351nm for machine-finished DKDP optics. The objective was to determine which methods could be applied to pre-initiated or retrieved-from-service optics, in order to stop further damage growth for large aperture DKDP optics used in high-peak-power laser applications. The test results, and the, evaluation thereof, are presented for several mitigation methods applied to DKDP surface damage. The mitigation methods tested were CW-CO2 laser processing, aqueous wet-etching, short-pulse laser ablation, and micro-machining. We found that micro-machining, using a single crystal diamond tool to completely remove the damage pit, produces the most consistent results to halt the growth of surface damage on DKDP. We obtained the successful mitigation of laser-initiated surface damage sites as large as 0.14mm diameter, for up to 1000 shots at 351nm and fluences in the. range of 2 to 13J/cm(2), similar to11ns pulse length. Data obtained to-date indicates that micro-machining is the preferred method to process large-aperture optics.

Targets to study high-energy density physics and inertial confinement fusion processes have very specific and precise tolerances that are pushing the state-of-the-art in mesoscale microsculpting technology. A significant effort is required in order to advance the capabilities to make these targets with very challenging geometries. Ultrashort pulsed (USP) Ti:Sapphire lasers and excimer lasers are proving to be very effective tools in the fabrication of the very small pieces that make up these targets. A brief description of the dimensional and structural requirements of these pieces will be presented, along with theoretical and experimental results that demonstrate to what extent these lasers are achieving the desired results, which include sub-µm precision and RMS surface values well below 100 nm. This work indicates that excimer lasers are best at sculpting the polymer pieces and that the USP lasers work quite well on metal and aerogel surfaces, especially for those geometries that cannot be produced using diamond machining and where material removal amounts are too great to do with focused ion beam milling in a cost effective manner. In addition, the USP laser may be used as part of the procedure to fill target capsules with fusion fuel, a mixture of deuterium and tritium, without causing large perturbations on the surface of the target by keeping holes drilled through 125 µm of beryllium below 5 µm in diameter.

Highly oriented pyrolytic graphite (HOPG) targets were irradiated by use of an ultra-short pulsed (pico to femtosecond) Ti:Sapphire laser operating at 825-nm wavelength. The morphology and quality of laser ablated surfaces were characterized by stylus profilometry, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy. The results were also compared with those obtained in nanosecond pulsed excimer laser ablation. The ablation rates expressed in depth per pulse were substantially higher for ultra-short pulsed lasers, and were in close agreement with the theoretical model predictions of laser–solid interactions in the short-pulse regime. Post-analysis of laser irradiated regions revealed a reduction in thermal effects and a decrease in the formation of diamond-like carbon as the pulse width was shortened. This work demonstrates the clean and precise machining capabilities of ultra-short pulsed lasers for HOPG that could be applied in the areas of thin-film deposition, nanotube synthesis, and dust-free machining.

We report an atomistic simulation study of laser-induced graphitization on the diamond (111) surface. Our simulation results show that the diamond to graphite transition occurs along different pathways depending on the length of the laser pulse being used. Under nanosecond or longer laser pulses, graphitization propagates vertically into bulk layers, leading to the formation of diamond-graphite interfaces after the laser treatment. By contrast, with femtosecond (0.2--0.5 ps) laser pulses, graphitization of the surface occurs layer by layer, resulting in a clean diamond surface after the ablation. This atomistic picture provides an explanation of recent experimental observations.

Precision microfabrication of diamond has many applications in the fields of microelectronics and cutting tools. In this work, an ultra-short pulsed Ti: Sapphire laser was used to perform patterning, hole drilling, and scribing of synthetic and CVD diamonds. Scanning electron microscopy, atomic force microscopy, profilometry, and Raman spectroscopy were employed to characterize the microstructures. A tight-binding molecular dynamics (TBMD) model was used to investigate atomic movements during ablation and predict thresholds for ablation. The ultra-short pulsed laser generated holes and grooves that were nearly perfect with smooth edges, little collateral thermal damage and recast layer. The most exciting observation was the absence of graphite residue that always occurs in the longer-pulsed laser machining. The ablation threshold for ultra-short pulsed laser was two orders of magnitude lower than that of longer-pulsed laser. Finite-difference thermal modeling showed that ultra-short pulses raised the electron temperatures of diamond in excess of 100,000 K due to multiphoton absorption, absence of hydrodynamic motion, and lack of time for energy transfer from electrons to the lattice during the pulse duration. TBMD simulations, carried out on (111) and (100) diamond surfaces, revealed that ultra-short pulses peel carbon atoms layer-by-layer from the surface, leaving a smooth surface after ablation. However, longer pulses cause thermal melting resulting in graphite residue that anchors to the diamond surface following ablation.

Three-dimensional (3D), periodic nanowriting on diamond clusters is reported in this letter. Concentric circular rings were observed on diamond microclusters, nucleated near the periphery of a laser-irradiated region, when chemical-vapor deposited diamond was processed in air, with laser pulses of 380 fs duration and at a wavelength of 248 nm. Periodic ripples also have been observed on single-crystal and polycrystalline diamond surfaces. Further, it is experimentally shown that the periodicity of these corrugated two-dimensional and 3D structures is shorter than that of the laser wavelength used (248 nm for the excimer fs laser and 825 nm for the Ti–sapphire fs laser).

Laser processing of diamond and chemical-vapor-deposited diamond thin films are important in the microelectronics and cutting tool industries because the manufacture of diamond films with low surface roughness and complex shapes has proven to be difficult. In this paper we present a review of current laser polishing and ablation processes followed by a discussion of ultrashort pulsed processing of diamond. Compared with the use of longer pulsed lasers, the use of 248-nm, 500-fs duration pulses at extreme intensity offered multiple advantages, including a lack of lateral thermal damage and significant improvements in the structural purity of the ablated surface. Experimental data, including Raman spectra and scanning electron micrographs, were presented to demonstrate the superior capabilities of this new class of lasers for diamond processing.

The use of ultrashort pulsed lasers in materials processing is an emerging technology. These lasers have the capability to ablate materials precisely with little or no collateral damage, even with materials that are impervious to laser energy from conventional pulsed lasers. The extreme intensities and short timescale at which ultrashort pulsed lasers operate differentiate them from other lasers. The means of ultrashort pulsed laser generation is discussed; included are a survey of pulse compressor techniques with solid state lasers and a brief discussion of excimer-dye lasers. This is followed by a discussion of specific examples of ultrashort pulsed machining of specific materials, along with mechanistic details. Optical breakdown mechanisms, including electron avalanche ionization and multiphoton absorption are discussed. It is shown that as pulse width increases and intensity decreases, laser damage becomes a stochastic process in which the ultrashort pulsed, high intensity light causes optical breakdown over a very narrow range. This, along with the lack of significant thermal conduction, greatly improves the precision of ultrashort pulsed lasers in micromachining applications. (C) 1998 Laser Institute of America. [S1042-346X(98)00401-X].

We have studied the effects of temporal pulse width and target density on the deposition of thin films of YBaCuO. A 248nm excimer laser and an 825nm Ti-sapphire laser were used to conduct the experiments with pulse widths of 27 ns, 16 ns, and 150 fs, and target densities of 80% and 90%. Scanning electron microscope photomicrographs and profilometer traces show a striking difference between nanosecond and femtosecond laser irradiation. Shortening the pulse width reduced particulate formation, provided stoichiometry, and improved the film properties. Decreasing the target density raised the ablation rate, produced thicker but nonuniform films, and reduced particulate formation.