R.C. Weingart

X-Ray-Induced Photoconductivity in Dielectric Films

We have measured the conductivity induced in films of polyethylene, epoxy, polytetrafluoroethylene, polyethylene terephthalate, polyimide, and glass by x rays at dose rates between 109 and 1010 rad/sec (dose in air). The films were 0.05 to 1.25 mm thick. The x-ray spectrum peaked in the vicinity of 10 keV, and the x-ray pulse width was about 40 nsec FWHM. X-ray induced photocurrents were found to obey Ohms law at low bias voltages (less than 1 kV). Above 1 kV, however, we observed that the peak photoconductivity signals from some of the 0.05-mm-thick materials began to increase at a slightly faster than linear rate with bias voltage. The glass samples exhibited no apparent delayed conductivity, while the other sample materials showed various amounts. The magnitude of the delayed conductivity in polytetrafluoroethylene, polyethylene terephthalate, and polyimide depended on the electric field, an effect that is consistent with Poole-Frenkel field assisted carrier generation. We have qualitatively described the magnitude and time dependence of the conductivity signals by a simple trapping model, using reasonable values for mobility, trap density, capture cross sections and trap depths.

Electric gun: a versatile tool for high-pressure shock-wave research

We have developed a versatile tool for generating planar shock waves. This system, which we call the electric gun, is capable of projecting thin flyer plates with velocities in the range 1–20 km/s. It is presently being used in high‐explosives‐initiation experiments and is being developed for equation‐of‐state measurements in the 1–5 TPa range. We describe the electric gun facilities that are operational at Lawerence Livermore Laboratory and discuss applications of electric gun technology to problems of interest to shock‐wave researchers.

Precision stress measurements in severe shock‐wave environments with low‐impedance manganin gauges

We describe new techniques that permit the use of low‐impedance manganin stress gauges in chemically reacting shock waves in the 1.0–40.0 GPa range. The rugged, small, and fast response gauge has reproducibility better than 2% when used in conjunction with a pulsed bridge circuit and adjustable, current‐regulated power supplies. Techniques are presented for fabricating the transducer package, calibrating the bridge circuit and oscilloscopes, designing the drive system, and reducing the data. Data are presented for planar impact experiments performed with a 102‐mm gas gun on high‐explosive samples. In particular, we directly measured the Chapman‐Jouquet pressure in the explosive RX03‐BB [92.5% triaminotrinitrobenzene (TATB)/7.5% polychlorotrifluoroethylene (Kel‐F binder)] as 28.2±0.6 GPa. These new developments open the possibility of applying low‐impedance manganin gauges in chemically reactive hydrodynamic flows such as the evolution of a shock wave into a detonation wave.

Operating characteristics and modeling of the LLNL 100-kV electric gun

In the electric gun, the explosion of an electrically heated metal foil and the accompanying magnetic forces drive a thin flyer plate up a short barrel. Flyer velocities of up to 18 km/s make the gun useful for hypervelocity impact studies. The authors review the technological evolution of the exploding-metal circuit elements that power the gun, describe the 100-kV electric gun designed at Lawrence Livermore National Laboratory (LLNL) in some detail, and present the general principles of electric gun operation. They compare the experimental performance of the LLNL gun with a simple model and with predictions of a magnetohydrodynamics code. >

Temperature Dependence of X-Ray-Induced Photoconductivity in Kapton and Teflon

We have measured the X-ray-induced photoconductivity in Kapton (Dupont polyimide) and Teflon over the temperature range 100-5000K. The observed temperature dependence of the photoconductivity was strikingly different for these two materials. The qualitative behavior of the Kapton samples was consistent with the predictions of a model where the delayed photoconductivity signal is due to thermal release of trapped charge. In the case of the Teflon samples, the observed prompt conductivity was almost temperature independent and we observed a pronounced peak in the delayed component of the photoconductivity at about 360°K. The decay time of the delayed photoconductivity for Teflon was also observed to be temperature dependent. We discuss changes in the occupation of deep trapping levels as a possible mechanism for the observed thermal quenching of the Teflon photoconductivity.

X-Ray-Induced Photoconductivity in Dielectric Materials

X-ray-induced photoconductivity has been measured in low-density polyethylene, Tefzel¿ (Dupont fluorocarbon), Durasan (Dupont fluorocarbon), polychlorotrifluoroethylene (Kel-F), polystyrene, polypropylene, polyvinylidene chloride, polyvinylidene fluoride, polyvinyl chloride and mica. Dose rates varied in the range 109 - 5 × 1010 rad/s. The observed photoconductivity showed no apparent dependence on the molecular structure of the materials. The photoconductivity of polyethylene and Kel-F was observed to depend on the degree of crystallinity in these materials.

Application of a 100-kV electric gun for hypervelocity impact studies

Abstract The Lawrence Livermore National Laboratory 100-kV electric gun has been used to launch flat-plate projectiles for use in studies of spall and hypervelocity impact penetration of thin plates. Impactors were 0.3-mm thick Kapton with dimensions and velocities ranging from 100 mm 2 at 4 km/s to 10 mm 2 at 18 km/s. A Fabry-Perot laser velocimeter, an electronic streak camera, and a flash x ray were used as diagnostics of the flyer-plate impact on the selected specimen. Experiments generally included the recovery of the remnant specimen and fragments for detailed examination, permitting a study of incipient spall, onset of melting, and fraction fragmented. Experiments to be described include spall measurements on simple and composite target walls at normal and oblique incidence and “reverse ballistics” impacts of the thin-plate impactor on a stationary penetrator (e.g., Kapton impactors at 15 km/s incident on rods of steel, aluminum, and lead) for calibration of hypervelocity impact codes.

Hypervelocity acceleration and impact experiments with the LLNL electric guns

Abstract In the electric gun, the explosion of an electrically heated metal foil and the accompanying magnetic forces are used to accelerate a thin flyer plate to velocities as high as 18 km/s. Here we report preliminary results of a study to extend this capability to the acceleration of projectiles or particles ranging in size from micrometeorites to chunky projectiles with a mass as high as 0.5 g. We also have started code calculations of projectile impacts on thick aluminum witness plates for comparison with observations of experimentally produced cratering.

Performance of a 100 KV, 78 KJ electric gun system

We have constructed a new electric gun system for use in high‐pressure EOS studies. The system is powered by a 100 kV, 15.6 μF capacitor bank. At 100 kV charging voltage the system inductance is 23 nH. This system has driven 0.3 mm‐thick Kapton projectiles to ≳20 km/s and 0.3 mm Kapton 30 μm Ta projectiles to ∼10 km/s. Projectile velocity is modeled phenomenlogically by an electrical Gurney model.

Shock Hugoniot measurements on Ta to 0. 78 TPa

Symmetric impact shock Hugoniot measurements have been made on Ta with an electrically exploded foil gun system. The results obtained to date for the Hugoniot of Ta cover the range 0.19 to 0.78 TPa (impact velocities from 4.0 to 9.7 km/s) and agree with data obtained by other researchers to within 2.7% rms. Recent improvements in the system include electromagnetic shielding of impactor and target, continuous measurement of impactor velocity with a Fabry-Perot interferometer and computer-aided analysis of shot film. Conservative extrapolation from current operating conditions indicate that pressures of 1.1 to 1.5 TPa could be achieved with little difficulty.