F. C. Perry

A radial ion diode for generating intense focused proton beams

A magnetically insulated light ion diode which has produced an ion beam with total current exceeding 400 kA for 25 ns and generated a proton current density approaching 500 kA/cm2 is described. This intense beam current is achieved in a noncurrent neutralized mode via geometric focusing and a balance between self‐magnetic field and space‐charge forces. A number of techniques are described which have been used to diagnose the beam production, transport, and focusing. These include observation of Kα emission due to beam‐induced atomic excitation, prompt‐γ radiation due to beam‐induced nuclear reactions, and thermal emission due to beam‐target heating.

Investigation of shell stability in imploding cylindrical targets

As part of a program to determine the feasibility of inertial confinement fusion (ICF), the physics of implosion stability is being studied. Ablatively‐driven double‐shell cylinders with and without initial periodic perturbations on the outer edge of the pusher were imploded using a single electron beam. Four‐pulse holographic shadowgraphy yielded spatially and temporally resolved images of the implosions. The experiments are in a regime where fluidlike behavior is expected to dominate. A comparison of experimental data on the free‐surface motion with two‐dimensional, planar‐geometry numerical calculations which include materials effects indicates shock‐accelerated unstable growth of fabrication irregularities at the perturbed material interface. Peak pressures of 0.26 TPa (2.6 Mbars) are inferred in the high‐density pusher material. Both the experiment and the calculation show a decrease in the amplitude of the free‐surface perturbations at late time. In the experiment this decrease in amplitude begins e...

Energy deposition of superpinched relativistic electron beams in aluminum targets

Dynamic response data, which traditionally have been used to obtain equation‐of‐state (EOS) information of materials, were instead used here to study energy deposition of an intense (∼1011 W/cm2) tightly focused relativistic electron beam (REB). Measurements of the REB‐induced shock‐wave transit time and average rear‐surface velocity were compared with two‐dimensional hydrodynamic calculations which contain well‐known EOS information for 6061‐T6 aluminum. The experimental results were consistent with classical electron deposition, i.e., a one‐dimensional Monte Carlo transport calculation. In addition, peak pressures in the range 1–2 Mbar (0.1–0.2 TPa) were implied. Two anomalous effects were observed: (i) a low‐amplitude (free‐surface velocity ∼104 cm/sec) precursor signal, preceding the REB‐induced shock wave and (ii) a velocity distribution of material behind the rear surface of the target following the arrival of the REB‐induced shock wave.

Shock focusing in implosions of inertial fusion model targets

Implosions of high‐gain targets for inertial confinement fusion must be highly spherically symmetric for efficient ignition. Using a single relativistic electron beam at low power (∼0.3 TW) and cylindrical targets, we have experimentally shown that loading asymmetries generate strong implosion asymmetries, resulting in poor convergence ratios. By introducing a nonspherical variation in the shell radius and thickness (shimming), the implosion symmetry is radically improved at one time during the implosion. The effect is explained by a shock focusing mechanism whereby ultrahigh pressure—0.9 Tpa (9 Mbar) —is achieved on one side of the target.

Dynamic response of pulse‐heated porous copper by the double interferometer technique

A new interferometer concept has been successfully applied to the measurement of dynamic response associated with thermally induced stress waves. The technique involves the use of two Michelson interferometers to simultaneously measure front and back surface displacements of a specimen exposed to an intense electron pulse. Application of the method to porous copper (86 and 70% of the theoretical density) has resulted in dynamic data over an energy range spanning elastic and plastic behavior of the porous material. These data were compared to the predictions based on a hydrodynamic model for stress wave generation and propagation in a porous medium. The results indicated semiquantitative agreement, complete agreement not being possible using a single set of porous material model parameters. The study demonstrated that some important differences exist in porous material response to constant volume heating and planar plate impact loading. Finally, the results indicated that the double interferometer techniqu...

Shock effects in particle beam fusion targets

At Sandia National Laboratorics we are assessing the response of fusion target materials to shock loading with the particle beam accelerators HYDRA and PROTO I and the gas gun facility. Nonlinear shock‐accelerated unstable growth of fabriction irregularities has been demonstrated, and jetting is found to occur in imploding targets because of asymmetric beam deposition. Cylindrical ion targets display an instability due either to beam or target nonuniformity. However, the data suggest targets with aspect ratios of 30 may implode stably. The first time‐ and space‐resolved measurements of shock‐induced vaporization have been made. A homogeneous mixed phase EOS model cannot adequately explain the results because of the kinetic effects of vapor formation and expansion.

Pressure response of a solid pulse heated through a polymorphic phase transition: α‐β inversion in quartz

Using 70‐ns bursts of 3‐MeV average energy electrons as the heating source, inertial thermomechanical stresses were produced in X‐cut quartz disks. The range of peak absorbed energy density was from 30 to 360 cal/g. The stress pulses propagated through the samples (direction perpendicular to the c axis), and the ensuing rear surface motion was recorded with a laser interferometer. At the highest dose, the peak temperature produced in the absorber was calculated to exceed 1600 K, well in excess of the α‐β transition temperature. Comparison of the experimental results with a model calculation, taking into account the shift of transition temperature with pressure, suggests that the phase transition occurred and that the response was not associated with a metastable state.

Implosion dynamics of a hemispherical target irradiated by an intense relativistic electron beam

An intense tightly pinched relativistic electron beam from the Hydra accelerator (250 kA, 800 kV, 80 nsec) was used to produce an ablation driven implosion of a dense hemispherical shell. A laser reflection measurement was used to measure the implosion time and the velocity of the jet of material produced following the implosion. X‐ray pinhole photography and optical holography were used to study the uniformity of target loading and ablator motion. Target dynamics was modeled by a two‐dimensional Eulerian hydrodynamic code. The results indicated 17–50% of the total beam energy was delivered to the target, where the total beam energies varied between 17 and 20 kj. The uniformity in target loading varied between 20 and 80% from pole to equator. Suggestions are given for improving beam accuracy and target loading uniformity.

Characterization of the dose from a pulsed electron beam using holographic interferometry

Energy deposition of a pulsed electron beam has been investigated using double‐pulse holographic interferometry. The technique has been shown to provide depth‐dose data over the illuminated area for a thermoelastic absorber. Possibilities of employing the method at higher doses are discussed.