When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2 kA, 18 MeV electron beam with more than 1500 ns current/energy “flat-top.” In initial tests DARHT-II has already accelerated beams with current pulse lengths from 500 ns to 1200 ns full-width at half maximum (FWHM) with more than 1.2 kA peak current and 12.5 MeV peak energy. Experiments will soon begin with a ∼ 1600 ns flat-top pulse, but with reduced current and energy. These pulse lengths are all significantly longer than any other multi-MeV LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated the robustness of the DARHT-II LIA to BBU, the < 1200 ns FWHM pulse lengths were too short to test the predicted protection against ion-hose instability. The present experiments are designed to resolve these and other beam-dynamics issues with a ∼ 1600 ns pulse length beam.

The DARHT-II linear-induction accelerator has been successfully operated at 1.2-1.3 kA and 12.5-12.7 MeV to demonstrate the production and acceleration of an electron beam. Beam pulse lengths for these experiments were varied from 0.5 mu s to 1.2 mu s full-width half-maximum. A low-frequency inductance-capacitance (LC) oscillation of diode voltage and current resulted in an oscillation of the beam position through interaction with an accidental (static) magnetic dipole in the diode region. There was no growth in the amplitude of this oscillation after propagating more than 44 m through the accelerator, and there was no loss of beam current that could be measured. The results of these initial experiments are presented in this paper.

This paper describes a low energy flash X-ray system that is ideal for radiographing a wide variety of experimental phenomenon on both capacitor-bank pulsed power facilities and explosively driven magnetic-flux compression experiments. The versatility of this system has allowed us to obtain both single X-radiographs of imploding liners and multiple, temporally resolved radiographic sequences of target evolution. The dynamic liner radiographs are acquired with radially oriented X-ray heads that are instrumental for observing and diagnosing liner shape and symmetry, Rayleigh-Taylor instability growth, and liner-glide plane interaction (see Fig. 1). Multiframe radiographs acquired along the axis of a cylindrical target are used to provide physical data on phenomena such as shock-driven target hydrodynamics, Richtmyer-Meshkov instability growth, spall, fiction, and equations of state. The flexibility of this X-ray system has also allowed it to be successfully fielded both at various gas and powder gun facilities and explosively driven shock physics experiments.

The second axis of the Dual Axis Radiographic Hydro-Test (DARRT) facility will provide up to four short (< 150 ns) radiation pulses for flash radiography of high-explosive driven implosion experiments[1]. To accomplish this the DARHT-II linear induction accelerator (LIA) will produce a 2-kA electron beam with 18-MeV kinetic energy, constant to within +/-0.5% for 2mus. A fast kicker will cleave four short pulses out of the 2mus flattop, with the bulk of the beam diverted into a dump. The short pulses will then be transported to the final-focus magnet, and focused onto a tantalum target for conversion to bremsstrahlung pulses for radiography. DARHT-II is a collaborative effort between the Los Alamos, Lawrence Livermore, and Lawrence Berkeley National Laboratories of the University of California.

The dual axis radiographic hydrotest facility (DARHT) at Los Alamos National Laboratory produces flash radiographs of hydrodynamic experiments using two linear induction accelerators situated on orthogonal axes. The first axis accelerator is operational and has produced radiographs of hydrodynamic experiments. The second axis accelerator, which is in the commissioning stage, will generate an 18-MeV, 2-kA, 2-/spl mu/s electron beam. These parameters are reduced during the initial phase of commissioning to /spl ges/12-MeV, /spl ges/1-kA, and /spl ges/350-ns. These reduced parameters allow lower voltages and fewer components than what will be necessary for the final machine. This paper presents experimental results of commissioning the second axis DARHT accelerator, including an overview of the pulsed power system plus electrical performance data from the 3-MV injector Marx generator and the 200-kV, 2-/spl mu/s induction accelerator cells. Beam transport data will also be presented.

We discuss the design, fabrication, and operation of a liner implosion system At peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of a axial current, rising in 8 mus. Fields; on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and. the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

Electromagnetically-driven implosion of solid-density, cylindrical liners can launch shocks with excellent precision at impact speeds exceeding 5 km/s. We discuss the design and operation of liner implosions driven at peak currents of 16MA, using the Shiva Star capacitor bank at the Air Force Research Laboratory. Liners of 1100 aluminum, with initial length, radius and thickness of 4 cm, 5 cm and 1 mm, respectively, implode under the action of an axial current, rising in 8 /spl mu/s. Fields on conductor surfaces exceed 0.6 MG. The inner surface of the liner achieves a speed of 6.25 km/s when it impacts a concentric target cylinder of tin at a radius of 2 cm. Magnetic probes and radially-aligned X-radiography follow the motion of the liner and its impact on the tin cylinder. This cylinder holds a solid cylinder of acrylic of 1.5 cm radius in which the motion of a converging shock is followed by optical shadowgraphy and axially-aligned, X-radiography. Design issues that were successfully addressed include: Pulsed Power - current joints at high magnetic fields in the vicinity of the liner and glide-plane/electrodes, where magnetic pressures quickly exceed values for mechanical pre-stress, requiring dynamic solutions; surface temperature enhancements at changes in current direction; possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; need for magnetic inhibition of breakdown (MIB) between liner surface and insulator; Liner Physics - angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; nonlinear magnetic diffusion into liner and associated melting; Diagnostics X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

The NTLX series of experiments are focused on measuring the shock induced hydrodynamic flow of a Sn-PMMA target. For these experiments multi-frame flash X-ray radiography is used to measure the position of the Sn-PMMA target interface and the location of shock in the PMMA as a function of time. Four radiographs are acquired at 700 ns intervals having a line-of-sight following the target's axis of symmetry. Because the X-ray spectrum from the sources has an end-point energy of /spl sim/300 keV with a strong component of /spl sim/60 keV tungsten K-line radiation, the Sn portion of the target is radiographically opaque. However, X-rays are transmitted through the PMMA portion of the target thereby allowing motion of the Sn-PMMA interface to be imaged. Also, the shock location is tracked as a function of time due to the density increase in the shocked PMMA. The resulting radiographs are analyzed to provide the trajectory and shape of both the shock and Sn-PMMA interface. In addition, the shock velocity in the Sn is determined for asymmetric target geometries.

Physics Division 94 Progress Report 1997–1998 Introduction In 1997 and 1998, a series of three subcritical plutonium experiments was conducted at the Nevada Test Site (NTS). These experiments, which are the first of their kind to be completed at NTS since the moratorium on underground nuclear testing in 1992, have several purposes. Foremost among these purposes is the study of plutonium physics and the maintenance of our readiness to resume underground nuclear testing should the need arise. These experiments were designed and fielded by the Dynamic Experiments (DX) Division in collaboration with the Physics Division and other Los Alamos divisions, as well as Sandia National Laboratory and Lawrence Livermore National Laboratory. The equation-of-state (EOS) measurement techniques were developed in DX Division, and they have been used by DX Division and its predecessors (GMX and M Divisions) for the past 40 years. Many of the diagnostic and recording techniques, especially for remote data collection, were developed by Physics Division for underground nuclear tests and pulsed power facilities. The first two experiments, called Rebound and Stagecoach, were executed in July 1997 and March 1998, respectively. Their focus was the plutonium EOS. The third experiment, Cimarron, was conducted in December 1998. Its focus was the ejecta produced when a shock in the plutonium releases into the surrounding vacuum. The data from all three experiments are classified; therefore, this discussion will focus on the experiments themselves and the roles of the Detonation Science and Technology (DX-1), Hydrodynamic and X-Ray Physics (P-22), and Neutron Science and Technology (P-23) groups.

The Rayleigh-Taylor Mix (RTMIX) project will attempt to diagnose and understand the growth of a mixing layer at the interface between an imploding metal liner and a polystyrene foam core in a series of pulsed power experiments on the Pegasus capacitor bank. Understanding the effects of material strength will be an important part of the study. During the initial phase of the implosion, the liner/foam interface is Rayleigh-Taylor (RT) stable; however, as the foam is compressed, it decelerates the liner causing it to bounce and to go RT unstable. In this paper, we report 1D and 2D MHD simulations and preliminary results from the first experiment in the series.

We have designed and are building a subpicosecond electron injector. The injector is based on an 8 MeV photoinjector, used previously at Los Alamos in the APEX experiment. The nominal design includes magnetically compressing a 20 ps long, 3 nC bunch to a FWHM bunch length of 2/3 ps (peak current in excess of 3 kA) using a four dipole chicane buncher. The geometrical averaged transverse normalized transverse emittance after compression is about 15 /spl pi/ mm mrad.

We have found that a potassium-iodide photocathode of an x-ray streak camera responds to UV light at λ=308 nm. The photocathode surface work function, 6.5 eV, is larger than the 4 eV energy of the UV photon, hence the source of the response is interesting. We will present results on the response of a transmission type potassium-iodide photocathode to the UV light from a λ=308 nm, subpicosecond XeCl laser and from a λ=325 nm HeCd laser. We will test for the nonlinearity of the yield by measuring the number of photons that are needed to be absorbed before a signal is recorded. We will present data on the effect of the UV irradiance on the yield, as well as on the temporal width of the recorded signal. We will give an explanation of the observation and its effect on the dynamic- range response of the streak-camera. We will show that the response is linear with the incident irradiance, up to an incident irradiance of 108 W/cm2 and we will explain the observation.