H.C. Ives

Z-Beamlet: a multikilojoule, terawatt-class laser system

A large-aperture (30-cm) kilojoule-class Nd:glass laser system known as Z-Beamlet has been constructed to perform x-ray radiography of high-energy-density science experiments conducted on the Z facility at Sandia National Laboratories, Albuquerque, New Mexico. The laser, operating with typical pulse durations from 0.3 to 1.5 ns, employs a sequence of successively larger multipass amplifiers to achieve up to 3-kJ energy at 1054 nm. Large-aperture frequency conversion and long-distance beam transport can provide on-target energies of up to 1.5 kJ at 527 nm.

Pulsed power performance of PBFA Z

PBFA Z is a new 60 TW/5 MJ electrical driver located at Sandia National Laboratories. The authors use PBFA Z to drive Z pinches. The pulsed power design of PBFA Z is based on a conventional single-pulse Marx generator, water-line pulse-forming technology used on the earlier Saturn and PBFA II accelerators. PBFA Z stores 11.4 MJ in its 36 Marx generators, couples 5 MJ in a 60 TW/105 ns pulse to the output water transmission lines, and delivers 3.0 MJ and 50 TW of electrical energy to the Z-pinch load. Depending on the initial load inductance and the implosion time, one attains peak currents of 16-20 MA with a rise time of 105 ns. Current is fed to the Z-pinch load through self magnetically-insulated transmission lines (MITLs). Peak electric fields in the MITLs exceed 2 MV/cm. The current from the four independent conical-disk MITLs is combined together in a double post-hole vacuum convolute with an efficiency greater than 95%. The authors achieved X-ray powers of 200 TW and X-ray energies of 1.9 MJ from a tungsten wire-array Z-pinch loads.

Particle-in-cell simulations of electron flow in the post-hole convolute of the Z accelerator

The three-dimensional, particle-in-cell code QUICKSILVER [J. P. Quintenz et al., Lasers Part. Beams 12, 283 (1994)] is now being used to simulate the inner region of the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The simulations model electron flow and anode losses in the double post-hole convolute, which couples four radial, magnetically insulated transmission lines (MITLs) in parallel to a single MITL that drives a Z-pinch load. To efficiently handle the large range in the magnetic field, 0<B<200 T, the particle pusher is modified to subcycle the electron advance relative to the field solver. Results from a series of simulations using a constant-impedance load are presented. The locations of electron losses to the anode in the convolute are in qualitative agreement with damage to the Z hardware. The electron energy deposited in these anode regions rapidly heats the surface to temperatures above 400 °C—the threshold at which anode plasma formation...

The ZR refurbishment project

ZR is a refurbished (R) version of Z aiming to improve its overall performance, reliability, precision, pulse shape tailoring and reproducibility. Z, the largest pulsed power machine at Sandia, began in December 1985 as the Particle Beam Fusion Accelerator II (PBFA II). PBFAII was modified in 1996 to a z-pinch driver by incorporating a high-current (20-MA, 2.5-MV) configuration in the inner ∼ 4.5 meter section. Following its remarkable success as z-pinch driver, PBFA II was renamed Z in 1997. Currently Z fires 170 to 180 shots a year with a peak load current of the order of 18–20 MA. The maximum z-pinch output achieved to date is 1.6-MJ, 170-TW radiated energy and power from a single 4-cm diameter, 2-cm tall array, and 215 eV temperature from a dynamic hohlraum. ZR in turn will, operating in double shift, enable 400 shots per year, deliver a peak current of 26 MA into a standard 4cm × 2cm Z-pinch load, and should provide a total radiated x-ray energy and power of 3 MJ and 350 TW, respectively, achieve a maximum hohlraum temperature of 260 eV, and include a pulse-shaping flexibility extending from 100ns to 300ns for equation of state and isentropic compression studies. To achieve this performance ZR will incorporate substantial modifications and upgrades to Marx generator, intermediate store capacitors, gas and water switches, water transmission lines and the laser triggering system. Test beds are already in place, and the new pulsed power components are undergoing extensive evaluation. The Z refurbishment (ZR) will be operational by 2006 and will cost approximately

Operation of a five-stage 40000-cm 2 -area insulator stack at 158 kV/cm

60M.

Status and plans for the next generation magnetically immersed diodes on KITS

We have demonstrated operation of a 3.35-m-diameter insulator stack at 158 kV/cm with no total-stack flashovers on five consecutive Z-accelerator shots. The stack consisted of five +45/spl deg/-profile 5.715-cm-thick crosslinked-polystyrene (Rexolite-1422) insulator rings, and four anodized-aluminum grading rings shaped to reduce the field at cathode triple junctions. The width of the voltage pulse at 89% of peak was 32 ns. We compare this result to a new empirical flashover relation developed from previous small-insulator experiments conducted with flat unanodized electrodes. The relation predicts a 50% flashover probability for a Rexolite insulator during an applied voltage pulse when E/sub max/e/sup -0.27/d/(t/sub eff/C)/sup 1/10/=224, where E/sub max/ is the peak mean electric field (kV/cm), d is the insulator thickness (cm), t/sub eff/ is the effective pulse width (/spl mu/s), and C is the insulator circumference (cm). We find the Z stack can be operated at a stress at least 19% higher than predicted. This result, together with previous experiments conducted by Vogtlin, suggest anodized electrodes with geometries that reduce the field at both anode and cathode triple junctions would improve the flashover strength of multi-stage insulator stacks.

Effect of self-magnetic field on large pulsed insulators operated at 4 megavolts and 5 megaamperes

Sandia National Laboratories is investigating and developing high-dose, high-brightness flash radiographic sources. We are in the process of designing; fabricating and conducting engineering tests on the next-generation magnetically immersed electron diodes. These diodes employ unique, large-bore (80–110 mm), high-field (28–45 T), cryogenically-cooled solenoid magnets to help produce an intense electron beam from a needle-like cathode “immersed” in the strong Bz field of the magnet. The diode designs and status of the engineering development are described. Later this year we plan to conduct experiments with these sources on the new Radiographic Integrated Test Stand (KITS) [1], now in operation at Sandia. In its present three-stage configuration, KITS provides a 4-MV, 150-kA, 70-ns pulse to the diode. Fully three-dimensional particle in cell LSP code [2] simulations are used to investigate relevant physics issues and the expected radiographic performance (spot size and dose) of this system. Preliminary results from these simulations are described.

D-dot and B-dot monitors for Z-vacuum section power-flow measurements

Experiments on a large, high voltage pulsed insulator will be described. The goal was investigation of the effect of the self-magnetic field associated with current flow toward the load, on the insulator voltage hold-off capability. In principle, the self-magnetic field could improve insulator hold-off by deflecting electrons away from the insulator surface. However, it appears that no controlled experiments have been done to measure the effect of magnetic flashover-inhibition (MFI) on a 45-degree insulator. Most modern insulator stacks, including the one used here, have the insulators at 45 degrees to the power flow direction, often with reduced electric field at the cathode triple point. Results from the experiments presented in this paper do not show a significant beneficial effect from current flow in this situation. This paper will also show calculations on the effect of magnetic field on electrons emitted from the capacitively- coupled grading rings in the insulator stack

High-voltage hold-off of large surface area metal electrodes with dielectric surface layers

New differential D-dot and B-dot monitors were developed for the Z vacuum section of an accelerator pulsed power supply. The D-dots measure voltage at the insulator stack. The B-dots measure current at the stack and in the outer magnetically-insulated transmission lines. Each monitor has two outputs that allow common-mode noise to be cancelled to the first order. The differential D-dot has one signal and one noise channel; the differential B-dot has two signal channels with opposite polarities. Each of the two B-dot sensors in the differential B-dot monitor has four 3 mm diameter loops and is encased in copper to reduce flux penetration. For both types of probes, two 2.2 mm diameter coaxial cables connect the outputs to a Prodyn balun for common-mode-noise rejection. The cables provide reasonable bandwidth and generate acceptable levels of Compton drive in the bremsstrahlung field of the Z accelerator. A new cavity B-dot is being developed to measure the total Z current 4.3 cm from the axis of the z-pinch load. All of the sensors are calibrated with 2-4% accuracy. The monitor signals are reduced with Barth or Weinschel attenuators, recorded on Tektronix 0.5 ns/sample digitizing oscilloscopes, and software cable compensated and integrated.

Design validation of the PBFA-Z vacuum insulator stack

Aluminum and stainless steel electrodes with dielectric surface layers have been tested for the avoidance of electron emission and plasma induced arcs that limit high voltage hold-off. The aluminum dielectric coating is a 50-/spl mu/m-thick hard-anodized surface layer. Tests of the layer were made with and without a chemical etch to improve bonding to the machined aluminum. The coating on stainless steel is a 1-/spl mu/m-thick chromium oxide surface layer formed during high humidity hydrogen firing. Tests were performed on 15 and 17-cm-diameter electrodes, with 1.8 to 6 mm gaps. Tests were made with a 150 to 500 kV, 160-ns-FWHM waveform, with 1-cos(/spl omega/)t) shape. The electrodes were cleaned with acetone and installed into the vacuum chamber in a filtered clean air environment with <100 particles/m/sup 3/. Tests were performed with < 3x10/sup -6/ Torr vacuum. Data were obtained for virgin surfaces and as a function of accumulated damage from breakdown arcs. The anodized coatings gave maximum high voltage hold-off of 1.4 MV/cm when new and showed little degradation with 150-/spl mu/m-diameter damage craters in the cathode. Craters larger than 1-mm-diameter sometimes caused a factor of two degradation in hold off. It was less necessary to electrically condition the anodized surfaces to get optimum results. Results from each of the dielectric surface coatings and uncoated machined surfaces will be discussed.