T. C. Wagoner

Wire array implosion characteristics from determination of load inductance on the Z pulsed-power accelerator

The time-dependent inductance of Z pinches and other loads on the Z pulsed-power accelerator at Sandia National Laboratories [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] is determined by using electrical measurements and a lumped-circuit analysis. One finds that ∑kαkVk−LI=d(LlIl)/dt, where ∑kαkVk is the weighted sum of the Z-insulator-stack voltages, L is the equivalent inductance of the magnetically insulated transmission lines connecting the stack to the load, I is the added current for those lines, Ll is the load inductance, and Il is the load current. Ll obtained from this expression is used to reconstruct the motion of the outer edge of wire-array Z-pinch loads, providing an estimate of the time at which the cores start moving significantly from their initial position. Results are consistent with previous optical measurements suggesting that core motion is delayed with respect to a zero-dimensional thin-shell model of the implosion. These results provide useful insights and constraints in ex...

Circuit Model for Driving Three-Dimensional Resistive MHD Wire Array

Compact tungsten wire array Z-pinches imploded on the Z generator at Sandia National Laboratories have proven to be a powerful reproducible X-ray source. Wire arrays have also been used in dynamic hohlraum radiation flow experiments and as an intense K-shell source, while the generator has been used extensively for isentropic compression experiments. A problem shared by all these applications is current loss, preventing the ~20-MA drive current from being reliably coupled to the load. This potentially degrades performance, while uncertainties in how this loss is described limit our predictive capability. We present details of a transmission line equivalent circuit model of the Z generator for use in driving 3-D resistive MHD simulations of wire array loads. We describe how power delivery to these loads is affected by multiple current losses and demonstrate how these may be calculated or reconstructed from available electrical data for inclusion in the circuit model. We then demonstrate how the circuit model and MHD load calculation may be combined to infer an additional current loss that has not been directly diagnosed for wire arrays.

Z

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.

-Pinch Calculations

PBFA Z, a new 60-TW/5-MJ electrical accelerator located at Sandia National Laboratories, is now the world’s most powerful z-pinch driver. PBFA Z stores 11.4 MJ in its 36 Marx generators, couples 5 MJ into a 60-TW/105-ns FWHM pulse to the 120-mΩ water transmission lines, and delivers 3.0 MJ and 50 TW of electrical energy to the z-pinch load. Depending on load parameters, we attain peak load currents of 16–20 MA with a current rise time of ∼105 ns with wire-array z-pinch loads. We have extended the x-ray performance of tungsten wire-array z pinches from earlier Saturn experiments. Using a 2-cm-radius, 2-cm-long tungsten wire array with 240, 7.5-μm diameter wires (4.1-mg mass), we achieved an x-ray power of 210 TW and an x-ray energy of 1.9 MJ. Preliminary spectral measurements suggest a mostly optically-thick, Planckian-like radiator below 1000 eV. Data indicate ∼100 kJ of x rays radiated above 1000 eV. An intense z-pinch x-ray source with an overall coupling efficiency greater than 15% has been demonstrated.

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

Very high current generators are being developed to drive compact loads leading to conductors carrying very high current densities. Losses in conductors include resistive, magnetic field diffusion, pdV work, and material motion contributions. We have designed and executed experiments on Sandias 100-ns rise time, 20 MA Z accelerator to quantify those losses at current densities reaching 10 MA/cm. In these experiments we delivered nearly 20 MA to both high-current density and low-current density short circuit loads. We used B-dot probes and VISAR techniques to measure the magnetic field near the load. A reduction in the delivered current of /spl sim/15% over the 20 MA peak current prediction made without resistive losses was observed. Comparisons of these data with radiation magneto-hydrodynamics codes (RMHD) are presented. Implications on the efficiency of next generation pulsed power drivers are discussed.

PBFA Z: A 60-TW/5-MJ Z-pinch driver

With the successful completion of its refurbishment the Z machine at Sandia is now routinely operating with currents over 26 MA into various loads. Now that the machine is operating we can measure current and voltage at various locations throughout the machine and compare with circuit code predictions. These measurements have led to improvements in the model that provide a more accurate predictive capability. In this paper we describe the full-machine circuit model of Z, and indicate how machine parameters are derived. Many were determined with commercially-available field calculation software, but parameters for switches and other non-linear elements were determined empirically. We show comparisons of circuit code predictions with machine performance. Finally, we show where improvements to the model can yet be made.

Conductor energy losses at 10 MA/cm on Z

MHD models of imploding loads fielded on the Z accelerator are typically driven by reduced or simplified circuit representations of the generator. The performance of many of the imploding loads is critically dependent on the current and power delivered to them, so may be strongly influenced by the generators response to their implosion. Current losses diagnosed in the transmission lines approaching the load are further known to limit the energy delivery, while exhibiting some load dependence. Through comparing the convolute performance of a wide variety of short pulse Z loads we parameterize a convolute loss resistance applicable between different experiments. We incorporate this, and other current loss terms into a transmission line representation of the Z vacuum section. We then apply this model to study the current delivery to a wide variety of wire array and MagLif style liner loads.

Circuit-Code Modeling of the Refurbished Z Accelerator: Comparison of Measurements with Predictions

The ZR upgrade to the Z accelerator at Sandia National Laboratories will replace much of the power flow components. The single most massive of these components is the 1.6 meter diameter, four-feed MITL system. This 14.5 Ton assembly transports electromagnetic energy from the vacuum interface to the load. The MITLs must cany 30 MA total with 99% reliability. Optimum performance requires careful control of the transmission line gaps. The MITLs must support the load while maintaining strict gap tolerance under their own weight and the weight of diagnostic assemblies. Dynamically they must be robust enough to withstand accelerations and impacts similar if not greater than those on Z today. Stress and deflection calculations will be shown for the self-deflection, diagnostic loading, and dynamic motion present in the system. The MITLs are built in multiple annular sections to reduce cost and allow replacement of damaged pieces. Reliable power delivery demands that the multi-piece MITLs be joined with current-carrying contacts that do not introduce plasma or neutrals into the power flow gap. The design of the ZR MITL system and the design of the various current contacts will be shown. The current contacts were designed to maintain a continuous static pressure on soft copper gaskets to achieve intimate conductor contact. Testing on Z will be done in order to validate FEA pressure calculations as well as the current contact concept itself.