H.C. Harjes

Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments

The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850μm aluminum (6061-T6) flyer plates to peak velocities of 34km∕s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during ac...

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

Results of initial testing of the four stage RHEPP accelerator

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The Repetitive High Energy Pulsed Power module

The low power checkout of the Repetitive High Energy Pulsed Power (RHEPP) pulse forming line (PFL) and linear induction voltage adder (LIVA) is complete. The accelerator has four LIVA cavities driven via coaxial cables from the PFL that utilizes magnetic switching to provide a 250-kV, 60-ns output pulse. The PFL is repetitively charged by a ten stage Marx generator to operate from single shot to five Hz. Results from these tests of the initial four stage RHEPP accelerator are presented and compared with design simulations. Data from a resistive cavity load and from preliminary electron diode experiments are included. While core temperatures remain low during five Hz operation, they are monitored and compared to extrapolated predictions from the design modeling. Performance of the Metglas magnetic switches and blocking cores, the voltage addition in the four LIVA cavities, and system efficiencies are discussed. Sources of discrepancies from the original design models are identified, and improved models that account for the discrepancies are presented. Improved performance potential based on these models is discussed. Plans for future testing of the 1-MV system up to 120 kW at 120 Hz and for the full system with ten LIVA cavities are presented.

A new laser trigger system for current pulse shaping and jitter reduction on Z

In the Repetitive High Energy Pulsed Power (RHEPP) module, pulse compression is done exclusively with magnetic switches (saturable reactors). Such switches have the potential of performing efficiently and reliably for >10/sup 10/ shots. The objective of the RHEPP project is to explore the feasibility of using magnetic pulse compression technology in continuous high average power applications. The RHEPP system consists of a compressor driving a linear induction voltage added with a diode load. Construction and initial testing in a bipolar mode of the first two stages of the compressor have been completed. This system has operated for a total of 332 min (4.8*10/sup 6/ pulses) at full power (600 kW) with an efficiency of 94+/-3%. The first stage magnetic switch has a pulse compression factor of 8.4 (4.2 ms to 500 mu s time to peak). It has two parallel-connected, 67 n turns copper coils and a 760 kg core of 2 mil silicon steel with a magnetic cross sectional area of 0.065 m/sup 2/. The second stage magnetic switch has a pulse compression factor of three (500 mu s to 170 mu s). It has two parallel-connected, 36 turn copper coils and a 361 kg core of field annealed 2605 CO Metglas with a magnetic area of 0.019 m/sup 2/.<<ETX>>

Design options for a pulsed-power upgrade of the Z accelerator

A new laser trigger system (LTS) has been installed on Z that benefits the experimenter with reduced temporal jitter on the X-ray output, the confidence to use command triggers for time sensitive diagnostics and the ability to shape the current pulse at the load. This paper presents work on the pulse shaping aspects of the new LTS. Pulse shaping is possible because the trigger system is based on 36 individual lasers, one per each pulsed power module, instead of a single laser for the entire machine. The firing time of each module can be individually controlled to create an overall waveform that is the linear superposition of all 36 modules. In addition, each module can be set to a long-pulse mode or short-pulse mode for added flexibility. The current waveform has been stretched from /spl sim/100 ns to /spl sim/250 ns. A circuit model has been developed with BERTHA Code, which contains the independent timing feature of the new LTS to predict and design pulse shapes. The ability to pulse-shape directly benefits isentropic compression experiments (ICE) and equation of state measurements (EOS) for the shock physics programs at Sandia National Laboratories. With the new LTS, the maximum isentropic loading applied to Cu samples 750 um thick has been doubled to 3.2 Mb without generating a shockwave. Macroscopically thick sample of Al, 1.5 mm, have been isentropically compressed to 1.7 Mb. Also, shockless Ti flyer-plates have been launched to 21 km.s/sup -1/, remaining in the solid state until impact.

Status of repetitive pulsed power at Sandia National Laboratories

We are presently considering an extensive modification of the Z accelerator at the Sandia National Laboratories to both increase the current and radiative power, and to improve the facility, diagnostics, and shot rate. Record-breaking peak x-ray powers and Hohlraum temperatures achieved in z-pinch experiments on this machine motivate this effort. The electrical design goal of the upgrade is to drive a 40-mm diameter, 20-mm long wire-array z-pinch load with a peak current of 26 MA with a 100-ns implosion. Several changes to the pulsed-power design of Z are being considered. They are to increase the energy and lower the inductance of the Marx bank, increase the capacitance of the intermediate-store water capacitor, increase the voltage hold-off capability of the laser-triggered gas switch, lengthen the first section of the water pulse-forming line, remove impedance mismatches in the pulse-forming line, and adjust field grading on the water side of the insulator stack. With these changes Z will be able to provide peak currents greater than 26 MA, and x-ray energies exceeding 2.7 MJ. We plan to use the existing oil and water tanks, use the existing insulator stack and MITLs, and as much of the existing Marx-bank hardware as feasible. Circuit-code calculations for one design option are shown. The results of these simulations, when applied to standard water-breakdown criteria, are used to determine the size of the intermediate store (IS) and pulse-forming-line (PFL) components. We also indicate where further component development is needed.

FANTM: the first article NIF test module for the laser power conditioning system

Multi-kilojoule repetitive pulsed power technology moved from a laboratory environment into its first commercial application in 1997 as a driver for ion beam surface treatment. Sandias RHEPP II (Repetitive High energy Pulsed Power), a repetitive 2.5 kJ/pulse electron beam accelerator, has supported the development of radiation treatment processes for polymers and elastomers, food products, and high dose-rate effects testing for defense programs since early 1996. Dos Lineas, an all solid-state testbed, has demonstrated synchronization techniques for parallel magnetic modulator systems and is continuing the development of design standards for long lifetime magnetic switches and voltage adders at a shot rate capability that exceeds 5/spl times/10/sup 6/ pulses per day. This paper describes progress in multi-kilojoule class repetitive pulsed power technology development, magnetic switching technology for modulator applications, and future research and development directions.

Pulse shaping of the load current on the Z accelerator

Designing and developing the 1.7 to 2.1-MJ power conditioning system (PCS) that powers the flashlamps for the National Ignition Facility (NIF), currently being constructed at Lawrence Livermore National Labs (LLNL), is one of several responsibilities assumed by Sandia National Labs (SNL) in support of the NIF Project. The test facility that has evolved over the last three years to satisfy the project requirements is called FANTM. It was built at SNL and has operated for about 17,000 shots to demonstrate component performance expectations over the lifetime of NIF. The final full NIF system will require 192 PCSs (48 in each of four bays). This paper briefly summarizes the final design of the FANTM facility and compares its performance with the predictions of circuit simulations for both normal operation and fault-mode response. A physics-based, semi-empirical amplifier gain code indicates that the 20 capacitor PCS can satisfy the NIF requirement for an average gain coefficient of 5.00 %/cm and can exceed 5.20 %/cm with 24 capacitors.

Investigations into the use of dielectric coatings in magnetic switches

The refurbishment project (ZR) for the Z accelerator at Sandia National Laboratories. McDaniel, DH. et al., (2002) is required to provide the capability to temporally shape the current pulse at the load, primarily for isentropic compression experiments (ICE) Hall, CA. et al., (2001). The baseline design to accomplish this uses staggered triggering of the 36 pulse-forming lines that are added in parallel. With relative timing differences between the lines, and each line run in either a short-pulse mode or a long-pulse mode, there are many degrees of freedom for achieving a required pulse shape at the load. This approach is currently being evaluated with circuit simulations, 3D, particle-in-cell (PIC) simulations, and experiments on Z. Results from these experiments have been very successful.