The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state (EOS) experiments at pressures up to 10Mbar in aluminum. Development of multidimensional radiation-magnetohydrodynamic codes, coupled with more accurate material models (e.g., quantum molecular dynamics calculations with density functional theory), has produced synergy between validating the simulations and guiding the experiments. Z is now routinely used to drive ICF capsule implosions (focusing on implosion symmetry and neutron production) and to perform HEDP experiments (including radiation-driven hydrodynamic jets, EOS, phase transitions, strength of materials, and detailed behavior of z-pinch wire-array initiation and implosion). This research is performed in collaboration with many other groups from around the world. A five year project to enhance the capability and precision of Z, to be completed in 2007, will result in x-ray energies of nearly 3MJ at x-ray powers >300TW.
We have developed a system of differential-output monitors that diagnose current and voltage in the vacuum section of a 20-MA 3-MV pulsed-power accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuum-insulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The inner-MITL monitors are located 6 cm from the axis of the load. Each of the stack and outer-MITL current monitors comprises two separate B-dot sensors, each of which consists of four 3-mm-diameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100-ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the B-dot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Common-mode-noise rejection is achieved by combining these signals in a 50-balun. The signal cables that connect the B-dot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single 50-cable transmits the output signal of each balun to a double-wall screen room, where the signals are attenuated, digitized (0:5-ns=sample), numerically compensated for cable losses, and numerically integrated. By contrast, each inner-MITL current monitor contains only a single B-dot sensor. These monitors are fielded in opposite-polarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for common-mode-noise rejection after digitization. All the current monitors are calibrated on a 76-cmdiameter axisymmetric radial transmission line that is driven by a 10-kA current pulse. The reference current is measured by a current-viewing resistor (CVR). The stack voltage monitors are also differentialoutput gauges, consisting of one 1.8-cm-diameter D-dot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MVon dedicated accelerator shots with a short-circuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outer-MITL B-dot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA...
We describe herein a system of self-magnetically insulated vacuum transmission lines (MITLs) that operated successfully at 20 MA, 3 MV, and 55 TW. The system delivered the electromagnetic-power pulse generated by the Z accelerator to a physics-package load on over 1700 Z shots. The system included four levels that were electrically in parallel. Each level consisted of a water flare, vacuum-insulator stack, vacuum flare, and 1.3-m-radius conical outer MITL. The outputs of the four outer MITLs were connected in parallel by a 7.6-cm-radius 12-post double-post-hole vacuum convolute. The convolute added the currents of the four outer MITLs, and delivered the combined current to a single 6-cm-long inner MITL. The inner MITL delivered the current to the load. The total initial inductance of the stack-MITL system was 11 nH. A 300-element transmission-line-circuit model of the system has been developed using the TL code. The model accounts for the following: (i) impedance and electrical length of each of the 300 circuit elements, (ii) electron emission from MITL-cathode surfaces wherever the electric field has previously exceeded a constant threshold value, (iii) Child-Langmuir electron loss in the MITLs before magnetic insulation is established, (iv) MITL-flow-electron loss after insulation, assuming either collisionless or collisional electron flow, (v) MITL-gap closure, (vi) energy loss to MITL conductors operated at high lineal current densities, (vii) time-dependent self-consistent inductance of an imploding z-pinch load, and (viii) load resistance, which is assumed to be constant. Simulations performed with the TL model demonstrate that the nominal geometric outer-MITL-system impedance that optimizes overall performance is a factor of $3 greater than the convolute-load impedance, which is consistent with an analytic model of an idealized MITL-load system. Power-flow measurements demonstrate that, until peak current, the Z stack-MITL system performed as expected. TL calculations of the peak electromagnetic power at the stack, stack energy, stack voltage, outer-MITL current, and load current, as well as the pinch-implosion time, agree with measurements to within 5%. After peak current, TL calculations and measurements diverge, which appears to be due in part to the idealized pinch model assumed by TL. The results presented suggest that the design of the Z accelerator's stack-MITL system, and the TL model, can serve as starting points for the design of stack-MITL systems of future superpower accelerators.
We have developed a semianalytic expression for the total energy loss to a vacuum transmission-line electrode operated at high lineal current densities. (We define the lineal current density j ' B= 0 to be the current per unit electrode width, where B is the magnetic field at the electrode surface and 0 is the permeability of free space.) The expression accounts for energy loss due to Ohmic heating, magnetic diffusion, j  B work, and the increase in the transmission line's vacuum inductance due to motion of the vacuum-electrode boundary. The sum of these four terms constitutes the Poynting fluence at the original location of the boundary. The expression assumes that (i) the current distribution in the electrode can be approximated as one-dimensional and planar; (ii) the current IðtÞ ¼ 0 for t < 0, and IðtÞ / t for t ! 0; (iii) j ' 10 MA=cm; and (iv) the current-pulse width is between 50 and 300 ns. Under these conditions we find that, to first order, the total energy lost per unit electrode-surface area is given by W t ðtÞ ¼ t B ðtÞ þ t B ðtÞ, where BðtÞ is the nominal magnetic field at the surface. The quantities
We have conducted dielectric-breakdown tests on water subject to a single unipolar pulse. The peak voltages used for the tests range from 5.8 to 6.8 MV; the effective pulse widths range from 0.60 to 1:1 s; and the effective areas tested range from 1:8 Â 10 5 to 3:6 Â 10 6 cm 2 . The tests were conducted on waterinsulated coaxial capacitors. Large-area water-insulated electrical components are often incorporated in the designs of multiterawatt pulsedpower accelerators, such as the Z [1-10] and ZR [11] machines. Water-insulated components are also proposed for use in future accelerators [12][13][14][15][16][17][18][19]. Optimizing the design of such an accelerator requires a knowledge of the conditions under which its water-insulated components can be operated reliably.Reference [20] proposes that the characteristic time delay delay between the application of a voltage to a water-insulated anode-cathode gap, and the completion of dielectric failure of that gap, can be approximated as follows:In this expression stat is the statistical component of the delay time; i.e., the characteristic time between the application of the voltage and the appearance of free electrons and ions that initiate the formation of streamers in the water. We define form to be the formative component: the time required for the streamers to propagate across the gap and evolve sufficiently to produce complete dielectric failure.To inhibit electrical breakdown, water-insulated components are usually designed to produce a nominally uniform electric field over most of the component's area. We assume that, when the area of a water-insulated system with a uniform field is sufficiently large, the appearance of free electrons and ions necessary to initiate a breakdown occurs somewhere in the system very early in the voltage pulse [20]. Under this condition the statistical time delay stat can be neglected, and the breakdown delay is dominated by its formative component:In principle, dielectric breakdown dominated by the formative component can be studied with an electrode geometry that consists of a point anode and a planar cathode [20][21][22]. Although measurements with an infinitely field-enhanced anode point and an infinitely extended flat cathode are not possible, a number of dielectric-breakdown measurements between a significantly field-enhanced anode electrode and a less-enhanced cathode have been described in the literature.Using these measurements, Ref.[20] finds that complete dielectric failure is likely to occur in water between a fieldenhanced anode and a less-enhanced cathode when In this expression E p V p =d is the peak value in time of the spatially averaged electric field between the anode and cathode (in MV=cm, where V p is the peak voltage difference and d is the minimum distance between the electrodes), and eff is the temporal width (in s) of the voltage pulse at 63% of peak. This relation is based on 25 measurements for which 1 V p 4:10 MV, 1:25 d 22 cm, and 0:011 eff 0:6 s.To develop a tentative design criterion for a large-area...
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