IEEE ICOPS 2000 Abstracts
Abstracts for the IEEE International
Conference on Plasma Science, June 2000
Factors Affecting the Expansion of Single 25 mm Wires
Driven by a Current Rise Rate of 1010 A/s for <100 ns
D. B. Sinars, Min Hu, K. M. Chandler, S. A. Pikuz*,
T. A. Shelkovenko*, J. B. Greenly,
D. A. Hammer and B. R. Kusse
Lab of Plasma Studies, Cornell University, Ithaca, NY 14853
In previous work we investigated the significant impact of thin insulating
coatings on the explosion of 25mm Ag and W wires driven by a pulsed current
source with a rate of current rise of 1010 A/s per wire [1]. This range is
similar to the prepulse conditions of wire-array experiments on the Saturn
and Z accelerators at Sandia National Laboratories in Albuquerque, NM. Our
studies have found that most of the energy deposition in wires under these
conditions occurs during an initial resistive heating phase lasting <100 ns.
This phase is terminated with the collapse of the voltage across the wire.
The effect of the insulating coatings is to postpone the voltage collapse,
thereby significantly increasing the amount of energy deposited during the
resistive heating phase. The increased energy deposition causes a significant
increase in the observed expansion rate of the dense wire cores as they explode.
In recent experiments, we have found that insulating coatings have a similar
effect on the expansion of 25mm Cu, Au, and Pt wires. In addition, we will
discuss the effect of other factors on the behavior of exploding wires, such
as (1) thin metal coatings, (2) differences in the initial purity or structure
of the wire material, and (3) different current driver pulse shapes.
The diagnostics used include: (1) X-pinch x-ray backlighters driven by the XP Pulser
[450 kA, 100 ns], (2) optical interferometry and schlieren imaging using an Nd:Yag
laser system [l = 532 nm, 4 ns], and (3) current and voltage monitors.
[1] D. B. Sinars, T. A. Shelkovenko, S. A. Pikuz, Min Hu, V. M. Romanova,
K. M. Chandler, J. B. Greenly, D. A. Hammer, and B. R. Kusse, "The Effect of
Insulating Coatings on Exploding Wire Plasma Formation." Physics of Plasmas 7,
in press (2000).
Sandia Contract BD-9356 and DOE contract DE-FG03-98DP00712 supported this research.
* Permanent Address: Lebedev Physical Institute, Moscow, Russia
Study of the Rapid Expansion Rates of 25mm Ag, Au,
and Cu Wires Driven by a Current
Rise Rate of 1010 A/s for <100 ns
K. M. Chandler, Min Hu, D. B. Sinars, T. A. Shelkovenko*,
S. A. Pikuz*, J. B.
Greenly, D. A. Hammer and B. R. Kusse
Lab of Plasma Studies, Cornell University, Ithaca, NY 14853
Investigations of the expansion rates of exploding wires composed of many different
materials are in progress using a pulsed current source with a rise rate of 1010 A/s
per wire. This range is similar to the prepulse phase of wire-array experiments on
the Saturn and Z accelerators at Sandia National Laboratories in Albuquerque, NM.
Materials investigated include Ag, Al, Au, Cu, Mo, Ni, Pd, Pt, Ti, W, Zn, stainless
steel, and others. Under identical pulser parameters, Ag, Au, and Cu wires exhibit
among the fastest expansion rates and most uniform wire explosions of all the
materials studied.
The characteristics of Ag, Au, and Cu wire explosions will be discussed, including
the effects of insulating coatings [1]. Comparisons will be made to materials
from nearby columns on the periodic table, such as Ni and Pt.
The diagnostics used in these studies include: (1) X-pinch x-ray backlighters
driven by the XP Pulser [450 kA, 100 ns], (2) optical interferometry and schlieren
imaging using an Nd:Yag laser system [l = 532 nm, 4 ns], and (3) current and voltage
monitors.
[1] D. B. Sinars, T. A. Shelkovenko, S. A. Pikuz, Min Hu, V. M. Romanova, K. M.
Chandler, J. B. Greenly, D. A. Hammer, and B. R. Kusse, "The Effect of Insulating
Coatings on Exploding Wire Plasma Formation." Physics of Plasmas 7, in press (2000).
Sandia Contract BD-9356 and DOE contract DE-FG03-98DP00712 supported this research.
* Permanent Address: Lebedev Physical Institute, Moscow, Russia
Optical Measurements of the Properties of Exploding Wires
B. R. Kusse, Min Hu, S. A. Pikuz*, D. B. Sinars,
K. M. Chandler, J. B. Greenly,
D. A. Hammer, and T. A. Shelkovenko*
Lab of Plasma Studies, Cornell University, Ithaca, NY 14853
The explosions of fine wires driven by a pulsed current source with a rise rate of
1010 A/s per wire were observed with optical diagnostics using a Nd:YAG laser
[l=532nm, 4ns]. Three simultaneous schlieren channels and two interferometry
channels separated by 50 ns were used. The schlieren channels consist of a
bright-field iris, a dark field strip and a dark-field knife-edge. The interferometry
was performed using a new version of a shearing interferometer, which is based on a
double-prism with an air wedge. The expansion rates of various materials
(Cu, Ag, Au, etc.) both with and without thin insulating coatings were measured.
A strong effect of the insulation coating on wire expansion was observed for all
those materials [1]. The interferometry images showed a plasma shell surrounding
an expanding neutral vapor column. From these measurements we obtained the
refractive index of the exploding wire vapor/plasma and used these results to find
values for the plasma electron and neutral vapor densities.
[1] D. B. Sinars, T. A. Shelkovenko, S. A. Pikuz, Min Hu, V. M. Romanova, K. M.
Chandler, J. B. Greenly, D. A. Hammer, and B. R. Kusse, "The Effect of Insulating
Coatings on Exploding Wire Plasma Formation." Physics of Plasmas 7, in press (2000).
Sandia Contract BD-9356 and DOE contract DE-FG03-98DP00712 supported this research.
*Permanent Address: Lebedev Physical Institute, Moscow, Russia
Spectroscopic Investigations of X-ray Radiation from X Pinches
T. A. Shelkovenko*, S. A. Pikuz*, D. B. Sinars,
I. Yu. Scobelev**, D. A. Hammer,
K. M. Chandler, Min Hu
Lab of Plasma Studies, Cornell University, Ithaca, NY 14853
Time-integrated x-ray spectroscopic measurements of the radiation produced by X
pinches have been made using a focusing spectrograph based on a spherically bent
mica crystal (R=100 mm). Spectra of Al, Ti, NiCr, Ni, Mo, and Pd X pinches have
been recorded using Bragg reflection angles near 45°. The large angle enabled us
to place the spectrograph a large distance from the X pinch, thereby preventing
damage to the crystal and allowing a larger spectral range (dl/l ~ 10-1) than is
normally possible for this method. In these experiments the film was not placed
exactly on the Rowland circle, but this did not significantly decrease the spectral
resolution because of the small size of the x-ray source. The minimum determinable
width was still much smaller than the spectral width of the lines studied. In
most experiments, the radiation from two X pinches was recorded simultaneously,
allowing the possibility of wavelength calibration.
Using ratios of resonance lines to the satellite lines of H- and He-like spectra
in the 0.6-1.5 keV range, we obtained the average electron temperature for different
elements. Unfortunately, it is not possible to measure the density of X-pinch hot
spots using standard techniques for densities exceeding 1021 cm-3 using
time-integrated spectra. Spectral line widths (dl/l ~ 1.8-3.8x10-3) correspond
to Doppler broadening of plasma expanding with velocities of 0.5-1.1x108 cm/s.
Our experiments indicate that time-resolved spectral measurements are required to
determine all of the hot spot parameters. The setup used for time-integrated
measurements is readily adapted to time-resolved measurements using an x-ray
streak camera.
This research was supported by DOE grant DE-FG02-98ER54496.
*Permanent Address: Lebedev Physical Institute, Moscow, Russia
**VNIIFTRI, Moscow region, Russia
Measurements of the Structural Evolution of X pinches and
the Formation of Radiating
Hot Spots
S. A. Pikuz*, T. A. Shelkovenko*, D. B. Sinars,
D. A. Hammer, K. M. Chandler,
Min Hu
Lab of Plasma Studies, Cornell University, Ithaca, NY 14853
An X pinch is composed of two or more fine wires arranged so that they cross and
touch at a single point, thus forming an "X" shape. When used as the load of a
high-current pulsed power generator, a complex pinching process occurs at the
cross-point of the X pinch, and soft x rays are emitted from this region in short
(£0.5 ns), intense bursts. Although X pinches have been successfully developed as
point sources of soft x rays for radiography [1], the physical processes that produce
the point source of radiation are not well understood.
In previous work on the XP facility (470 kA, 100 ns), we found that an on-axis z
pinch forms in the cross point region during the final few ns prior to the radiation
bursts. The rapid collapse of the z pinch results in a gap in the observable mass
distribution, which we refer to as a minidiode. Improved measurements of the collapse
of this z pinch have been made, allowing more reliable estimates of the pinch
velocities prior to the radiation burst, and the expansion velocities after the
radiation burst. We have also observed "self-similar" structure in different stages
of the z pinch collapse. In addition, the source of the radiation bursts was
localized to within 10 microns within the observed z pinch structure using a
technique reported during ICOPS 1999. Using three X pinches to backlight one
another, two images of each X pinch can be obtained. Thus, the hot spot of one
of the X pinches and the z-pinch structure both prior to and after its radiation
burst can be determined. These results should enable us to better understand the
role of the x-ray source in the dynamics of the X pinch.
[1] T. A. Shelkovenko, S. A. Pikuz, A. R. Mingaleev, and D. A. Hammer, Rev. Sci.
Instrum. 70, 667 (1999).
This research was supported by DOE grant DE-FG02-98ER54496.
* Permanent Address: Lebedev Physical Institute, Moscow, Russia
Polarizability Measurements of Exploding Wire Vapor
using Optical Interferometry
and X-pinch Backlighting
Min Hu, D. B. Sinars, S. A. Pikuz*, K. M. Chandler,
J. B. Greenly, D. A. Hammer,
B. R. Kusse and T. A. Shelkovenko*
Lab of Plasma Studies, Cornell University, Ithaca, NY 14853
Most of the data for the atomic polarizability of neutral metal vapor is theoretically
calculated [1] as opposed to experimentally measured. In this paper we present
experimentally measured values for the polarizability of neutral vapor generated
by exploding wires. The polarizability P, index of refraction h and the neutral
vapor density n can be related by the expression
h=1 + 2p n P.
X-pinch backlighting results give the line-integrated neutral vapor density and,
assuming azimuthal symmetry, yield the neutral density as a function of radius.
Again assuming azimuthal symmetry, the line-integrated interferometry measurements
are inverted to give h as a function of position.
Several factors complicate these measurements: 1. For interferometry, wires must
be completely vaporized and symmetrically expanded enough to be transparent to the
laser. We found this to be the case with Ag, Cu, and Au, for example, but not for
W wires. 2. The expanding wires are observed to have plasma shells surrounding
the neutral vapor. This plasma is created by current through the wire as well as
radiation from the x-pinch discharge. Consequently, the conversion from a local
index of refraction to an index of refraction only due to the neutral vapor can only
be done in the central region where it is assumed that the plasma density is negligible. 3. The plasma created by the x-pinch discharge is asymmetric and consequently symmetry assumptions for the plasma are not valid. Since the interferometric phase shifts caused by the neutral vapor are approximately a order of magnitude greater than those introduced by the plasma this effect should be small.
[1] CRC Handbook of Chemistry and Physics, 78th Ed., edited by David R. Lide
(CRC, New York, 1997), pp. 10-201.
Sandia Contract BD-9356 and DOE contract DE-FG03-98DP00712 supported this research.
*Permanent Address: Lebedev Physical Institute, Moscow, Russia