DESIGN OF FOIL IMPLOSION SYSTEM FOR PIONEER I EXPERIMENTS*
D. J. Erickson, B. L. Barthell, J. H. Brownell,
R. S. Caird, D. V. Duchane, B. L. Freeman, C. M. Fowler,
J. H. Goforth, A. E. Greene, W. T. Leland, I. R. Lindemuth,
T. Oliphant, H. Oona, R. H. Price, B. Suydam, R. J. Trainor,
D. L. Weiss, A. H. Williams and J. B. VanMarter
Los Alamos National Laboratory
Los Alamos, New Mexico 87545
Abstract
A foil implosion system is described that integrates
an explosive flux-compression generator, a flat plate
feed section with power conditioning switches, and a
vacuum electrode region containing a cylindrical
foil/plasma load. Power conditioning, obtained with
an explosive-driven plasma compression opening switch
and explosive-actuated closing switches, provides a
submicrosecond multimegampere pulse for the implosion
of an aluminum plasma. The flat plate section is con¬
figured for bidirectional feed to the coaxial vacuum
electrodes. Important considerations in the design of
the vacuum power flow region include gap failure, feed
symmetry, and radial diagnostic access. The system
presently accommodates a foil radius of 3 cm. Innova¬
tive foil insertion and clamping techniques are also
described.
Introduction
Experiments are being conducted that employ an active,
compact inductive driver for the fast x 15 implosion
of a thin cylindrical plasma. The driver consists of
an explosive powered flux compression generator and a
fast opening/closing switch combination. These
Pioneer I experiments are our first attempts at cou¬
pling an explosive generator to a fast dynamic load
using intermediate pulse conditioning techniques. The
Pioneer I system is a close-coupled expendable system.
It combines a proven driver with flat plate symmetry
and a higher-symmetry coaxial feed and load. The ex¬
periments, when fully optimized, should be capable of
peak load currents of 4-5 MA at input voltages of
120-150 kV for submicrosecond implosions. The
Pioneer I system is being used as a test bed for the
development of techniques, the validation of codes,
the exercising of diagnostics and the identification
of systems problems. Such experiments are preliminary
to more ambitious ones that will use higher energy
flux compression and switching components in cylindri¬
cal geometry for the development of an intense, pulsed
soft x-ray source.
The potential for the high energy application of in¬
ductive storage/compression has been demonstrated by
the Air Force Weapons Laboratory in their SHIVA pro¬
gram.* Their approach,^ which uses capacitive storage
as a primary source, is responsible for much of the
relevant power flow technology and load physics. The
Pioneer I system described below uses much of that ex¬
perience .
The discussion here concentrates on the design and
related issues for the Pioneer I system. Companion
papers by Greene et al. and Lee et al. discuss sys¬
tem expectations, diagnostics, and test results.
Inductive Driver: Components and Feed
The Pioneer I system is shown in Fig. 1. The
experimentally determined source characteristics of
the explosive plate generator' 1 are shown in Fig. 2.
The lower parallel plate section is terminated by a
*Work supported by the US Department of Energy.
716
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1. REPORT DATE
JUN 1985
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4. TITLE AND SUBTITLE
Design Of Foil Implosion System For Pioneer I Experiments
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13. SUPPLEMENTARY NOTES
See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and
Abstracts of the 2013 IEEE International Conference on Plasma Science. Held in San Francisco, CA on
16-21 June 2013. U.S. Government or Federal Purpose Rights License.
14. ABSTRACT
A foil implosion system is described that integrates an explosive flux-compression generator, a flat plate
feed section with power conditioning switches, and a vacuum electrode region containing a cylindrical
foil/plasma load. Power conditioning, obtained with an explosive-driven plasma compression opening
switch and explosive-actuated closing switches, provides a submicrosecond multimegampere pulse for the
implosion of an aluminum plasma. The flat plate section is configured for bidirectional feed to the coaxial
vacuum electrodes. Important considerations in the design of the vacuum power flow region include gap
failure, feed symmetry, and radial diagnostic access. The system presently accommodates a foil radius of 3
em. Innovative foil insertion and clamping techniques are also described.
15. SUBJECT TERMS
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unclassified
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unclassified
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ABSTRACT
OF PAGES
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19a. NAME OF
RESPONSIBLE PERSON
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
plasma compression opening switch, also driven by ex¬
plosive. The switch geometry and characteristics are
described in detail by Goforth et al. elsewhere in
these proceedings. The upper section is brought into
the circuit through a pair of curved transitions that
contain multichannel detonator-driven closing
switches. The upper section also provides
bidirectional feed to the coaxial vacuum power flow
and load regions.
a
Fig. 2. Inductance and source impedance of plate
generator.
The operating sequence for the system begins with the
discharge of a capacitor bank through the lower part
of the assembly. The discharge establishes conducting
plasma in the opening switch cavities and primes the
generator with magnetic energy. After injection of
initial current, the explosive generator is actuated,
first trapping flux in the generator volume and then
compressing it to amplify current in the circuit. At
an appropriate time, the plasmas in the opening switch
are compressed, giving rise to a fast increase in
resistance. As voltage rises across the opening
switch, the closing switches are actuated to direct
current into the load located in the upper part of the
experiment.
The overall length of the lower biplate section is
about 1.5 m. The geometry of the upper biplate is a
square with dimension 0.76 m, which is also the work¬
ing width for inductance considerations. These dimen¬
sions are a compromise between our desire for a low
inductance, close coupled feed (< 5 nH in any direc¬
tion) and convenience in fabrication. The experiment
is designed to be driven at negative potential with
reference ground connected to the top member of the
upper blplate. High voltage is therefore confined to
the inner members of the assembly.
The closing switches are shown in cutaway In Fig. 1
and in profile in Fig. 3. These switches are linear
multichannel arrays, up to 20 channels per line. The
arrays employ detonators which Initiate explosive
pellets (2 mm long and 6.3 mm dlam) placed over
3.2-mm-diam holes bored in 6.4-mm-thick aluminum. The
closing mechanism results from a jet-like stem,
produced by the explosive Interaction, that penetrates
1 mm of polyethylene sheet In the switch gap. Limited
testing has been done to quantify simultaneity. Two
20-channel arrays were tested at 20 kV DC with low
current transfer. The jitter associated with closure
was 20 ns for one array and 32 ns for the other.
Standard deviations were 10 and 12 ns respectively.
Similar testing has not been done under high current
pulsed conditions, although we know that closing time
is voltage dependent.
UPPER FEED
NESTED
INSULATORS
LEXAN
LOWER FEED
CLAMPING STUD
Fig. 3. Transition region from lower blplate to upper
biplate showing location of closing switch
array and insulated clamp.
Also shown in Fig. 3 is one of the four insulated
mechanical clamps used to align the assembly to high
precision. The lower blplate is separated from the
upper by Lexan spacers through which threaded, steel
rods are passed. Nested polyethylene insulators
Isolate the rods within each biplate. After careful
alignment, the assembly is pulled together and locked
against heavily torqued steel nuts. The air within
the spacers is replaced by SFg, which flows through
holes provided in each rod. When properly assembled,
axial concentricity between the inner and outer vacuum
electrodes is within 75 pm; planarity between
horizontal electrode surfaces at regions near the foil
radius is within 25 pm.
The lower blplate is insulated with 2.5 mm of Mylar
sheet. The curved transitions also have 2.5-mm gaps
that are filled with various thicknesses of
polyethylene switch Insulation and Mylar. The upper
biplate is Insulated with the 2.5-mm-thick brim of the
vacuum insulator and 1.25 mm of Mylar.
Vacuum Power Flow: Design and Fabrication
The design challenge for this part of our experiment
is to prevent gap failure at minimum inductance in a
current-density regime where magnetic insulation is
not yet working to advantage. Principal failure
mechanisms are conventional vacuum breakdown, ablative
closure, and insulator flashover. The latter two
mechanisms are driven by the ultraviolet radiation
from the plasma load. Various inputs have guided the
design. Electric field strengths are estimated from
the two-dimensional code LAPLACE; ablative closure Is
predicted from MHD modeling; and ultraviolet attenua¬
tion is examined with the Air Force Weapons Laborato¬
ry's ray-tracing code STREAM.
The design of the vacuum power flow geometry as
presently used In our Pioneer I testing is shown in
Fig. 4. The geometry is constrained radially by a
3-cra foil radius, selected to reasonably shorten im¬
plosion time, and a 17.8-cm radius for the vacuum
interface. The geometry is further constrained by our
intent to view implosion dynamics radially.
717
The convoluted vacuum gap is formed from nested
aluminum electrodes. Input to the gap is across the
insulator fabricated from high-density polyethylene.
The current-carrying surfaces were machined, using
numerical control, and hard anodized. The mounting
cap, which completes the outer electrode, is fabri¬
cated from copper. An annular array of radial vanes
at 10° intervals is machined in the cap using a spark
cutting technique. Each vane is 1.5 mm wide. The
minimum gap in the power flow region is 1.0 cm. It
opens to 1.5 cm at the top of the feed and to 2.5 cm
between the foil and the i.d. of the vane structure.
The inductance calculated for this power flow geometry
is 10.2 nH. We have considered the perturbations that
this periodic vane structure would cause in the
magnetic field that drives the implosion. Effects due
to asymmetry in the current feed have also been
examined. On the basis of an analytic model used to
estimate Rayleigh-Taylor instabilities, perturbations
from the vane periodicity and the feed asymmetry are
negligibly small for the load geometry of Fig. 4.
Fig. 4. Vacuum power flow and load regions of
Pioneer I system.
The diagnostic chamber, shown in Fig. 1, is equipped
with multiple radial ports and an upper axial port.
Pumping for the system is provided by a small
turbomolecular pump connected directly to one of the
radial ports. Vacuum quality is better than 1 x 10 -5
torr.
Load Foil: Fabrication and Insertion
Seamless aluminum foils of 200-nm thickness are
presently being used in our implosion experiments.
The hardware used to both fabricate the foil and
insert it into the Pioneer I electrode configuration
is shown in Fig. 5. Assembly of the hardware prior to
foil fabrication begins by attaching the lightweight
aluminum spacer to the upper mounting ring. The lower
mounting ring is added and the clamp screw inserted
through the assembly. A set screw is driven against
the flat in the clamp screw. The clamp nut is added
and lightly tightened to draw the assembly together.
The essential feature of the hardware is the set screw
that prevents rotation. Rotational torque tends to
rip a foil during the insertion process.
Foil fabrication involves the evaporation of an
aluminum/aluminum oxide composite onto a seamless,
polyvinylalcohol mandrel.® The mandrel is first drawn
onto the fabrication hardware and positioned in an
evaporation chamber. A 100-nm layer of aluminum,
monitored by an Auger technique, is evaporated onto
the rotating mandrel. Oxygen is then pulsed into the
chamber to form a 10-nm layer of A1 X 0 for
strengthening. The additional 100-nm thickness of
aluminum is evaporated over the oxide layer. The foil
assembly is removed, the mandrel dissolved in water,
and the assembly dried. Final foil thickness is de¬
termined from alpha step measurements on an adjacent
witness slide. Oxygen content in a foil is estimated
at about 1%. Catenation for a typical foil is
estimated at < 1 mm.
Fig. 5. Assembly for fabrication and insertion of
ultrathin aluminum foils.
After fabrication of the foil, the hardware in Fig. 5
is lowered into the Pioneer I diode on guide posts not
shown. Two techniques have been used to uniformly
contact the lower mounting ring to the inner elec¬
trode. The first involves the expansion of a Tygon
tube that is installed in the circular channel of the
inner electrode immediately adjacent to the lower ring
(see Fig. 4). The tube when energized with externally
supplied air pressure acts as a bladder to expand a
segmented copper strip, which has been spring loaded
into the circular channel, against the lower ring.
Our preferred technique, for the time being, employs a
coil spring Inserted in the circular channel which
grabs the lower ring upon insertion. Both techniques
center the foil. After constraining the lower ring,
the clamp plate in Fig. 4 is installed to lock the
upper mounting ring. Screws holding the aligning
spacer to the upper ring are removed, and the clamp
nut is retracted. The set screw is loosened allowing
the clamp screw and washer to drop. The aligning
spacer and guideposts are then withdrawn. With the
hardware in Fig. 5 and the procedures just described,
our insertion process has become routine and trauma
free.
718
Conclusions
We have designed the compact but expendable Pioneer I
system to drive a cylindrical plasma implosion. The
system, which features explosive pulsed power, has
been successfully demonstrated. ’ y A scheme to convert
a single-point flat plate drive into a bidirectional
coaxial load has been successfully
Innovative techniques for the insertion
of ultrathin aluminum foils have been
put into routine use. Future
will push the system for increased
Additional
feed for a
incorporated,
and clamping
developed and
experiments
performance.
design effort will
be
directed at the vacuum power flow region.
References
[1] R. E. Reinovsky, W. L. Baker, Y. G. Chen,
J. Holmes and E. A. Lopez, "SHIVA STAR Inductive
Pulse Compression System," in Digest of Technical
Papers - 4th IEEE Pulsed Power Conference,
ed. by M. F. Rose and T. H. Martin. Lubbock:
Texas Tech Press, 1983, pp. 196-201.
[2] W. L. Baker, J. H. Degnan and R. E. Reinovsky,
"High Energy Pulse Power Development and Applica¬
tion to Fast Imploding Plasma Liners," in
Ultrahigh Magnetic Fields-Physics, Techniques and
Applications , ed. by V. M. Titov and G. A.
Shvetsov. Moscow. Nauka, 1984, pp. 39-49.
[3] A. E. Greene, J. H. Brownell, R. S. Caird,
D. J. Erickson, J. H. Goforth, I. R. Lindemuth,
T. A. Oliphant and D. L. Weiss, "System Expecta¬
tions for Pioneer I Foil Implosion Experiments,"
to be published in Digest of Technical
Papers - 5th IEEE Pulsed Power Conference , 1985.
[4] P. H. Y. Lee, R. F. Benjamin, J. H. Brownell,
D. J. Erickson, J. H. Goforth, A. E. Greene,
J. S. McGurn, R. H. Price, H. Oona, J. F. Pecos,
J. L. Reay, R. M. Stringfield, R. J. Trainor,
L. R. Veeser and A. H. Williams, "Diagnostics for
Pioneer I Imploding Plasma Experiments," ibid .
[5] R. S. Caird, D. J. Erickson, C. M. Fowler,
B. L. Freeman and J. H. Goforth, "A Circuit Model
for the Explosive-Driven Plate Generator," in
Ref. [2], pp. 246-253.
[6] J. H. Goforth, D. J. Erickson, A. H. Williams and
A. E. Greene, "Multimegampere Operation of Plasma
Compression Opening Switches in Planar Geometry,"
to be published in Ref. [3].
[7] B. R. Suydam, "Currents and Fields in an
Asymmetrically Driven Coaxial Line," to be
published in Ref. [3].
[8] D. V. Duchane and B. L. Barthell, "Unbacked Cy¬
lindrical Metal Foils of Submicron Thickness,"
Thin Solid Films 107 , pp. 373-378, 1983.
[9] In the Pioneer 1-2 event, a 1.0 ys implosion was
achieved at a peak current of about 2 MA. Peak
implosion velocity exceeded 10 cm/us with kinetic
energy in the implosion of 15-20 kJ. Peak plasma
temperature was 30 eV.
Acknowledgements
We are grateful to R. E. Reinovsky and E. A. Lopez of
the AFWL for many helpful discussions and suggestions,
to J. Lupo of the AFWL for STREAM calculations, to
A. R. Martinez for supervising the assembly and firing
of Pioneer I experiments, and to G. Heltne for
mechanical design contributions.
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