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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. 


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JUN 1985 


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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. 


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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. 


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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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