HIGH CURRENT, LOW JITTER,EXPLOSIVE CLOSING SWITCHES*

Douglas G. Taskert, James H. Goforth, Dennis H. Herrera, David T. Torres,James C. King, and Henn Oona

Los Alamos National Laboratory, DX-2, MS J566,Los Alamos, New Mexico, 87545, USA

Abstract closing to near zero voltage; of conducting currents of theorder of 10 MA; and of closing with a timing precision of

Isentropic compression experiments (ICE) using a high 25 to 50 ns. The original ICE circuit design used Procyonexplosive pulsed power (HEPP) system have been devel- switches, which had been designed for another applica-oped to obtain isentropic equation of state data for metals tion. We incorporated four of these switches in parallel toat megabar pressures [1][2]. The HEPP system com- optimize the current transfer. In practice we found switch

prises a magnetic flux compressor, an explosively-driven timing jitter of the order of 600 ns, poor current transfer to

opening switch and a series of closing switches; fast ris- our load and the voltage did not drop to zero across the

ing current pulses are produced, with rise times of switch when it closed. Thus we had two problems: tim--500 ns at current densities exceeding many MA/cm. ing; and closure impedance. We will describe the switchThese currents create continuous magnetic loading of the design; our analysis of the switch behavior; and a simplemetals under study. The success of these experiments physical model that was shown to accurately describe the

depends on the precise control of the current profile, and switch performance. Based on the model we have de-that in tur depends on the precise timing of the closing signed a new switch that has successfully met the re-

switches to within 50 ns at currents of the order 10 MA quirements of the HEPP-ICE system.and voltages of 150 kV. We first used Procyon closingswitches [3] but found their timing to be unacceptably H TIMING PRECISIONimprecise for ICE with a jitter of typically 600 ns. We II. SWITCsuspected that the switch timing was sensitive to appliedvoltage; this was subsequently confirmed by experiment, A. Procyon- switch design.as we will show. A simple shock model was developed Two PBX-9407, 12.7-mm diameter x 12.7-mm longto explain the voltage sensitivity of closure time, dt/dV, explosive pellets are detonated from the top in Figure la.and from the model we designed a low jitter switch that The detonation wave travels downward and a shock waveuses the shock-induced electrical conduction of polyim- is produced in the bottom of the aluminum metal cupide. The predicted dt/dV was exactly equal to the meas- which impacts a small ring air cavity. This develops aured value, thus confirming the model. This new switchdesign proved successful and met the 50 ns criterion; it is (a) Dnow used routinely in HEPP-ICE experiments.

I. INTRODUCTION

The ICE technique was first reported by Asay [4]. Ityields accurate isentropic equation of state (EOS) datafor various materials at Mbar-pressures in a planar ge- kLJJometry by magnetic loading. We have demonstrated the PWL ) DHEPP version of the ICE technique can also be used toobtain accurate (within 0.2% in pressure) isentropic EOS (HE)data at Mbar pressures in a planar geometry, and that thesystem is reliable, reproducible and predictable. Withthis system we may ultimately reach pressures of X-20 Mbar (2 TPa) with an advanced system [5].For the precise control of pressure profiles in ther

HEPP-ICE experiment we needed closing switches that Figure 1. The two closing switch designs showing:are capable: of withstanding ~150kV without failure; of the detonator D and the insulator I. a) Procyon

_________________________________switch: explosive pellets P; ring groove in the alumi-* num housing R. b) Polyimide switch: plane waveThis work was supported by the U.S. Department of Energy ln W;PX90 ikH;auiu o nemail: taskerglanl.gov bottom electrodes Al.

0-7803-9189-6/05/$20.00 ©2005 IEEE. 517

circular ring-shaped jet of aluminum, the slug, which is dt -1projected across an air gap into the 1.5-mm thick polyes- dV -{UEbk} (2)ter insulation [6]. The circuit is closed when the slug haspenetrated enough of the insulation for electrical conduc- D. Procyon-switch modeltion to occur, see section II.C. For the Procyon switch the penetration velocity U hasIt wasn't clear what was causing the poor timing but been calculated to be 1 km/s [7] and polyester has awe suspected that the switch may be sensitive to voltage,i.e., the time to switch closure depended on the applied brea stength of 1 MV/in Hencewparedito a' . . ~~~~~~~~~~voltagesensitivity dt/dV =-5.56 ns/kV compared to thevoltage. We therefore devised a series of tests where we -3.4 ns/kV we measured (this predicted line was arbitrar-applied a controlled voltage across a switch, via acharged~~ ~~~~~~~~~~~~.trnmsinln,te frdtesic n ly drawn through the centroid of the data). The modelcharged transmission line, then fired the switch andmeasured the elapsed time from firing the detonator to . (a)when the current flow started. 0.5 Pre

PS

B. Procyon-switch timing data 0.4The results of the timing experiments with the Procyon 0.3 . \

switches are shown in Figure 2a, where the normalizedtime (measured - mean time) to switch closure is plotted Best

fitagainst applied voltage. The switch closure times were 0.1 fclearly very sensitive to the applied voltage. The best kV X(least squares) fit of the slope was dt/dV -3.4 ns/kV 0 50 100 - 150with a standard error (SE) of the slope c7, 0.68 ns/kV; 01the standard deviation (SD) of the data from the fit Gd -0.2 -

107 ns. Clearly, such large uncertainties were unaccept-able when 50 ns precision was sought. For example, thetime to closure was increased by 480 ns as the voltage 60 (b)was reduced from 140 kV to zero. The consequences of Pdtthese timing uncertainties for the ICE experiment are 40--e

d

discussed in section III.20-

C. Explosive switch breakdown modelTo understand the sensitivity to applied voltage we _

formulated a simple model of the switch. Consider the 20 40 60 80 1i0 kV 120 1 0slug or conducting region penetrating the insulator at a 20- \velocity U, see Figure 3. As it does so the thickness ofthe insulation ahead of it is progressively reduced, and 40the electric field across it, E2(t), is progressively in- 0

V+l - -60

Figure 2. Normalized switch timing data vs. appliedvoltage in kV. a) Procyon switch, times in pts. b)

IWWE2 _ Polyimide switch, times in ns. The black squares arethe data; the dotted lines are the best fits; and the solid

ut lines are the predictions.- h- bpredicts a larger but comparable sensitivity than we

Figure 3. Simple switch model. A voltage V is ap- measured, but, even with the imprecision of the measuredplied across the electrodes. This establishes electric dt/dV (i.e., G, = 0.68 ns/kV), the fit is beyond 3G from thefields E2 and El ahead of the conducting region and theory. As the slug penetration is subsonic (i.e., 1 km/sbehind it, where El = 0. The conducting region vs. 2.4 km/s for the ambient pressure sound speed ingrows with time from left to right at a velocity U. polyester) it projects a divergent precursor wave ahead of

creased as a function of time t. Given an initial insula- it which pre-compresses the polyester. We ignored this

tion thickness h, we have E2(t) = V/(h - Ut). When for the simple model, but it may induce conduction in theE2(t) > Ebk, the dielectric breakdown strength, the switch polyester [8], which would reduce dt/dV.breaks down and currents flows in the switch. The..brakow votg is F. Polyimide-switch design

Vbk (t)=Ebkh-Ut) ( l ) Fromn Eq. (2) it is clear that in order to reduce dt/dV weVl3k (t)= Ebk(hUt) (1) must increase the product of Ebk, and U. From previousSo the sensitivity of the timing to the applied voltage is work we knew that electrical conduction can be induced

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by pure shock in polyimide plastics [8][9][10]. Also, In other work we had developed an accurate dynamicthe shock velocity (U) is >6 km/s when shocked to pres- simulation of the ICE circuit [16]. Based on our switchsures in excess of 20 GPa [11] and the dielectric strength model (II.D) we incorporated three parallel circuits repre-is high, Ebk =275 MV/m [12]. Another design consid- senting the closing switches into our ICE simulation. Theeration was that we wanted to increase the conducting switch simulation was based on the simple model (II.C)surface area of the switch to minimize the circuit imped- where the breakdown voltage varied according to Eq. (1).ance and improve its high current performance.We therefore designed the polyimide closing switch A. Effects ofdt/dVon switch closure

shown in Figure lb. It comprises a 100-mm diameter 1) Procyonplane wave lens and PBX-9501 explosive disk inserted From experimental results we estimated the inductancein a well of the top aluminum electrode; the bottom of and resistance of each Procyon switch on closure wasthe well is a 0.125-in (3.175 mm) thick aluminum disk. 8 nH and 3 mohm at the pertinent current levels, and weThe top and bottom electrodes are insulated from each set the jitter between the three parallel switches to 100 nsother by a 1-mm layer of polyimide insulation made (the experimental value) to show the effects of dt/dV.from 2-mil and 3-mil (50-ptm and 75-ptm) thick sheets. The value of dt/dV we used was -5.56 ns/kV from II.D.

The calculations showed that as soon as the first switch1) Polyimide-switch explosives manufacture (Xl) closed the voltage across all three switches fell byOne minor drawback of the switch is that we must en- -50 kV in 10 ns (we needed it to fall by -100 kV for good

sure that the explosive drive system (plane wave lens and current transfer). Consequently the other two switchesexplosive disk) is made to high precision from the same (X2 and X3) were delayed by -50 kV x -5.56 ns/kV =

batches of explosives. Moreover, we must test every 278 ns in addition to the jitter (100 ns). The results of thebatch to ensure accurate timing. All this makes the poly- simulation are seen in Figure 4a. Xl was the only switchimide switches more expensive to use than the Procyon to conduct for the first 357 ns, and this resulted in poorswitches. (a)

LoadF. Polyimide-switch timing data and model 4 /The polyimide switch timing data are shown in Figure 3

2b. The best fit is dt/dV = -0.58 ns/kV, c, = 0.1 ns/kV, 2and the SD of the data from the line (7d = 13 ns. Compar- /ing these results with the Procyon switch (dt/dV =

-3.4 ns/kV, G, = 0.68 ns/V, and Gd = 107 ns) this new °switch has significantly improved precision. 0.0 0.2 0.4 Ps 0.6 0.8 1.

We calculated the shock velocity in the polyimide to 6 (b)be 6.2 km/s using shock-impedance mismatch calcula- M Load

tions [13], the known JWL equation of state of the PBX- 4 -MA9501 explosive products [14], and the shock Hugoniots 3of 6061-T6 aluminum and polyimide [11]. Using Eq. Xi(2), and given Ebk= 275 MV/m, the predicted dt/dV = 2-0.59 ns/kV; in fortuitously close agreement with the fit 1 X3

to the data (-0.58 ns/kV, G, = 0.1 ns/kV). It is important 0to note that, unlike the Procyon switch, the shock front 0.0 0.2 0.4 PS 0.6 0.8 1.traveling through the insulation in this switch is super- Figure 4. Simulation of the total ICE load currentsonic, so there is no acoustic precursor [15]. This proba- (Load) with currents in the three parallel switchesbly accounts for the excellent agreement between the (Xl to X3): a) Procyon; b) Polyimide. Currents insimple model predictions and the results. MA; times in 1ts; the plot scales are the same.

current transfer to the load. (The results of our simula-III. PARALLEL SWITCH EFFECTS tions with 10-ns delays were similar.) The initial rate of

rise of the current, dI/dt, to the load was 6 TA/s and theThe Procyon switch was not designed to take more current reached only 3.5 MA in 500 ns.

than a few MA per switch, so we had to fire several of This clearly demonstrates that the effects of the largethem in parallel to transfer the desired 5 to 7 MA to the dt/dV are sufficient to allow only one switch to effec-ICE load in -500 ns. To do this we had to balance the tively conduct. Moreover, the effect of dt/dV is muchcurrents between the switches, which meant that all the more significant than the jitter between switches (at theswitches must close simultaneously; yet the chances of same voltage), i.e., a 10 ns jitter still led to a 300 ns delay.simultaneity were small because the SD of Procyonswitch closures was ~100 ns (independent of voltage 2) Polyimidesensitivity), see IIF. We needed to estimate the effects We performed the same circuit simulation on three par-on non-simultaneity of current balance between switches. allel polyimide switches [17]. Here we used the polyim-

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ide switch model and, from experimental data, 0.5 nH V. REFERENCESand 100 ptohm for the switch impedance and 25 ns forthe jitter. All other circuit parameters were the same as [1] D.G. Tasker, et al., "Equation of State Experiments infor the Procyon simulation. The results in Figure 4b Extreme Magnetic Fields," in Procs. Of Xth Internationalshow a significant improvement over the Procyon Conference On Megagauss Magnetic Field Generationswitch. With polyimide the switch currents are more And Related Topics, Berlin, July 18 - 23, 2004.closely matched to each other, and the initial dI/dt in the [2] D.G. Tasker, et al., "Advances in Isentropic Compres-load is doubled to 12 TA/s. The load current reached sion Experiments Using High Explosive Pulsed Power,"5 MA in 500 ns as required. (As the magnetic pressure in Procs. of Fourteenth IEEE International Pulsed Powervaries as the current squared the pressure doubled from Conference, Dallas, TX, 2003, p.Il1.the Procyon to polyimide simulations.) The voltage [3] J.H. Goforth, et al., "Procyon Experiments Utilizingacross the first switch fell by -120 kV in 5 ns to near Explosively-Formed Fuse Opening Switches," in Procs.zero, so the calculated effect of dt/dV is -120 kV x Eighth IEEE International Pulsed Power Conference, San-0.59 ns/kV = 71 ns. The combined effect of this delay Diego, CA, 1991, p.273.and the jitter was 99 ns and 125 ns in the simulation. [4] J.R. Asay, Shock Comp. ofCondensed Matter - 1999,When performing the actual ICE experiments we Am. Inst. Phys., p.261.

found that the polyimide switch worked better than pre- [5] J.H. Goforth, in Procs. Of IXth International Confer-dicted. We measured currents in each switch and found ence On Megagauss Magnetic Field Generation And Re-that they were balanced to within 500 of each other. This lated Topics, July 2002, Moscow-St. Petersburg, Russia,appears to be due to high frequency ringing at the time of p. 137.switch closure, which reduces the effect of dt/dV. [6] The polyester insulation was DuPont Mylar® film in

5-mil thick sheets.[7] S.P. Marsh, Private communication, LANL, 1998.

IV. SUMMARY AND CONCLUSIONS [8] R.A. Graham, "Shock-Induced Electrical Activity inPolymeric Solids," J. Phys. Chem., 83 (23), p.3048.

For isentropic compression experiments (ICE) using a [9] The polyimide insulating film used in this study washigh explosive pulsed power (HEPP) system we needed DuPont type HN Kapton®.to transfer currents of 5 to 7 MA or more with risetimes [10] D.G. Tasker, R.J. Lee, and P.K. Gustavson, "An Ex-of -500 ns. To do this we had first tried using Procyon plosively-Actuated Electrical Switch Using Kapton Insu-closing switches, which operate by projecting a jet of lation," Tech. Rep. NSWCDD/TR-92/124, Naval Surfacealuminum through a polyester insulator, but we found Warfare Center, March 1993.their timing to be unacceptably imprecise for ICE with a [11] S.P. Marsh, LASL Shock Hugoniot Data, Univ. Cali-jitter of typically 600 ns. We suspected that the switch fornia Press, 1980.timing was sensitive to applied voltage and this was sub- [12] The breakdown strength of Kapton type HN is thick-sequently confirmed by experiment. We developed a ness dependent; Ebk= 7700, 6100, 5200, and 3900 V/milsimple shock model to explain the voltage sensitivity, for 1, 2, 3 and 5-mil thick sheets.dt/dV, and the predicted sensitivity was found to be simi- http://www.dupont.com/kapton/general/spelec.html.lar but larger than the measured values. The difference is [13] G.E. Duvall, "Some Properties and Applications ofthought to be due to a shock precursor in the insulation. Shock Waves," in Procs. Response of Metals to High Ve-We incorporated the model into simulations of the ICE locity Deformation, July 1960, vol. 9, 165-203.

circuit. The results matched those of the experiment [14] LLNL Explosives Reference Guide, Lawrence Liv-closely, and confirmed that the poor current transfer was ermore National Laboratory, UCRL-WEB-207690, Octmost likely due to the dt/dV timing sensitivity. 2004.Based on the model we designed a faster switch based [15] We inserted a 10-ptm thick aluminum foil as a shock

on the shock induced conduction of polyimide [15]. arrival detector between the bottom of the top electrodeWithin the precision of the data the predicted dt/dV was and the top of the polyimide (not possible with the Pro-equal to the measured value, thus confirming the model. cyon switch). This verified that conduction started withinOne minor drawback of the polyimide switch is that, 160 ns, i.e., during the first wave transit through the insu-

for predictable and accurate timing, we must ensure that lation, and was shock induced.the explosive drive system is made to high precision [16] Using a generic SPICE circuit code by Top-from the same batches of explosives. Moreover, we SPICE/Win32 v7.0 circuit code, Penzar Development,must test every batch to ensure accurate timing. All this P.O. Box 10358, Canoga Park, CA 91309, U.S.A. Themakes these switches expensive. details of our model are omitted for lack of space.We have successfully used the polyimide switch in [17] We may not need more than one polyimide switch in

many HEPP-ICE experiments and obtained the desired the ICE experiment, but dare not risk a shot failure bycurrent profiles and isentropic equation of state data. It using just one.is now used routinely in HEPP-ICE experiments.

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