Capacitive pulse forming network

The invention features a capacitive pulse forming network including at least one capacitor having a substantially linear discharge characteristic and at least another capacitor having a nonlinear discharge characteristic. The capacitive pulse forming network allows the duration and shape of pulses generated by the network to be accurately controlled.

BACKGROUND OF THE INVENTION 
The invention relates to high energy pulse-forming networks. 
In certain applications where high power sources (e.g., power lines, 
batteries) are unable to deliver high levels of peak power, pulse forming 
networks having high-energy density capacitors are often used. In these 
applications, the capacitors are slowly charged from the power source and 
then quickly discharged for short time periods to provide pulsed energy at 
high peak power levels. The capacitors are typically used with large 
inductors to store energy from the external power source and to establish 
the frequency of the period and shape of the output pulse from the 
network. 
Large high power inductors are often used with capacitors to shape the arc 
passing through the plasma. One technique for controlling the pulse shape 
of the arc is called the "multiple trigger" or "programmed discharge" 
approach. In this approach, a pulse forming network includes a number of 
capacitors each having an associated switch that is sequentially triggered 
to discharge its capacitor at a predesignated time. Each capacitor and 
associated switch generally requires an isolating diode to prevent 
feedback between the capacitors. Because the isolating diodes must be 
capable of handling the large values of peak power they are relatively 
large and expensive. 
SUMMARY OF THE INVENTION 
The invention features an approach for controlling the shape and duration 
of pulses generated by a capacitive pulse forming network. The pulse 
forming network includes capacitors connected to an external load, at 
least one of the capacitors having a substantially linear discharge 
characteristic and at least another having a nonlinear discharge 
characteristic. 
The linearity characteristics of the capacitors are selected and combined 
to shape the pulse and control its duration for a given application. As a 
result, a relatively simple and physically small circuit provides pulse 
shape control without the use of high power inductors that are relatively 
large and heavy. 
In preferred embodiments, the nonlinear discharge capacitor includes a 
polyvinylidene fluoride dielectric layer. The capacitors may be connected 
to each other in parallel or series to provide storage sections capable of 
generating high energy pulses greater than several hundred joules. These 
storage sections may be, in turn, combined to provide even higher energy 
capacitor banks capable of generating energy pulses greater than 1 
megajoule. The discharge element may be a high coulomb spark gap switch 
which is in the open state during the charging period and in the closed 
state during the discharge period. 
In another embodiment, the linear and nonlinear discharge capacitors may be 
used as part of a programmable discharge circuit with each capacitor 
having an associated switch. The switches are triggered sequentially to 
discharge the capacitors and provide the desired pulse shape. Each 
capacitor may be connected to an impedance network having an inductor 
and/or a resistor for providing additional pulse shaping. 
The linear and nonlinear capacitors may be fabricated independently in a 
single or separate canister housings. Alternatively, the capacitors may be 
fabricated in a single winding as an integral capacitor unit having 
conductive electrodes separated by at least a pair of dielectric sheets 
with at least one of the dielectric sheets having polyvinylidene fluoride. 
In another embodiment, the pair of dielectric sheets may include differing 
amounts of polyvinylidene fluoride.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a pulse forming network 10 includes a capacitor bank 
12 having three linear discharge capacitors 14-16 and a nonlinear 
discharge capacitor 17 all connected in parallel to each other. Each of 
the capacitors is charged from a DC voltage level charging power supply 20 
having a transformer 22 (which steps up a low AC voltage signal from an 
external source to a higher AC voltage) and a rectifier diode 24 (for 
converting the AC from the transformer to DC). A charging switch 26 is 
initially placed in its closed position to allow the DC signal from power 
supply 20 to charge capacitors 14-17. A voltmeter 28 is used to monitor 
the voltage across the capacitors and upon reaching a desired voltage 
level, it is removed and switch 26 is opened to protect power supply 20 
during discharge of capacitors 14-17. The time required to charge the 
capacitors may be as long as several minutes. The pulse forming network 
also includes a high coulomb spark gap switch 30, (e.g., Model T150, 
Physics International Company, San Leandro, Calif.) having a pair of 
terminals 32, 34, separated by a gap 36 which upon triggering causes an 
arc to be generated to allow capacitors 14-17 to discharge into a load 38. 
Capacitance may be defined by the relationship: 
EQU C=dq/dV. 
where: C=capacitance in farads (coulombs/volt) 
q=stored charge in the capacitor in coulombs 
V=potential across the capacitor in volts 
Linear discharge capacitors 14-16 have a characteristic where a change in 
charge (or discharge) provides a linear change in voltage. It is 
appreciated that "linear" discharge capacitors 14-16 includes those 
capacitors that may not be perfectly linear, but are highly linear 
relative to the nonlinear discharge characteristic of capacitor 17. Linear 
discharge capacitors 14-16 are high energy density metal film capacitors 
(e.g, Product No. KM532YWO53D, Aerovox Corporation, New Bedford, Mass.) 
each having an energy content of 0.754 KJ and a capacitance of 53 .mu.F. 
On the other hand, nonlinear discharge capacitor 17 has a characteristic 
in which the charge and voltage have a nonlinear relationship. At a 
certain charge level, the rate of voltage change with respect to 
increasing charge decreases across the nonlinear capacitor and the 
capacitor loses its linear characteristic. Nonlinear discharge capacitor 
17 is also a high energy density metal film capacitor (e.g., Product No. 
LM532YWO50D, Aerovox Corporation, New Bedford, Mass.) having an energy 
content of 0.740 KJ and a capacitance of 52 .mu.farads. Both linear and 
nonlinear discharge capacitors 14-17 are electrostatic, as opposed to 
electrolytic, and are fabricated as two separate solid foil electrodes 
wound around two separate dielectrics in a convoluted manner. The solid 
foil electrodes are fabricated from sheets of aluminum having a thickness 
of 0.22 mil with a metallized layer of 300 angstroms of zinc. Linear 
discharge capacitors 14-16 include as part of the dielectric system 
separating the electrodes a combination of linear dielectrics, for 
example, polypropylene and kraft tissue, while nonlinear discharge 
capacitor 17 include a layer of nonlinear dielectric, such as 
polyvinylidene fluoride (PVDF). Both linear and nonlinear discharge 
capacitors 14-17 are impregnated with a dielectric fluid, for example, 
castor oil. It is believed that the long chain molecular structure of PVDF 
provides the nonlinear charge/discharge characteristic of capacitor 17. 
One explanation is that as the nonlinear discharge capacitor begins to 
absorb charges within the PVDF layer, the long chain molecules begin to 
twist to align the charges. This twisting action results in the stored 
charge in the PVDF to be released at a nonlinear rate. 
Referring to FIG. 2, the resultant output waveform from pulse circuit 10 is 
shown. In particular, curves 40, 42 represent typical output current 
characteristics (Y-axis) in units of kiloamps as a function of time in 
seconds (X-axis) for linear and nonlinear discharge capacitors 14-17, 
respectively. Note that at t=0, when the capacitors are discharged, curve 
40 decreases rather sharply from a value of 0 kiloamps to a peak 44 at 
-0.46 kiloamps before rising and settling into a damped sinusoidal 
pattern. Curve 42, on the other hand, discharges at a much slower rate so 
that the current reaches a peak 46 about one microsecond after peak 44 of 
the linear discharge capacitor before it too settles into a damped 
sinusoidal waveform. The total output current curve 48 (the superposition 
of curves 40, 42) of the capacitors is shown having a peak 50 between 
component peaks 44 and 46. It has been observed that connecting nonlinear 
discharge capacitor 17 in parallel with the linear discharge capacitors 
14-16 causes current peak 44 to occur slightly earlier than if measured 
without capacitor 17. Curve 52 represents the voltage (Y-axis) in units of 
10,000 volts across the capacitors. Although a single nonlinear discharge 
capacitor may be used to provide longer duration pulses than linear 
discharge capacitors, by adjusting the values of capacitors 14-16 and 17, 
the shape of the total output current pulse can be "tuned" to have a 
duration between that of the combination of linear discharge capacitors 
14-16 and nonlinear discharge capacitor 17. 
Pulse circuit 10 may be configured in other topologies. For example, 
referring to FIG. 3, a pulse forming circuit 60 includes a nonlinear 
discharge capacitor 62 arranged in series with a pair of parallel 
connected linear discharge capacitors 64, 66 so that the current through 
the capacitors are the same but the voltage across the capacitors varies. 
In this configuration, the voltage waveform rather than the current 
waveform 50 of FIG. 2 is shaped. 
Referring to FIG. 4, the invention is shown used in conjunction with a 
programmed discharge circuit 80 to provide additional flexibility and 
control in shaping the pulse output. Discharge circuit 80 includes 
capacitors 82-88 each having a switch 90-96 and inductor 98-104 
(represented as impedances Z.sub.1 -Z.sub.4, respectively). Capacitors 
82-88 may be any combination of linear discharge and nonlinear discharge 
capacitors. A switch 106 is used to connect DC charging power supply 108 
to the capacitors. In operation, switches 90-94 and 106 are all closed 
until capacitors 82-88 are fully charged at which time the switches are 
opened. During discharge, switch 96 is closed to allow capacitor 88 to 
discharge into a common load 110. Switches 94, 92, and 90 are then closed 
in succession to provide the proper pulse shape characteristics. 
The individual linear and nonlinear capacitors may be wound independently 
and disposed in separate canisters or within a single canister. 
Alternatively, both a linear and a nonlinear discharge capacitor may be 
fabricated as a single wound unit within a housing. Referring to FIG. 5, 
wound capacitor 112 includes a first aluminum foil electrode 114 connected 
to a top portion 115 of housing. A second aluminum foil electrode 116 is 
likewise connected to a bottom portion 118 of housing, with a dielectric 
spacer 120 used to isolate the top and bottom portions. Sandwiched between 
electrodes 114, 116 are a pair of dielectric layers 122, 124. In this 
embodiment, dielectric layer 122 is formed of polypropylene and kraft 
tissue while portion 124 is formed of PVDF as described above. Thus, as 
shown schematically in FIG. 6, a linear discharge capacitor is formed by 
electrodes 114, 116, separated by dielectric layer 122, while a nonlinear 
discharge capacitor is formed by the same electrodes 114, 116 separated by 
dielectric layer 124. In other embodiments, dielectric layer 122 may 
include a relatively small amount of PVDF combined with the polypropylene 
and kraft tissue to provide a capacitor having a nonlinear characteristic 
much less than non linear characteristic of the capacitor having 
dielectric layer 124. The nonlinearity characteristic is generally 
proportional to the amount of PVDF in the particular layer. Other 
approaches for winding combinations of capacitors within a single unit are 
described in U.S. Pat. No. 4,856,112, assigned to the assignee of the 
present invention, and hereby incorporated by reference. 
Other embodiments are within the claims. For example, it is appreciated 
that any combination of series and parallel capacitors may be combined, 
either separately, within the same capacitor housing, or, in some cases, 
within the same winding (see FIG. 5) to provide the desired shape of 
voltage or current waveform. Furthermore, in applications where a spark 
gap switch 30 is used to discharge the capacitors, it may be any of a 
variety of high action spark gap switches including, as is well known in 
the art, a mid-plane switch.