Method and Apparatus for Eliminating Reflected Energy Due to Stage Mismatch in Nonlinear Magnetic Compression Modules

1. Field of the Invention

The present invention relates generally to pulse power sources and more particularly, to an apparatus and method for eliminating secondary pulses due to stage mismatch in nonlinear magnetic compression pulse power sources for gas discharge lasers.

2. Description of the Prior Art

In applications where pulsed lasers are operated for extended periods of time, there is a need for energy efficiency, long term reliability and high component lifetimes. In these applications, magnetic compression circuits in conjunction with solid state switches have recently been utilized to supply pulse power because of the reliability and useful life of the magnetic compression circuits and solid state switches.

Such magnetic compression circuits generally utilize a multi-stage LC network to provide power compression. They are generally in accordance with U.S. Pat. No. 5,313,481.

One particular difficulty with using multiple stages of magnetic compression is the inability to perfectly match one stage to the next, so that complete energy transfer from one stage to the next cannot be accomplished. This problem is compounded when one or more stages are located in different thermal environments. The high temperature sensitivity of the capacitors typically used in these high voltage systems causes mismatch between stages as the temperature of each stage reaches a different value.

The mismatch between successive compression stages will cause some of the energy in the forward traveling pulse to be left behind at each compression stage. This energy is temporarily stored in the capacitors of each stage. Under certain conditions, this left-over energy can form a second electrical pulse which is launched toward the load some time after the main pulse. Such secondary pulses can, for example, cause damage to the electrodes in a gas discharge laser.

FIG. 1 illustrates a typical pulse power system which implements a magnetic pulse compression circuit with stages that exhibit some small mismatch. In the system of FIG. 1, C.sub.0, L.sub.0, and C.sub.1 form one C-L-C circuit while C.sub.1, L.sub.1 and C.sub.2 form another C-L-C circuit. In the first C-L-C circuit, C.sub.0 is the source capacitor, and in the second C-L-C circuit, C.sub.1 is the source capacitor. During normal operation, a command charge power supply is used to charge the DC storage capacitor, C.sub.0, to the desired voltage. Upon command, a semiconductor switch such as an Isolated Gate Bipolar Transistor (IGBT), or silicon controlled rectifier (SCR) switch will discharge C.sub.0 (through L.sub.0, which limits the peak current), into the capacitor C.sub.1 at the input to the nonlinear magnetic pulse compression network. The nonlinear magnetic compression network provides temporal compression of the stored energy via the large inductance changes associated with the saturation of the nonlinear magnetic core material which is contained within the induction cores.

Once the compressed pulse reaches the load, such as a gas discharge laser, the mismatch between the load and the output stage leads to some reflected energy traveling backward through the compression stages. This reflected energy is captured on C.sub.0 through the use of a semiconductor switch which will carry current in only one direction. This keeps the energy on C.sub.0 from again traveling forward through the compression stages. In an ideal system, the capacitance values in each stage are accurately matched so that there is no overshoot or undershoot in the voltage of the source capacitor in a C-L-C circuit that is provided by an adjacent stage.

As mentioned above, the values of C.sub.0, C.sub.1, and C.sub.2 do not remain at the same values as the temperature of each stage changes. When capacitive mismatch exists, the voltage on the source capacitor will not ring down to zero voltage upon the transfer of energy to an adjacent stage. If the source capacitor is slightly larger than desired, the voltage on the source capacitor will ring down to a non-zero positive value. If the source capacitor is slightly smaller than desired, the voltage will overshoot the zero voltage point and end up with a negative value.

This overshoot or undershoot voltage will occur for both the main pulse traveling toward the load and for the backward traveling reflected energy pulse. The voltage on the capacitor of a particular stage will create a potential across its neighboring inductor. Since each inductor, except for L.sub.0, is a saturable inductor, the saturable inductors in the compressor circuit will eventually saturate in the direction necessary to allow energy transfer of the secondary pulse toward the load.

FIG. 2 is a graph of the voltage on capacitors C.sub.0 and C.sub.1 as a function of time, when C.sub.0 is slightly undersized relative to C.sub.1. In this case, the voltage on C.sub.0 will overshoot negatively during the initial energy transfer from C.sub.0 to C.sub.1. The reflected energy pulse travels from the load, back through the various stages, to C.sub.1, and finally back to C.sub.0. During the transfer of charge from C.sub.1 back to C.sub.0, the voltage on C.sub.1 will undershoot the zero voltage point because its capacitance value is slightly larger than that of C.sub.0. This results in leaving a negative voltage on C.sub.1 since the reflected energy pulse is a negative voltage pulse.

This leftover negative voltage will remain on C.sub.1 until L.sub.1 saturates in the reverse conduction direction, either due to the potential difference between C.sub.1 and C.sub.2, or because of the bias current used to reset each saturable inductor to its proper point on the B-H curve in preparation for the next pulse. Once L.sub.1 reverse saturates, a negative voltage pulse is launched toward the load which will be compressed by the remaining stages. In extreme cases, this pulse can have a magnitude of 1 to 2 kV compared to the main pulse peak of 15-20 kV. A pulse of such a magnitude can cause significant erosion to the electrodes in a gas discharge laser and result in lower electrode lifetimes.

Great care is taken to prepare the laser for the main 15-20 kV pulse. The gas medium has been cooled and homogenized, or in some cases replaced since the previous main discharge event. Just prior to the main discharge, care is taken to uniformly pre-ionize the medium. The end result is a discharge which initially will uniformly fill the discharge chamber. The plasma draws current from a large electrode area. Through processes such as carrier attachment, the discharge begins to localize into streamers over time periods measured in tens of nanoseconds. These processes are capable of reducing the discharge to a single localized arc in a few 100 nanoseconds. A few millijoules delivered into such a localized area can be more destructive than a few joules delivered to a uniform plasma which fills the discharge volume. Consequently, it is very important to minimize any energy pulses which might be delivered to the discharge other than at the desired main discharge time.

Accordingly, there is a need in the technology to provide an apparatus and method for eliminating reflected energy due to stage mismatch in pulse power lasers, so that electrode erosion may be minimized.

A high voltage pulse power generating circuit capable of eliminating secondary pulses due to stage mismatch in the stages of its magnetic pulse compressor circuit is disclosed. The pulse generating circuit source comprises a high voltage charge storing element which is periodically discharged into a nonlinear magnetic compressor circuit. A mismatch correction circuit coupled to each stage of the compressor circuit discharges the reflected energy due to stage mismatch and thus eliminates the reflected energy from being launched back into the load. In a preferred embodiment, the pulse power generating circuit of the present invention utilizes a diode and a resistor that is coupled to each stage of the magnetic pulse compressor circuit. Various embodiments are disclosed.

FIG. 1 illustrates a typical pulse power system which implements a magnetic pulse compression circuit with stages that exhibit a small mismatch.

FIG. 2 is a graph illustrating the voltage on capacitors C.sub.0 and C.sub.1 of FIG. 1 as a function of time, when C.sub.0 is slightly undersized relative to C.sub.1.

FIG. 3 is a schematic diagram of one embodiment of a magnetic pulse compression circuit which implements the mismatch correction circuit of the present invention.

FIG. 4 is a schematic diagram of a second alternate embodiment of the magnetic pulse compression circuit of the present invention.

FIG. 5 is a graph illustrating the voltage on capacitors C.sub.0 and C.sub.1 of FIG. 3 or FIG. 5 as a function of time, when C.sub.0 is slightly undersized relative to C.sub.1.

FIG. 6 is a schematic diagram of a third alternate embodiment of the magnetic pulse compression circuit of the present invention.

FIG. 7 is a schematic diagram of a fourth alternate embodiment of the magnetic pulse compression circuit of the present invention.

The present invention relates to a magnetic pulse compression circuit which is capable of eliminating reflected energy due to stage mismatch in pulse power lasers.

FIG. 3 is a schematic diagram of one embodiment of a pulse power generating circuit 10 which implements the magnetic pulse compression circuit of the present invention. The pulse power generating circuit 10 comprises a high voltage DC power supply 12, an inductor L, a capacitor C.sub.0, a solid state switch S, a second inductor L.sub.0 and a pulse compressor circuit 16 for compressing the duration and increasing the peak power of pulses applied. The switch S may be a silicon controlled rectifier (SCR), an insulated-gate bipolar transistor (IGBT), a metal-oxide semiconductor field-effect transistor (MOSFET), a MOSFET controlled thyristor (MCT), or any other suitable switching element as known in the technology.

The pulse generating circuit 10 preferably comprises a diode D.sub.s connected in series with the switch S. The inclusion of D.sub.s in the circuit 10 ensures that the pulse compressor circuit 16 is nonconductive with respect to reverse current. In one preferred embodiment, the circuit 10 further comprises an inductor L.sub.0 selected in conjunction with C.sub.1 to provide a time constant for the LC combination to slightly slow the buildup of current flow through the switch S so that the switch may become completely closed prior to high current flow through the switch S. The pulse compressor circuit 16 is coupled to a load 18, typically a pulse discharge load such as an excimer laser.

The compressor circuit 16 comprises banks of capacitors C.sub.1, C.sub.2, . . . C.sub.n-1 and C.sub.n and one or more saturable core inductors L.sub.1, L.sub.2, . . . , L.sub.n connected in a ladder network. As shown in FIG. 3, a first saturable core inductor L.sub.1 is connected between C.sub.1 and C.sub.2, a second saturable core inductor L.sub.2 may be connected in series between C.sub.2 and C.sub.3, . . . , and a saturable core inductor L.sub.n-1 is connected in series between C.sub.n-1 and C.sub.n. In the circuit 16 of FIG. 2, C.sub.0, L.sub.0 and C.sub.1 form one C-L-C circuit while C.sub.1, L.sub.1 and C.sub.2 form another C-L-C circuit. In the first C-L-C circuit, C.sub.0 is the source capacitor, and in the second C-L-C circuit, C.sub.1 is the source capacitor. This extends throughout the pulse compressor circuit 16, so that in essence, C.sub.n-1, L.sub.n-1 and C.sub.n forms the (n-1)th C-L-C circuit, with C.sub.n-1 being the source capacitor.

The pulse compressor circuit 16 further comprises a plurality of mismatch correction circuits CR.sub.1, CR.sub.2, . . . , CR.sub.n each comprising a high voltage diode and a resistor placed in parallel with C.sub.1, C.sub.2, . . . , C.sub.n respectively. In particular, each mismatch correction circuit CR.sub.n comprises diode D.sub.n and resistor R.sub.n placed in parallel with C.sub.n. The polarity of each of the diodes D1, D2, . . . , Dn is chosen so that each diode acts as an open circuit to the positive voltage main pulse, and thus has no effect on the main pulse.

However, each diode D.sub.1, D.sub.2, . . . , D.sub.n will conduct during the negative voltage swing caused by the reflected energy pulse. For example, when D.sub.1 is conducting, R.sub.1 discharges a negative voltage on C.sub.1 at a rate determined by the RC time constant of R.sub.1 and C.sub.1. The value of each of the resistors R.sub.1, R.sub.2, . . . , or R.sub.n is chosen to be small enough so that the corresponding voltage on capacitor C.sub.1, C.sub.2, . . . , C.sub.n, respectively, is substantially discharged after the main pulse and before the associated inductor L.sub.1, L.sub.2, . . . , L.sub.n, respectively, reverse saturates. However, the value of each resistor is also large enough to minimize the amount of charge discharged from the corresponding capacitor C.sub.1, C.sub.2,. . . , C.sub.n respectively, during the passage of the negative voltage energy pulse reflected by the load (laser).

FIG. 4 is a schematic diagram of a second alternate embodiment of the magnetic pulse compression circuit of the present invention. As shown in FIG. 5, a step up transformer 29 may be coupled to the output of the compression circuit 16. Alternatively the transformer 20 may be coupled to the pulse compressor circuit 16 of FIG. 3 at one of several nodes (for example, nodes 1, 2, . . . n) depending on design requirements. A transformer 20 coupled to the circuit 10 at a node closer to the power supply 12 will subject downstream components of the pulse compressor circuit 16 to a higher voltage but lower current than the embodiment coupled to the output of the compression circuit 16, and of course will effect the design of such components because of the different impedance and other operating conditions they will be subjected to. Conversely, a transformer 20 coupled to the circuit 10 at a node closer to the load 18 will allow use of low voltage but high current components supplying the transformer. Furthermore, the transformer 20 itself can be somewhat different, depending on its placement, as stepping the voltage up to the desired high level before the final pulse compressor reduces the required peak current capabilities of the transformer 20.

As stated before, for the mismatch correction circuit CR to work properly under all conditions, the values of stage capacitance must be chosen so that the voltage remaining on each capacitor after the passage of the main pulse undershoots during the transmission of the negative voltage energy pulse reflected by the load. This is accomplished by intentionally nominally sizing C1 slightly larger than C0, C2 slightly larger than C1, etc., so that manufacturing tolerances and temperature variations will not be enough to cause any capacitor C.sub.n to actually be less than its preceding capacitor C.sub.n-1. In this way, both the manufacturing tolerances and any temperature variations can be accommodated.

The circuit of FIG. 4 is identical to that of FIG. 3 with the exception of the addition of the step up transformer 20 shown between the output of the pulse power source and the load 18, for the preferred embodiment an excimer laser, though the step up transformer 20 may be inserted in circuit at C.sub.1 or any of the capacitors between C.sub.1 and C.sub.n. The presence and location of the transformer 20 does not effect the function of the other components of the pulse power supply, though being an impedance transforming device (transforming voltages and currents), the values, current capacities and voltage capabilities of the components from and including the power supply 12 to the primary side of the transformer 20 will differ substantially from the corresponding components of FIG. 3, even though the same components will normally be chosen to have substantially the same time constants as the embodiment of FIG. 3.

The operation of the pulse power generating circuit 10 of the present invention will be described with reference to the embodiments of FIGS. 3 and 4 and the graph illustrated in FIG. 5. For discussion purposes, only one mismatch correction circuit CR.sub.1 will be considered. It is understood however, that the use of other mismatch correction circuits in the other stages of the pulse compressor circuit 16 may be implemented. With the D.C. power supply 12 on prior to the time to (when switch S is open), capacitor C.sub.0 is charged by the current supplied by the D.C. power supply 12. When fully charged, the voltage across capacitor C.sub.0 is V.sub.C0 (see FIG. 5). At t.sub.0, the switch S closes, and the energy stored in capacitor C.sub.0 is transferred to C.sub.1 by action of the LC circuit comprising L.sub.0, C.sub.0 and C.sub.1 with a time constant of approximately ##EQU1## C.sub.1 becoming nearly fully charged and the current in inductor L.sub.0 reducing substantially but still remaining positive at time t.sub.1. Inductor L.sub.0 is included in the circuit to purposely provide a sufficiently large time constant in this energy transfer to ensure that S is fully switched before high currents are encountered, and to limit the maximum current through the switch for a given energy transfer to a value manageable by a solid state switch by extending the time of the transfer. As capacitor C.sub.0 starts to discharge through switch S and capacitor C.sub.1 begins to charge during this transfer, the current in the coil of saturable inductor L.sub.1 rapidly changes from a negative bias current to a low positive current, at which point the inductance jumps to a relatively high value to limit temporarily the rate of further current increase through L.sub.1.

In addition, since the negative terminal of the diode D.sub.1 is coupled to the positive terminal of C.sub.0, D.sub.1 acts as an open circuit to the positive voltage main pulse, and thus has no effect on the main pulse. However, due to the mismatch between C.sub.0 and C.sub.1, the voltage on C.sub.0 will overshoot negatively during the energy transfer from C.sub.0 to C.sub.1.

At time t.sub.1, while the current in inductor L.sub.0 is low but still positive, the saturable core of inductor L.sub.1 saturates, at which time the inductance of inductor L.sub.1 jumps to a low value, allowing the current through L.sub.1 to rise to a very high value to transfer the energy from capacitor C.sub.1 to capacitor C.sub.2 with a time constant approximately equal to ##EQU2## where L.sub.1 is the inductance of the inductor L.sub.1 with the core in saturation. Note that this time constant may be substantially shorter than the time constant ##EQU3## because may be made much less than L.sub.0.

In the manner described above, energy is transferred through each stage of the compressor circuit 16. When C.sub.n-1 is substantially fully charged and the core of inductor L.sub.n-1 saturates, the energy is coupled through the saturated inductor from C.sub.n-1 to C.sub.n in a short time period pulse, with the voltage across C.sub.n being applied to the laser 18.

When the voltage across C.sub.n builds to the level required to initiate discharge in the laser, the laser 18 fires. Energy stored in C.sub.n and still being fed to C.sub.n through the inductor is delivered to the laser discharge in the compressed time period of the pulse power supply 12. Once the compressed pulse reaches the load 18, the mismatch between the load 18 and the output stage of the pulse compressor circuit 16 leads to some reflected energy traveling backward through the compression stages of the pulse compressor circuit 16. The reflected energy has been recovered in C.sub.1 at time t.sub.2.

As the reflected energy is transferred from C.sub.1 back to C.sub.0 (between time t.sub.2 and t.sub.3), the voltage on C.sub.1 will be negative as compared to ground and the diode D.sub.1 will conduct, causing R.sub.1 to discharge C.sub.1 at a rate determined by the RC time constant of R.sub.1 and C.sub.1, effectively bringing the voltage of C.sub.1 to near zero. The value of the resistor R.sub.1 is selected to be small enough so that the corresponding capacitor C.sub.1 is substantially discharged before the associated inductor L.sub.1 reverse saturates. However, the value of R1 must be large enough to minimize the amount of charge discharged from the corresponding capacitor C.sub.1 during the passage of the negative voltage reflected energy pulse.

As shown in FIG. 5, the voltage on C.sub.1 (as shown with the solid line) is less when the mismatch correction circuit CR1 is added to the pulse compression circuit 16 (FIGS. 3 and 4), than when it is not implemented (as shown with the dashed line). By implementing the mismatch correction circuit of the present invention, the intentionally imposed undershoot voltage on C.sub.1 due to mismatch between C.sub.0 and C.sub.1 is substantially discharged before inductor L1 reverse saturates. As a result, when the inductor L1 does reverse saturate, the negative voltage pulse that would have been launched toward the laser chamber is effectively eliminated.

As discussed earlier, the mismatch correction circuit CR may be applied to each stage of the pulse compression circuit 16 with properly chosen values of resistance in accordance with the size of each stage capacitor and the reverse saturation time of each stage inductor.

It has been observed that the embodiments of FIGS. 3 and 4 may be effectively used for pulse compression circuits operating in the 1 kV range using commonly available 1 kV high voltage diodes and resistors.

FIG. 6 is a schematic diagram of a third alternate embodiment of the magnetic pulse compression circuit of the present invention. This embodiment may be used in pulse compression circuits which operate in the 10-30 kV range or higher. Although the embodiments of FIGS. 3 and 4 may, theoretically be applied in pulse compression circuits operating in this range or higher, high voltage diodes that are commercially available are of unknown reliability, and series stacks of low voltage devices require additional components to ensure proper voltage division under both dc and transient conditions. The embodiment of FIG. 6 provides a simple alternative and solution to this problem. In this embodiment 16a of the pulse compression circuit, a diode D.sub.1T and resistor R.sub.1T are placed in parallel with the primary windings of the step-up transformer 20. In this manner, the charge on the high voltage capacitor, for example, C3, can be discharged using more robust 1 kV high voltage diodes instead of having to implement the high voltage diode stacks which have unknown reliability.

If additional compression stages exist after the step-up transformer 20, then a step-up transformer 22 may be placed across the high voltage capacitor (such as C.sub.4 or C.sub.n) of each stage with a 1 kV high voltage diode D.sub.2T and resistor R.sub.2T across the primary windings of the transformer 22. This embodiment is shown in FIG. 7.

By implementing the magnetic compression circuit of the present invention, reflected energy due to stage mismatch in pulse power lasers may be eliminated, so that electrode erosion is minimized.

The above-described embodiments of the present invention are especially suited to providing high voltage pulses for producing electrical discharges between the anode and the cathode of an excimer laser. The invention could also be applied to provide high voltage pulses in many other devices including lasers.

Although the invention has been described in terms of a certain preferred embodiment, other embodiments apparent to those skilled in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims which follow.