IEEE TRANSACTIONS ON zyxwvutsrqponmlk POWER ELECTRONICS. VOL. zyxwvutsrqp 10, NO. 2, MARCH 1995 239 Transmission Line Based Energy Recovery Circuits and Their Application in Copper Vapor Laser Systems Sigmunt Singer, Doron Shmilovitz, Y. Ifrah, and I. Cohen Abstract-Energy recovery of narrow, high power pulses based on loss free resistor topology is described. A novel regenerative snubber realization based on transmission lines is presented. The purpose of this snubber is to recycle high voltage, high power (up to 20 KV, 1 MW) parasite pulses which arise in a copper vapor laser. These pulses consume up to 40% of the total power of the system. Currently, a low voltage (170 V) circuit of this type has been constructed and operated at recycling efficiency up to 85%. I. INTRODUCTION HE operation of a copper vapor laser (CVL) is followed T by the creation of narrow, high voltage, high power parasite pulses which contain up to 40% of the total power consumed by the CVL system. Those pulses arise due to the oscillatory discharge of the energy, stored in the plasma tube of the laser. The tube is usually driven by a resonant circuit [I] which generates the narrow, high power pulses needed for the excitation and plasma generation. The lasing occurs at high value of the tube current, at the vicinity of the (negative) driving pulse. At the end of the lasing a large amount of energy is still stored in the inductance of the tube. The copper vaporation, which results from the driving pulse, implies reduction of the resistance in the loop, which includes the tube and the driver capacitance. This results in oscillatory discharge of the tube. This oscillation is highly nonlinear due to the high rate of the variation of the inductive characteristic of the tube, implied by the time variation of the plasma. The pulses which are generated by this process are reflected to the driver and dissipated by its components. It has been found that the energy of those pulses must be absorbed by a resistive element, in order to eliminate the arcing effects which would otherwise appear at the plasma tube and thereby shorten the life span of the switching elements (thyratrons). Replacement of the conventional resistors by loss free resistors (LFR's) seems a convenient idea. The LFR is a two-port element with a resistive characteristic at the input terminals. This element was first realized by the combination of a controlled two-port network (characterized by a transformer matrix) and a signal-processing circuit. It has been found that, by suitable driving of the two-port network, Manuscript received March 29, 1993; revised July 17, 1994. S. Singer and S. Shmilovitz are with the School of Engineering, Tel Aviv T. Ifrah and I. Cohen are with the Nuclear Research Center, Negev, Beer IEEE Log Number 9407244. University, Tel Aviv, Israel. Sheva, Israel. zyxwvutsrqponm . - ... .. ..-- ..I ......... zyxw i ................... i ......... i ....... I . . ..:.. ............ z -L ................................................................ . . -. -.. ..... ._--. .... ................... .... I zyxwvu 6 zyxw 0 ' 5 ~ * Fig. zyxwvuts 1. Shape of the pulse generated by the laser. it is possible to achieve resistive characteristics [2]. Since the transformer is a loss-free network, all the power absorbed at input is transferred to the output (in principle). Practically, this realization was based on a switched-mode converter operated at high switching frequency. Another realization, by means of a family of converters operated at discontinuous conduction mode, is described in [3]. In both of the cases the output exhibits a power source characteristic [3]; therefore it is possible to parallel several outputs when high power is required [4]. In the case of the copper vapor laser (CVL), the application of LFR (based on a switched mode converter) is not practical because of the high voltage, high power (up to 20 KV, 1 MW) of the pulses and, mainly, because of their short duration (about 0.4 psec, see Fig. l), so a totally new type of LFR based on a transmission line has been developed. 11. TRANSMISSION LINE BASED LFR The fact that the ideal transmission line (TL) is loss free (in principle only) and has a resistive input impedance, leads to the idea of basing the LFR realization on this element. The uniform transmission line equations, neglecting the losses, are given as follows: where zyxwvuts L, C are the inductance and the capacitance per a unit of length z (see Fig. 2). Substituting ZO+K I 1 'U=-- zyxwvu m 0885-8993/95$04.00 0 1995 IEEE