DESIGN OF MICROWAVE PULSE COMPRESSION CIRCUITS FOR THE EXCITATION OF PULSED HIGH POWER GAS LASERS

Abstract

During the last decade, the microwave discharge was investigated as an alternative technique avoiding some of the disadvantages associated with the transversely excited atmosphere (TEA-) discharge such as preionization, instabilities and pulse length limitations due to the metallic electrodes. However, up to now one drawback of the pulsed high power microwave discharge was the lack of commercially available high power microwave tubes with pulsed output powers in the region of 100 MW to 1 GW. There are two methods for achieving output powers comparable to relativistic sources. One way is the combining of several conventional tubes, which is rather complicated especially for magnetrons. The other method is the compression of the microwave pulse of a conventional tube by means of a microwave cavity or a network of delay lines and hybrids. This paper describes the application of microwave pulse compression using Q-switching for discharging the storage cavity. Under optimum conditions, up to 80 % of the microwave pulse energy can be stored in a cavity and delivered to a matched load via triggering a suited switching element. The design discussed in this paper uses the laser discharge as switch. The plasma tube is integrated in the storage cavity and the discharge is triggered when the electric field strength exceeds the holdoff field strength of the gas mixture. The microwave source was operated at power levels of 4.5 - 5 MW and at repetition rates of 10 - 20 Hz. With a stored energy of 1.4 J a XeCl laser featuring a pulse energy of 4 mJ and a pulse length of 16 ns could be realized without any preionization. The discharge structure was in no way optimized. Principal limitations in laser pulse energy and repetition frequency are arising from the fact that the discharge acts both as a switch and as laser amplifier. Considerable improvements in the performance of the complete system can be expected, if the cavity is discharged by an optimized switch while the laser discharge acts only as a load. Some theoretical aspects of the pulse compression circuit will be presented together with experimental results. The results show the potential of realizing microwave discharge pumped excimer lasers with output powers ranging from 10 to 100 d. A laser head powered by a S-band (3 GHz) magnetron would be rather compact, because it comprises only a storage cavity and a discharge structure without any high voltage component. The feeding could be done via a flexible waveguide. In this way using microwave pulse compression techniques, the microwave discharge is an interesting alternative to TEA-discharges.