High-repetition Rate, High-voltage Pulser For Multi-atmosphere C0/sub 2/ Laser

Abstract

A high-repetition rate, high-voltage excitation system has been built for a multi-atmosphere CO;! laser, employcd in the Molecular Laser Isotope Separation (MLIS) process. The system has been designed to the demanding industrial reliability standards required by the MLIS process and will be capable of driving the-high pressure CO;! laser at repetition rates of up to 2 kHz. The excitation system consists of a modular, microprocessor controlled pulse power supply and a thyratron switchcd LC inversion laser pulser eniploying a single stage of magnetic pulse compression. , The pulse power supply chargcs a single stage LC inversion pulser to energy levels of 60 J in 150 psec at 70 kV peak voltage. Each module can individually generate pulses at a repetition rate of 250 Hz and 8 modules are time multiplexed to achieve an overall repetition rate of 2 kHz. The pulser supplies voltage pulses of 120 kV with risc timcs of 120 ns to the laser. Application background Molecular laser isotope separation (MLIS) requires for its initial selective excitation step a continuously tuneable laser source with a wavelength in the 16 pm range'. High repetition rate, pulsed CO;! lasers emitting in the 10 pm range can be employed as primary laser sources for this step and the requircd 16 pm radiation is then crcated via a four-wave mixing process in a multipass Raman cell. Tuneability of a CO;! laser can be achieved by utilising the pressure broadening of the CO;! emission lines at high gas pressurcs. Continuous tuneability of the laser requires operating pressures of 8-10 bar. Furthermore, the MLIS process demands laser pulse repctition rates in excess of 1 kHz. Excitation system parameters There are several problcms associatcd with higlipressure, high-repetition rate opcration of CO2 lasers. High excitation voltagcs of 10-14 kVcnr'bar' and voltage rise times in the order of 100 ns are necessary for the creation of stable, arc-frce discharges. The generation of homogeneous glow discharges becomes generally more diflicult at pressures in excess of 1 bar, 63 demanding eficient pre-ionization techniques. Furthermore, specific discharge input energies required for the generation of suflicicnt laser gain, scale directly with the operating pressure and typically 100-150 J litre'bar' have to be supplied. At high repetition rates a fast transverse gas flow has to be provided to replace the gas in the discharge volume between successive laser excitations and performance of the lasers can be severely limited by acoustically initiated discharge instabilities. Optical performance and required excitation parameters of the laser were established on an experimental, low repetition rate device. In order to keep excitation parameters of the laser at a manageable level, a small electrode separation of 10 mm arid an active discharge volume of 50 cm3 were selected. Experiments indicated that an electrode breakdown voltage of 100 kV was necessary to generate stable discharges and an input energy to the laser of 60 J was needed to produce the required small signal gain of the laser. It was soon rcaliscd that at the projected repetition rate of 2 kHz and an assumed over-all power supply eficiency of 75 %, 160 kW of power would be ncedcd. The precise control of these power levels docs present a significant technological challenge.