The performance of recently developed hollow-anode thyratrons in high-energy uniform-beam XeCX lasers has been studied. Using a single thyratron, a peak laser pulse energy of 1.24 J was demonstrated. Reliable long-term operation at 100 W average power and 100 pulses per second has been achieved at a peak forward thyratron current of 15 kA and reverse current of 6 kA. A dual thyratron driver has been developed and achieved a peak output energy of 1.61 J from the same laser device. This scheme lends itself to simple length scaling of excimer lasers to higher pulse energies. Thus, high power excimer lasers can be designed using commercially available thyratrons, while still maintaining uniform, diffuse gas discharge quality.

For modulators driving high-power discharge lasers, themajor system issues are system performance, efficiency, output voltage and risetime, and required electrical output characteristics dictated by the design of the laser cavity. From the system issues, modulator trade-offs are determined; these design trade-offs include shape of the output waveform from the modulator, switching approach for the modulator, grounding and shielding of the modulator with respect to the laser load, and interaction between the laser and the modulator.

Nova is a 100 TW Nd++ solid state laser designed for experiments with laser fusion at LLNL. The pulsed power for Nova includes a 58 MJ capacitor bank driving 5336 flashlamps with millisecond pulses and subnanosecond high voltages for electro optics. This paper summarizes the pulsed power designs and the operational experience to date.

The energy output and reliability of the multi-joule, injection-locked KrF laser used to trigger the PBFA II accelerator gas switches were improved through modifications identified in modeling the Blumlein driver circuit for the power oscillator. A combination of the SCEPTRE¹ network solver code and JASON² electrostatic field code were used to model the laser pulse-forming circuit in its single-channel rail gap configuration and modified versions with three or five discrete switches across the 1.45-m-wide, water-insulated transmission line. Three regularly spaced trigatron spark gaps resulted in a more uniformly driven laser volume with lower variations in voltages (10%) and rise times (9%) along its length. With the new configuration, over 3000 shots have been recorded without a single misfire compared to an average of ---25 shots before a prefire with the original design. The gas mix and pressure had to be optimized to match a given driver pulse voltage and rise time to achieve maximum performance from the laser. We summarize the model results which led to our decision to change the Blumlein switch configuration.

The Universal Voltronics type BAX-1.6-15,000-RC-LANL flashlamp modulator built for Los Alamos National Lahoratories in 1984 has proved a highly reliable unit. It has delivered over limo hours of failure-free service. This unit develops 15 kW average output power; it contains two separate PFN subsystems, each with 30 microfarad capacitance, charged to 1414 volts, which deliver 1000-ampere pulses to two flashlamp loads at 250Hz. The efficiency of the unit is in the 90-percent range. About one kW is dissipated inside the modulator. The entire modulator is packaged in a 4 ft. long, 2 ft. high, 2 ft. wide enclosure, and is water cooled. It consists of solid-state controls (shot counters, digital voltmeters, LED indicators), an SCR-regulated DC supply with water-cooled transformer, an SCR command charger and water-cooled resonant charging choke, and two PFN assemblies, each containing a 30-microfarad capacitor, an 1800-volt inverter-class SCR output switch, a pulse shaping inductor and a "simmer" supply to sustain the flashlamp discharge between pulses. Each PFN is mounted on a removable tray for easy service and maintenance.

Transverse discharge gas lasers exhibit a wide range of requirements in terms of operating voltage, discharge duration, energy deposition, and details of breakdown mechanism. The negative slope of the impedance characteristic, coupled with a large impedance range during a discharge cycle, can yield some unexpected results. Waveforms taken from several different lasers, including HP, XeCl, and KrF are presented, and practical consequences of the dynamic impedances are considered. These include: choices of components in final pulse forming networks; charging system requirements; and calculation of predicted performance, preionization, impedance matching, and circuit losses. mhese lead to a discharge system optimized for the particular application.

A primary consideration for operation of many high power gas lasers is the generation of a high voltage, large area electron beam. Beam parameters are dictated by laser requirements. Historically the electrical system typically has consisted of a Marx generator with pulse shaping stages or a pulse forming line. In the latter case, one or more high voltage switches (spark gaps) are required. Generally, these systems have been single pulse with cold cathode diodes. For repetitive systems, there is a trend to replace the Marx generator with a pulse transformer. For long pulses and large diode impedances, the pulse shaping elements can be placed in the transformer primary circuit. Otherwise, a PFL and high voltage switch are still required. The trend in pulsed power systems is to attempt to increase the range over which the PFL is not required. For short pulses (10's of nanoseconds), cold cathodes provide a stable impedance for at least hundreds of thousands of shots at rates to several tens of hertz. For longer pulses, where impedance collapse becomes a problem, the shot life of cold cathodes becomes short and the use of hot cathodes may be necessary. In either case, issues of beam current uniformity over large areas (including control of pinching) and mitigation of arc formation must be addressed. This paper will review on-going work relevant to the electrical systems and beam sources. State-of-the-art will be described and a modest attempt will be made to extrapolate current trends.

A pulse forming network for excitation of high pressure TE-lasers using a large number of paralleled waterline capacitors is described. The low inductance PFN can be arranged in different circuits and has to be treated as a lumped circuit. At 33 kV charging voltage the XeCl-laser has emitted 4.0 J optical energy. A scaling of the PFN to a storage capacity of 7 kJ electrical energy is presented.

An ideal laser power supply should bring a capacitive storage medium to a programmable voltage level at a constant rate. This voltage level must be maintained until the laser is fired, at which time the charging source must be immune to severe transients. Considerations include efficiency, size, cost, and reliability. A switched mode charging source is described which has been in commercial production for several years, and which will transfer 5KW of average power to a value of capacitance ranging from 20 to 100nF at approximately 32KV with repetition rates to 500Hz.

We report on an electric discharge laser pulser that used three parallel thyratrons to drive a large, meter-long HgBr laser. By placing a magnetic sharpening switch between the thyratrons and the laser load, we were able to generate a 60 kV voltage,Rpike, and a 30 kA, 120 nS long current pulse with a rising front of approximately 1 x 10¹² A/S at the laserhead. This led to an optical output exceeding 2 J and an energy transfer efficiency of 1.7 percent.

Gating parameters for using thyristors in laser power supplies are discussed. Manufacturer's recommendations for improving turn-on in SCRs are reviewed. Measurements of gate current, voltage, and power, in excess of specified device gate maximums, indicate that peak gate current is the primary factor controlling the rate of rise of anode current.

A new switch for laser applications is described. Experiments are reported wherein it is shown that according to voltage operation range, switching precision, trigger efficiency, voltage reversal, and recovery time the pseudo-spark switch is superior to high pressure spark gap switches and comparable to thyratrons. Several pseudo-spark trigger methods are presented. The discharge can be initiated by a dielectric surface breakdown trigger. A dif ferent trigger method is based on a pulseslilL low-current gas discharge, which has practically unlimited lifetime and allows repitition rates up to 100 kHz. Several modifications of switches triggered in this manner have been tested in different types of gas lasers (copper va pour, N₂-laser), which are normally driven by thyratrons or high-pressure spark gap switches, respectively.

The molecular processes that increases a discharge current by photoelectron detachment of negative ions and decreases the current by photodissociation products could be applied for the development of fast and high repetition-rated opening switches. To demonstrate this principle, a switching of electron conduction current by laser irradiation on a DC discharge medium was investigated. The discharge current sharply increased after the discharge medium was irradiated by a laser pulse, and then decreased to a small fraction of the original DC current. Our results show that the magnitude and pulse duration of a current switching can be controlled by SOC1₂ pressure, laser power, and laser interaction volume.

Two concepts for externally controlled bulk semiconductor switches are discussed. One concept is based on electron-beam ionization of direct semiconductors with a negative differential electron mobility region in their velocity-field strength characteristic (e.g. GaAs). The e-beam is injected through one of the contacts in direction of the applied electric field, similar to e-beam controlled diffuse discharge switches¹. Because of the very high value of the source function (number of electrons generated per second and cubic centimeter) compared to diffuse discharge switches, the ratio of switch current to sustaining e-beam current can reach values of 10⁵. Generation of excess charge carriers in the region beyond the range of the electrons is provided by absorption of recombination radiation and bremsstrahlung emanating from the e-beam excited zone. Opening of the switch is obtained by turning the e-beam off. If the switch is part of an inductive discharge circuit, the increasing field strength during opening drives the semiconductor through a negative differential mobility region into a low conductivity range, a mechanism which supports switch opening. A second switch concept is based on optical control of semiconductors, where lasers are used to drive the switch into and out of the conductive state using two different wavelengths. The direct semiconductor is doped with material which generates deep acceptor levels and is compensated with donors in shallow levels. Increase of conductivity - closing of the switch - is obtained through excitation of electrons from the occupied deep traps. Reduction of conductivity - opening of the switch - is achieved through optical hole excitation from the deep centers and subsequent direct recombination of electron-hole pairs.

A type-A pulse forming network is used to simulate a pulse generator for the leading edge of an electrical pulse to drive a load. Various scaling relationships are developed which indicate laser design trade-offs in relationship to the state of the art in switching capabilities. It is concluded that dielectric materials are the primary factor limiting technology in fast, low impedance pulse generation.