An annular electron beam with less than 30 mrad of angular velocity spread, a radius of 1 cm, and a current density exceeding 0.4 MA/cm2 has been generated with a magnetized foilless diode. The diode current loss is limited to less than a few percent by careful design of the tapered transition region connecting a self-magnetically insulated vacuum transmission line to the externally magnetized foilless diode. Details of the transition section design and operating characteristics of the electron beam generator are given.

We report observations of heating of a dense (∼1017 cm−3) plasma using a 3-MeV electron beam with a current density of 5×105 A/cm2. The measured, strong dependence of heating on beam transverse temperature and the short saturation distance (<4 cm) are evidence that the beam-to-plasma energy transfer takes place via a fast growing microinstability such as the relativistic electron-electron two stream.

A high-energy, low-inductance, low prefire rate, low trigger jitter, high-voltage, pulsed-power supply was needed to drive the gas discharge in the Antares laser power amplifier. This report describes the design and testing of a Marx generator that meets these requirements, the development and testing of a high-capacity spark gap, and the selection of suitable capacitors and resistors. I. INTRODUCTIONThe power-amplifier design for the Antares 100-kJ CO : laser system at the Los Alamos Scientific Laboratory (LASL) required a new pulsed-power supply to drive the gas discharge. Analysis of the system showed that the most cost-effective approach to the pulsed-power supply design was a single-mesh pulse-forming network using twenty-four 1.2-MV, 3OO-kJ Marx generators, each with an inductance less than 3 uH. 1 None of these parameters was considered beyond the state of the art. However, in the event of a short circuit on the output, caused, for example, by an output-cable failure, the circuit would ring with high reversal, giving a peak current of 400 kA, with charge transfer of 6 C. Because this peak current would push the state of the art in pressurized spark-gap technology, a spark-gap development program was instituted. A spark-gap prototype was run for 2000 shots without maintenance under conditions (500 kA, 10 C) even more severe than the Marx short-circuit conditions. This program is described in detail in Appendix A. Table I shows the Marx specifications. TABLE I MARX SPECIFICATIONS Open circuit voltage Load voltage Stored energy Inductance Load current, charge Short-circuit current, charge Prefire rate Jitter, rms Reliability Maintenance schedule 2.2 MV 550 kV 300 kJ <3uH 250 kA, 1 C 400kA,6C <0.001 <20ns >0.999 1000 shotsA Marx design was selected which we felt would meet these specifications. The capacitor chosen was the 2.8-uF, 60-kV, castor-oil-impregnated paper design which was life tested for the Scyllac program. To give the voltage and energy required, 3 of these are used in parallel and 20 in series. Bipolar charging produces a 120-kV spark-gap voltage. A double-folded geometry. with the capacitor terminals facing each other and the spark gaps between (Fig. la), was selected to give the required inductance. Figure lb is a close-up of the spark-gap region. By operating the spark gaps with a high safety factor (far below self-breakdown), we felt that the prefire rate would be low, while good trigger coupling would still give low jitter.A prototype of the Marx was built so that the design concepts could be tested, and reliable electrical and mechanical components could be developed. Partway through the testing program, we decided that the Marx generator system should be built by a contractor because of limitations on in-house engineering personnel. Although the contractor chose a different geomety for the Marx, the testing program provided valuable information, which later was used by the pulser contractor. II. SUMMARY OF RESULTSA full-sized prototype was built and discharged a few thousand times into a d...

A large CO2 laser being developed at Los Alamos Scientific Laboratory for fusion research requires a high-voltage, high-power pumping pulse of several microseconds duration. A pulse generator has been developed which provides a flat-topped pulse of 120 kA at 300 kV into a matched impedance load, with a 2.5-μsec pulse duration. The design is based on a combination of the Guillemin and Marx circuits. Eight of these will be used to pump an eight-beam laser system. There are applications for this circuit in other fields, such as electron beams for plasma heating. Design techniques for the pulse-forming network are discussed, including physical layout and triggering techniques.

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