V. E. Scherrer

Development of a sodium-pump/neo-lasant photopumped soft X-ray laser

An intense source of sodium pump-line radiation has been created and used to photopump a neon plasma for application to a pulsed-power driven sodium/neo X-ray laser. Properties of the sodium-pump plasma and the neon-lasant plasma required to optimize fluorescence and lasing are determined. The implosion of a sodium-bearing plasma with a megampere pulsed-power driver (Gamble II) is used to produce a linear Z-pinch with up to 25 GW of sodium-pump-line radiation. A separate neon plasma, driven by part of the return current from the imploding sodium plasma, is created parallel to the sodium line source at a distance of 5 cm. Evidence for population inversion is indicated by fluorescence enhancement of the 11-AA resonance line from the n=4 level of neon when pumped by sodium. >

Recovery characteristic of exploding wire fuses in air and vacuum

The dielectric strength of exploded wire fuses during the vaporization and after recovery period and fuse resistivity are functions of time and depend on the medium surrounding the fuse channel. When fuses are used as opening switches in inductive storage systems, the above parameters determine the efficiency of power flow from the storage to the load. Specifically, design of experiments using loads with variable impedance, such as imploding plasma, requires detailed information on the fuse characteristics during the time when the load impedance is changing. To provide data that determine the interaction between fuses and variable impedance loads, inductive and recovery electric fields and fuse resistance in air and vacuum were studied. The results show that inductive field amplitude follows the dependence on time needed to vaporize the fuse in a manner similar to that established for fuses in other media. The characteristics of recovery rates of fuses in air and vacuum differ drastically due to the early...

Filtered x‐ray diodes for imploding plasma experiments

Vacuum x‐ray diodes with μm‐thick filters have been developed to measure specific x‐ray line emissions from plasmas imploded by megampere driving currents. Detector–filter combinations are presented for argon L‐shell radiation (0.25–0.46 keV), for the 0.92‐keV neon He‐α line, and for the 1.13‐keV sodium He‐α line. For example, a power of 50 GW is measured in the neon He‐α line.

Some Aspects of Surface Boiling

An experiment is described in which the growth of vapor bubbles at a surface is followed in detail. Growth curves are obtained and shown to agree, within experimental error, with current theories of growth under idealized conditions, provided an average superheat present at the initiation of growth is used in the calculation. A criterion is proposed to permit an estimate of a delay time for vapor formation. The estimated delays are found to be in qualitative agreement with those measured.

Implosion of sodium‐bearing capillary‐discharge plasmas for x‐ray laser experiments

A NaF plasma from a capillary discharge has been imploded with a 1.2‐MA driving current to produce an intense source of sodium K‐shell x rays. A peak power of 25 GW in a 20‐ns pulse was measured for the sodium He‐α line which can be used as the pump radiation for a Na/Ne XUV laser scheme.

Electron-Beam Controlled Discharges

When an electron beam (e-beam) is injected into a gas located between two electrodes, a volume discharge, which turns on and off in association with the beam, can be generated. We present a review of the theory and new experimental data for such a discharge relevant to potential applications for high-power switching. The data suggest that an optimum balance between the contradictory requirements of low resistivity and short opening time may be realized by proper choice of gas mixture.

Inductively generated, high voltage pulse using an electron beam controlled opening switch

An electron beam controlled opening switch with a conduction period of ∼1 μs has been used in an inductive store system to generate a 280‐kV, 60‐ns full width at half‐maximum voltage pulse across an open circuit by interrupting a 10‐kA discharge. The switch was pressurized to 5 atm with a 99:1 mixture of CH4:C2F6. Degraded voltage performance was obtained with pure CH4 or N2. These results support the conjecture that a judicious choice of gas mixture can lead to optimization of such an opening switch in a parameter regime of interest to pulsed power applications.

High‐power electron‐beam controlled switches

The application of an electron‐beam controlled diffuse discharge to high‐power (>109 W), repetitive opening switches is analytically formulated under a set of assumptions. Basic physics considerations are combined with energy transfer requirements to obtain analytical estimates of the e‐beam controlled switch parameters for given circuit requirements. The switch design is optimized by minimizing the switch pressure subject to the constraint of system efficiency. The result of this optimization is that each of the major energy losses—conduction, opening, and electron‐beam production—are roughly equal to each other. This formulation is used to relate the switch parameters to the desired operating characteristics for an arbitrary number of pulses. As an example, the formalism is utilized in outlining the design of a single pulse, high‐power (≂1010 W) inductive storage system. A judicious choice of gas or gas mixture results in desirable changes in the system design or efficiency.

Pulsed-Power Driven, Photopumped X-Ray Laser Research At The Naval Research Laboratory

Resonant phOtopumping is an attractive and potentially very efficient technique for achieving x-ray lasing in plasmas. At the Naval Research Laboratory we are pursuing the realization of one of the most promising photopumped schemes wherein heliumlike sodium pumps heliumlike neon. The theoretical side of this research has revealed the conditions required for the sodium and neon plasmas to maximize the possibility of photopumped fluorescence and lasing. On the experimental side, a powerful (25 GW) z-pinch source of sodium line pumping radiation has been developed and characterized. A separate neon plasma, driven by part of the return current from the sodium-bearing plasma, has been deployed side-by-side with respect to the sodium line source at a distance of 5 cm. The presence of photopumping has been indicated in fluorescing spectra. The remaining steps toward achievement of an x-ray laser are reducing the spatial separation of the pumped and pumping plasmas and increasing the pump line power of the pumping, sodium-bearing plasma.

Repetitive Operation of an Inductively Driven Electron-Beam Diode.

Recovery of a pulsed electron beam diode operating in the 50 A/cm2 range has been studied using an inductive storage source producing two∼150 kV pulses with pulse‐to‐pulse separation ranging from 10–500 μsec. The diode cannot instantaneously support a second voltage pulse because of the short circuit provided by the interelectrode plasma associated with the first pulse. The properties of this plasma, including its effective lifetime, have been studied. The diode is observed to recover for pulse separation times≳100 μsec. The results can be extended to repetitively pulsed electron beam generation as needed in many physics experiments such as those related to beam propagation in gases, opening switches, and laser phenomenon.