In a series of experiments on the Naval Research Laboratory's Electra generator, we have measured the dependence of the laser output in the principal (1.733μm) transition of the Ar–Xe laser upon both initial gas temperature and Xe concentration. The data show that the laser output is less sensitive to gas temperature when the laser gas contains more Xe. The destruction rate of the molecular ion ArXe+ increases rapidly with gas temperature, but that of Xe2+ does not. Interpreted with a kinetics model, these data indicate that both Xe2+ and ArXe+ contribute to the pumping of the ArXe laser.
Abstract:An overview of experiments is presented, in which the physical dimensions, emittance and perveance are scaled to explore driver-relevant beam dynamics. Among these are beam merging, focusing to a small spot, and bending and recirculating beams.The Virtual National Laboratory for Heavy Ion Fusion (VNL) is also developing two driver-scale beam experiments involving heavy-ion beams with I beam ~ 1 Ampere to provide guidance for the design of an Integrated Research Experiment (IRE) for driver system studies within the next 5 years. Multiple-beam sources and injectors are being designed and a one-beam module will be built and tested. Another experimental effort will be the transport of such a beam through ~100 magnetic quadrupoles. The experiment will determine transport limits at high aperture fill factors, beam halo formation, and the influence on beam properties of secondary electron production and neutral atom desorption from the vacuum pipe wall. Research into driver technology will be briefly presented, including the development of ferromagnetic core materials, induction core pulsers, multiple-beam quadrupole arrays and plasma channel formation experiments for pinched transport in reactor chambers.
The plasma-filled rod-pinch diode is a new technique to concentrate an intense electron beam to high power and energy density. Current from a pulsed power generator (typically ∼MV, MA, 100 ns pulse duration) flows through the injected plasma, which short-circuits the diode for 10–70 ns, then the impedance increases and a large fraction of the ∼MeV electron-beam energy is deposited at the tip of a 1 mm diameter, tapered rod anode, producing a small (sub-mm diameter), intense x-ray source. The current and voltage parameters imply 20–150 μm effective anode-cathode gaps at the time of maximum radiation, much smaller gaps than can be used between metal electrodes without premature shorting. Interferometric diagnostics indicate that the current initially sweeps up plasma in a snowplow-like manner, convecting current toward the rod tip. The density distribution is more diffuse at the time of beam formation with a low-density region near the rod surface where gap formation could occur. Particle simulations of the beam formation phase are dominated by rapid field penetration along the anode and radial J×B forces leading to gap formation and high-energy beam propagation to the rod tip. Beam deposition at the rod tip produces a high thermal energy-density (∼0.75 MJ/cm3), highly ionized (Z∼10, T∼25 eV) expanding tungsten plasma. Potential applications of this technique include improved radiography sources, high-energy-density plasma generation, and intense 10–100 keV x-ray production for nuclear-weapon-effects testing.
Self-magnetic-pinched diode behavior at 1.5-2 MV was diagnosed using a variety of electrical, radiation, and optical diagnostics. Results are compared with predictions of the LSP particle-in-cell code, and shown to be in good agreement. A practical diagnostic of electron incidence angles is demonstrated. A quadrature interferometer is shown to be capable of measuring the time-dependent position of the effective electrode-plasma boundaries. Both one-dimensional (1-D) and two-dimensional (2-D) interferometry show the importance of anode plasma expansion in such diodes with high anode-power concentration. Not only does the anode plasma contribute significantly to gap closure, but there is evidence that anode plasma expansion results in a distortion of the effective anode shape, which can significantly affect the diode performance.Index Terms-Electron beams, particle beams, pinched-beam diodes, pulsed power, radiography.
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