Browsing by Author "Valenzuela, J. C."

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    A compact ultrafast capillary plasma discharge as an intense XUV source
    (2014) Valenzuela, J. C.; Wyndham, Edmund
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    Bayesian inference of plasma parameters from collective Thomson scattering technique on a gas-puff near stagnation
    (2023) Escalona, M.; Valenzuela, J. C.; Avaria, G.; Veloso, F.; Wyndham, E. S.
    The Collective Thomson scattering technique has been implemented to study the stagnation of a single liner gas-puff. The plasma parameters are determined by theoretically modelling the scattering form factor in combination with Bayesian inference to provide the set of the most probable parameters that describe the experimental data. Analysis of the data reveal that incoming flows are able to interpenetrate partially. Estimation of the mean free path shows a gradual transition from a weakly collisional to a collisional regime as the plasma gets to the axis. Furthermore, we find that the ion energy at r = 2.5 mm is 13.6(-0.9)(+1.0) keV and is mostly kinetic in nature and represents 98(+10) (-9) % of the total energy. This kinetic energy is far greater than the value on axis of 3.7(-0.5)(+0.4) keV which is 84(-14)(+15) % of the total energy. Energy transfer to the electrons and radiation losses are found to be negligible by this time. A possible explanation for this energy imbalance is the presence of an azimuthal magnetic field larger than similar to 4.7 T that deflect the ions vertically. The uncertainties quoted represent 68% credible intervals.
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    Observations of the emission processes of a fast capillary discharge operated in nitrogen
    (2012) Valdivia, M. P.; Wyndham, E. S.; Favre, M.; Valenzuela, J. C.; Chuaqui, H.; Bhuyan, H.
    We present observations of the emission characteristics of the plasma processes of a low inductance, sub Joule, compact capillary discharge, when operated in nitrogen at up to 600 Hz. A quarter period of under 10 ns is achieved allowing currents of order 5 kA. Four geometries are explored: two lengths, 21 and 36 mm, and two internal diameters, 1.6 and 3.2 mm. Transient hollow cathode fast electrons are associated with enhanced soft x-ray emission at shorter wavelengths with measured output energies of N VI at 28.8 angstrom as compared with a Maxwellian plasma. Time-integrated spectroscopy together with filtered diode signals, and with spatial resolution, reveals a small axial emitting plasma close to the anode. Optical time-resolved spectroscopy gives larger scale plasma parameters both of the anode and the cathode plasmas. This volume plasma covers the range 2-8 eV, while the x-ray emitting plasma covers 10-20 eV, according to geometry. Nitrogen metastable emission is also observed in the hollow cathode volume prior to breakdown. Both internal wall diameter and capillary length affect both the spectrum and the x-ray emitting volume as does the axial pressure gradient. Electron beams from the transient hollow cathode are associated in model spectra with the observed N VI, O VI and Al VII-X ionization stages. Operating conditions that affect the spectral purity and discharge characteristics include the internal pressure gradient and nitrogen to helium mix ratio. We discuss the suitability of the capillary geometries as a soft x-ray source and in the context of available computer models.
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    Study of stability in a liner-on-target gas puff Z-pinch as a function of pre-embedded axial magnetic field
    (2020) Conti, F.; Aybar, N.; Narkis, J.; Valenzuela, J. C.; Rahman, H. U.; Ruskov, E.; Dutra, E.; Haque, S.; Covington, A.; Beg, F. N.
    Gas puff Z-pinches are intense sources of X-rays and neutrons but are highly susceptible to the magneto-Rayleigh-Taylor instability (MRTI). MRTI mitigation is critical for optimal and reproducible yields, motivating significant attention toward various potential mitigation mechanisms. One such approach is the external application of an axial magnetic field, which will be discussed here in the context of recent experiments on the Zebra generator (1 MA, 100ns) at the University of Nevada, Reno. In these experiments, an annular Kr gas liner is imploded onto an on-axis deuterium target with a pre-embedded axial magnetic field Bz 0 ranging from 0 to 0.3T. The effect of Bz0 on the stability of the Kr liner is evaluated with measurements of plasma radius, overall instability amplitude, and dominant instability wavelength at different times obtained from time-gated extreme ultraviolet pinhole images. It was observed that the external axial magnetic field does not affect the implosion velocity significantly and that it reduces the overall instability amplitude and the presence of short-wavelength modes, indicating improved pinch stability and reproducibility. For the highest applied Bz0=0.3 T, the stagnation radius measured via visible streak images was found to increase. These findings are consistent with experiments reported in the literature, but here, the Bz0 required for stability, Bz0=0.13Ipk/R0 (where I-pk is the driver peak current and R-0 is the initial radius), is lower. This could be attributed to the smaller load geometry, both radially and axially. Consistent with other experiments, the cause of decreased convergence cannot be explained by the additional axial magnetic pressure and remains an open question. (C) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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