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Title: Puzzling differences in bismuth and lead plasmas: evidence for thesignificant role of neutrals in cathodic vacuum arcs

Abstract

Time-dependent ion charge state measurements for Pb and Bicathodic arc plasmas revealed unexpectedly strong differences: the meanBi ion charge state dropped much stronger and with a longer time constantthan the mean Pb ion charge state. It is shown that the difference inthermal conductivity led to much higher vapor pressure for Bi, which inturn much more effectively caused charge exchange collisions. The resultshave implications beyond Pb and Bi plasmas, most importantly that the"true" charge states as emitted from the cathode spot are higher thanwhat is generally measured and published.

Authors:
;
Publication Date:
Research Org.:
Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
Sponsoring Org.:
USDOE. Department of Energy. Initiatives for ProliferationPrevnetion (IPP) project IPP-LBNL-196
OSTI Identifier:
918637
Report Number(s):
LBNL-62639
R&D Project: Z2PA196
DOE Contract Number:
DE-AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 102; Journal Issue: 4; Related Information: Journal Publication Date: 08/20/2007
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Cathodic vacuum arc Bi Pb ion charge states

Citation Formats

Anders, Andre, and Yushkov, Georgy Yu. Puzzling differences in bismuth and lead plasmas: evidence for thesignificant role of neutrals in cathodic vacuum arcs. United States: N. p., 2007. Web. doi:10.1063/1.2776858.
Anders, Andre, & Yushkov, Georgy Yu. Puzzling differences in bismuth and lead plasmas: evidence for thesignificant role of neutrals in cathodic vacuum arcs. United States. doi:10.1063/1.2776858.
Anders, Andre, and Yushkov, Georgy Yu. Thu . "Puzzling differences in bismuth and lead plasmas: evidence for thesignificant role of neutrals in cathodic vacuum arcs". United States. doi:10.1063/1.2776858. https://www.osti.gov/servlets/purl/918637.
@article{osti_918637,
title = {Puzzling differences in bismuth and lead plasmas: evidence for thesignificant role of neutrals in cathodic vacuum arcs},
author = {Anders, Andre and Yushkov, Georgy Yu.},
abstractNote = {Time-dependent ion charge state measurements for Pb and Bicathodic arc plasmas revealed unexpectedly strong differences: the meanBi ion charge state dropped much stronger and with a longer time constantthan the mean Pb ion charge state. It is shown that the difference inthermal conductivity led to much higher vapor pressure for Bi, which inturn much more effectively caused charge exchange collisions. The resultshave implications beyond Pb and Bi plasmas, most importantly that the"true" charge states as emitted from the cathode spot are higher thanwhat is generally measured and published.},
doi = {10.1063/1.2776858},
journal = {Applied Physics Letters},
number = 4,
volume = 102,
place = {United States},
year = {Thu May 10 00:00:00 EDT 2007},
month = {Thu May 10 00:00:00 EDT 2007}
}
  • Time-dependent ion charge state measurements for Pb and Bi cathodic arc plasmas revealed unexpected differences: the mean Bi ion charge state dropped much stronger and with a longer time constant. It is shown that the differences in thermal conductivity and vapor pressure led to much higher neutral density for Bi, which in turn can cause charge exchange collisions. The results have implications beyond Pb and Bi plasmas: most importantly, they imply that the 'true' ion charge states, as emitted from the cathode spots, are higher than what is generally measured and published.
  • Cathodic arc plasmas are considered fully ionized and theycontain multiply charged ions, yet, gaseous and metal neutrals can bepresent. It is shown that they can cause a significant reduction of theion charge states as measured far from the cathode spots. Several cathodematerials were used to study the evolution the mean ion charge state as afunction of time after arc ignition. The type of cathode material, arccurrent amplitude, intentionally increased background gas, additionalsurfaces placed near the plasma flow, and other factors influence thedegree of charge state reduction because all of these factors influencethe density of neutrals. In all cases, it wasmore » found that the mean ioncharge state follows an exponential decay of first order, Q(t) = A *exp(t/tau) + Qss, where A is a parameter describing the importance of thedecay, tau is the characteristic decay time, and Qss is a steady-statevalue approached for continuous arc operation. The extrapolated valuesQ(t-->0) indicate surprisingly high mean charge states as produced atcathode spots and not "skewed" by charge exchange collisions withneutrals.« less
  • Cathodic arc plasmas are considered fully ionized and they contain multiply charged ions, yet gaseous and metal neutrals can be present. It is shown that they can cause a significant reduction of the ion charge states as measured far from the cathode spots. Several cathode materials were used to study the evolution of the mean ion charge state as a function of time after arc ignition. The type of cathode material, arc current amplitude, intentionally increased background gas, additional surfaces placed near the plasma flow, and other factors influence the degree of charge state reduction because all of these factorsmore » influence the density of neutrals. In all cases, it was found that the mean ion charge state follows an exponential decay of first order, Q(t)=A exp(-t/{tau})+Q{sub ss}, where A is a parameter describing the importance of the decay, {tau} is the characteristic decay time, and Q{sub ss} is a steady-state value approached for continuous arc operation. The extrapolated values Q(t{yields}0) indicate surprisingly high mean charge states as produced at cathode spots and not ''skewed'' by charge exchange collisions with neutrals.« less
  • Current fluctuations of cathodic arcs were recorded with high analog bandwidth (up to 1 GHz) and fast digital sampling (up to 5 Gsamples/s). The power spectral density of the arc current was determined by fast Fourier transform clearly showing material dependent, nonlinear features in the frequency domain. These features can be associated with the nonlinear impedance of the conducting channel between cathode and anode, driven by the explosive nature of electron emission and plasma formation. The characteristic times of less than 100 ns can be associated with individual explosive processes, 'ectons', and therefore represent the short-time physical cutoff for themore » fractal model of cathodic arcs.« less
  • Charge-state-resolved ion energy distribution functions were measured for pulsed cathodic arcs taking the sheath into account that formed between the plasma and the entrance of a combined energy and mass spectrometer. An electron emitting probe was employed to independently determine the plasma potential. All results were obtained by averaging over several individual measurements because the instantaneous energy distributions and the plasma potential show large amplitude fluctuations due to the explosive nature of the arc plasma generation. It was found that the ion energy distribution functions in the plasma were independent of the ion charge state. This is in contrast tomore » findings with continuously operating, direct-current arcs that employ a magnetic field at the cathode to steer the cathode spot motion. The different findings indicate the important role of the magnetic steering field for the plasma properties of direct-current arcs. The results are further supported by experiments with 'biased plasmas' obtained by shifting the potential of the anode. Finally, it was shown that the ion energy distributions were broader and shifted to higher energy at the beginning of each arc pulse. The characteristic time for relaxation to steady state distributions is about 100 {mu}s.« less