NASA Technical Reports Server (NTRS) 20040161130: Plasma in a Pulsed Discharge Environment

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Piasma in a puked discharge environment
1. Rem!. L. Biennizr. F. Salama
.-lhstract - The plasma generated in a pulsed slit discharge sheaths at the electrodes [j]. Fufiher derailed model
nozzle is used to form molecular ions in an astrophysicallj simulations based on rhe micro-discharge 2D (md2d) code
rcleiant eniironmcnt The plasma has been characterized as a
have confirmed the presence of these three zones in the
glow discharge in the abnormal regime. Laboratoq studies help
discharge [q.
understand the fonnatian processes of polycyclic aromatic
hydrocartwn (PAW ions that are thought to be the source of the In ~picalr unning conditions, the visible light that
ubiquitous unidentified infrared bands. emerges from the slit in the expansion plane originates from
photons emitted by de-excitation of the metastable .4r atoms
Pol>c>c hc aromatic hydrocarbons (P.4Hs) play an and ions produced in the negative glow. Electrons rhat have
important role in astrophysics. P.LvIs are found in meteorites been accelerated in the cathode region have gained enough
and Interplanetaq dust particles and they are thought to can) energy. once in the negative glow region. to excite Ar aioms
some of the 300 diffuse interstellar Sands (DIBs) seen in (and e\en ionize a fi-action ofthe gas).
absorption in difYme interstellar clouds as well as the This has indeed been observationail?; confirmed:
ubiquitous unidentified ;nE-ared emission bands [I] The Figs.;(% b) display static images that were captured in
plasma produced in a pulsed slit discharge nozzle (PDN) various gases with a digital camcorder. The color of the
source. see Fig.]. is a perfectly suited environment for mined radiation matches the color of the negative glow,
generating high column densities of P.4H ions [l]d ue to the which changes from gas to gu (blue for .4r, orange-red for I
combination of two factors: the significant dissociation and Ne and green for He [7]). The combined effecv of the small
ionization yields that can be reached in the discharge inter-electrode gap with the steep pressure =-adient of the
combined to the low temperature and density regime that are supersonic jet expansion explain why the radiation is visible
reached in the supersonic jet flow. at the slit’s exit.
The experimental set-6 has been described in detail mrmr
elsewhere [3].T he pulsed discharge node (PDA7 consists of
Fig. I. Cross-seaion of
a supersonic slit fiee jet mounted in a vacuum chamber and
the PDY source.
TWO knife-edge electrodes that create a disdharge in the stream A solenoid-activated
of the p!anar expansion. The plasma is thus generated after mature iiljects a 1.2 ms-
’ the gas expands. Two negati\)ely biased ja\vs forming the pulse of -gas inro the
plasma source. (I) points
cathode are mounted on each side of the slit and insulated
to rhe 700 p X 10 cm
fiom the PDX assembly (anode) . The applied voltage (-400 slit: (2) indicates !he 400
IO -600 X‘ through 1 Id2 ballast resistors) ensures that pm Fap berwm the IWD
electrons flow against the supersonic stream and that the jaws forming the cathode.
Note that the distance
discharge remains confined as well as uniform within the slit.
henieen anode and
,4t typical backing pressure of 760 Torr. detailed flow cathode is 1.5 mm.
simulations show that the average pressure within the thin
plasma region is of he order of 40 Tom [4]. rsmi:wai mew:s
The typical visible light emission of the Ar plasma The plasma expansion generated in the PDN source
-ze nerated in the PDX sowce is shown in Fig.?. The discharge is probed either in absorption or in emission. In absorption
generated in the PDN has been identified as an abnormal mode, the plasma expansion is probed transvenally several
glow discharge. characterized by a negative glow, and bvo mm downstream by Cavity Ring I3ow-1 Spectroscopy. In
Manuscript submitted 02 July 2004.
The work described here Has performed at X4SA Ames Research Center. ’Moffett Field, C-4 94035-10 00, USA.
F Salama is with the X4SA Ames Research Center. Mail Stop 245-6. hloffett Field, C.4 94035-10 00. US.4
L Biennier’s current address: Laboratoire de Physique des Atomes. Lasers, !ilolecules et Surfaces (PALhlS). LWR 6627.1;niversitE
de Rennes 1,35042 Rennes Cedex, France, EU
J Remy’s current address: %-,e EindhoLen. Applied Physics Dept. Den Dolech 2.5600 MB Eindhol en, The Fetherlands, EU
emission niodt. :lie pizsnia espansiL)n is probed frontally in
the plane of expansion b! Optical Emission Spectroscopy.
'4bsorption and emission measurements of the .4rI" lines in
the 700 nm range point to a gas excitation temperature in the
expansion in the order of 1.3 e\' (1 5000 K) i3].
This design makes also possible the investigation of
carbonaceous nanoparticles (analogs of interstellar dust)
generated in significant quantities in the plasma from P.4H
molecules seeded in the expansion. .4ccuniulation of soot on
the electrodes is observed minutes afier igniting the plasma.
It indicates that complex cheniistry (fragmentation,
aggregation of precursors, dehydrogenation, formation of Fig. 3. Pictures of the (a) Ar and (b) Ne plasma that is expanding in
the vacuum chamber at a residual pressure of 153 mTorr. Clearly
large particles. among others) is taking place in the source.
recognizable are the cathode jaws mounted 400-pm apart on each
Furthermore, the variation of the extinction in the cavity with
side of the IO-cm long slit. The plasma is homogeneous along the
increased voltage (;.e.. at higher plasma energy) is a clear slit and its color is characteristic of the carrier gas: blue dischzrge in
indication of the formation of small nanoparticles in the dusty Ar (a) and orange-red discharge in Ne (b).
plasma. Based on a particle growth model by ion-neutral an3
neutral-neutral reactions, these measurements are interpreted A pulsed discharge nozzle is a complex plasma
as indicating the presence of small siz,e nanoparticles source that represents an invaluable tool for astrophysics
(maximum diameter of the order of 7 nni) formed during the experiments. As shown in this contribution, the nature of the
short residence time (2.5 ps) of the niolecular precursors in gas mixture injected in the soul-ce strongly influences the
the plasma region. The discharge creates a high number of properties (color, energy, ) of the pla.sma emerging from the
positive and negative ions that act as nucleation sites for the slit. In addition, the plasma is well confined and probing the
condensation of the precursor molecules. spectroscopy downstream provides a quantitative estimate of
the excitation temperature of the gas and electron and ion
densities. Simulations of the pulsed source are in process to
further characterize the bu!k of the plzsma .
ACKNO WEDGMENTS
This work is supported by the NASA APR4 Progani and by
the Center for Plasma Physics and Radiation Technology
(CPS). The authors wish to thank Robert Walker of NASA
Ames for his invaluable technical support.
REFEEENCES
Fig. 2. Picture of an Ar plasma generated in the pulsed discharge
F.Salama, "Solid Interstellar Matter: the IS0 revolution". Les
nozzle source.
Houches, France (1 999)
D.T.Anderson, S.Davis, T.S.Zwier. D.J.Nesbitt, ".&I intense
slit discharge source ofjet-cooled molecular ions and radicals
(T,,, < -30 Kj'.. Clieni. Pii~sL. eft. Vol. 258 (I->), (1996)
L.Biennier. F.Salama L.J.Allamandola. J.J.Scherer, "Pulsed
discharge nozzle cavity ring-down spectroscopy of cold
polycyclic aromatic hydrocarbon ions". J. Clieni. Pli~s..V ol.
I 18 (1 7). pp 7863-7872. (2003)
A. Benidar. private communication, Unit-. Rennes 1 (2004)
J.Remy, L.Biennier, F.Salama, "Plasma structure in a pulsed
discharge environment", Plosnia Sources Sei. Teciiiiol.. Vol.
I?. pp 295-301 (2003)
B.Broks. private communication. TUIe Eindhoven (2004)
S.C.Brown. "Introduction to electrical discharges in gases",
Netv-York:Wile>. pp 214. (1966)