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Langmuir Probe Diagnostics and Spectroscopic Measurements in the Post-Discharge of a
Dinitrogen/Methane Microwave Plasma
A. Diamy, J. Legrand, V. Hrachová
To cite this version:
A. Diamy, J. Legrand, V. Hrachová. Langmuir Probe Diagnostics and Spectroscopic Measurements in the Post-Discharge of a Dinitrogen/Methane Microwave Plasma. Journal de Physique III, EDP Sciences, 1995, 5 (4), pp.435-445. �10.1051/jp3:1995138�. �jpa-00249321�
Classification Physics Abstracts
52.25L 52.70D 52.80P
Langmuir Probe Diagnostics and Spectroscopic Measurements in the Post-Discharge of a Dinitrogen/Methane Microwave Plasma
A-M- Diamy (~)~ J-C- Legrand (~) and V. Hrachov£ (~)
(~) Laboratoire de Chimie G4n4rale et de Chimie des Surfaces (CNRS URA 1428), Universitd Pierre et Marie Curie, Case 196, 4 place Jussieu, 75252 Paris Cedex 05, France
(~) Department of Electronics and Vacuum Physics, Charles University, V Hole§oviEkhch 2, 18000 Praha 8, Czech Republic
(Received 29 September 1994, revised 13 December1994, accepted 24 January 1995)
R4sumd, L'influence de l'addition de m6thane h un plasma microonde (2450 MHz) de di- azote est dtudide. La tempdrature des dlectrons (T~) et la concentration des ions (n+) sort
ddtermindes darts la post-ddcharge du plasma quand le mdthane est introduit soit
en amont de la d6charge soit darts la post-ddcharge. On dtudie l'influence de la puissance microonde, de la quantitd de mdthane ajout6e au diazote et de la position d'introduction du m6thane. La concentration des ions est ddterminde en utilisant la thdorie de Su et Kiel (modble continuum basd sur l'dquation de Poisson et sur les dquations de continuit6). Les valeurs obtenues sont
compardes avec celles calculdes en utilisant la thdorie de (icha et al. (modAle collisionnel). Les deux modAles donnent des densitds ioniques du mAme ordre de grandeur dans nos conditions expdrimentales. L'introduction du mdthane provoque une diminution de la concentration des ions. Des mesures spectroscopiques de I'dmission de N) (systbme premier ndgatif) et de CN (systbme violet) confirment la diminution de la densitd ionique simultandment h la formation de
nouvelles espbces chimiques.
Abstract. Measurements of electron temperature (T~) and ion density (n+)
are reported for mixtures of dinitrogen with methane in the afterglow of a high frequency discharge (2450 MHz).
Methane is added upstream or downstream the discharge. Ion density is derived by means of the Su and Kiel theory (continuum theory based
on Poisson's equation and continuity equations).
Results
are compared with values obtained by means of the theory of (icha et al. (collisional theory). Both theories give results of the same order of magnitude in our experimental condi-
tions. The influence of the location of the methane introduction, of the ratio of methane in the mixture and of the microwave power is studied. A decay ofion density is observed when methane is added to the plasma. Spectroscopic emission measurements of N) (first negative) and CN
(violet system) show simultaneous ion density decrease and new chemical species formation.
© Les Editions de Physique 1995
1. Introduction
Reaction of methane with a dinitrogen plasma is the subject of a great number of studies in various domains such as: simulation of Titan's atmosphere [1], detoxification of gases generated by combustion [2], properties and reactions of CN radical [3], metal coating [4], conversion of
natural gas [5]. We have used such a medium to obtain methane valorization in more useful chemicals [6] and also to study nitrogen and carbon atom formation along the post-discharge [3]. In the last two researches to carry out kinetic study of methane decomposition, it is
necessary to know plasma characteristics (electric, spectroscopic, chemical...). This study is an attempt to determine some spectroscopic and electronic properties of the near post-discharge
of a dinitrogen plasma with methane added upstream or downstream the discharge.
2. Experimental
The experimental set-up is shown in Figure 1. A microwave generator (Thomson- CSF, 2450
MHz, adjustable power up to 1500 W) is connected to a rectangular waveguide cavity of about
one meter long. A plasma is produced in a cylindrical reactor crossing the cavity. The reactor is made of fused silica and its inner diameter is 2.8 cm (o.d.
= 3 cm). Microwave power
coupling to the gas medium is obtained by adjusting the position (0 -12 cm) of a sliding short located at the end of the cavity. Incident (Hi) and reflected (IIr) powers are measured by
a powermeter (HP 432A) connected to thermistors (HP 478A). Energy losses in the line are supposed to be negligible, so the energy absorbed by the gas is II
= Hi Hr. At low pressure
(P < 30 mbars), a surface wave propagates along the reactor, and the discharge looks like a
ring when the position of the sliding short is about 10 12 cm (standing wave ratio SWR
> 1.8) or looks like a croissant near the wall when the sliding short position is about 3 4 cm
(SWR
+~ 0). When the pressure rises (P > 30 mbars), the discharge moves off the wall to occupy the central part of the reactor.
Methane can be introduced in the reactor either upstream or downstream the dinitrogen plasma. In the first instance, methane is mixed with dinitrogen before the cavity; in the second one, it is introduced downstream the plasma gas issuing the cavity by means of four ports distributed all around the reactor (Fig. 1). Two other ports are used to measure the pressure P and to introduce thermocouple or Langmuir probe. The distance d between the ports and the cavity can be modified between 0.9 and 4 cm by moving the reactor. A floating
double probe (Pt/Rh wires, L = 4 mm, r
= 0.1 mm, 3 mm apart) is used to determine electron temperature and ion density; a point by point method is used to obtain the probe
characteristic. As the plasma is chemically active, experiments are carried out with small ratio of methane (0% 5%) to prevent important chemical deposition on the probe. Probe and
discharge tube wall were regularly cleaned by using an air plasma.
Optical emission from the post-discharge is analysed by a monochromator (Jobin- Yvon
HRSI, 1220 lines/mm, focal length 588 mm) and a photomultiplier (Hamamatsu, R928) by
means of an optical fiber.
Gas temperature is obtained by means of a Pt-Pt /10% Rh thermocouple, coated with a silica sheath. Surface temperature Ts of silica is not the gas temperature because of reactions on the wall but this temperature gives an order of the maximum value of gas temperature Tg (Fig. 2) [7]. Ion temperature Tj is assumed equal to neutral species temperature Tg(Ts +~ Tg = 2j).
The experimental conditions are: pressure range 2 14 mbar, flow-rates range 0.03 1
L(STP).min~~, methane ratio in dinitrogen range 0.025 5% and absorbed power range 90 340 W.
MICROWAVE GENERATOR 24S0MHz
c<RCULATOR OPTICALF<BER
PRESSURE~
cH~
i I
PLASMA L<QUID NITROGEN
TRAP SLIDING
p~~~~ To pUhiP
SHORT
N~FLOWMETER
PRESSURE
and ~j
~°~T GAUGE
SCHARGE
~~4~ fi / cH~
' ~
~) ( ~~<NTROUUCT>ON
CH~/(~~~ ~~~~~~~~~~~
~H4 DOUBLE
pROBE
Fig. I. Experimental set-up.
) (
~ a
o ~~
a w~
~
~~
" a
500 ~
.~
'. ~~
o '
' ~ a
j~
~,
d
Fig. 2. Gas temperature (Tg) versus distance (d), pure dinitrogen, Tg
+~ Ts; (o) P
= 13.2 mbar, F = 350 cm~.min~~ (STP) and H
= 200 W, (.) P
= 13.2 mbar, F
= 350 cm~.min~~ (STP) and H
= 340 W, (.) P
= 4 mbar, F
= 30 cm~.min~~ (STP) and H
= 340 W, (A) P
= 2 mbar, F = 30 cm~.min~~ (STP) and H = 340 W.
3. Electron Temperature and Ion Density Determination
A floating Langmuir double probe system was used for the characterisation of the electron temperature and ion density of the plasma. In our experimental conditions, we must take into account that measurements are in the pressure range 2 14 mbar. The accuracy of the plasma parameter determination depends not only on the experimental devices and reproducibility of the experiments but also on the theory used to derive the electron temperature and ion density
from current-voltage curves obtained with the double probe system.
3.I. DETERMINATION oF ELECTRON TEMPERATURE. The electron temperature, Te, is derived from the classical equivalent resistance method developed by Johnson and Malter [8]
for a Maxwellian distribution of the electron energy. In this method, the space charge sheath around the probe is assumed without collisions of the charged particles (low pressure), but it
was shown [9, 10] that this theory can be used for higher pressures. Generally, the electron energy distribution function (EEDF) in microwave discharges is not Maxwellian but several investigators have reported that the classical equivalent resistance method can be applied. Fer- dinand [11] finds a good agreement between results of spectroscopic measurements of electron
temperature and results of the collisionless equivalent resistance method up to atmospheric
pressure in the case of non Maxwellian plasmas. Heidenreich et al. [12] measured the elec-
tron energy distributions in dioxygen microwave plasmas. The EEDF is intermediate between Maxwellian and Druyvesteyn forms but the average electron energies were in good agreement with values from the standard Langmuir probe electron temperatures T~. Then, the classical
equivalent resistance method is not too sensitive to the type of distribution so the electron tem-
perature derived from double probe characteristics by this method is shown to be reasonably accurate; the error due to non-Maxwellian distribution does not exceed 5% [10,13].
3.2. ION DENSITY DETERMINATION BY CONTINUUM THEORY. Theories for evaluation of
ion densities from probe characteristics are numerous, and the choice of the theory depends on
the probe geometry and on the parameters of the plasma such as the Debye length or the ion
mean free path [14]. Evaluation of ion densities is carried out by using the theory, developed for pressures above 1 torr, by Su and Kiel [15] which is a continuum theory based on the Poisson's equation and the continuity equations for positive ions and electrons. This theory assumes the thin sheath of space charge around the probe. If it is not the case, other analytical expressions
have been given by Kiel for large sheaths [16]. Although, in our experimental conditions, the sheath is not quite thin, we choose the theory of Su and Kiel because the aim of this
paper is to compare values obtained with a very simple usefull expression and values obtained with a more elaborated theorie which takes into account the collisions in the probe sheath and which is described later. Moreover, in similar conditions (pressure, Debye length, probe radius), Nguyen Cao and Gagne [17] who compared results obtained by several theories with
experimental results obtained by microwave interferometry, concluded that the most suitable
expression at pressures over torr is the Su and Kiel's one. So, ion density n+ is calculated from the measured ion saturation current Is by means of the following expression:
n+ = Is.Ln(7rL/4r)/[27r.Lk(T~ + f)./J+]
The Langevin formula has been used to calculate the ion mobility /J+ [18]. T is the ion temperature defined above and shown in Figure 2. L and r are the length and the radius of the probe; k is Boltzmann's constant.
3.3. ION DENSITY DETERMINATION BY A COLLISION THEORY EXTENDED FOR DOUBLE
PROBE. For comparison, some values of ion densities have been calculated with a the- ory which takes into account the collisions in the probe sheath (Fig. 6). There are several
possibilities how to describe the collision influence. For the description of the probe working regime, usually two parameters are used. That is the Knudsen number for ions and electrons:
Kj,e = lj,eIT (lj,e is the mean free path for ions and electrons respectively) and Debye number D = r/lD where ID is the Debye length (lD
" (kTeeole~ne)~/~ where eo is the permitivity of
vacuum, e and ne are the charge and density of electrons).
The theory which takes into account the decrease of both of ion and electron cylindric probe
current due to scattering of the particles in the collisions with neutrals in the probe sheath
was derived by Chou et al. [19]. These collisions are assumed only as a correction, calculated in detail by Talbot and Chou [20], to the collisionless ion and electron currents.
Another theory of Zakrzewski and Kopiczynski [21] assumes not only the decrease of cylin-
dric probe ion current due to the scattering, but also increase of this current due to the orbital motion destruction by collisions in the probe sheath. This theory is limited for pressure corre-
sponding to Kj > 1.
New model of the ion collection was developed by Tichf et al. [22] by connecting both theories described above:
the increase of the ion current is computed according Zakrzewski and Kopiczynski [21]
the decrease of the ion current due to scattering is estimated according Talbot and Chou (2°).
This model was extended by (icha et al. [23] to the double probe system. It is possible to obtain Debye number by numerical calculations from directly measurable parameters (satu-
rated ion current for normalized voltage between two probes, electron temperature, geometrical probe parameters) in a large range of Knudsen number.
' 1%CH4 '~ (~
)rj before I [-)-'~
'=l cavity ' '-~~
,b
2fl,1,
-~-
l 10mVlcm
~i T>~
t i~
'
h
j- T-
_ji 1%CH4 fi
I"I downstream
~-.
; 5mV/cm 1
"~~~~- 'd$2.3cri
if ;
_L
r pure N2
flSQ 5mV/Cm 3~
~>_r$_
~j_
~_
[ __~=
)__I )Z~~~,~ l
@
'~ 'j~ 'fm
j~@
,--
[-~,~
380nm 390nm
Fig. 3. Emission spectra of N2, N) and CN, P
= 3.2 mbar, H
= 200 W, F = 90 cm~.min~~ (STP),
d = 2.3
cm and optical fiber in d
= 2.3 cm; 1
,
2 and 3 second positive system of N2 (Au
= -2
and -3), 4
,
5 and 6 first negative system of N) (A~
= 0), 7 violet system of CN (Au = 0).
3.4. AccuRAcY OF DETERMINATIONS. The gas mixture where measurements are carried out is a very complex one. In our plasma, radicals (CH~, N [2], [6], [24]) and ions (CHj
[25], N+, N2+, N3+ [26], [27] are produced by electron impact, chemical reactions or charge exchange. Ion density computed by both methods depends on the nature of the ion through
the mobility or the mean free path. In the experimental conditions used in this study, N) is the dominating ion at the end of the discharge [27] moreover, we use small ratios of methane in the mixture, so all calculations have been carried out for N).
We must notice that accuracy in the determination of ion density by means of both methods is 20 30% due to limits of using the theory and due to uncertainties in the evaluation of mean free paths, ion mobility, etc...
4. Results and Discussion
4. I. PosT-DISCHARGE EmissioN. Emission from the post-discharge is shown in Figure 3 in the range corresponding to second positive system of N2 (Au
= -2 and -3) and first negative
CN (B,0 ---> X,0)
.
m
~
/ 10
~~
~.
->X,0)
0
0
%
of N) (Au
= 0) and also violet band of CN (Au
= 0) radical. It appears that CH4 addition leads to a decrease of the first negative emission especially when CH4 is added upstream the
discharge and that CN violet band is more important for CH4 upstream addition. These results
are corroborated in Figure 4 where a N) (391.4 nm) emission intensity decrease is observed
as methane concentration increases in the mixture. Simultaneously, CN (388.3 nm) emission
intensity increases with CN formation as expected; the same observation is done for the CN red system. Methane addition has only a weak influence on the second positive system in our
experimental conditions (Fig. 3).
4.2. ION DENSITY AND ELECTRON TEMPERATURE. The great influence of the sliding short position is seen on Figure 5. For a position of about 4.5 cm, the discharge is no more sustained.
The different positions of the sliding short explain the discrepancy observed between the values of ion density in Figure 6 and in Figure 7 (sliding short position: 11.5 and 3.5 cm respectively).
For our conditions (it means probe geometry and degree of ionisation), the values of ion den- sity obtained by both methods (continuum theory and collision theory) are in good agreement.
Example of comparison is shown in the Figure 6 for pure dinitrogen and for 1% methane (filled symbols represent the results obtained from collision theory and open symbols correspond to the values from the continuum theory). Similarly, this agreement was found for all measured
probe characteristics so that on further figures are only represented the values obtained from the continuum theory. Figure 6 shows the radial distributions of the ion density both for the
afterglow of pure dinitrogen and for the afterglow with 1% of methane. The radial profile of
o .1
20 1"
-
-~ j
1
~ _
r~
E (
"
~
# )
~
+
~ 0
0 6 12
Sliding Short Position (cm)
Fig. 5. Ion density (n+), electron temperature (T«) and incident power (H,) versus sliding short position. Methane introduced after the plasma; probe located at the centre of the tube (R
= 0),
P = 3.5 mbar, d
= 2.3 cm, fl
= 200 W, F(N2)
" 100 mL.min~~ (STP) and F(CH4) " 0.25 mL.
min~~ (STP).
the gas temperature is not known so calculations of ion concentration have been done with a constant temperature 2j = Tg = 550 K. Nevertheless, if we take, for example, a value of 300 K for the next point (R
= 1.4 cm), the radial distribution of n+ is not so sharp. This density profile does not seem a symetric one; this is probably due to the difficulty to obtain a quite symetrical discharge because, as explained above, the geometrical aspect of the discharge de-
pends on the position of the sliding short. A distribution with a diffusion character is described by a Bessel function; here there is a well pronounced maximum near the center and a decrease
along the reactor radius indicating ion diffusion towards the wall but with deviation from the
case which corresponds to a true diffusion distribution and the deviation is more pronounced
with methane addition.
Influence of methane ratio in the mixture is shown on Figure 7 for upstream CH4 addition. A decrease of ion density and electron temperature is observed as a function of CH4 ratio. When
methane is introduced downstream the discharge, a decrease is observed also. However, ion
density decrease is more important with upstream addition than with downstream addition;
this is in good agreement with results obtained for first negative emission of N) (Fig. 3).
In Figure 8, the role of the input power on the ion density is shown for pure dinitrogen and mixture of dinitrogen and methane; it appears that increasing power acts on ion density in the
10.109 n+<part./Cm3)
~~
Te K
~~ O
~
~O
N~ . /o
~
~ ~
~ fl
/
.
~ . ~~
. .
~ N2/CH4 ~ '
g 5.109
I 10 000
", I /
O
D '
'
~
~
~
~
/ m
N2/CH4 ~
"
/
0 0
0 +1 1 0 +1
R <cm) R <cm)
Fig. 6. Ion density (n+), electron temperature (Te) versus (R). P
= 4.2 mbar, d
= 2.3 cm,
H = 200 W, F
= 100 mL.min~~ (STP) and sliding short position
= 11.5 cm with a) and b) (.), (O)
pure dinitrogen, (.), (ll) 1% CH4 (introduced before the plasma). b) (.), (.) probe collision theory,
(O), (11) probe continuum theory (Su and Kiel).
2.
0 0.5 2 3 4 5 0 0.5 1 2 3 4 5
% CH4 % CH4
Fig. 7. Electron temperature T~ and ion density (n+)
versus methane ratio. CH4 introduced before
the cavity, probe location at R = 2 mm, P
= 4.2 mbar, d
= 2.3 cm, H = 200 W, F
= loo mL. min~~
(STP), T, = Tg = 550 K and sliding short position = 3.5 cm.
4.1010
/
Q . '
E N2 /
C
f ~ .
« .
/b / ~/
j ~/
fl~N2/CH4
/
~
/ fN2/CH4
/ "
m~
0 /
0 100 200 300
nov) Fig. 8. Ion density (n+) versus absorbed power (H). F
= loo mL.min~~ (STP), d
= 2.3 cm.
(.) pure N2, P
= 4.2 mbar; (.) 0.25% CH4 P
= 3.5 mbar and CH4 introduced after the cavity (d = 2.3 cm); (A) 0.25% CH4, P
= 4.2 mbar and CH4 introduced before the cavity.
