Download Low pressure (rare gas + water vapor)

28th ICPIG, July 15-20, 2007, Prague, Czech RepubliF
Topic number: 15
Low pressure (rare gas + water vapor)-discharge as a light
source: (2) electrical characteristics
E. Artamonova1, T. Artamonova1, A. Beliaeva1, D. Gorbov1, D.Michael2,
M. Khodorkovskii1, A. Melnikov1, V. Milenin1, S. Murashov1, L. Rakcheeva1,
N. Timofeev1, G. Zissis3
1
Saint-Petersburg State University, Ulyanovskaya 3, 198504, Russia
2
General Electric Global Research Center, Niskayuna, USA
3
Paul Sabatier University, Toulouse, France
The strength of the electric field, electrode fall voltage, light emission and light efficiency
of an (Ar + H2O) DC discharge as functions of water vapor content, argon pressure and
electric current are presented. Obtained data make it possible to conclude that the light
efficiency of the plasma under study can reach 30-40 Lum/Wt.
1. Introduction
The first part of the study [1] devoted to the
investigation of spectroscopic and energetic
characteristics of a low pressure low current
discharge in the mixture of a rare gas (Ne, Ar, Kr or
Xe) and water vapor showed that the (Ar + H2O)discharge, in comparison with the discharges with
the other rare gases, has some prospects as a light
source. It was shown that under studied discharge
conditions the light efficiency of the discharge in
question reached (20-22)% of a mercury fluorescent
lamp efficiency, the lifetime of an (Ar + H2O)-lamp
being several weeks. The UV radiation exciting a
phosphor and defining discharge lighting properties
was OH 306.4 nm. Unfortunately, the way (ways) of
the water molecule destruction and the hydroxyl
molecule excitation in the plasma in question are not
clear wholly. The present paper is the continuation
of [1] and describes the data of probes measurements
of the strength of the electric field in the positive
column of an (Ar + H2O)-discharge, electrodes fall
voltage and the dependencies of some electric,
spectroscopic and energetic characteristics on the
discharge parameters. In addition to the new
information about the discharge these data could
also make it possible to estimate light efficiency of a
positive column and make more clear the picture of
plasma-chemical processes taking place in the (Ar +
H2O)-plasma. The latter is very important due to the
fact that the further progress is surely connected
with the elucidation of these processes.
discharge two molybdenum cylindrical probes are
inserted into a tube. Probes are 0.2 mm in diameter
and 1.5 mm in length. The probes are placed in the
axis of the tube perpendicular to its axis.
The scheme of the E-measurement is shown in
Fig.1. The every probe is connected to the “ground”
potential with the help of two resistances, one of
them having the large value 5.4 MΩ, so that the
probe potentials are almost the floating ones. Two
signals, proportional to the probe floating potentials,
come to the input of a differential amplifier. The
output signal of this amplifier is the subtraction of
input signals. It comes to the millivoltmeter and to
the analog-digital converter (PUPSIK), and then to a
computer. The floating potential does not coincide
with the plasma potential, so such measurements
give the strength of the electric field E in the
positive column and make it possible also to derive
the total electrodes fall voltage Uelectrode, i.e. the sum
of cathode and anode fall voltages.
2. Experimental
In general the experimental equipment and the
procedure of (Ar + H2O)-mixture preparation and
further discharge operation is similar to that of
described in [1]. For the measurement of the strength
of the electric field E in the positive column of the
Fig.1. The scheme of the set-up for the measurement of
the strength of the electric field E.
3. Results
Discharge characteristics – the strength of the
electric field E in the positive column, electrodes fall
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28th ICPIG, July 15-20, 2007, Prague, Czech RepubliF
voltage Uelectrodes, phosphor emission I and light
efficiency I/W of a discharge (W is the discharge
consumed electric power) – have been measured in
dependence on the argon pressure in the range (0.520) Torr, value of the electric current in the range
(0.1-0.6) A, and quantity of water molecules added
to the discharge volume. The water vapor pressure is
defined by the tube wall temperature and is in the
range (0-80) mTorr.
The strength of the electric field as a function of
the argon pressure and the tube wall temperature is
shown in Fig.2. At 20C, when the effect of water
molecules is negligible, E is in a good agreement
with the known data (see e.g. [2, 3]).
products begin to define plasma properties. At the
lowest investigated argon pressure the rise of PAr
from 0.5 Torr till 1.0 Torr leads approximately to
twice diminishing of E at all studied T. It is possibly
connected with the fact that, in spite of the essential
increase of water molecule concentration, argon
atoms mainly define the ionization processes in the
plasma as before. The rise of E as a function of T is
different at different argon pressure and is
considerable: from ~ (1-2) V/cm at 20C till ~ (1326) V/cm at (70-80) C.
The electrodes fall voltage Uelectrodes dependences
on the argon pressure and on the tube wall
temperature shown in Figs.3 are in general similar to
that of the E dependencies.
20C
30C
40C
50C
60C
70C
30
25
20C
30C
40C
50C
60C
70C
110
100
90
20
15
Uelectrodes, V
E, V/cm
80
10
70
60
50
40
5
30
20
0
0
5
10
15
20
0
PAr, Torr
5
10
a)
30
26
24
22
Ar0.5Torr
Ar1.0Torr
Ar3.5Torr
Ar10Torr
Ar20Torr
110
100
90
20
18
80
16
70
14
Uelectrodes, V
E, V/cm
20
a)
0.5 Torr
1.0 Torr
3.5 Torr
10 Torr
20 Torr
28
15
PAr, Torr
12
10
8
6
60
50
40
4
30
2
0
20
20
30
40
50
60
70
80
10
t, C
20
30
40
50
60
70
80
T, C
b)
Fig.2. The strength of the electric field E as a function of
argon pressure (a) and the tube wall temperature T (b)
i = 0.3 A.
Addition of water vapor makes the curve E(PAr) as a
whole to move up, the bigger the tube wall
temperature the higher the curve position. The
tendency of E to be weakly changed for PAr > 10
Torr means that water molecules or their dissociative
b)
Fig.3. The electrodes fall voltage Uelectrodes as a function of
argon pressure (a) and the tube wall temperature T (b).
PAr. i = 0.3 A.
However its rise is considerably less – in (2-4)
times, while E changes approximately in an order at
corresponding changes of the discharge conditions.
Most likely, similarly to a mercury fluorescent lamp
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28th ICPIG, July 15-20, 2007, Prague, Czech RepubliF
discharge as a whole. The light efficiency of the
discharge as a whole is ~ (20-22) Lum/Wt, so the
reachable value of the efficiency is (30-40) Lum/Wt.
Relative to the mercury fluorescent lamp efficiency
this gives 0.06-0.09 that is very close to the
estimation made in [1].
Fig.5 shows E as a function of i at different tube
wall temperature. The dependency is as expected: E
falls with the increase of the electric current i. The
similar dependence has been obtained in the whole
argon pressure diapason investigated in the work.
One could notice that at the low water content (at the
low tube wall temperature) the relative decrease of E
with the increase of the electric current value i is
more noticeable than at the higher tube wall
temperatures. Most likely, the reason is connected
with the Maxwellization of the electron energy
distribution function and increasing role of collisions
with water molecules when T rises.
o
12
11
10
9
8
7
6
5
T8 with probes.
Ar. i=300 mA.
Wac/Wcol
T=10 C
o
T=20 C
o
T=30 C
o
T=40 C
o
T=50 C
o
T=60 C
o
T=70 C
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E, V/cm
[4], it is explained by prevailing of argon atoms
ionization over the ionization of additive atoms
(mercury) or molecules (water, hydroxyl) nearby
electrodes. Moreover the identity of E(PAr, T) and
Uelectrodes(PAr, T) proves that the main ionization
mechanism in the plasma in question is also the
ionization of argon atoms. Note that in spite of the
common opinion (see e.g. [4, 5]) water vapor does
not influence crucially on the operation of oxide
electrodes. It is true both for the value of the
electrodes fall voltage and the tube lifetime.
Fig.4 presents the ratio of the power Wac
consumed by a discharge as a whole to the power
Wcol consumed by a positive column in dependence
on the total discharge voltage Uac. The discharge
voltage rises with the rise of the tube wall
temperature and, as an experiment shows, is a very
good indicator of the water vapor influence on the
discharge characteristics. Wcol is obtained in the
assumption that the length of the cathode and anode
regions is much less than the length of a positive
column. The general tendency of the obtained
behavior of this ratio is falling with the discharge
voltage rise. It is explained by sufficiently bigger
rise of the electric field strength in comparison with
the rise of the electrodes fall voltage when the tube
wall temperature is increasing.
4
3
2,6
2,5
2,4
2,3
2,2
2,1
2,0
1,9
1,8
1,7
1,6
1,5
1,4
1,3
1,2
1,1
2
Wac/Wcol
0,5 torr Ar
1 torr Ar
3,5 torr Ar
10 torr Ar
20 torr Ar
1
100
200
300
400
500
600
i, mA
Fig,5. The strength of the electric field E as a function of
the electric current i at the different tube wall
temperature. The argon pressure is 1 Torr.
o
0
50
100
150
200
250
300
350
400
450
T=10 C
o
T=20 C
o
T=30 C
o
T=40 C
o
T=50 C
o
T=60 C
o
T=70 C
34
Uac, V
32
Fig.4. The ratio of the discharge consumed electric power
Wac/Wcol to the electric power dissipated in the discharge
positive column in dependence on the discharge voltage
at different argon pressure. i = 300 mA.
28
Uelectrode, V
These data make it possible to define the
efficiency of the positive column in the mixture of
argon and water vapor and estimate the maximum
efficiency that can be reached with lengthening a
discharge tube. The highest discharge light
efficiency for the tube configuration in question is
obtained when the water vapor pressure is so that the
total discharge voltage is about (60-80)V. At this
discharge voltage the efficiency of the positive
column is ~ (1.5-1.8) times higher then that of the
30
26
24
22
20
18
100
200
300
400
500
600
i, mA
Fig.6. The electrodes fall voltage Uelectrodes as a function of
the electric current i at the different tube wall
temperature. The argon pressure is 1 Torr.
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28th ICPIG, July 15-20, 2007, Prague, Czech RepubliF
The electrodes fall voltage Uelectrodes as a function
of the electric current i is shown in Fig.6. It also
changes rather weak with the increase of the electric
current, so that in the first assumption one can
consider Uelectrodes as independent on i. Similar data
have been obtained for the whole investigated
diapason of argon pressure.
The phosphor emission caused by the UV
radiation of the discharge positive column is shown
in Fig.7. At the low tube wall temperature the role of
water molecules is negligible, and the light emission
of a phosphor is low and even slightly decreases
with the rise of the electric current. Addition of
water molecules into the discharge leads to the
appearance of OH 306.4 nm band radiation which
excites a phosphor. The light emission increases and
its current dependence becomes rising and close to
linear at T > 40C. Usually the rise of i causes the
proportional rise of the electron density, so one can
conclude that the phosphor emission becomes
proportional to the electron density
o
T=1,2 C
o
T=5 C
o
T=20 C
o
T=30 C
o
T=40 C
o
T=50 C
o
T=60 C
o
T=70 C
250
I, rel. un.
200
150
100
50
0
100
200
300
400
500
600
i, mA
Fig.7. Light emission of (Ar + H2O)-discharge as a
function of the electric current i. PAr = 6 Torr.
The light efficiency at T ≤ 20C falls down with
the increase of the electric current due to the rise of
the consumed electric power and merely constant
light emission (see Fig.7). Addition of water vapor
leads to the rise of the light phosphor emission that
makes the efficiency merely independent on the
electric current at T ≥ 40C.
4. Discussion
In addition to the study of some electric
discharge characteristics the important task of the
study is to inquire the processes that could be
responsible for the excitation of hydroxyl molecules.
In this aspect the current dependencies of plasma
characteristics can be fruitful to make some
conclusions about the question.
Analysis of the balance equations for the main
participants of the plasma processes (argon
metastable atoms, water molecules, hydroxyl
molecules in the ground and excited states, hydrogen
atoms) that take into account the main processes
with their participation (quenching of excited argon
atoms by electrons and water molecules, diffusive
losses of hydroxyl molecules and hydrogen atoms
that, as simple estimations show, are their main
disappearance processes in the plasma in question,
and radiative destruction of excited hydroxyl
molecules) shows that the linear increase of the
phosphor emission at the water vapor presence in the
plasma can be explained only by two processes: 1)
quenching of an argon excited (metastable) atom by
a water molecule and 2) dissociation of a water
molecule by an electron collision with simultaneous
excitation of OH. The latter is less probable [1], so
the role of argon metastable atoms in the plasma is
defining.
The solution of the balance equations shows that
when the role of water molecules in quenching of
argon metastable atoms is sufficiently weaker than
that of the electron one, the concentration of argon
metastable atoms does not depend on the electron
concentration. This is in a good agreement with the
experimental data [2, 3] and the data of Fig.7 where
the weak current dependence of I at the low tube
wall temperature is noticeable. When, oppositely,
the water molecule destruction of argon metastable
atoms prevails the electron one, the concentrations
of hydroxyl molecules in the ground and excited
states and the concentration of atomic hydrogen
become independent on the water molecule one. The
phosphor emission I of the discharge is caused by
the hydroxyl radiation 306.4 nm and hence in this
case the phosphor emission is independent on the
water concentration, i.e. on the tube wall
temperature T. Fig.3a [1] confirms this conclusion –
the phosphor emission changes negligible at T ≥
60C.
The study is partly supported by RFBR, grant No
06-02-17326a.
5. References
[1] E. Artamonova et al, Proc. 28th ICPIG.
[2] V. Granovsky, Electric current in a gas.
Steady-state current, “Nauka” Publ., Moscow
(1971).
[3] Yu. Kagan, R. Liagustchenko, A. Khakhaev,
Optika i spectroskopia 14 (1963) 538; 15 (1963) 13.
[4] G. Rokhlin. Discharge light sources,
Energoatomizdat, Moscow (1991).
[5] J. F. Waymouth. Electric discharge lamps,
The M. I. T. Pres, Cambridge, Massachusetts and
London, England (1971).
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