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Spectral intensity measurements from high-pressure nitrogen plasmas
SPECTRAL INTENSITY HIGH-PRESSURE
MEASUREMENTS FROM NITROGEN PLASMAS
DAVID M. COOPER Ames Research
Center,
NASA,
(Received
Moffett
Field, California
15 Deccmher
94035, U.S.A.
197 I)
Abstract-Spectral measurements of the equilibrium radiation from mtrogen at high pressures have been obtained. Experiments were performed in a ballistic range and an explosively driven shock tube. The tests covered a temperature range from 9000 to 17,200”K and a pressure range from 1.3 to 63.0atm. Averaged electronic absorption f-numbers for Ni B2~~~xzZ~, NzC3rI-B3Q, and N,B3Fl-A”EJ bands are reported. N- continuum radiation was not observed.
INTRODUCTION
SEVERAL studies”P6) of the equilibrium radiation from nitrogen at pressures between 0.2 and 2 atm and over a temperature range from 8000 to 13,OOO”K have been performed in shock tube and arc facilities. In the majority of the studies!‘- ” the measured absolute radiation exceeded the theoretical predictions which included molecular band, atomic line, and continuum radiation from electron-ion interactions. The excess radiation has been interpreted as arising from the production of the negative ion N- by the attachment of an electron to a neutral atom. The resulting experimental estimates of the cross section for photon capture of the NP ion (for the ‘D state) differ significantly in magnitude and wavelength dependence.‘4’ In an additional high pressure (5&60atm) study,‘7’ where it was expected that continuum radiation from electron-atom interactions would dominate that from electron-ion interactions, excess radiation that could be attributed to the existence of NP was not observed. There exists, then, a serious contradiction between the low pressure measurements (less than 2 atm) which suggest the existence of an N- contribution to the continuum radiation and the high pressure measurement which does not. The purpose of the present paper is to present additional measurements of the spectral intensity from high pressure nitrogen plasmas in order to resolve the anomaly surrounding the existence of N -, and consequently to verify the theoretical predictions of the radiation at these conditions.
BALLISTIC
Experimental
RANGE
TESTS
description
Measurements of the radiation from the shock layers of models were performed prototype of the Ames Hypervelocity Free-Flight Facility operated as a ballistic 1175
in the range.
This facility consists of a light gas gun that serves as a model launcher. an instrumented test section, and a catcher that terminates the projectile flight. A complete description of this facility may be found in Ref. (8). For the present tests, the models flew through quiescent nitrogen in the instrumented test section. The model trajectories were determined from 11 spark shadowgraph stations spaced at I .22 m intervals along the test section length. Electronic chronographs recorded the time at which each shadowgraph picture was obtained, thus allowing a velocity determination. The models were launched at a velocity of approximately 8.X km/set into pure nitrogen at initial pressures of 25 and 50 torr. At these test conditions, the computed ratio of continuum radiation from electronPatom interactions (based on the measurements of MORRIS rf ~1.‘~‘) to that from electronGon (N’) interactions should be approximately a factor of 10. Furthermore, prior tests(“~‘O’ have confirmed that at these test conditions the pressure and temperature dependence of the radiation from the model gas cap is effectively that for thermodynamic equilibrium. For nitrogen, shock-layer pressures must be reduced to below 1Oatm before excess radiation due to nonequilibrium radiation is important. The models used in these tests were Apollo-shaped and made of copper. This material was used because the increased surface temperature due to convective heat transfer remains below the melting temperature of copper until the model is past the station at which the radiation measurements were taken. (The onset of melting is easily recogniTable” ‘) when present, as there is a sudden increase in luminosity from the model and its wake when melting occurs.) Since the wake luminosity from the nonablating copper models is small compared to that from the shock layer, it can safely be neglected in the data reduction. ln.striunrnlation
The absolute levels of spectral emission from the model shock layer were measured by a four-channel scanning spectrometer ‘I” that views the model flight path from the side. Figure 1 is a schematic sketch of this spectrometer. As the model flies past the instrument, TEST
SECTION
--+JN
OSCILLOSCOPE
-FOCUS COLLIMATING
MIRROR
MIRROR
FIG. I. Diagram
of moving
source scanning
spectrometer.
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1177
the luminous gas cap acts as a moving entrance slit. The divergent beam of radiation from this source is collimated and directed onto the grat;ng. The diffracted rays leaving the grating are focused by a spherical mirror onto a plane containing the exit slit. As the spectrum of the hot gas is swept past the exit slit, it is sensed by a photomultiplier tube, the output of which is displayed on an oscilloscope and photographed. The spectrometer has a spectral resolution of approximately 25 A and has been calibrated”” for determining the wavelength as well as the absolute intensity of the radiation. A typical oscillogram from one of the channels of this instrument covering the spectral region from 0.32 to 0.52 /* is displayed in Fig. 2. Increasing voltage corresponds to increasing intensity. The tests were also instrumented with a dual prism, f/3.5 spectrograph (Huet UV 24) to obtain time-integrated spectra of the shock-layer luminosity. Theoretical
predictions
A synthetic spectrum was calculated for comparison with the experimental results. Molecular band, atomic line, and free-free and free-bound continuum radiation were combined to form this synthetic spectrum. Molecular
hands und atomic lines
The computer program of WHITING et al.” 3’ was used to predict the spectra resulting from electronic transitions of diatomic molecules and atoms. This program produces a spectrum by accounting for the contribution of each rotational and atomic line considered. The integrated intensity of each line (given by equations (1) and (2) for rotational and atomic lines, respectively) is distributed in the spectrum by an approximate Voigt prohle.“4’
In the above
N hvA,, EC_“_ IO-‘, W/cm3-sr. 4Tr equations, IRdY,.,,.)12 = square
of the electronic
q,.,,, = Franck-Condon S
J’ A’ J” A ”
=
line intensity
transition
moment
factor factor
N, = number
of molecules
A,, = Einstein
A coefficient.
in the upper state
All other symbols are defined in the Notation. The integrated intensity of each line is dependent on the number of atoms or molecules in the upper state of the transition. Ordinarily, the number in the upper state is determined from the following equations for molecules and atoms, respectively, N
=
11
N4(2K’+ 1)
N, = F
Q exp( - hcTb/kT)
where d, = electronic
Q= partition
multiplicity
of upper state
function
7;, = electronic G(a)= vibrational F(k) = rotational
term energy term energy term energy.
However, since the gas cap of the model is nonhomogeneous. a singtc tcmpcraturc and density cannot be assigned to describe the thermodynamic properties of the gas cap. This complicates the calculation of species concentrations in the gas cap which are required to make meaningful theoretical predictions. For the present tests, the determination of the total number of each radiating species in the gas cap was accomplished as follows: First, the equilibrium temperature and density distributions immediately behind the shock and along the body surface wcrc computed From these distributions, the number density of each using the method of KAATTARI.“” species as welt as the number in each excited state was calculated for positions along the body and behind. the shock. Figure 3 shows the temperature. pressure, and some of the excited level number density distributions for a typical test. The number density distributions behind the shock were terminated by the first Mach line of the corner expansion fan and assumed to be negligible thereafter. This is reasonably justified since auxiliary calculations’“’ demonstrate that the corner expansion fan rapidly quenches the radiation. Next. the gas cap was divided into small volume elements. selected by allowing the cxcitcd
0
0
16
I
I
I
24
32
40
8, deg k-K,. 3. Trmper -atule. pr~s~u
V = 8.84 km;sec. P, = 50 tow.
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
I179
number density to vary less than 10 per cent between adjacent elements. This typically resulted in a A0 (see Fig. 3) of 2.5-S to delineate each element while a value of 15” was selected for A4. The fraction of each small volume element visible to the scanning spectrometer was then determined. The total number of radiating particles is then given by (Ni)7. = i
(Ni)jy,fcj
j= 1
where
(Ni)T.= total number
of radiating
(Ni)j = average number “;.=
volume
.f, = fraction
density
particles
of species i
in the volume
element
Vj
of element ,i of the volume
element
visible to the spectrometer.
The input to the computer program of Whiting, Arnold, and Lyle was modified to allow the number of particles in the upper state of the transition to be an input quantity, thereby eliminating the necessity of specifying a temperature. An additional feature of their program is that a measured instrument slit function can be combined with the synthetic spectrum to obtain a prediction that accounts for the instrument response and broadening. This option was used to generate the theoretical spectra presented herein. Four molecular bands were included in the predictions : the First Positive and Second Positive systems of N, and the First Negative and Meinel systems of Nz. Additional molecular systems”” were omitted because their contribution to the radiation was predicted to be negligible or the spectroscopic data (flnumbers, FranckkCondon factors, etc.) were unavailable. The electronic absorption ,f-numbers used in the calculations are listed in Table 1. The corresponding arrays of Franck-Condon factors as well as the basic spectroscopic constants can be found in Ref. (13).
Molecular N, N, N; NI
system
First positive Second positive First Negative Meinel
Electronic absorption f-number used m theoretical predictions 0.0034 0.057 0.053 0.00x25
Reference (29) (10) (tw (37)
All of the allowed bound-bound transitions for atomic nitrogen in the wavelength region of 0.221 .O p listed by WIESE et ul.” 7, were included in the program. The.f-numbers for these lines were also taken from Wiese’s paper. Continuum
radiation
The theoretical predictions of the continuum radiation resulting from the interaction of electrons with nitrogen ions (N+) including both free-free and free-bound transitions
t)A\WI),“I. C‘OOHK
I I x0
vary in magnitude by as much as a factor of 2. “I The largest prediction of the continuum radiation represents less than IO per cent of the predicted spectral radiation in the region between 0.2 and 1.0~~. Consequently, the theory used to predict this radiation is not critical to the interpretation of the results of the prcscnt experiment. The theory of W~I.SO\ and NICOLET('~' for free-free and free-bound transitions was used to predict this continuum radiation. The calculated number densities described earlier and a mean temperature based on equations (I) and (3) of Ref. (19) were used in the computation of the theoretical continuum radiation. A typical error of &100”K in the mean temperature represents an error of only *I! per cent in the continuum radiation. This is. of course. negligible since the continuum radiation represents only a smail part of the total radiation. The predictions. to be shown subsequently, do not include any contribution from radiation produced by the radiative attachment of electrons to neutral nitrogen atoms (N radiation). The greatest optical depth at the core of any of the lines was computed to bc 0.067 for the nitrogen atomic line at 8680~%. This optical depth is based on the width of the line before it was broadened by the instrument function. Consequently. the assumption of an optically thin theory is justified. Rc.slr1i.s A measured shock-layer spectrum from one of the tests is displayed in Fig. 4. The corresponding temperature and pressure profiles wcrc those given in Fig. 3. The spectra from both the scanning spectrometer and the spectrograph showed that the test gas was generally free of any contamination. The only contaminant emission
4.2
-THEORY ---EXPERIMENT N
3.6
N
.6
0
h L
2
.3
.4
.5
.6
WAVELENGTH,
FIG 4. Experimental
and theoretical
spectra;
.7
.f
3
1.0
p
I; = 8.84 km/set.
P, = 50 torr.
Spectral intensity measurements from high-pressure nitrogen plasmas
II81
observed in any of the tests was from oxygen. This was evidenced by the presence of the oxygen multiplet at 7773 A, which is the strongest radiating (excluding the vacuum ultraviolet) oxygen multiplet at the test conditions. Since the strongly radiating hydrogen lines do not appear in the spectra, there was apparently no water vapor present; thus, the oxygen probably entered the test section through a small air leak in the chamber walls. The amount of oxygen in the test gas was inferred by integrating the observed energy in the multiplet and comparing the result with theoretical predictions of the radiation at the test conditions, The mole fraction of oxygen so obtained was always less than 0.4 per cent. Using the cross sections measured by SMITH (20)for O- continuum radiation and the predictions of WnsoN and NICOLET(‘~) for 0 continuum radiation, this amount of oxygen can account for only 0.3 per cent of the total measured radiation at any given wavelength. Therefore, contaminant radiation should not affect the results of the present experiment. The theoretical predictions for the radiation are also shown in Fig. 4. The overall agreement between theory and experiment is quite good. A comparison of the experiment and the theoretical predictions for the N,f( 1 -) and N,(2 +) systems (0.28-0.45 ~1)indicates that the.f-numbers used in the calculations are approximately correct. The present measurements can be used to obtain values of the averaged electronic transition moments. This was possible for the AC = - 1. 0, + 1 sequences of N:(l -) and Ar = + 1, +2 sequences of N,(2 +). These transition moments were deduced from the experimental data by selecting those features of the spectrum dominated (greater than 80 per cent) by a single Aa sequence of a single molecular band system. The experimentally determined values of the square of the electronic transition moments 1 IR,/ru,l& are then given by
where
c l~rl4:heo = square
of ,f-numbers I LX, = experimental lilhro = theoretical broadening 1AoCher = theoretical
The electronic
transition
the electronic transition moment corresponding to the listed in Table 1 spectral intensity spectral intensity, including instrument response and spectral
moment
intensity
from other known
and theflnumber
sources.
are related by
c IW~~o12 = 3he2WL,(4 87c2m,c where d, = electronic f,,(A) = electronic
multiplicity
of lower state
absorptionf-number.
The square of the averaged electronic transition moment is plotted as a function of internuclear separation for the Ni( 1 -) system and for the N,(2+) system in Figs. 5 and 6,
I IX2
M. (‘OOl’t,H
DAVIII
6 0 KECK ET AL 0 REIS A ALLEN ET AL
0
4
0 o 0 n
WRAY AND CONNOLLY BENNETT AND DALBY JEUNEHOMME VALUE USED IN THEORETICAL PREDICTION -WALLACE AND NICHOLLS . PRESENT EXPERIMENT
I
1.0 .8 I -1
.6 t
‘1
.4x+
‘l._
t
.4 _t .21-
t
AV=-I 0 I.--i_ 1.0 I. I
I .9
---___
i
I3
1 +I
I
-I
1.2
1.3
1.4
INTERNUCLEAR SEPARATION, fi FIG. 5. Square
of the dimensionless
electronic
transition
moment
for Ni
(I -)
respectively. (The internuclear separation assigned to the experimental data is based on a weighted average of the F,,.,...centroid values. The theoretical vibrational band intensities in each Ar sequence were used as the weighting factors.) The values used in the theoretical prediction in Fig. 4 are shown for comparison. Also included in the figures are the absolute
0 0 b 0 0
IO 863_- <
KECK ET AL BENNETT AND DALBY JEUNEHOMME REIS VALUE USED IN THEORETICAL PREDICTIONS
-WALLACE AND NICHOLLS . PRESENT EXPERIMENT
4r
2\
-.5 \
- .4 - .3 x, ru-
,j-j2----
I I 1.0
I I.1
I 1.2
I 1.3
I 1.4
I 1.5
.2 1.6
INTERNUCLEAR SEPARATION, 8, FIG. 6. Square of the dlmensionless
electronic
transition
moment
for N,(2 +)
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1183
of KECK et ~1.;‘~” REIS,.(l”’ ALLEN et al.;‘22’ WRAY and CONNOLLY;(23’ BENNETT and DALBY ;(24’ JEUNEHOMME;(~” and the relative variation given by WALLACE and NICHOLLS.(‘~,‘~’ The measurements of the present experiment agree reasonably well with the measurements listed in the literature. However, the measured variation of the transition moment for Ni(l-) seems to take a different shape than that reported by Wallace and Nicholls. The dashed curve in Figs. 5 and 6 represents the wavelength as a function of internuclear separation. The electronic $numbers corresponding to the present measurements of the electronic transition moments are listed in Table 2. Also included in Table 2 is an estimate of the accuracy to which the ,flnumbers have been determined. The prediction of the radiation shown in Fig. 4 at wavelengths exceeding 0.5 p is composed of atomic line multiplet radiation and N,(l +) and N: Meinel molecular band radiation. The good agreement between experiment and the prediction of the atomic line multiplet radiation indicate that the,flnumbers reported by WIESE et ~1.“~’ are indeed very close to the ones being observed here experimentally. measurements
Molecular system
Sequence (AU
N,t(l-)
0
+I N#+)
lv,(‘+’
+I
+2 +2 +3 +4
Electronic absorption f-number measured in this study
0.053f 0.004 0.046+ 0.006 0.0527 0.007 0.058* 0.005 0.0036) 0.0008 0.0041f 0.00IO 0.0044f 0.00I 2
The radiation, excluding atomic line radiation, at wavelengths greater than 0.5 11 is slightly higher than the theoretical predictions. If it is assumed that this radiation results from the N2( 1 +) system, then the electronic transition moments shown in Fig. 7 result. Because of the large contributions of atomic line multiplets at wavelengths greater than 0.8 11,it was not possible to obtain electronic transition moments for sequences other than those listed in Fig. 7. The measurements of KECK ef a/. ;(28’ WRAY and CONNOLLY ;‘2y’ WURSTER ;t30’A~~~~ et al. ;(22)JEUNEHOMME;(~~) and TURNER and NICHOLLS(~~’are included in the figure. Once again the present measurements of the square of the electronic transition moments agree with the other experimental values. Table 2 contains the corresponding electronic,flnumbers. It is probable that part of the difference between theory and experiment in Fig. 4 results from other molecular systems such as Herman’s infrared bands, the Green bands, or the 6536 cm ’ band, which are not included in the present theory. Also shown in Fig. 4 is the predicted values of the NP radiation based on the calculated number of N particles, the cross section deduced experimentally by MORRIS et ~l.,‘~’ and the calculated mean temperature in the model gas cap. The radiation is assumed to result from the ‘D state only. While the predicted level of N- radiation is not overwhelming, it would contribute a substantial amount to the total theoretical predictions if included.
M. (‘OOPFK
k)AVII)
AV=+2
4-
+3
+4
.9 .8 .7 .6 +
---.
J.5 0 KECK ET AL 0 WRAY AND CONNOLLY o WURSTER
2-
zl&l’
A ALLEN ET AL 0 VALUE USED IN THEORETICAL PREDICTIONS
‘.O .8 -
-TURNER
.6 -
l
AND NICHOLLS
PRESENT EXPERIMENT
.4 -
.2 I.1
I
I
1
I
\
1
1.6 1.2 1.3 1.4 1.5 INTERNUCLEAR SEPARATION, 8.
I
1.7
FIG. 7. Square of the dimensionless electronic transition moment for N,(l +)
However. in the spectral region greater than 0.4 /L. the experimental values of the total radiation with the exception of a few atomic lines are less than the predicted values of the N radiation alone. If the difference between the experimental measurements and the theory for all other radiative processes is assumed to be due to N radiation, then the resulting upper limit of the cross section for N -( ‘D) is shown in Fig. 8. This curve is based on the assumption that the only molecular systems contributing any radiation arc those listed in Table 1, and that the,fnumbers for these systems are those given in Table 1. l‘hc upper limit of the cross section differs in both magnitude and shape from the cross sections reported in Refs. (I), (2). (4) and (5) and shown in Fig. 8. The cross-sectional values deduced from the present tests are strongly dependent on the molecular radiation intensity. Furthermore. only small changes occur in the electronic transition moments of NJ1 +) (Fig. 7) by assuming the radiation to be all molecular. These facts suggest that the small differences in certain spectral regions between the theory and experiment in the present tests is due to molecular radiation rather than N continuum radiation. SHOCK-TUBE
MEASUREMENTS
The experimental work just described was dominated by molecular radiation. Since some uncertainties exist in the molecular radiation, additional tests were performed in a shock-tube facility capable of shock-heating high pressure gas samples to temperatures greater than those obtainable with the ballistic range. At these conditions. molecular radiation represents a minor part of the total radiation.
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1185
0-l PRESENT TESTS
---
1
.4
,
1
1
I
.6
.8
1.0
MORRIS (REF 2) BOLDT (REF I) CIFFONE ( REF 4) ASINOVSKII (REF5)
I
1.2
I
1.4
I
1.6
WAVELENGTH, p FIG;. 8. Comparison
Experimental
of reported
values of the N-(ID) tests.
cross section
with the results of the present
description
The tests were performed in the Ames High-Explosive Shock-Tube Facility. This facility consists of a glass shock tube 7.3 m long and 4.7 cm in diameter, which is fully confined by containment chambers to allow safe operation in the laboratory. A detailed description of this facility and its operation can be found in Refs. (33) and (34). A sketch of the facility with its accompanying instrumentation is shown in Fig. 9. The glass tube is imploded at one end by an explosive charge (approximately 2 kg of composition C-4) that generates a high-speed jet of glass particles. This jet forms a piston that drives a shock wave ahead of it as it proceeds down the shock tube. The shock speeds of the tests performed and reported here ranged between 10.7 and 11.8 km/set. Initial loading pressures varied between 20 and 40 torr of pure nitrogen. Absolute intensities of selected spectral regions of the radiation were measured behind the incident shock wave as a function of time by four calibrated narrow pass-band radiometers.‘3s’ The radiometers consisted of photomultipiier tube detectors coupled to Bausch and Lomb 1/4-m monochromators adjusted to a 50 A half-intensity bandpass. The output of each detector was amplified, displayed on an oscilloscope, and photographed. As shown in the insert in Fig. 9, the glass tube terminates in a small stainless steel test section containing copper splitter plates through which the radiometers view the flow. These plates are used to exclude from the radiometer viewpath the relatively thick variabletemperature boundary layer on the shock-tube wall, replacing it by the thinner splitterplate boundary layer. As a result of the splitter plate, weak oblique shocks which appeared calculations using the oblique shock relain the viewpath were formed. (34) Equilibrium tions showed these shocks to have a negligible effect on the intensity measurements.
1186
DAVID
M. CCKIPEK
RADIOMETERS
Measurements at two temperatures of the continuum radiation at 3357,620O. 7200 and 7750 A are plotted in Fig. IO. The measurements at 3357 A were the only ones having any significant molecularcontribution.Tocorrectforthis, thedataat 3357 Aoftests 1and II wcrc reduced. using the predictions of Ref. (13). by approximately 40 and 3 per cent, respectively. Also displayed on the figure (lower two curves) is the theoretical prediction of the free free and free bound radiation by WILSON and NICOLET.('w The InglissTeller theory for line mcrging’“h’ was used to compute the wavelength shift of the absorption edge at 2940 A. The experimental results of test I compare closely with Wilson and Nicolet’s predictions in Fig. 10; however, the experimental data of test II fall substantially below the theory. A probable reason for this will be discussed below. Also included in Fig. 10 are the predictions for the total continuum radiation when the N -(ID) contribution based on the cross section reported by MORRIS rt al. “I is added. The measurements lie considerably below the sum. As was the case with the ballistic range data, the shock-tube measurements do not support the existence of N at these high pressures. The high pressure tests performed in the High-Explosive Shock Tube were characterized by disturbed (nonplanar) shock fronts. Photographs of this luminous flow (e.g. Fig. I I) gave evidence that the glass particle piston was irregular in form and was the sour-cc of the disturbance to the shock front. In the majority of the tests (e.g. test I in Fig. IO), the photographs of the flow showed that the disturbance was slight and off to one side. In these tests, the splitter plates excluded this disturbance and the measurements between the plates should not have been affected by the presence of the nearby disturbances. In other tests (e.g. test II in Fig. IO), a portion of the disturbance passed between the splitter plates and
Spectral intensity measurements from high-pressure nitrogen plasmas
1187
WILSON AND NICOLET + N- PREDICTION TEST I, T=12,550”K TEST 2, T= 14,360”K
T, OK 14,360 12,550 14,360
12,550 IO31 .2 Fw.
10. Comparison
I .3
I .4
I I .5 .6 WAVELENGTH,
I .7
I .6
I .9
/_L
of theory and experimental data; test I: T, = l2,550”K, PI = 63.0atm: test II : T2 = 14,36O”K,P2 = 38.3 atm.
consequently the measurements were probably affected. This conclusion is supported by the results shown in Fig. 12. The ratio of the experimental measurements to the theoretical predictions of WILSON and NICOLET t’*) for free-free and free-bound continuum radiation is plotted as a function of the computed postshock equilibrium temperature. Each data point is labeled with the computed postshock pressure (P/P,) at which it was obtained. The measurements were made at a wavelength of approximately 6OOOA. The diamonds in Fig. 12 represent tests in which the photographs of the flow showed the shock fronts to be disturbed. From the data in this figure, which cover a temperature range of 12,55017,200”K and a postshock pressure range of 1.3-63.0 atm, it is evident that the theory of Wilson and Nicolet adequately predicts the continuum radiation at this spectral location. Specifically, the theory appears to be about 10 per cent higher than the present measurements. The data also show that the higher pressure results obtained with disturbed shocks have given reasonable results in the cases where the disturbance is excluded from the field of view. The two solid symbols represent the data from Fig. 10. The correlation shown here supports the earlier conclusion that the data from test I of Fig. 10 (the point at 12,550”K) are reasonably free of disturbances.
CONCLUSIONS
Spectral measurements of the radiation from nitrogen at high pressures and temperatures agree well with theoretical predictions. Averaged values of the electronic f-number for the Au = - 1, 0, + 1 sequence of Nl(l -), the Au = + 1, + 2 sequence of N,(2+), and the Au = + 2, + 3, +4, sequence of N,( 1 +) have been measured.
I IXX
M. C~OI’FK
L)AVII)
c 3 p
0 DISTURBED
1.6 -
SHOCK
FRONTS
z w” 4 B : a:
1.2 .a-
43.9 0
’
I”3 p3
1.4 0 205 2.60 27 o
63.0
g
z
,“6
‘b’ ;9 6%
01.4
z z fz 2
0 5.1
*
.4-
026
p2.363 PO
o1 12
Y b IG. I?. Katio of experimental
13
14
15
TEMPERATURE spectral intensity
17
16 X 10-3,
to theoretical
18
“K
intensity
as a function
of temperature.
The present tests reveal no evidence to support the existence of a continuum process from capture of an electron by a neutral nitrogen atom to form N -.
KOl‘,A
flON
Einstein A coefhctent. bet ‘-part ’ speed of light, 2.9379 x IO” cm-set ’ electronic multiplicity. dimensionless integrated intensity. W;cn’-stelectrontc charge, 4.80298 A IO I0 dyne-cm’ (esu) product of electronic charge and radius of first Bohr o&t. 2.S4ln i, IO In dyne-cm’ rotational term function for the vibrational level r. cm ’ electronic absorption ./-number. dimensionless fraction of the volume visible to the apectromcter. dimensionlw vibrational energy. cm I Planck’s constant. 6.623 x IO ‘- erg-set \pectral intensity, W:cm’-j-sr total rotational quantum number. dimensionless rotational quantum number without spin, dimenaionle~a Boltlmann constant. I.38054 x 10 ” erg-OK ’ massof the electron. 9.1091 x 10~‘” G number density. particles/cm’ total number of radiating particles of species i. particle\ average number density. particles’cm ’ pressure, atm partition function, dimensionless FranckkCondon factor, dimensionles\ electronic transition moment. (esu-cm)’ r-centroid, A line strength factor. dimensionless temperature, “K term energy of electronic state. cm ’ volume. cm3 vibrational quantum number. dnnensionlcss wavelength, ,& frequency of emitted radiation. set ’ spherical coordinates
(esu-cn))
radiative
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1189
upper state lower state upper state lower state reference conditions; PO = I atm, To = 288°K conditions behind incident shock wave
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO. I I. 12.
13. 14. 15. lb. 17. 18. 19.
G. BOLDT. 2. Phys. 154, 330 (1959). J. C. MOKIIIS. R. U. KKEV and R. L. GARRISOU, Aerospace Res. Lab., AVSSD-0049-6%RR (1968). G. M. THOMAS and W. A. MENARD. AIAA Jl5,2214 (1967). D. L. C~tbo~~and J. G. BORUCICI,JQSRT II, 1291 (1971). Z. J. ASINOSKII, A. V. KIRILLIN and G. A. KOUSEV,JQSR7’ 10, 143 (1970). D. E. BU~~REY and H..R. MC‘HESNEY, AFWL-TR-69-29 (1969). A. A. KONKOV, A. P. RYAZIN and V. S. RUDNEV, JQSRT7, 345 (1967). A. SEIFP, AIAA Paper 63-162 (1963). W. A. PAGE and J. 0. ARNOLD. NASA TR R-193 (1964). V. H. REIS. JQSRT4, 783 (1964). D. L. COMPWN and D. M. COOPER, NASA TN D-2871 (I 965). E. E. WIII~IN~;, A New Four-Channel Scanning Spectrometer for Balhstic-Range Radiometry, Proceedings of IEEE Second International Congress on Instrumentation in Aerospace Simulation Facilities. Stanford Univ., Aug. 29 3;. (1966) pp. 14-I-14-5. E. E. WHIIIN~;, J. 0. AKNOLD and G. C. LYLE, NASA TN D-5088 (1969). J. 0. ARNOI.D, E. E. WHlrlx and G. C. LYLE, JQSRT9, 775 (1969). G. E. KAATTAKI, NASA TN D-1423 (1962). L. WALLACE, Asrwphw. J. VI, Suppl. 62, 445 (1962). W. L. WIESE, M. W. SZU~H and B. M. GI.FUUOF;, National Standard Reference Data Series. ,Ytrr/. Bur. S/c&. 4, I (1966). K. H. WII.SOU and W. E. NI(.OLET. JQSRT7, 801 (1967). D. M. COOPER. AIAA J/4, 2125 (1966). International
2 I. 22. 23. 24. 25. 26. 27. 2X. 2Y. 30. 31. 32. 33. 34. 35. 36. 37.
Conference
on Ionization
Phenomena
in Gases.
Uppsala
(1960), pp. 2 I9 225
J. C. KWK, J. C. CAMM, B. KIVEL and T. WEYTINK. Ann. PIz>‘.s.7, I (1957).
R. A. AI.I.ES. J. C. CAMM and J. C. Ktc~. JQSRT I, 269 (1961). K. L. WRAY and T. J. CONNOLLY, JQSRT 5, 633 (1965). R. G. PEN%rT and F. W. DALBY, J. chm. Phys. 31, 434 (1959). M. JEUNEHOMME,J. chum. Phys. 44, 2672 (1966). L. V. WAI.I.ACE and R. W. NICHOLLS, J. Ammos. Twr. P/Iv.s. 23, 749 (1962). L. V. WALLACE.and R. W. NICHOLLS. J. Armo.s. Twr. Ph,vs. 7, 101 (1955). J. C. K~(.K. R. A. ALLEN and R. L. TAyt.oR. JQSRT3, 335 (1965). K. L. WKAV and T. L. CONNOLLY, JQSRT5, I I I (1965). W. H. WuasrEK, J. cl7em. Phys. 36, 21 I 1 ( 1962). M. JEU~EHO~IM~, J. chrn~. Phy.y. 45, 1805 (1966). R. G. TURNEK and R. W. NICHOLLS, Con. J. Pll>ls. 32, 468; 475 (1954). R. E. BERGGREN, D. L. CO%II>TO~~, T. N. CANNING and W. A. PAGE, Proceedings of the Seventh International Shock Tube Symposium, Toronto, Canada, June 23-25 (196Y) pp. 353 365. W. J. BOKUC‘KI, D. M. COOPEK and W. A. PAGE, Proceedings of the Seventh International Shock Tube Symposium, Toronto, Canada, June 23%25, (1969) pp. 708-720. J. 0. ARNOLD, JQSRTI, 1781 (1968). D. R. INGLIS and E. TELLER, Astrophys. J. 90, 439 (1939). E. B. KURPIANOVIA, V. N. K~LESN~KOVand N. N. SOB~LEV, JQSRT9, 1025 (1969).
MEASUREMENTS FROM NITROGEN PLASMAS
DAVID M. COOPER Ames Research
Center,
NASA,
(Received
Moffett
Field, California
15 Deccmher
94035, U.S.A.
197 I)
Abstract-Spectral measurements of the equilibrium radiation from mtrogen at high pressures have been obtained. Experiments were performed in a ballistic range and an explosively driven shock tube. The tests covered a temperature range from 9000 to 17,200”K and a pressure range from 1.3 to 63.0atm. Averaged electronic absorption f-numbers for Ni B2~~~xzZ~, NzC3rI-B3Q, and N,B3Fl-A”EJ bands are reported. N- continuum radiation was not observed.
INTRODUCTION
SEVERAL studies”P6) of the equilibrium radiation from nitrogen at pressures between 0.2 and 2 atm and over a temperature range from 8000 to 13,OOO”K have been performed in shock tube and arc facilities. In the majority of the studies!‘- ” the measured absolute radiation exceeded the theoretical predictions which included molecular band, atomic line, and continuum radiation from electron-ion interactions. The excess radiation has been interpreted as arising from the production of the negative ion N- by the attachment of an electron to a neutral atom. The resulting experimental estimates of the cross section for photon capture of the NP ion (for the ‘D state) differ significantly in magnitude and wavelength dependence.‘4’ In an additional high pressure (5&60atm) study,‘7’ where it was expected that continuum radiation from electron-atom interactions would dominate that from electron-ion interactions, excess radiation that could be attributed to the existence of NP was not observed. There exists, then, a serious contradiction between the low pressure measurements (less than 2 atm) which suggest the existence of an N- contribution to the continuum radiation and the high pressure measurement which does not. The purpose of the present paper is to present additional measurements of the spectral intensity from high pressure nitrogen plasmas in order to resolve the anomaly surrounding the existence of N -, and consequently to verify the theoretical predictions of the radiation at these conditions.
BALLISTIC
Experimental
RANGE
TESTS
description
Measurements of the radiation from the shock layers of models were performed prototype of the Ames Hypervelocity Free-Flight Facility operated as a ballistic 1175
in the range.
This facility consists of a light gas gun that serves as a model launcher. an instrumented test section, and a catcher that terminates the projectile flight. A complete description of this facility may be found in Ref. (8). For the present tests, the models flew through quiescent nitrogen in the instrumented test section. The model trajectories were determined from 11 spark shadowgraph stations spaced at I .22 m intervals along the test section length. Electronic chronographs recorded the time at which each shadowgraph picture was obtained, thus allowing a velocity determination. The models were launched at a velocity of approximately 8.X km/set into pure nitrogen at initial pressures of 25 and 50 torr. At these test conditions, the computed ratio of continuum radiation from electronPatom interactions (based on the measurements of MORRIS rf ~1.‘~‘) to that from electronGon (N’) interactions should be approximately a factor of 10. Furthermore, prior tests(“~‘O’ have confirmed that at these test conditions the pressure and temperature dependence of the radiation from the model gas cap is effectively that for thermodynamic equilibrium. For nitrogen, shock-layer pressures must be reduced to below 1Oatm before excess radiation due to nonequilibrium radiation is important. The models used in these tests were Apollo-shaped and made of copper. This material was used because the increased surface temperature due to convective heat transfer remains below the melting temperature of copper until the model is past the station at which the radiation measurements were taken. (The onset of melting is easily recogniTable” ‘) when present, as there is a sudden increase in luminosity from the model and its wake when melting occurs.) Since the wake luminosity from the nonablating copper models is small compared to that from the shock layer, it can safely be neglected in the data reduction. ln.striunrnlation
The absolute levels of spectral emission from the model shock layer were measured by a four-channel scanning spectrometer ‘I” that views the model flight path from the side. Figure 1 is a schematic sketch of this spectrometer. As the model flies past the instrument, TEST
SECTION
--+JN
OSCILLOSCOPE
-FOCUS COLLIMATING
MIRROR
MIRROR
FIG. I. Diagram
of moving
source scanning
spectrometer.
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1177
the luminous gas cap acts as a moving entrance slit. The divergent beam of radiation from this source is collimated and directed onto the grat;ng. The diffracted rays leaving the grating are focused by a spherical mirror onto a plane containing the exit slit. As the spectrum of the hot gas is swept past the exit slit, it is sensed by a photomultiplier tube, the output of which is displayed on an oscilloscope and photographed. The spectrometer has a spectral resolution of approximately 25 A and has been calibrated”” for determining the wavelength as well as the absolute intensity of the radiation. A typical oscillogram from one of the channels of this instrument covering the spectral region from 0.32 to 0.52 /* is displayed in Fig. 2. Increasing voltage corresponds to increasing intensity. The tests were also instrumented with a dual prism, f/3.5 spectrograph (Huet UV 24) to obtain time-integrated spectra of the shock-layer luminosity. Theoretical
predictions
A synthetic spectrum was calculated for comparison with the experimental results. Molecular band, atomic line, and free-free and free-bound continuum radiation were combined to form this synthetic spectrum. Molecular
hands und atomic lines
The computer program of WHITING et al.” 3’ was used to predict the spectra resulting from electronic transitions of diatomic molecules and atoms. This program produces a spectrum by accounting for the contribution of each rotational and atomic line considered. The integrated intensity of each line (given by equations (1) and (2) for rotational and atomic lines, respectively) is distributed in the spectrum by an approximate Voigt prohle.“4’
In the above
N hvA,, EC_“_ IO-‘, W/cm3-sr. 4Tr equations, IRdY,.,,.)12 = square
of the electronic
q,.,,, = Franck-Condon S
J’ A’ J” A ”
=
line intensity
transition
moment
factor factor
N, = number
of molecules
A,, = Einstein
A coefficient.
in the upper state
All other symbols are defined in the Notation. The integrated intensity of each line is dependent on the number of atoms or molecules in the upper state of the transition. Ordinarily, the number in the upper state is determined from the following equations for molecules and atoms, respectively, N
=
11
N4(2K’+ 1)
N, = F
Q exp( - hcTb/kT)
where d, = electronic
Q= partition
multiplicity
of upper state
function
7;, = electronic G(a)= vibrational F(k) = rotational
term energy term energy term energy.
However, since the gas cap of the model is nonhomogeneous. a singtc tcmpcraturc and density cannot be assigned to describe the thermodynamic properties of the gas cap. This complicates the calculation of species concentrations in the gas cap which are required to make meaningful theoretical predictions. For the present tests, the determination of the total number of each radiating species in the gas cap was accomplished as follows: First, the equilibrium temperature and density distributions immediately behind the shock and along the body surface wcrc computed From these distributions, the number density of each using the method of KAATTARI.“” species as welt as the number in each excited state was calculated for positions along the body and behind. the shock. Figure 3 shows the temperature. pressure, and some of the excited level number density distributions for a typical test. The number density distributions behind the shock were terminated by the first Mach line of the corner expansion fan and assumed to be negligible thereafter. This is reasonably justified since auxiliary calculations’“’ demonstrate that the corner expansion fan rapidly quenches the radiation. Next. the gas cap was divided into small volume elements. selected by allowing the cxcitcd
0
0
16
I
I
I
24
32
40
8, deg k-K,. 3. Trmper -atule. pr~s~u
V = 8.84 km;sec. P, = 50 tow.
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
I179
number density to vary less than 10 per cent between adjacent elements. This typically resulted in a A0 (see Fig. 3) of 2.5-S to delineate each element while a value of 15” was selected for A4. The fraction of each small volume element visible to the scanning spectrometer was then determined. The total number of radiating particles is then given by (Ni)7. = i
(Ni)jy,fcj
j= 1
where
(Ni)T.= total number
of radiating
(Ni)j = average number “;.=
volume
.f, = fraction
density
particles
of species i
in the volume
element
Vj
of element ,i of the volume
element
visible to the spectrometer.
The input to the computer program of Whiting, Arnold, and Lyle was modified to allow the number of particles in the upper state of the transition to be an input quantity, thereby eliminating the necessity of specifying a temperature. An additional feature of their program is that a measured instrument slit function can be combined with the synthetic spectrum to obtain a prediction that accounts for the instrument response and broadening. This option was used to generate the theoretical spectra presented herein. Four molecular bands were included in the predictions : the First Positive and Second Positive systems of N, and the First Negative and Meinel systems of Nz. Additional molecular systems”” were omitted because their contribution to the radiation was predicted to be negligible or the spectroscopic data (flnumbers, FranckkCondon factors, etc.) were unavailable. The electronic absorption ,f-numbers used in the calculations are listed in Table 1. The corresponding arrays of Franck-Condon factors as well as the basic spectroscopic constants can be found in Ref. (13).
Molecular N, N, N; NI
system
First positive Second positive First Negative Meinel
Electronic absorption f-number used m theoretical predictions 0.0034 0.057 0.053 0.00x25
Reference (29) (10) (tw (37)
All of the allowed bound-bound transitions for atomic nitrogen in the wavelength region of 0.221 .O p listed by WIESE et ul.” 7, were included in the program. The.f-numbers for these lines were also taken from Wiese’s paper. Continuum
radiation
The theoretical predictions of the continuum radiation resulting from the interaction of electrons with nitrogen ions (N+) including both free-free and free-bound transitions
t)A\WI),“I. C‘OOHK
I I x0
vary in magnitude by as much as a factor of 2. “I The largest prediction of the continuum radiation represents less than IO per cent of the predicted spectral radiation in the region between 0.2 and 1.0~~. Consequently, the theory used to predict this radiation is not critical to the interpretation of the results of the prcscnt experiment. The theory of W~I.SO\ and NICOLET('~' for free-free and free-bound transitions was used to predict this continuum radiation. The calculated number densities described earlier and a mean temperature based on equations (I) and (3) of Ref. (19) were used in the computation of the theoretical continuum radiation. A typical error of &100”K in the mean temperature represents an error of only *I! per cent in the continuum radiation. This is. of course. negligible since the continuum radiation represents only a smail part of the total radiation. The predictions. to be shown subsequently, do not include any contribution from radiation produced by the radiative attachment of electrons to neutral nitrogen atoms (N radiation). The greatest optical depth at the core of any of the lines was computed to bc 0.067 for the nitrogen atomic line at 8680~%. This optical depth is based on the width of the line before it was broadened by the instrument function. Consequently. the assumption of an optically thin theory is justified. Rc.slr1i.s A measured shock-layer spectrum from one of the tests is displayed in Fig. 4. The corresponding temperature and pressure profiles wcrc those given in Fig. 3. The spectra from both the scanning spectrometer and the spectrograph showed that the test gas was generally free of any contamination. The only contaminant emission
4.2
-THEORY ---EXPERIMENT N
3.6
N
.6
0
h L
2
.3
.4
.5
.6
WAVELENGTH,
FIG 4. Experimental
and theoretical
spectra;
.7
.f
3
1.0
p
I; = 8.84 km/set.
P, = 50 torr.
Spectral intensity measurements from high-pressure nitrogen plasmas
II81
observed in any of the tests was from oxygen. This was evidenced by the presence of the oxygen multiplet at 7773 A, which is the strongest radiating (excluding the vacuum ultraviolet) oxygen multiplet at the test conditions. Since the strongly radiating hydrogen lines do not appear in the spectra, there was apparently no water vapor present; thus, the oxygen probably entered the test section through a small air leak in the chamber walls. The amount of oxygen in the test gas was inferred by integrating the observed energy in the multiplet and comparing the result with theoretical predictions of the radiation at the test conditions, The mole fraction of oxygen so obtained was always less than 0.4 per cent. Using the cross sections measured by SMITH (20)for O- continuum radiation and the predictions of WnsoN and NICOLET(‘~) for 0 continuum radiation, this amount of oxygen can account for only 0.3 per cent of the total measured radiation at any given wavelength. Therefore, contaminant radiation should not affect the results of the present experiment. The theoretical predictions for the radiation are also shown in Fig. 4. The overall agreement between theory and experiment is quite good. A comparison of the experiment and the theoretical predictions for the N,f( 1 -) and N,(2 +) systems (0.28-0.45 ~1)indicates that the.f-numbers used in the calculations are approximately correct. The present measurements can be used to obtain values of the averaged electronic transition moments. This was possible for the AC = - 1. 0, + 1 sequences of N:(l -) and Ar = + 1, +2 sequences of N,(2 +). These transition moments were deduced from the experimental data by selecting those features of the spectrum dominated (greater than 80 per cent) by a single Aa sequence of a single molecular band system. The experimentally determined values of the square of the electronic transition moments 1 IR,/ru,l& are then given by
where
c l~rl4:heo = square
of ,f-numbers I LX, = experimental lilhro = theoretical broadening 1AoCher = theoretical
The electronic
transition
the electronic transition moment corresponding to the listed in Table 1 spectral intensity spectral intensity, including instrument response and spectral
moment
intensity
from other known
and theflnumber
sources.
are related by
c IW~~o12 = 3he2WL,(4 87c2m,c where d, = electronic f,,(A) = electronic
multiplicity
of lower state
absorptionf-number.
The square of the averaged electronic transition moment is plotted as a function of internuclear separation for the Ni( 1 -) system and for the N,(2+) system in Figs. 5 and 6,
I IX2
M. (‘OOl’t,H
DAVIII
6 0 KECK ET AL 0 REIS A ALLEN ET AL
0
4
0 o 0 n
WRAY AND CONNOLLY BENNETT AND DALBY JEUNEHOMME VALUE USED IN THEORETICAL PREDICTION -WALLACE AND NICHOLLS . PRESENT EXPERIMENT
I
1.0 .8 I -1
.6 t
‘1
.4x+
‘l._
t
.4 _t .21-
t
AV=-I 0 I.--i_ 1.0 I. I
I .9
---___
i
I3
1 +I
I
-I
1.2
1.3
1.4
INTERNUCLEAR SEPARATION, fi FIG. 5. Square
of the dimensionless
electronic
transition
moment
for Ni
(I -)
respectively. (The internuclear separation assigned to the experimental data is based on a weighted average of the F,,.,...centroid values. The theoretical vibrational band intensities in each Ar sequence were used as the weighting factors.) The values used in the theoretical prediction in Fig. 4 are shown for comparison. Also included in the figures are the absolute
0 0 b 0 0
IO 863_- <
KECK ET AL BENNETT AND DALBY JEUNEHOMME REIS VALUE USED IN THEORETICAL PREDICTIONS
-WALLACE AND NICHOLLS . PRESENT EXPERIMENT
4r
2\
-.5 \
- .4 - .3 x, ru-
,j-j2----
I I 1.0
I I.1
I 1.2
I 1.3
I 1.4
I 1.5
.2 1.6
INTERNUCLEAR SEPARATION, 8, FIG. 6. Square of the dlmensionless
electronic
transition
moment
for N,(2 +)
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1183
of KECK et ~1.;‘~” REIS,.(l”’ ALLEN et al.;‘22’ WRAY and CONNOLLY;(23’ BENNETT and DALBY ;(24’ JEUNEHOMME;(~” and the relative variation given by WALLACE and NICHOLLS.(‘~,‘~’ The measurements of the present experiment agree reasonably well with the measurements listed in the literature. However, the measured variation of the transition moment for Ni(l-) seems to take a different shape than that reported by Wallace and Nicholls. The dashed curve in Figs. 5 and 6 represents the wavelength as a function of internuclear separation. The electronic $numbers corresponding to the present measurements of the electronic transition moments are listed in Table 2. Also included in Table 2 is an estimate of the accuracy to which the ,flnumbers have been determined. The prediction of the radiation shown in Fig. 4 at wavelengths exceeding 0.5 p is composed of atomic line multiplet radiation and N,(l +) and N: Meinel molecular band radiation. The good agreement between experiment and the prediction of the atomic line multiplet radiation indicate that the,flnumbers reported by WIESE et ~1.“~’ are indeed very close to the ones being observed here experimentally. measurements
Molecular system
Sequence (AU
N,t(l-)
0
+I N#+)
lv,(‘+’
+I
+2 +2 +3 +4
Electronic absorption f-number measured in this study
0.053f 0.004 0.046+ 0.006 0.0527 0.007 0.058* 0.005 0.0036) 0.0008 0.0041f 0.00IO 0.0044f 0.00I 2
The radiation, excluding atomic line radiation, at wavelengths greater than 0.5 11 is slightly higher than the theoretical predictions. If it is assumed that this radiation results from the N2( 1 +) system, then the electronic transition moments shown in Fig. 7 result. Because of the large contributions of atomic line multiplets at wavelengths greater than 0.8 11,it was not possible to obtain electronic transition moments for sequences other than those listed in Fig. 7. The measurements of KECK ef a/. ;(28’ WRAY and CONNOLLY ;‘2y’ WURSTER ;t30’A~~~~ et al. ;(22)JEUNEHOMME;(~~) and TURNER and NICHOLLS(~~’are included in the figure. Once again the present measurements of the square of the electronic transition moments agree with the other experimental values. Table 2 contains the corresponding electronic,flnumbers. It is probable that part of the difference between theory and experiment in Fig. 4 results from other molecular systems such as Herman’s infrared bands, the Green bands, or the 6536 cm ’ band, which are not included in the present theory. Also shown in Fig. 4 is the predicted values of the NP radiation based on the calculated number of N particles, the cross section deduced experimentally by MORRIS et ~l.,‘~’ and the calculated mean temperature in the model gas cap. The radiation is assumed to result from the ‘D state only. While the predicted level of N- radiation is not overwhelming, it would contribute a substantial amount to the total theoretical predictions if included.
M. (‘OOPFK
k)AVII)
AV=+2
4-
+3
+4
.9 .8 .7 .6 +
---.
J.5 0 KECK ET AL 0 WRAY AND CONNOLLY o WURSTER
2-
zl&l’
A ALLEN ET AL 0 VALUE USED IN THEORETICAL PREDICTIONS
‘.O .8 -
-TURNER
.6 -
l
AND NICHOLLS
PRESENT EXPERIMENT
.4 -
.2 I.1
I
I
1
I
\
1
1.6 1.2 1.3 1.4 1.5 INTERNUCLEAR SEPARATION, 8.
I
1.7
FIG. 7. Square of the dimensionless electronic transition moment for N,(l +)
However. in the spectral region greater than 0.4 /L. the experimental values of the total radiation with the exception of a few atomic lines are less than the predicted values of the N radiation alone. If the difference between the experimental measurements and the theory for all other radiative processes is assumed to be due to N radiation, then the resulting upper limit of the cross section for N -( ‘D) is shown in Fig. 8. This curve is based on the assumption that the only molecular systems contributing any radiation arc those listed in Table 1, and that the,fnumbers for these systems are those given in Table 1. l‘hc upper limit of the cross section differs in both magnitude and shape from the cross sections reported in Refs. (I), (2). (4) and (5) and shown in Fig. 8. The cross-sectional values deduced from the present tests are strongly dependent on the molecular radiation intensity. Furthermore. only small changes occur in the electronic transition moments of NJ1 +) (Fig. 7) by assuming the radiation to be all molecular. These facts suggest that the small differences in certain spectral regions between the theory and experiment in the present tests is due to molecular radiation rather than N continuum radiation. SHOCK-TUBE
MEASUREMENTS
The experimental work just described was dominated by molecular radiation. Since some uncertainties exist in the molecular radiation, additional tests were performed in a shock-tube facility capable of shock-heating high pressure gas samples to temperatures greater than those obtainable with the ballistic range. At these conditions. molecular radiation represents a minor part of the total radiation.
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1185
0-l PRESENT TESTS
---
1
.4
,
1
1
I
.6
.8
1.0
MORRIS (REF 2) BOLDT (REF I) CIFFONE ( REF 4) ASINOVSKII (REF5)
I
1.2
I
1.4
I
1.6
WAVELENGTH, p FIG;. 8. Comparison
Experimental
of reported
values of the N-(ID) tests.
cross section
with the results of the present
description
The tests were performed in the Ames High-Explosive Shock-Tube Facility. This facility consists of a glass shock tube 7.3 m long and 4.7 cm in diameter, which is fully confined by containment chambers to allow safe operation in the laboratory. A detailed description of this facility and its operation can be found in Refs. (33) and (34). A sketch of the facility with its accompanying instrumentation is shown in Fig. 9. The glass tube is imploded at one end by an explosive charge (approximately 2 kg of composition C-4) that generates a high-speed jet of glass particles. This jet forms a piston that drives a shock wave ahead of it as it proceeds down the shock tube. The shock speeds of the tests performed and reported here ranged between 10.7 and 11.8 km/set. Initial loading pressures varied between 20 and 40 torr of pure nitrogen. Absolute intensities of selected spectral regions of the radiation were measured behind the incident shock wave as a function of time by four calibrated narrow pass-band radiometers.‘3s’ The radiometers consisted of photomultipiier tube detectors coupled to Bausch and Lomb 1/4-m monochromators adjusted to a 50 A half-intensity bandpass. The output of each detector was amplified, displayed on an oscilloscope, and photographed. As shown in the insert in Fig. 9, the glass tube terminates in a small stainless steel test section containing copper splitter plates through which the radiometers view the flow. These plates are used to exclude from the radiometer viewpath the relatively thick variabletemperature boundary layer on the shock-tube wall, replacing it by the thinner splitterplate boundary layer. As a result of the splitter plate, weak oblique shocks which appeared calculations using the oblique shock relain the viewpath were formed. (34) Equilibrium tions showed these shocks to have a negligible effect on the intensity measurements.
1186
DAVID
M. CCKIPEK
RADIOMETERS
Measurements at two temperatures of the continuum radiation at 3357,620O. 7200 and 7750 A are plotted in Fig. IO. The measurements at 3357 A were the only ones having any significant molecularcontribution.Tocorrectforthis, thedataat 3357 Aoftests 1and II wcrc reduced. using the predictions of Ref. (13). by approximately 40 and 3 per cent, respectively. Also displayed on the figure (lower two curves) is the theoretical prediction of the free free and free bound radiation by WILSON and NICOLET.('w The InglissTeller theory for line mcrging’“h’ was used to compute the wavelength shift of the absorption edge at 2940 A. The experimental results of test I compare closely with Wilson and Nicolet’s predictions in Fig. 10; however, the experimental data of test II fall substantially below the theory. A probable reason for this will be discussed below. Also included in Fig. 10 are the predictions for the total continuum radiation when the N -(ID) contribution based on the cross section reported by MORRIS rt al. “I is added. The measurements lie considerably below the sum. As was the case with the ballistic range data, the shock-tube measurements do not support the existence of N at these high pressures. The high pressure tests performed in the High-Explosive Shock Tube were characterized by disturbed (nonplanar) shock fronts. Photographs of this luminous flow (e.g. Fig. I I) gave evidence that the glass particle piston was irregular in form and was the sour-cc of the disturbance to the shock front. In the majority of the tests (e.g. test I in Fig. IO), the photographs of the flow showed that the disturbance was slight and off to one side. In these tests, the splitter plates excluded this disturbance and the measurements between the plates should not have been affected by the presence of the nearby disturbances. In other tests (e.g. test II in Fig. IO), a portion of the disturbance passed between the splitter plates and
Spectral intensity measurements from high-pressure nitrogen plasmas
1187
WILSON AND NICOLET + N- PREDICTION TEST I, T=12,550”K TEST 2, T= 14,360”K
T, OK 14,360 12,550 14,360
12,550 IO31 .2 Fw.
10. Comparison
I .3
I .4
I I .5 .6 WAVELENGTH,
I .7
I .6
I .9
/_L
of theory and experimental data; test I: T, = l2,550”K, PI = 63.0atm: test II : T2 = 14,36O”K,P2 = 38.3 atm.
consequently the measurements were probably affected. This conclusion is supported by the results shown in Fig. 12. The ratio of the experimental measurements to the theoretical predictions of WILSON and NICOLET t’*) for free-free and free-bound continuum radiation is plotted as a function of the computed postshock equilibrium temperature. Each data point is labeled with the computed postshock pressure (P/P,) at which it was obtained. The measurements were made at a wavelength of approximately 6OOOA. The diamonds in Fig. 12 represent tests in which the photographs of the flow showed the shock fronts to be disturbed. From the data in this figure, which cover a temperature range of 12,55017,200”K and a postshock pressure range of 1.3-63.0 atm, it is evident that the theory of Wilson and Nicolet adequately predicts the continuum radiation at this spectral location. Specifically, the theory appears to be about 10 per cent higher than the present measurements. The data also show that the higher pressure results obtained with disturbed shocks have given reasonable results in the cases where the disturbance is excluded from the field of view. The two solid symbols represent the data from Fig. 10. The correlation shown here supports the earlier conclusion that the data from test I of Fig. 10 (the point at 12,550”K) are reasonably free of disturbances.
CONCLUSIONS
Spectral measurements of the radiation from nitrogen at high pressures and temperatures agree well with theoretical predictions. Averaged values of the electronic f-number for the Au = - 1, 0, + 1 sequence of Nl(l -), the Au = + 1, + 2 sequence of N,(2+), and the Au = + 2, + 3, +4, sequence of N,( 1 +) have been measured.
I IXX
M. C~OI’FK
L)AVII)
c 3 p
0 DISTURBED
1.6 -
SHOCK
FRONTS
z w” 4 B : a:
1.2 .a-
43.9 0
’
I”3 p3
1.4 0 205 2.60 27 o
63.0
g
z
,“6
‘b’ ;9 6%
01.4
z z fz 2
0 5.1
*
.4-
026
p2.363 PO
o1 12
Y b IG. I?. Katio of experimental
13
14
15
TEMPERATURE spectral intensity
17
16 X 10-3,
to theoretical
18
“K
intensity
as a function
of temperature.
The present tests reveal no evidence to support the existence of a continuum process from capture of an electron by a neutral nitrogen atom to form N -.
KOl‘,A
flON
Einstein A coefhctent. bet ‘-part ’ speed of light, 2.9379 x IO” cm-set ’ electronic multiplicity. dimensionless integrated intensity. W;cn’-stelectrontc charge, 4.80298 A IO I0 dyne-cm’ (esu) product of electronic charge and radius of first Bohr o&t. 2.S4ln i, IO In dyne-cm’ rotational term function for the vibrational level r. cm ’ electronic absorption ./-number. dimensionless fraction of the volume visible to the apectromcter. dimensionlw vibrational energy. cm I Planck’s constant. 6.623 x IO ‘- erg-set \pectral intensity, W:cm’-j-sr total rotational quantum number. dimensionless rotational quantum number without spin, dimenaionle~a Boltlmann constant. I.38054 x 10 ” erg-OK ’ massof the electron. 9.1091 x 10~‘” G number density. particles/cm’ total number of radiating particles of species i. particle\ average number density. particles’cm ’ pressure, atm partition function, dimensionless FranckkCondon factor, dimensionles\ electronic transition moment. (esu-cm)’ r-centroid, A line strength factor. dimensionless temperature, “K term energy of electronic state. cm ’ volume. cm3 vibrational quantum number. dnnensionlcss wavelength, ,& frequency of emitted radiation. set ’ spherical coordinates
(esu-cn))
radiative
Spectral
intensity
measurements
from high-pressure
nitrogen
plasmas
1189
upper state lower state upper state lower state reference conditions; PO = I atm, To = 288°K conditions behind incident shock wave
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO. I I. 12.
13. 14. 15. lb. 17. 18. 19.
G. BOLDT. 2. Phys. 154, 330 (1959). J. C. MOKIIIS. R. U. KKEV and R. L. GARRISOU, Aerospace Res. Lab., AVSSD-0049-6%RR (1968). G. M. THOMAS and W. A. MENARD. AIAA Jl5,2214 (1967). D. L. C~tbo~~and J. G. BORUCICI,JQSRT II, 1291 (1971). Z. J. ASINOSKII, A. V. KIRILLIN and G. A. KOUSEV,JQSR7’ 10, 143 (1970). D. E. BU~~REY and H..R. MC‘HESNEY, AFWL-TR-69-29 (1969). A. A. KONKOV, A. P. RYAZIN and V. S. RUDNEV, JQSRT7, 345 (1967). A. SEIFP, AIAA Paper 63-162 (1963). W. A. PAGE and J. 0. ARNOLD. NASA TR R-193 (1964). V. H. REIS. JQSRT4, 783 (1964). D. L. COMPWN and D. M. COOPER, NASA TN D-2871 (I 965). E. E. WIII~IN~;, A New Four-Channel Scanning Spectrometer for Balhstic-Range Radiometry, Proceedings of IEEE Second International Congress on Instrumentation in Aerospace Simulation Facilities. Stanford Univ., Aug. 29 3;. (1966) pp. 14-I-14-5. E. E. WHIIIN~;, J. 0. AKNOLD and G. C. LYLE, NASA TN D-5088 (1969). J. 0. ARNOI.D, E. E. WHlrlx and G. C. LYLE, JQSRT9, 775 (1969). G. E. KAATTAKI, NASA TN D-1423 (1962). L. WALLACE, Asrwphw. J. VI, Suppl. 62, 445 (1962). W. L. WIESE, M. W. SZU~H and B. M. GI.FUUOF;, National Standard Reference Data Series. ,Ytrr/. Bur. S/c&. 4, I (1966). K. H. WII.SOU and W. E. NI(.OLET. JQSRT7, 801 (1967). D. M. COOPER. AIAA J/4, 2125 (1966). International
2 I. 22. 23. 24. 25. 26. 27. 2X. 2Y. 30. 31. 32. 33. 34. 35. 36. 37.
Conference
on Ionization
Phenomena
in Gases.
Uppsala
(1960), pp. 2 I9 225
J. C. KWK, J. C. CAMM, B. KIVEL and T. WEYTINK. Ann. PIz>‘.s.7, I (1957).
R. A. AI.I.ES. J. C. CAMM and J. C. Ktc~. JQSRT I, 269 (1961). K. L. WRAY and T. J. CONNOLLY, JQSRT 5, 633 (1965). R. G. PEN%rT and F. W. DALBY, J. chm. Phys. 31, 434 (1959). M. JEUNEHOMME,J. chum. Phys. 44, 2672 (1966). L. V. WAI.I.ACE and R. W. NICHOLLS, J. Ammos. Twr. P/Iv.s. 23, 749 (1962). L. V. WALLACE.and R. W. NICHOLLS. J. Armo.s. Twr. Ph,vs. 7, 101 (1955). J. C. K~(.K. R. A. ALLEN and R. L. TAyt.oR. JQSRT3, 335 (1965). K. L. WKAV and T. L. CONNOLLY, JQSRT5, I I I (1965). W. H. WuasrEK, J. cl7em. Phys. 36, 21 I 1 ( 1962). M. JEU~EHO~IM~, J. chrn~. Phy.y. 45, 1805 (1966). R. G. TURNEK and R. W. NICHOLLS, Con. J. Pll>ls. 32, 468; 475 (1954). R. E. BERGGREN, D. L. CO%II>TO~~, T. N. CANNING and W. A. PAGE, Proceedings of the Seventh International Shock Tube Symposium, Toronto, Canada, June 23-25 (196Y) pp. 353 365. W. J. BOKUC‘KI, D. M. COOPEK and W. A. PAGE, Proceedings of the Seventh International Shock Tube Symposium, Toronto, Canada, June 23%25, (1969) pp. 708-720. J. 0. ARNOLD, JQSRTI, 1781 (1968). D. R. INGLIS and E. TELLER, Astrophys. J. 90, 439 (1939). E. B. KURPIANOVIA, V. N. K~LESN~KOVand N. N. SOB~LEV, JQSRT9, 1025 (1969).