Radiation temperature measurements of nitrogen afterglow plasm

Holt, E. H.

Noon, J. H.

English

NASA Technical Reports Server

t 
. Plasma Research Laboratory 
Technical Report No. 66 - 4 
June, 1966 
RADIATION TEMPERATURE MEASUREMENTS 
OF THE NITROGEN AFTERGLOW PLASMA 
by J.H.Noon and E.H.Holt  
Sponsored by the National Aeronautics and Space Administrat ion 
under grant no. NsG-48. 
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a o  - -0 & .u 2 :  3 Rensselaer Polytechnic Institute 
1 2 ; :  
o m  In Troy, New York Lo 
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RADIATION ~~~ bIl3AS-S OF THE 
NITROGEN AFTERGLOW PLASMA* 
J. H. Noon and E. H. Holt 
Rensselaer Polytechnic Inst i tute ,  Troy, New York 
ABSTRACT 
Measurements a t  microwave frequencies of the radiation temperature of a 
nitrogen afterglow plasma show that  a t  early post-discharge times a secondary 
sowce of electron heating exists,  and tha t  microwave conductivity values 
calculated on the basis of a Maxwellian form for  the electron velocity d i s t r i -  
bution function are anomalous. The evidence p i n t s  t o  non-Maxwellian electron 
velocity distributions i n  the early afterglow. 
INTRODUCTION 
We have carried out measurements on a pulsed discharge i n  nitrogen, 
using microwave techniques, which indicate tha t  an equilibrium state is  not 
achieved early i n  the afterglow, that a secondary source of electron heating 
exists,  and %hat estim+,es of electron temperature from radiometer readings, 
based on an assumed Maxwellian electron velocity distribution, are  questionable. 
2 An X-band hollow-cathode waveguide cell,' gated microwave radiometer 
and sensit ive microwave bridge3 were used t o  neasure the microwave conductivity 
and the radiation temperature as a f'unction of t i m e  i n  a nitrogen afterglow 
plasma. 
3 t o  30 microseconds i n  duration and repeated a t  100 c/s rate,  and energy 
input varied from Figure 1 shows measured radiation 
temperature versus time i n  the afterglow for different  gas pressures and 
different discharge conditions. 
discharge period radiation temperature drops rapidly as expected from electron 
col l is ions with the neutral  molecules. 
rises again with t i m e ,  reaching a maximum and then fa l l ing  off slowly. 
The active discharge w a s  in i t ia ted  by a voltage pulse variable from 
t o  1O-I joules. 
In  the first f e w  microseconds of the post- 
However the radiation temperature 
36 
Preprint of an a r t i c l e  accepted for  publication i n  the Physical Review. 
4 -  
-2- 
7 A  
3 p e c  
c 
6 
- 1.2 
0.45 
.- no I 
W - I  ct! 
- 
Y 2 -  
P) Pressure 
0 
Y 
 
1 1 
100 200 
5 01 
I - 0  
6.4 Torr 
Figure 1. Radiation temperatures 
measured f o r  d i f fe ren t  gas pressures 
and d i f f e ren t  energy input conditions 
i n  the ac t ive  discharge. Pulse widths 
from 3 microseconds t o  20 microseconds 
and discharge currents from 0.5 t o  
17'. 5 amps were used. 
7.5A 
2 0 p e c  
17.2A : I' 5 usec 
I ' 
20 40 60 
P ob 
n 
a 4  
p! 
2.7 Torr 
2 
0 
0 20 40 60 
TIME (pSBC)  
ELECTRON COLLISION PROBABILITY 
Calculations by other workers4' of t he  probabi l i ty  f o r  momentum t ransfer  
pC 
based on microwave conductivity measurements, have not included d i r e c t  measure- 
ment of the lectron temperature i n  the  p lasm.  
transmission 
low energies. 
i n  the afterglow, both sets of calculat ions included the assumption of a 
Maxwellian d is t r ibu t ion  of e lectron ve loc i t i e s  whether or  not the  mean electron 
energy i s  greater than that of the  neutrals.  
of low energy electrons (4 l ev)  i n  nitrogen as a function of energy, 
Results obtained by waveguide 
and resonant cavi ty  techniques5 differ by a fac tor  of f i v e  a t  
Although the  measurements were car r ied  out a t  d i f f e ren t  times 
Our data indicate that calculat ion of Pc based on the assumption tha t  Only 50 microseconds a re  required fo r  the e lec t ron  
plasma t o  reach the temperature of he neu t r a l  gasf; is  questionable. This point 
temperature i n  nitrogen but did not carry out measurements a t  times close t o  
the  termination of the ac t ive  discharge. 
i n  the  nitrogen afterglow 
W a s  r a i sed  by Formato and GileJ1.dini 8 who observed a slow relaxat ion of e lectron 
+ -3- 
t 
b We have calculated P as a function of energy by equating the measured 
C 
radiation temperature t o  the electron temperature and have obtained the values 
of Pc shown i n  Figure 2. Although the resul ts  are generally close t o  those 
+ O  
O 
I 1 I 
0.2 0.4 0.6 0.8 
ELECTRON VELOCITY (F 
Figure 2. Electron collision probability for  momnt transfer (P,) 
versus electron velocit  . AG Anderson and Goldstein, 
)t 1.75 to r r  0 5  t o r r  0 5.6 t o r r  +6.4 torr .  
k? 9 Phelps, Fundingsland and Brown, s EPR Engelhardt, Phelps and R i s k  
derived from transport coefficients7 and resonant cavity measurements5 they 
show a wide scatter.  Furthermore investigation of the behavior of the con- 
ductivity r a t io  rr/wi, (Figure 3), reveals tha t  it is  a multi-valued f'unction 
of the radiation temperature instead of decreasing monotonically. A calcula- 
t ion  shows that electron-ion collisions cannot explain the effect  a t  these 
temperatures (approximately 2000%). The mst l ikely explanation is  that the 
radiation temperature cannot be equated t o  the electron temperature and that 
the form of the electron velocity distribution is  not Maxwellian i n  the early 
post-discharge period, as assumed in  the calculations. 
DISCUSSION C F  RADIATION TEMPERATURE RESUUPS 
The fact  that the energy input into the active discharge determines the 
amount of electron heating i n  the afterglow is  s h o 2  clearly i n  Figure 1. 
This may help t o  explain why Formato and G i l a r d i n i ,  using 10 microsecond 
Pulse-widths and currents of order 1 amp. observed a slow decay i n  the radia- 
t ion  t e m  rature (which they cal led the "electron temperature") w h i l s t  Mentzoni 
and Row,rusing w h a t  they refer t o  as a mildly driven discharge found much 
more rapid electron relaxation times i n  agreement w i t h  a calculation based 
on both the e l a s t i c  electron-molecule collisions and inelast ic  electron 
excitation of low-lying rotational levels of the molecule. 
0.20 
F 
,QT 
-
0.1 5 
0 
I- 
G 
ci 0.10 
> 
I- 
> 
!- 
0 
3 
0 
0 
- 
- - 
0.05 
-4- 
5 TORR 
x 
x 
x 
6 5  psec 
x 
% 
x 
0 
0 1 2 3 4 
RADIATION TEMPERATURE ( lo3  OK) 
Figure 3. 
( rr/ ci) plotted as a function of radiat ion temperature f o r  post-discharge 
times ranging from 30 t o  65 microseconds. 
but does not do so, indicating t h a t  the calculat ions based on an assumed 
Maxwellian d is t r ibu t ion  are incorrect.  
Ra t io  of real  and imaginary par t s  of the  microwave conductivity 
This should f a l l  off monotonically 
A s igni f icant  feature of Figure 1 is the r ise i n  rad ia t ion  temperature 
which occurs some microseconds after the  i n i t i a l  f a l l  following termination 
of the act ive discharge. 
Source of electron heating i n  the afterglow of the pulsed nitrogen discharge 
since departures from a Maxwellian electron ve loc i ty  d i s t r ibu t ion  could w e l l  
be the  explanation. 
electrons and other plasmas consti tuents i s  not c lear .  
It i s  not necessary t o  postulate a delayed secondary 
However the  mechanism of energy interchange between 
Electron-ion recombination, although removing l o w  energy electrons 
preferen t ia l ly  and thus increasing the average e lec t ron  energy, could not 
explain the observed ef fec t .  
w i t h  nitrogen afterglow have been suggested: (1) energy from molecular 
dissociat ion being fed back t o  the  electrons as neu t r a l  atoms recombine 
(2)  vibrationally excited molecular s t a t e s  coupling with electrons,lOili (3) 
energy given t o  electrons from col l i s ions  involving metastable s ta tes .  6~12 
Further experiments a re  i n  progress which are intended (a) t o  gain d i r ec t  
information about the ac tua l  form of the  electron ve loc i ty  d is t r ibu t ion  function 
Three possible sources of energetic electrons 
9 
I 
T -5- 
f ' 
i n  the afterglow, using a microwave technique,13 and (b) t o  study the effect  
,of a small proportion of a foreign gas, such as C02, on the radiation temper- 
ature of the nitrogen plasma and thus establish which of the suggested 
mechanisms i s  the source of energetic electrons i n  the nitrogen afterglow. 
i REFEIiENcEs 
1- E. H. Holt, K. C. Stotz, "Plasma Waveguide Cell for  Afterglow Measureents," 
Rev. Sci. Inst. 1285 (1963). 
2-  W .  C. Taft, K. C. Stotz, E. H. Holt, "A tiated aaaiometer f o r  Piasma 
Afterglow Studies," IEEE Trans. IM L2, 90 (1963). 
3. J. Ajello, 
Electrons i n  a Nitrogen Plasma Afterglow," NASA CR - 104 (1964); 
"Investigation of Plasma Afterglows with Application in Nitrogen," 
"The Probability of Collision for  Momentum Transfer of Slow 
K. C. Stotz, 
NASA. m D-2226 (1963). 
4. J. M. Anderson, L. Goldstein, 
Radio-frequency i n  Isothermal Plasmas,," Phys. Rev. 100, 1037 (1955). 
"Interaction of Electromagnetic Waves of 
I 5. A. V. Phelps, 0. T. Fundingsland, S. C. Brown, "Microwave Determination 
of the Probability of Collision of Slow Electrons i n  Gases," Phys. Rev. 
84, 559 (1957). 
6. Do Formto, A. Gilardini, Ionization Phenomena in Gases, "Microwave 
Determinations of Afterglow Temperatures and Electron Collision Frequencies 
i n  Nitrogen, 
Holland (1960). 
(Proc. of Fourth Internat. Conf.) Vol. 1, 99, North 
7. A. G o  Engelhardt, A. V. Phelps, C. G. Risk, "Determination of Momntum 
Transfer and Inelastic Collision Cross-Sections for Electrons i n  Nitrogen 
Using Transport Coefficients," Phys. Rev. 
M. H. Mentzoni, R. V. Row, 
i n  Nitrogen," Phys. Rev.= 2312 (1963). 
~ 1 5 6  (1964). 
8. "Rotat ional  &citation and Electron Relaxartion 
9. E- A. McLennan, L. Goldstein, 
and H Afterglow Plasmas," AFCRL 66-88 (1966); 
E. A. McIennan, 
Hydrogen Afterglow Plasmas," Bull. Am. Phys. SOC. 1l, 504 (1966). 
"Atomic Density Measurements i n  02, N , 
"Atomic-Density Measurements i n  Nitrogen, Oxygen, and 
2 
2 
10- I. R. Hurle, 
Molecular Vibration," J. Chem. Phys. 41, 3592 (1964). 
'' On the Thermal Energy Transfer between Free Electrons and 
11. A- V. Phelps, private communication. 
12. J. A. Van Lint, "Ionization Afterglow Measurements on Nitrogen," lEEE 
Trans. NS-U, 266 (1964). 
13. H. Fields, G. Bekefi, S. C. Brown, 
pbsmas," Phys. Rev. 129, 506 (1963). 
'%icrowave Emission from Non-Maxuellian 


Radiation temperature measurements of the nitrogen afterglow plasma

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