Acoustic apparatus for study of acoustic wave pressure-disturbance propagation in plasm

Calvert, J. B.

English

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NsG-518 
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* DRI #2359 
FINAL REPORT 
IONIC SOUND WAVES IN PLASMAS 
A Resea rch  Project Supported by NASA Grant  
in  the Space -Related Sciences 
Project No.  DRI-615 
, GPO PRICE S 
\ CFSTI PRICE(S) S 
I 
I September 1966 
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(163 July 65 
UNIVERSITY OF DENVER 
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. 
FINAL REPORT 
IONIC SOUND WAVES IN PLASMAS 
A Research  Project  Supported by NASA Grant 
in  the Space-Related Sciences 
NsG- 5 18 
Project  No. DRI-615 
University of Denver 
September 19 66 
SUBMITTED BY: 
James B. Calver t  
Associate P ro fes so r  of Physics 
ii 
ABSTRACT 
/ A S S T  
Apparatus for  the study of collision-dominated p res su re  waves 
in  a plasma has  been designed. 
discharge in ul t rapure helium. 
diaphragm and detected by an electron-mobility type microphone. 
The plasma is a d i rec t -cur ren t  glow 
Acoustic pulses a r e  generated by a 
i 
iii ~ 
I. 
TABLE O F  CONTENTS 
P a g e  
INTRODUCTION . . . . . . . . . . . .  1 
11. EXPERIMENTAL APPARATUS. . . . . . . .  3 
1. Discharge Tube . . . . . . . . . . .  3 
2. Acoustic Apparatus . . . . . . . . . .  6 
111. DISCUSSION . . . . . . . . . . . . .  10 
IV. BIBLIOGRAPHY . . . . . . . . . . . .  11 
1 
I. INTRODUCTION 
The propagation of p re s su re  disturbances in an ionized medium 
is of considerable geophysical interest .  At altitudes between 50 km. 
and 100 k m . ,  corresponding to the ionospheric D-layer ,  the daytime 
atmosphere is a very weak plasma with a fraction ionized that may  
approach lo-' ' at the upper boundary. The p r e s s u r e  range in this 
region is f r o m  about 1 mm. Hg down to 
path increases  f rom c m  to 20 cm. 
upper altitude limit for  ordinary acoustic propagation. 
mm. Hg while the mean f r ee  
This region corresponds to the 
Between 100 km. and 150 k m . ,  which is the lower E-region of 
the ionosphere,  the fraction ionized r i s e s  to  about 
f r ee  path increases  to  about 45 m. 
can  propagate under these conditions without practically complete 
absorption. 
increase  as collisions between the molecules become l e s s  frequent. 
while the mean 
Only low-frequency acoustic waves 
At still g rea t e r  altitudes the fraction ionized continues to 
With respect  to pressure-dis turbance propagation, therefore ,  
the atmosphere presents  a continuous gradation of conditions f r o m  
those in which molecular collisions a r e  dominant and ordinary acoustic 
propagation predominates to those in which collisionless ion waves 
predominate. 
wave propagation a r e  important is naturally of grea t  interest .  
The transit ion region in which both collisions and ion- 
Collisionless ion-waves have been studied by Wong, Motley, 
and d'Angelo' in a ces ium plasma, and by Alexeff and Jones' in noble- 
gas plasmas.  The velocity V of these waves is given by the formula 
v =  
where k i s  Boltzmann's constant, Te is the electron tempera ture ,  mi 
is the ion mass, and y the compression coefficient. 
found that y N 1 under the conditions of their  experiments ,  which 
correspond to isothermal  waves. 
Alexeff and Jones 
F o r  collision-dominated waves,  the velocity is 
where,  in a monatomic g a s  y = 5/3 and T i  i s  the ion and neutral  t em-  
perature .  The ion waves and the normal  acoustic waves a r e  coupled by 
3 
Y 
ion-neutral  collisions. 
there  will be two strongly-interacting propagation modes. 
One expects that in the intermediate range 
We therefore were  interested in the development of apparatus to 
study the propagation of p ressure  pulses in the transit ion region be- 
tween the collisionless and coiiision-dominated processes ,  with 
par t icular  in te res t  in  the behavior of normal  acoustic waves. 
experimental  apparatus was designed to allow the propagation of 
acoustic pulses through the plasma medium and consists of a discharge 
tube with an acoustic source and receiver  mounted in s ide -a rms .  The 
discharge tube and acoustic tube were  built and tested separately.  
These tubes were  to be combined af ter  each operated successfully by 
itself. The project terminated for  lack of funds before this was done, 
but not before the feasibility of the design of the separate  components 
was established. 
The 
Proposals for  fur ther  support  have been made. 
The next section will discuss the apparatus a s  constructed. 
3 
11. EXPERIMENTAL APPARATUS 
The discharge tube and associated equipment will be described 
f i r s t ,  followed by a description of the acoustic apparatus.  
1 .  Discharge Tube 
The plasma medium is  provided by a direct  cur ren t  glow dis -  
charge in ul t ra-pure helium. 
descr ibed by Persson ,  
that  our cathode is a plain, flat stainless s tee l  plate. The fraction 
ionized is about lo- ' ,  and the electron temperature  is in the range 
1000-1800°C. The discharge tube i s  i l lustrated in Figure 1 .  Details 
of the vacuum sys t em a r e  given in Figure 2. 
This discharge is identical to that 
called by h im a "brush cathode plasma' '  except 
The helium is introduced through a home-built quartz leak. A 
section of thin wall quartz tube is wound with resis tance wire  and 
sealed into the system. 
surround the quartz tube with an atmosphere of commercial  helium at 
atmospheric p re s su re .  Upon heating, the quartz becomes permeable 
only to helium. 
An outside pyrex tube provides a means to 
The vacuum pump is a three-s tage glass diffusion pump (CVC 
Model GF-25) with polyphenyl e ther  ("Convalex- 10") working fluid. 
The pump is trapped only with a glass sorbent f i l ter .  
sys t em except the pump and the discharge tube itself a r e  contained in a 
bzkeout oven. 
approximately 12 hours of bakeout a t  450°C, which proved sufficient to 
insure  a suitable discharge. 
discharge tube for pa r t  of the bakeout t ime. 
All the vacuum 
This sys tem reaches a p res su re  below 10-*mm. af ter  
A portable bakeout hood is used over the 
The power supply is a Harr ison Labs Model 6522A, rated at  
0-2000 volts and 0-100 ma ,  connected direct ly  ac ross  the discharge 
tube. 
allows this type of connection. 
is a l so  current-l imiting and no difficulty is experienced. 
The discharge has  a positive current-voltage charac te r i s t ic  that 
Should an a r c  appear ,  the power supply 
When first built, the glassblower allowed a sha rp  point to r e -  
main when connecting the anode wire  to the wire  through the anode lead 
seal .  This limited the voltage that could be applied to the tube by 
causing an a r c  f rom cathode to envelope, along the surface of the 
envelope, and finally f r o m  the envelope to  the sha rp  point. This 
. 4 
5 
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U c 
0 c 
0 .- 
6 
difficulty is easi ly  cor rec ted  by eliminating the sharp  point and 
cleaning the envelope. 
In operation, the discharge is ve ry  stable.  Beyond the bluish 
cathode glow, which appears as a thin region paral le l  to  the cathode 
and about 1 cm.  in f ront  of it, the tube is filled with the character is t ic  
pink glow. With normal  illumination, the discharge f i r s t  appears  at a 
potential of about 650 volts when the p r e s s u r e  is about 0. 1 mm. 
(cur ren t  l e s s  than 5 ma).  
2. Acoustic Apparatus 
The acoustic apparatus that was constructed is shown in  Figure 
3 ,  and a block d iagram of its connections appears  in Figure 4. 
f o r m  of the apparatus was relatively unsat isfactory on two counts. 
First, the s tee l  diaphragm could be attached only with sof t  solder  be- 
cause the sh im s tee l  stock could not be heated above its annealing 
tempera ture  without wrinkling. 
proper  bakeout necessary  to insure purity in  the discharge.  Second, 
the receiver  was ve ry  sensitive to  noise and required considerable 
shielding to increase  the signal-to-noise ra t io  to  a n  acceptable level. 
P a r t  of this difficulty was due to  an insufficiently intense signal f r o m  
the diaphragm. 
This 
This property would in te r fe re  with the 
The microphone consists of a type 15E triode with the gr id  
removed. 
i ~ m z a t i o n  ?&entia] of the gas. The plate cur ren t  is controlled by the 
electron mobility in the gas between filament and anode, which, 
because it is pressure-dependent,  allows the detection of p r e s s u r e  
pulses.  
The resulting diode is operated at a plate voltage below the 
i .  
This device is described by Dayton, Verdeyen, and V i r ~ b i k . ~  
The operation of the acoustic apparatus to determine t ime delay 
between emission and detection of the pulse is straightforward. 
pulse generator  drives the diaphragm by means of an earphone e lec t ro-  
magnet, at the s a m e  t ime triggering the oscilloscope sweep. Variations 
in the plate cur ren t  of the microphone diode a r e  amplified and displayed 
on the oscilloscope. 
sweep of the oscilloscope. 
The 
The time delay is measured  by the calibrated 
The speeds measured cannot be interpreted a s  f ree-space  values 
unless  the r i se - t ime of the pulse i s  sho r t e r  than about 0.  01  m s e c ,  con- 
sidering the diameter  of the acoustic tube and the speed of sound (about 
920 m s e c  a t  22°C). The t ravel  t ime of the leading edge of the pulse in 
7 
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helium is about 0 . 2 5  msec .  
repetition rate of 1 kc/sec with a pulse of 0. 1 m s e c  duration. 
able values of the sound speed were  determined in argon and helium, to  
an  accuracy of about 10%. 
the beginning of the pulse. 
of the high attenuation. 
mm. of mercury .  
The measurements  were  made a t  a pulse 
Accept- 
The e r r o r  was due chiefly to noise masking 
Reflected pulses could not be seen because 
The pressures  used were  in the vicinity of 1 
10 
111. DISCUSSION 
The most  difficult problem in the development of this type of 
As we have seen ,  
To 
apparatus is the design of the acoustic apparatus.  
the diaphragm that was used was not only of unsatisfactory construction 
(soft solder  necessary)  but yielded a ra ther  low puise intensity. 
c o r r e c t  the constructional difficulty, it would be necessary  to turn the 
diaphragm on a lathe f rom a mater ia l  that could ei ther  be sealed into 
glass  o r  s i lver-soldered to Kovar. 
enough to  obtain a thin diaphragm. F r o m  pract ical  considerations,  it 
would s e e m  difficult to tu rn  a diaphragm a s  thin a s  the one in use  
(0. OOZ"), and therefore the driving force mus t  be increased.  
answer could be a non-magnetic diaphragm driven by a fe r r i te  core  
and electromagnet.  
Kovar itself does not machine well 
A possible 
Two methods a r e  available to avoid the diaphragm in the wall of 
the tube: a piezoelectric source o r  a ribbon-type speaker .  The 
peizoelectric source  requires  a high voltage for  operation and is hence 
unsuitable due to breakdown. 
choice, but mounting it in  the tube s o  that it will be compatible with the 
ultrahigh vacuum must  be considered. 
a l so  be baffled to eliminate back-radiation. 
s e e m  to have no advantages over the wall diaphragm. 
The ribbon-type speaker  is a possible 
A speaker  inside the tube must  
Speakers  inside the tube 
The microphone that was  used seems  to  be the mos t  sat isfactory 
available. 
diaphragm could couple well enough with the medium. One important 
factor could not be investigated: the effect on the signal of the 
electrodes drawing plasma cur ren t  when operated in conjunction with 
the discharge.  
by the changes in cur ren t  drawn f r o m  the plasma,  and a s imple probe 
would yield the s a m e  information. 
the microphone sufficiently t o  reduce greatly the plasma cur ren t  drawn 
(for example, by a glass-wool plug). 
At the low pressures  used in  the experiments ,  no mater ia l  
It is possible that the p r e s s u r e  effect would be masked 
There is a possibility of isolating 
Proposals  for fur ther  support  of this work have been issued.  
11 
e IV. BIBLIOGRAPHY 
1. A. Y. Wong, R. W.  Motley, a n d N .  d'Angelo, Phys. Rev. - 133,
A436 (1964). 
2. I. Alexeff and W. D. Jones, Phys. Rev. Letters, - 15, 286 (1965). 
3. K. B. Persson ,  "The Brush Cathode Plasma - A Well-Behaved 
Plasma, ' I  NBS Report  No. 8452, September,  1964. 
4. J. A. Dayton, J r . ,  J. T. Verdeyen, and P. F. Virobik, Rev. 
Sci. Inst. 34, 1451 (1963). -


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