Dissociators used to obtain an RF plasma discharge for hydrogen masers and the test system used for operation and evaluation of the dissociators are described. A compact sorption cartridge using a graphite matrix is tested as part of a hydrogen scavenging system. Testing of a vacuum enclosed hydrogen dissociator suitable for long term operation in space is described

Vessot, R. F. C.

English

NASA Technical Reports Server

- 
(NASA-C1.1-Iti184Y) STUCY UE T H E  DXSSOCIAT.LLJN N81-31537 
OF BOLECULAh HYE610GEN F lndP lieport, 2 Jan. 
1980 - JO Apr.  1981 (Smithsonian 
A s t r o p h y s i c a l  Observatory) 12 p Unclas 
HC A ~ ~ / H F  A O I  CSCL JOE ti3/J6 27385 
STUDY OF THE DISSOCIATION OF MOLECULAR HYDROGEN 
Contract NAS8-33521 
Final Report 
For the period 2 January 1980 to 30 April 1981 
Princi pa? Invest igator  
Dr. Robert F.C. Vessot 
July 1981 
Prepared f o r  
National heronauti cs and Space Admini stration 
Marshall Space F7 i g h t  Center, A1 abama 3581 2 
Srni thsoni an Institution 
Astrophysical Observatory 
Cambridge, Massachusetts 021 38 
The Smi thsonian Astrophysical Observatory 
and the Harvard Co1 Iege Observatory 
a r e  members of  the 
Center f o r  Astrophysics 
https://ntrs.nasa.gov/search.jsp?R=19810022994 2020-03-21T11:15:52+00:00Z
STUDY OF THE DISSOCIATION OF MOLECUL.4R HYDROGEN 
Contract NAS8-33521 
Final  Report 
For the period 2 January 1980 t o  30 April 1981 
Principal Investigator 
Dr. Robert F.C, Vessot 
Juty 1981 
Prepared f o r  
Nat iona l  Aeronautics and Space Admini stration 
Marshall Space F l  i g h t  Center, A1 abama 35812 
Smi thsonian I r \ s t i t u t i o n  
Ast rophys ica l  Observatory 
Cambridge, Massachusetts 021 38 
Center f o r  Astropl~ysics 
TABLE OF CONTENTS 
I n t r o d u c t i o n  
The Test  System 
Some Comments on Hydrogen Scavenging Systems 
Ongoing Tests  of Hydrogen Scavenging - July 1981 
Ongoing Tests o f  a Vacuum Enclosed Dissoc ia to r  - 
A p r i l  10 - July 1981 
PAGE 
-
1 
2 
4 
6 
6 
Plans  Fo r  t h e  Continuation o f  t h e  S t u d y  of 
Atomic llydrogen Sources  
Early in 1980 the Smithsonian Astrophysical Ob~eavatoxy~ 
with support from the National Aeronautics and Space 
Administration under Contract YAS8-33521, began a ;program t o  
study various types of r - f .  discharge plasma-type dissociator 
systems for hyd,ogen masers to be used i n  long-term applications 
i n  space. This report will describe the progress to date and 
outline d i r e c t i o n s  that the  program c o u l d  take i n  future 
investigations. 
The chief problem i n  praducing atomic hydrogen efficiently 
and reliably i.9 laboratory masers has been the deterioration of 
the wall surfaces i n  the  vessel  confining t h e  low pressure r - f .  
plasma discharge and the reliability of the r e f .  power transfer 
t o  t h e  ionized gas. The r - f .  d i s soc ia tors  in general use for 
hydrogen masers consist of a cylindrical pyrex glass tube about 3 
cm diameter with an inlet for ~nolecular hydrogen and an outlet 
leading to a collimator to produce a directed beam of atomic 
hydrogen. Hydrogen flow rates are small, generally no larger 
than 1015 molecules per second. The pressure required to sustain 
an r - f .  plasma discharge roughly depends on the dimensions of the 
vessel and the choice of frequency of t h e  I.£. excitation which 
is generally kept higher than- or at l e a s t  commensurate with, t h e  
reciprocal of the mean free time for an electron t o  traverse the 
plasma going from one wall to another  across  the tube- A 
Page 2 
t h e o r e t i c a l  study of t h e  processes i n  t h e  r - f .  hydrogen gas 
* 
d i s s o c i a t o r ,  recently made a t  J .P .L . ,  discusses t h e  r e l a t i o n s h i p  
of pressure-  frequency, and d i s s o c i a t o r  dimensions. 
It is c l e a r  t h a t  by minimizing t h e  rate  of  electron 
c o l l i s i o n s  a t  the  walls, t h e  walls can be spared erosion t h a t  can 
d e t e r i o r a t e  t h e i r  a b i l i t y  t o  prevent atomic recombination. I n  
addi t ion .  it is des i reab le  t o  reduce or e l imina te  contamination 
by elasomer s e a l s  which can cause t h e  wal l  t o  become c a t a l y t i c  
su r faces  and recombine atomic hydrogen. We can see severa l  areas 
where improvements can be made: (1) The na tu re  of t h e  s to rage  
vessel walls; ( 2 )  t h e  manner i n  which t h e  plasma i s  sus ta ined  by 
a n  external r - f .  field; ( 3 )  t h e  manner i n  which hea t  is removed 
from t h e  d i s s o c i a t o r ;  and ( 4 )  e l imina t ion  of possible sources of 
contamination i n  t h e  d i s s o c i a t o r  gas inlet connection. 
Our approach was to bu i ld  a two-station t e s t  system t o  
opera te  r - f .  d i s s o c i a t o r s  and evalua te  t h e i r  performance. One 
s t a t i o n  ( S t a t i o n  2 )  opera tes  a s  a test bed t o  study d i s s o c i a t o r s  
of d i f f e r e n t  ma te r i a l s  t o  observe the  surface erosion under 
adverse operating condit ions.  The other  s t a t i o n  (Station 21 
provides a workplace t o  test various r-f. conf igura t ions .  Fi5ure  
1 shows the t e s t  system a s  it is now configured and s-*cceasful ly  
operat ing.  
The two main ob jec t ives  we expect t o  achieve w i r . 5  t h e  new 
d i s s o c i a t o r s  a r e :  ( l l t o  opera te  t h e  d i s s o c i a t o r  totally within 
the vacuum envelope of t h e  maser and t h u s  e l imina te  a l l  forms of 
ex te rna l  air cooling, and (2 )  t o  e l iminate  elastomer 0 rings 
* L. Maleki 
Telecommunications and Data Acquisition P r o g r e s s  Report 4 2 - 5 9 ,  
July-August 1 9 8 0 ,  P.68 
Page 3 
and o ther  vacuum s e a l s  capable  of producing contaminants that can 
accumulate over long pe r iods  of opera t ion .  There i s  a l s o  a 
requirement f o r  t h e  d i s s o c i a t o r  t o  be s e l f - s t a r t i n g  and no t  
r e q u i r e  a d d i t i o n a l  adjustment  or m a n i ~ u l a t i o n  once t h e  r . f .  power 
is appl ied .  
The p r e s e n t  system has two s e p a r a t e  and completely 
independent hydrogen p r e s s u r e  c u n t r o l s .  S t a t i o ~ i  1 is c u r r e n t l y  
set up to o p e r a t e  a dissociator under convent ional  cond i t i ons  i n  
ambient a i r  w i t h  no forced  a i r  c i r c u l a t i o n ,  with a v a r i e t y  of 
excitation c i r c u i t s  to support an  r - f .  plasma d i schaxge-  It is 
ope ra t ing  a t  r e l a t i v e l y  c o n s t a n t  hydrogen flux and also se rves  a s  
a test bed f o r  va r ious  hydrogen scavenging methods. 
Cur ren t ly ,  t h e  most promising r . f .  c o n f i g u r a t i o n  i s  a low 
impedance 3-turn loop  B t r u c t u r e  that g e n e r a t e s  s u f f i c i e n t  r e f .  
e l e c t r i c  f i e l d  t o  s t a r t  t h e  plasma very r e l i a b l y  and ma in t a in s  
t h e  d i scharge  w i t h  very l i t l e  evidence of e ros ion .  An alternate 
conf igurak ion  of a c o i l  made i n  a p r i n t e d  c i r c u i t  con f igu ra t ion  
as a 3-turn s p i r a l  p r i n t e d  c i r c u i t  ha s  also proved t o  be 
e f f e c t i v e  and can be very r i g i d l y  supported.  The ch i e f  drawback 
of a lower impedance one-turn s t r u c t u r e  i s  t h e  d i f f i c u l t y  i n  
ob t a in ing  a h i g h  enough v o l t a g e  t o  s t a r t  t h e  d i s s o c i a t o r  by 
i n i t i a t i n g  i o n i z a t i o n ,  and a requirement f o r  somewhat more r . f .  
power than r equ i r ed  by t h e  e l e c t r i c  f i e l d  mode of e x c i t a t i o n .  We 
p lan  t o  imp w e  s t a r t i n g  with an i n i t i a t o r  i n  t h e  form of a 
minute amount of an alpha p a r t i c l e  emitter such a s  polonium or  
americium s e a l e d  i n s i d e  the tube. T h i s  technique has long been 
used i n  t h e  hydrogen plasma tubes of high power radar 
t r a n s n i t - r e c e i v e  switches .  
Page 4 
T e s t  s t a t i o n  2 (shown t o  the  r i g h t  i n  Figure 1) is used to 
test the performance of r , f .  dissociators t h a t  are  completely 
vacuum enclosed, This system is pumped by a turbo-molecular 
vacuum pump (TRP) and is  kept well below torr  during  t e s t s .  
It is equipped w i t h  r - f .  and d.c. feedthrough ports .  The 
glassware now i n  u s e  is made of Corning  7070 Low-loss glass and 
h a s  been running for 3 months. 
Both s t a t i o n s  can, by va lve s ,  be connec ted  t o  the TMP, 
through a manifold t h a t  contains a residual gas analyser  and a 
Varian Smart gauge. The former i s  u s e d  to determine the r e l a t i v e  
abundance of outgassing products; t h e  l a t t e r  i s  used  a s  a 
pressure gauge and with its nitrogen detecting featurer is used 
t o  leak check the system during  .perat ion.  
The first use  of sorpt ion  car tr idges  i n  hydrogen masers was made 
in 1974-1976 dur ing  the  development of t h e  spaceborne maser f o r  
the  SAO/NASA Gravitat ional  Redshif t  experiment. One of t h e  
dominant constraints i n  this program was weight;  it was c l e a r  
from t h e  outset that  khe conventional  large capac i ty  i o n  pump 
we igh ing  over 206 pounds and i ts  requirement for large magnetic 
s h i e l d s ,  could not be accommodated by the  Scout r o c k e t  system 
used  t o  f l y  t h e  experiment. Al ternat ive  hydrogen scavenging 
systems using chernisorption were t r i e d  and, after c o n s i d e r a b l e  
testing, a combination of sorpt ion  car tr idge  and low voltage ion 
pump was adopted. 
Page 5 
The s o r p t i o n  c a r t r i d g e  we used was made by SAES Getters 
Company and consisted of zirconium and aluminum i n  t h e  form of a 
powder s i n t c r e d  t o  the s u r f a c e  of a t h i n  s t r i p  of stainless steel 
pleated back and f o r t h ,  and conf ined i n  a c a n i s t e r  l i k e  an 
automobile o i l  f i l t e r .  Once a c t i v a t e d  by hea t ing  under vacuum at 
well over 700°c, t h i s  dev ice  has an e x t r a o r d i n s r y  a p p e t i t e  f a r  
hydrogen a t  room temperature  and t o  a l i m i t e d  e x t e n t ,  can cope 
with outgassed oxygen, n i t rogen .  CO. and COZ. However, 
hydrocarbons and nob le  gases were n o t  pumped and we found that by 
us ing  a very small. ion pump ope ra t ing  a t  low voltage (<2kV), t h e  
ion pump would ~ u c c e s s f u l l y  dissociate hydrocarbons and capture 
t h e  carbon and the noble gases without  being g l u t t e d  by hydrogen, 
which a t  low vo l t age  is no t  e f f e c t i v e l y  pumped by s p u t t e r i n g .  
Our t e s t s  showed that a s i n g l e  car t r idge about 3" diameter  
by 5"  long  would ope ra t e  the  maser fo r  one year when opera ted  
w i t h  a s i n g l e  0 . 1  l i ter /sec.  ion pump (about t h e  s i z e  of a 
walnut I .  
Since that time- SAES has  developed new cartridges us ing  a 
graphi te  mat r ix  with zirconium and o t h e r  metals .  These aye more 
compact and according to repor t s ,  have even more c a p a c i t y  fo r  
hydrogen than t h e  o lder  vers ions .  
The chief  ques t ion  remaining i s  t h e  a b i l i t y  of t h e  new 
c a r t r i d g e  t o  cope with non-hydrogenic spec ies  l i k e  N2, U2, CO and 
C O Z ,  normal ly  evolved by metal  vacuum systems that a r e  not 
bakeable  a t  temperatures  much beyond 150'~ 
Page 6 
ONGOING TESTS OF_ HYDROGEN SCAVENGING - J U L Y  1981  
Ile a r e  now o p e r a t i n g  S t a t i o n  1 of t h e  t e s t  s y s t e m  u s i n g  one 
o f  t h e  new sorption c a r t r j  dges and o b s e r v i n g  hydrogen scaveng ing  
c a p a b i l i t y  i n  the p r e s e n c e  o f  con taminan t s  n o r m a l l y  e x i s t i n g  i n  
t h e  masers. T h i s  t e s t  has been r u n n i n g  s i n c e  May 18 ,  1981 and 
w i l l  continue u n t i l  the hydrogen p a r t i a l  p r e s s u r e  r e a c h e s  
1 x 10.' t o r r .  We will t h e n  r e a c t i v a t e  t h e  c a r t r i d g e  and 
de te rmine  i t s  o u t g a s s i n g  p r o d u c t s  u s i n g  t h e  r e s i d u a l  gas ana lyser  
i n  t h e  system. The t e s t  w i l l  b e  r e p e a t e d  w i t h  two o r  more 
c a r t r i d g e s  t o  s ee  i f  they behave c o n s i s t e n t l y ,  
O N G O I N G  TESTS OF A VACUUM ENCLOSED DISSOCIATOR - APRIL 10 - 
J U L Y  1981 
A t e s t  i s  now i n  p r o g r e s s  t o  o b s e r v e  t h e  p o s s i b l e  
d e t e r i o r a t i o n  o f  a c o m p l e t e l y  vacuum e n c l o s e d  a tomic  hydrogen 
d i s s o c i a t o r  s u i t a b l e  f o r  use i n  l o n g - t e r m  o p e r a t i o n  i n  s p a c e .  
The thermal d e s i g n  o f  t h e  p r e s e n t  d i s s o c i a t o r  i s  based  on t h e  
u s e  of  a  low d i e l e c t r i c  l o s s  b o r o s i l i c a t e  g l a s s  (Corning 7 0 7 0 )  
w i t h  thermal  conduc t ion  t h r o u g h  t h e  t o p  and bot tom surfaces t o  
t h e  main mount ing f l a n g e .  F i g u r e  2 shows t h e  d e s i g n  a s  i t  would 
b e  adapted  f o r  use i n  an  a c t u a l  maser. The d i s s o c i a t o r  now 
under t e s t  i s  r e a s o n a b l y  c l o s e  t o  t h i s  d e s i g n .  
Our t e s t  p rocedure  f i r s t  i s  s imply  t o  r u n  t h i s  d i s s o c i a t o r  
w i t h  v a r i o u s  l e v e l s  o f  r . f .  e x c i t a t i o r .  and measure t h e  
t e m p e r a t u r e  r i s e  o f  t h e  g l a s s w a r e  t o  o b t a i n  power d i s s i p a t i o n  
i n f o r m a t i o n .  This  w i l l  b e  r e p e a t e d  u s i n g  t h e  c o n v e n t i o n a l  
Corning 7740 g la s sware  t o  s e e  i f  t h e r e  i s  any b e n e f i t  i n  t h e  
low l o s s  Corn ing  7070 g l a s s .  The n e x t  s t e p  i s  t o  a c c e l e r a t e  the 
d e t e r i o r a t i o n  o f  t h e  d i s s o c i a t o r  by o p e r a t i n g  i t  a t  a  
Page 7 
substant ia l ly  higher power level and at twice the normal fiource 
pressure and visually observe  any noticeable eros ion  as time 
progresses .  
To date,  after about 3 months of operat ion ,  the 7 0 7 0  
dissociator is operating flawlessly. The temperature rise vs. 
power input results are fairly well understood and we have 
dT/dP 5'c/watt. 
lz!xwxs 
The current  e f f o r t  will proceed a s  a life t e s t i n g  program. 
S t a t i o n  I will con t inue  to be used for  di s soc ia tor  l i f e  testing 
and Station 2 will be used for t h e  development of a cold hydrogen 
d i s soc ia tor ,  under NASA/MSFC Grant No. NAG-8012. 
Data are taken d a i l y  to monitor pressures, voltages. 
temperatures and e x c i t a t i o n  power levels. We will continue to do 
t h i s  as long as poss ib l e  in an effort to simulate some of t h e  
aspects of long-term operation in space. We are now optimistic 
that a source of atomic hydrogen can be developed that w i l l  
operate f o r  a t  l e a s t  5 years i n  the 'vacuum of space. 

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Study of the dissociation of molecular hydrogen

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