Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10~563~
This invention relates to a Process for the Fabri-
cation of Thermonuclear Fuel Pellets and the Product thereof.
Background Of The Invention
Much work is presently being done in connection
with the utilization of laser energy to create a condition
of thermonuclear fusion with a unique fuel configuration in
a very small dimension. This fuel configuration, which can
be referred to as a pellet, has dimensions which may vary all
the way from 1/16 millimeter (mm) in diameter to approximately
2mm or larger, but the preferred dimensions are 1/8 to 1 mm.
It is possible to make these pellets of thermonuclear fuel
such as deuterium-tritium utilizing cryogenic conditions.
At the present time, solidified hydrogen isotopes have been
used in fusion experiments in the form of slabs, cubes, or
droplets. These simple geometries have been adequate for
experimental tests, but it is desirable to provide a method
for manufacturing special fuel configurations in large quan-
tities for purposes of extensive use in commercial fusion
reactor operations.
The problem before the art is to provide fuel
pellets which can be accurately regulated in dimension and
thus more efficient in the ultimate fusion process and also
to provide fuel pellets with a special configuration which
lends itself to an efficient fusion implosion by the input
of laser energy.
-1- i~
~9563~P
Q~ects of the Invention
It is, therefore, an object of the present invention to
disclose a thermonuclear fuel pellet configuration which conforms
to the prescribed requirements of the nuclear physics involved,
and also to disclose a method for manufacturing these fuel pellets
which can be easily regulated and which permits manufacturing at
low cost with uniform results under circumstances which are
conducive to commercialization and production.
According to a first aspect of the invention there is
provided a method of making a thermonuclear fuel pellet which
comprises; (a) providing a hollow microsphere having a continuous
wall of glass having a permeability rate for hydrogen isotopes
which decreases with decreasing temperature and is low enough to
retain hydrogen isotopes within said hollow microsphere during
subsequent handling of the fuel pellet including handling under
conditions of temperature and external pressure of about room
temperature and one atmosphere, and (b) introducing through the
continuous wall and into the interior of said hollow microsphere a
quantity of a gaseous fuel having at least one isotope of hydrogen,
said quantity of gaseous fuel introduced into said hollow micros-
phere being insufficient to cause the tensile strength of said
continuous wall to be exceeded during subsequent handling of the
fuel pellet, thereby forming a thermonuclear fuel pellet
According to a second aspect of the invention there is
provided a method of making a thermonuclear fuel pellet for use
in a nuclear fusion reactor said method comprising; (a) providing
a hollow microsphere having a continuous wall of a material which
consists essentially of at least one of the group of glass, ceramic,
and plastic material and which is suitable for containing
fuel in a said reactor, said material having a permeability rate
~or hydrogen isotopes which decreases with decreasing temperature
and is low enough to retain hydrogen isotopes within said hollow
microsphere during subsequent handling of the fuel pellet including
2 -
~5~3~g
enabling handling under normal conditions of temperature and
external pressure of about room temperature and one atmosphere, and
(b) introducing into the interior of said hollow microsphere by
diffusing through the continuous wall thereof under pressure and
at an elevated temperature, a quantity of a gaseous fuel having
at least one isotope of hydrogen, said quantity oE gaseous fuel
introduced into said hollow microsphere being insufficient to
cause the tensile strength of said continuous wall to be exceeded
during subsequent handling of the fuel pellet under said normal
conditions, thereby forming a thermonuclear fuel pellet.
The invention will now be described in more detail, by
way of example only, with reference to the accompanying drawings,
in which:-
FIGURE 1 is a diagrammatic view of a thermonuclear fuelpellet.
FIGURE 2 is a diagrammatic view showing a fuel pellet with
a solid interior spherical lining of thermonuclear fuel.
FIGURE 3 is a flow chart showing the various steps in
the process.
FIGURE4 isa view of a pressure vessel heater used in
the process.
!~t - 2a -
1~:)9S~3~
Detailed Disclosure of the Invention
. . _ . . .
Briefly, the invention contemplates the use of
small, hollow shells of a material such as glass having the
dimensions desired for the final product. These shells are
exposed to the hydrogen isotopes in the form of gas such as
deuterium-tritium under conditions of pressure and heat such
that the gas will permeate the walls of the hollow shells or
microspheres and be entrapped within. The pressurized mi-
crospheres may then be used either with the fuel in gaseous
form or the gas may be deposited on the inner walls of the
sphere by subjecting the sphere to a cryogenic temperature.
The hollow shells to be used for the fuel con-
figuration and the process will be referred to as micro-
spheres and may be formed of glass, ceramic, carbon, or
plastic as a basic initial structure. These hollow spheres
are available commercially and are sometimes identified by
the trademarks MICROBALLOON, ECCOSPHERES, and CARBO-SPHERES.
They can be obt~ined in diameters from 10 micrometers to
1000 to 2000 micrometers. These spheres are presently
used commercially in syntactic foams, low density structure
materials, dielectrics, and thermal insulation. Pertinent
U.S. patents describing the processes and the product of
glass and other microspheres are listed in the following
Table I.
1~3563~
TABLE I
Patent No. Issued Inventors Title
2,797,201 June 25, 1957 Veatch et Process of Pro-
al ducing Hollow
Particles and
- Resulting Product
3,138,444 June 23, 1964 Searight et Method and Apparatus
al For Manufacturing
Glass Beads
3,161,468 Dec. 15, 1964 Walsh Process for Pro-
ducing Hollow
Spheres of Silica
3,365,315 Jan. 23, 1968 Beck et al Glass Bubbles Pre-
pared by Reheating
Solid Glass Par-
ticles
3,441,396 Apr. 29, 1969 D'Eustachio Process For Making
et al Cellular Materials
3,615,792 Oct. 26, 1971 Morehouse Expansible Thermo-
et al plastic Polymer
Particles
The invention will be described in connection
with glass microspheres although it will be appreciated
that other materials might be utilized.
Basically, the microspheres of glass (which is
the principal material proposed) are formed by pulverizing
a glass containing volatile compounds and injecting the pul-
verized glass into a gas stream which passes through a hot
zone such as that formed by a torch. While in the hot
zone, the volatile compounds are vaporized and expand and
this, when coupled to the inherent surface tension of glass,~
causes the molten glass to take the form of a hollow sphere.
--4--
~,.
1~5~3~
The process is controlled by the selection of the glass com-
position, the hot zone temperature, the velocity at which
the gases are passed through the hot zone, and the cooling
provided thereafter. The commercially available micro-
spheres have been designed to give a relatively low density
and therefore have had walls in the neighborhood of 1 to 2
microns thick. The processes may be used to produce micro-
spheres with much thicker walls if this is desired. The
following Table II gives example properties of commercially
available glass microspheres produced by Emerson and Cuming,
Inc.
TABLE II
PROPERTIES OF COMMERCIALLY AVAILABLE GLASS MICRO~ALLOONS
G R A D E
Property IG 101 SI FTL200
Sodium Silica >95% SiO2
CompositionBorosilicate
Particle Size Range,
Microns (% By Weight)
~ 175 5 O 12
149-175 10 14 18
125-149 12 10 112
100-125 12 12 ~13
62-100 44 40 52
44-62 10 15 11
~ 44 7 9 2
Average Particle
Diameter Microns 80 80 90
Average Wall Thick-
ness Microns 2 1.5 ¦ 1.5
Softening Tempera-
ture or Melting
Temperature ( F) 900 1800 2000
Compressive Strength
Volume % Survivors
at Pressure (psi)
500 96.3 -
1000 66.4
1500 46.2
--5--
" .. ~
,.~ - - -
3'~
It will be seen from the above that it is within
the state of the art to independtly regulate not only the
size of the microsphere but the wall thickness and, of course,
the chemical composition of the glass. There are two thermo-
nuclear pellet designs which are of immediate interest with
respect to the present disclosure. These are shown in FIG-
URES 1 and 2 respectively.
In FIGURE l, there is-shown the structural shell
20 in the form of a cross-section of a sphere and this con-
tains a gaseous thermonuclear fuel 22 which, of course, is
invisible in the drawing. The hollow solid shell may be
made of glass as indicated above or ceramic, plastic,
or carbon and it will contain a gaseous mixture of hydrogen
isotopes. such as deuterium or a deuterirm-tritium mix.
In FIGURE 2, the structural shell 20 is shown but
in this case, it has layered inside of it a solid coating of
the thermonuclear fuel 24 leaving a vacuum centrally of the
sphere. In the forming of the above pellet designs, there
are certain steps in the process which are illustrated in
FIGURE 3 showing a flow chart.
Step #l in the process involves the procurement
and selection of the microspheres. As has been indicated
above, these microspheres are available commercially and
the selection involves a consideration of the chemical
-6-
~9563~3
composition, the size, the wall thickness, the wall
strength and the residual gas content. The preferred
structural shell material is glass because of its in-
herent strength, its availability in desired sizes at
nominal costs in large quantities, and its relative
uniform sphericity and wall thickness.
The selection of a specific glass composition is
based upon its permeability to hydrogen isotopes (here -
after including all isotopes of hydrogen individually or
in mixtures and in the ortho or para spin states) and its
softening temperature. High silica contents favor high
permeabilities and high softening temperature. The dilu-
tion of silica with other glass formers tB203~ ~a20, K20,Li20, A1203, PbO and others) lowers both the permeability
and the softening temperature.
Any desired glass microsphere size can be sorted
out by well-known techniques, wet or dry sieving, cyclone
separation, hydraulic elutriation, microscopic hand sort-
ing or micro-radiography to mention a few. The sphericity
of the microspheres can be evaluated by micro-radiographic
techniques.
The wall thickness dimensions and uniformity can
be evaluated by quantitative, micro-radiography and by em-
ploying mass separation techniques (such as gas suspension
or carefully sized microspheres).
.
:, - -
~, ' '- . .
3~
Compressive and tensile wall strengths can be
determined by first applying a hydraulic compressive
pressure. Separation is performed by introducing the
microspheres into a fluid bath. The sealed microspheres
float and the broken parts or leaky microspheres sink and
thereby separate. Then tensile load may be applied to the
microsphere walls by permeation filling the microspheres
by the procedure described later to a pressure higher than
desired ultimately. Survivors can be recovered by the sink-
float technique mentioned above.
There are residual gases within the selected
microspheres which are derived from the blowing agents
used in the manufacture. These may be C02, H20, or S02
or mixtures thereof. In some instancesO it may be de-
sirable to evacuate these gases by a permeation process,
but the present process contemplates a selective lamina-
tion of these gases to reduce the effect on the fusion
process.
The first step of the process also involves a
proper cleaning of the microspheres to remove the particu-
late matter which may be present on the exterior surface.
A typical cleaning process utilizes hot trichloroethylene
for the removal of organic material and water detergent
--8--
10~5~3~
solution wash in ultrasonic agitated bath for the removal
of salts and other water soluble material. These steps may
be ~ollowed by a rinse in pure water, an etch in l~/o solu-
tion of hydrofluoric acid to strengthen the microsphere, a
further rinse in pure water to remove the acid residue, and
a drying step in methanol.
The second basic step in the process is permeation
filling of the microspheres. This involves loading the se-
lected microspheres into a commercially available pressure
vessel as illustrated in FIGURE 4 and pressurizing the vessel
with deuterirm-tritium gas while heating the pressure vessel
to temperatures ranging from 150C to 800C, as limited by
the sintering temperature of the specific glass used to
prevent adherence of the particles to each other. The
pressure vessel in FIGURE 4 has a suitable interior chamber
surrounded by heater coils connected to a power supply.
Suitable temperature indicators and thermocouples are pro-
vided. A lock-on cover has a gas pressure inlet and a
pressure gauge. The gas filling pressure is predetermined
to provide the desired quantity of deuterium-tritium within
the sphere. First of all, it must be appreciated that the
permeation rates of hydrogen or hydrogen isotopes such as
deuterium-tritium and similar materials through glass is
known, and data is available in a book entitled Vacuu
Techniq~ by Saul Dushman, published by John Wiley & Sons,
Inc., ~ew York, 1949. The following Table III indicates
diffusion data taken from this publication.
1`~
10~5~;3~3
TABLE III
DIFFUSION DATA COMPII~TION
__ _ _.
System Permeability Data
S 2 H2 log10 ~2 . 08 x 10 K = 1. 735 - T
SiO2-H2 at 700C K = 2.1 x 10
SiO2-D2 at 700C K = 1.7 x 10 9
Si02-H2 at 900C K = 6.4 x 10
Pyrex-H2 at 520C K = 2 . O x 10
K = permeability (cc of gas (STP) per sec per cm
area per mm thickness per cmm of Hg pressure head)
T = temperature ( K)
* N. W. Taylor and W. Rast, "The Diffusion of Helium and
of Hydrogen Through Chemically Resistant Glass,ll
Journal of Chemical Physics, 6, Octo~er 1938, p. 619
. . _
From the above equation in Table III, it has been
determined that with respect to H2, silica has a permeability
K factor of 3 x 10 7 at 750C while sodium borosilicate has
a K factor of 2 x 10 11 at 520C. Silica has a K
factor of 2 x 10-11 at 25C.
--10--
10~563~
Using the published information, it will be
!3ee~ that hydrogen permeation through silica (Sio2) shows
permeability at 520 C. to be 1.1 x 10 . The permeability
~Eor Pyrex (typically Corning-code 7740) at the same temp-
erature is 1.97 x 10 . Integrating the we~l-known Fick's
law equation for diffusion under the proper conditions for
filling the hollow spheres, the following equation was de-
erived:
to 99 = 5.50 xr (Equation 1)
where
to 99 = time for the inside hydrogen pressure
to reach 99% of the external hydrogen
pressure (sec.)
x = thickness of glass wall (mm)
r = radius of small hollow sphere (cm)
T = temperature of filling (K)
K = permeability (cc of gas (STP) per sec.
~ per cm area per mm thick~ess per cm
of Hg pressure head
Using Equation (1) and the gas permeabilities
given above, the specific time required to raise the in-
ternal hydrogen pressure to 9g~0 of the external pressure
is between 50 and 2600 sec. for a temperature of 520C.
~95~3~
The external hydrogen pressure and, therefore, the inter-
nal hydrogen pressure attained are not limited by the per-
meation requirements. The strength of the glass wall limits
the pressure gradient across the glass wall during filling
and later during storage where the external pressure is re-
duced to one atmosphere.
The ultimate pressure contained in a hollow glass
microsphere is limited by the bursting strength of the
shell. Using the familiar hoop stress formula (Equation 1),
the pressure across the wall (~ P) can be related to the
tensile strength (T.S.) of the microsphere wall.
(T.S.)t
~ P = r (Equation 2)
The tensile strength of glasses has received a
great deal of attention in the literature~ It has been es-
tablished that surface imperfections greatly affect the
tensile strength. Normally, a strength of 1000 psi is used
for structural design. However, it is also known that
glass fibers having 1 - 2 microns diameter exhibit tensile
strengths between 700,000 and 2,000,000 psi. From all pub-
lished data, it is very likely that the strength of the
glass used for glass microspheres has high strength.
Using Equation 2, it has been calculated that internal
-12-
~-
1~5G;3S~
pressures of 15,000 psi or roughly 1000 atmospheres could
be contained in glass microspheres. Current requirements
for thermonuclear fuel pellets indicate 100 atmospheres is
adequate.
The pressure and temperature of the deuterium-
tritium gas surrounding the microspheres is maintained
for a period sufficient to cause the pressure inside the
microsphere to rise to 99/O of that of the external pressure.
This period has been precisely established by a series of
experiments in which the particular microspheres are treated
at various temperatures for various periods. After these
tests, and to confirm the hydrogen isotope pressure inside
the diffusion filling, the microspheres are broken and the
quantities of released gases are measured by (1) mass spec-
trometer gas analysis, and (2) actual size of the gas bubble
in a fluid. As one example, a batch of microspheres was
treated at 300C under 100 atmospheres of pressure for a
period of 36 hours to obtain an internal pressure of approxi-
mately 99 atmospheres and a contained gas quantity of 10-2
grams per cubic centimeter.
After holding the temperature and pressure for
the desired time period, the temperature of pressure vessel
is then lowered to room termperature and finally the gas
;
-13-
~c
~9563'~
pressure external to the microsphere is relieved. Inas-
much as the permeation rate of the gas at room tempera-
tures is typically 104 times less than that at elevated
temperatures, the contained gases will remain trapped
In the microspheres during subsequent handling operations.
Using the SiO2-H2 equation (Table III) for
permeability, the permeability diminishes to 1.41 x 10 13
at room temperature and that for Pyrex at least an order
of magnitude more lower. The leakage rate can be
predicted by the same formulae (Equation 1) developed for
the filling prediction. Using Equation 1, the time
required to reduce the internal hydrogen pressure to 1%
of the original in a quartz microsphere is over 100 hours,
and in a Pyrex microsphere over 1000 hours. Even further
reductions in differential gas pressure and permeability
can be realized by storing the spheres at lower tempera-
tures (dry ice or liquid nitrogen temperatures).
After the pressure filling step, it may be
desirable to separate microspheres which have been broken
in the process or by mechanical handling from those that
- are intact after the filling operation. This can again
be easily accomplished by utilizing a flotation process.
The completion of Step #2, that is, the filling
with the hydrogen isotope, will provide a thermonuclear
fuel pellet which may be utilized under some circumstances
in a fusion process. An optional third step in the process
involves applying a coating on the exterior of the
-14-
. i
filled microspheres. There may be a variety of reasons for
providing this coa~ing. In the first place, it may serve to
provide a better seal for the microsphere as a diffusion barrier
to improve storage. For example, a lead or bismuth glass, a
soda-lime glass, copper, or aluminum coating may serve this
function. Secondly, a coating may be applied to improve the surface
properties with respect to increasing the laser light absorption.
Thirdly, a coatlng may thicken the structural wall to improve
the pellet implosion properties; and, fourthly, a coating may
serve as an energy channel to provide a more uniform fuel pellet
illumination. These layers may be applied by vacuum vapor
deposition, ion sputtering, chemical vapor deposition, electrolytic
or electroless plating, or physical vapor deposition.
An optional though usually preferred fourth step in
the processing of the fuel pellets involves subjecting the pellets
to a cryogenic temperature. There are two purposes for this
procedure. First, any impurity gases within the pellet will be
selectively frozen out on to the inside surface of the structural
shell by selecting a temperature intermediate between the
freeze-out temperature of the impurity gas and the freeze-out
temperature of the hydrogen isotopes. For example, at liquid
nitrogen temperatures CO2 and H2O will freeze out leaving a
purified hydrogen isotope gas in the pellet core. Under some
circumstances this alone will provide a pellet which may be
utilized in fusion operations if suitable laser input power is
awailable.
- 15 -
563~
The second reason for the use of the cryogenic
temperatures would be to freeze out the hydrogen isotope
gas as a solid layer on the inside of the microsphere.
There are basic and important reasons from the point of
view of the fusion process for having the nuclear fuel in
this particular configuration.
Freeæing of the gases within the microspheres may
be accomplished in a number of different ways. It is known
that liquid hydrogen, deuterium, and tritium will wet a glass
surface which insures that condensation on the glass will
form a continuous uniform layer as the temperature decreases.
The rate of temperature decrease is also important to insure
an even layer of fuel. The freezing rate can either be such
that any liquid deposit cannot conglomerate unevenly in the
sphere, or it is also possible to hold the microspheLes in
suspension while tum~ling them during the cooling operation,
thus providing uniform cooling on all sides so as to achieve .-
even distribution of the freezing gas within. There is con-
siderable information in the literature relative to the
freezing of hydrogen isotopes. The following Table IV
summarizes the critical points regarding the liquification
and solidification of hydrogen isotopes and their mixtures.
5S3~
___ ~
U~ . _
W S ,i o ~ .
o ~. ~ ~ ~o ~ 0o ~:r
~ ~ ~ ,~ N N
O U~ ~ . . . , , .
U~ O ~O O OO O O O
H 1~ . .
._ ~
~ ~ I` ~ r` ~ ~ o
O ~ O~1 ~1 ~I
C~ .,~ ~.
P O ~ O O O O
.~ ~ . . .
.
s~ o cr~ U
.,~ ~ . .. . .. . -
S~ ~ ~D ~ a) ~ 'I
O ~ E~ y
0
~ ~ ~ ~ o -~ r~
~ ~ P ~ a~ , .
~ E :C ~ ~ c~ o
a) ~
n S
X -- Q~ o a~ 1 I
a) ~ . . .. . . ..
t!~ --1 h ~ lO ~D O a:~ r` ~ I .
a~o t~ ~D ~~D Ir) ~0U~' Ul 11
~1 ~ E
Q
~ ~ t) U~
H ~1_, ~ G~
a ta u~ . . .
W :~:> ~ D ~ O l
~ ~ D
s u~ l
U.? ~J ~J R~ d'r~ ~I G?
H ~ 2 ~ J , . . ~ . .. .
~; ~ -- ~ ~, u~ a) o
E~ ~? u ~ ~ ~ ~ 1
U~ C' ~: O
O ~1 ~ E
E~~ ) ~; ~ O O
~ ~1 _~ r~ ~
E~ ~ U ~~ ~ 0
Z ~ U ~ ~ t-~ ~ ,~
H C) ~1 ~ ~. .
O ~) :J X ~ -~
u 5 ~
~ C) .
H P~ S
U~~? r~
. . . . . .. .
O ~ ~ ~D ? ~ a? ~ O ~-
,_ ~ ~") ~ d~') ~) ~ ~
æ uq X-
o
o
` I,q ~ ~~ ~
rl E~ ~ t~ l N
C!l .. . .
H tl:l X -r~?Ltl a? OD
O ~ ' ~?~'? ~ ?
Z J~ ~
H ~ ~ d' ~ ~D ~ ~) G~
~ ,1 ~ . . . . .
H O ~ o ~ ~ r? ~ d~
O ~ ^ ~ ~ N ~ N t~
m ~o~
~ rl R C~~1?1-- N
rS ~ E~ IJ ~ ~ ul 6
t~ .,~ ~, .
H O :~ O ~ ~ ~
~ ~ C) ~
I
...
-
--17--
S3a~
This table is taken from an article by A. S. Fried-
man, D. White, and H. L. Johnson entitled ~Critical Constants,
Boiling Points, Triple Point Constants and Vapor Pressure on
Fixed Isotopic Hydrogen Molecules Based on Simple Mass Rela-
tionship, Journal of Chemical Physicsl, Vol. 19, No. 1,
January 1951. It will be seen from this table that while
the various isotopes of hydrogen vary in detail from one
another, in general, their critical constants and triple
point are about the same and, therefore, the discussion of
one isotope would approximate the other isotopes.
Additional information on the characteristics of
deuterium is shown in an article by R. Prydz, K. D. Timmerhaus,
and R. B. Stewart entitled "The Thermal Dynamic Properties of
Deuterium,~l published in CrYoqenic Engineering, Vol. 13 (1968),
pages 384 to 396. This article includes temperature-enthalpy
diagrams. These diagrams are used to predict the behavior of
the deuterium during cooling. They indicate that cooling under
equilibrium conditions and starting with D2 gas pressures be-
tween 10-100 atmospheres in a sealed microsphere, the gas will
condense to liquid deuterium, then to solid ice. Standard
calculations on condensing liquids indicate that it would take
greater than .3 of a second for appreciable flow to occur.
Best results are obtained by freezing the hydrogen isotope
to solid ice in a fraction of a second to prevent liquid
flow within the microsphere. This is accomplished by pla-
cing the microballoon in a liquid helium pool so that the
cooling is the result of thermal conductivity through the
-18-
~3~ljf~3~
silica. Standard heat transEer calculations indicate that
it takes 8 x 10 5 seconds to go t'nrough the freezing process.
This freezing time is very favorable from the point of view
of retaining a uniform liquid layer on the inside of the
microsphere during the cooling period. It is highly probable
that intermediate cooling mechanisms providing a slightly
slower freezing time might be quite adequate to retain the
liquid hydrogen in place while solidification is completed~
The presence of tritium gas in the microsphere will con-
tribute an insignificant amount of energy by radioisotopic
heating.
Filling of microballoons with H2, D2, and T2
by permeation has been accomplished under the above-
suggested conditions of heat and pressure. Cleaned and
sized microballoons were placed in a pressure vessel
illucitrated in FIGURE 4 filled with hydrogen at a pressure
of 100 atmospheres (1500 psi), heated to 300C, and held
for 96 hours. The pressure at temperature was 2800 psi.
After pressurization, one of the techniques for measuring
the pressure in the surviving individual microspheres was
by breaking a filled microsphere, while submerged in
mineral oil, and observing the size of the resultant gas f
bubble with a microscope. The results indicated 90 at-
mospheres of included gas depending on the wall thickness
of the microsphere as compared with 1/3 atmosphere prior
to pressurization. After 18 months at ambient temperatures,
there was no appreciable change in the determined internal
pressure.
--19--
- 10~563~
It will thus be seen that the disclosed process
may be utilized to produce enclosed minute and predeter-
mined quantities of fusion fuel in gaseous form, and also
the same fuel may be provided in the form of a hollow
spherical shell of solid material within the glass micro-
sphere held at cryogenic temperatures.
With respect to details of selection, we recom-
mend particularly the use of sodium borosilicate as a mate-
rlal for the glass spheres with a diameter which is matched
to that of the laser beam. This is preferably within 50 to
500 micrometers with a wall thickness of .6 to 3 microme-
ters. The variations in wall uniformity should be less
than 10% of the wall thickness.
The pressure in the spheres should range between
10 to 100 atmospheres. A permeation temperature of 300C.
for a period of 96 hours is preferred.
We can point out that the reason for the wall
thickness above delineated lies in target survival. In an
effective laser system, a pre-pulse precedes the main laser
pulse. The target must be so constructed that it survives
this pre-pulse so it retains the proper configuration and
fuel for the main laser pulse.
-20-
