Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
i~'I`F.~SE TE~ IE~T `1}~GNETlC ~'TEI.,D
G `r~ERA'I'IO~1 BY ri~ SEP~-PI ~.C'~.A
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The p.r:esent in~ention rela~'es ~en!-rally to gene.-r~tion
of intense transienL mayne~ic fields and n-,ore particularly
.o the generation of an intense transient m2anetic field
through a laser-plasma method and app3r~tus.
One field in wh.ich such a generation of an intense
transient Inagnetic fiela ~o~ld be rc)st us~ l is t.he field
of laser-initiated thermonuclear rusion power generation.
One techniq~e proposed to generate electrical po~er from a
controlled thermonuclear event involves injecting a small
deuterium-tritium pellet into a vacuum chamber and trig-
gerilly a fusion reaction by ~Idriving~ t:he deuterium and
trit;~m ions toyether with the energy supplied by las,er
beams to produce a heliuM ion and a neutron. Since the
heliurn ion and neutron have slightly less mass than the
deuterium and tritium ions a small amount of mass is con-
verted into energy in accordance with the fanl.ous Einstein
eq~ation E = mc2 where E equals the energy ~rod-~ced, m
equals the nnass converted, and c equals the speed of
light.
Absorption of the laser pulse q~1ickly hea.s the outer
region of .he pellet co form an ioni.~ed ~35 or plasma that
expands outward (blows off) rapidly. rrhe recorl impulse
from the very rapid bl.owing off of the o~ter pellet layer
compresses the pellet core in the same way .hat the
impulse from a roc~et's exhaust p~shes the rocket forward,
or a rifle shot recoil pushes the rifle ag~ir.st one's
shoulaer.
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Theory predicts that the center of the pellet core
will be compressed to superdensities; one to ten thousand
times the normal solid densi~y, or about ten times the
density of the center of the sun (about one hundred times
as dense as lead~. Such pellet core densities are impor-
tant beca~se they greatly increase the li~elihoOd that
energetic deuterium and tritium ions will collide with one
another. Also~ they enable still unfused d~uterium and
tritium ions to recapture, or share, some of the energy of
~ fusion product helium particle before the high-velocity
helium particle can escape ~he core region. This is anal-
ogous to one fast moving billard ball striking and giving
energy of motion to others. This energy sharing with
unburned fuel gives rise to so-called ~'bootstrap" heating
that further increases the reaction rate. Achievement of
core compression is crucial to the laser fusion process.
In practice there are forces which oppose or hinder
core compression. One problem just recently being recog-
nized and studied is the effect of energetic or hot elec-
trons generated in the pellet surface area plasma. Thehot electrons produced move rapidly around and penetrate
the pellet core causing heating thereof which makes core
compression more difficult and thus for a given laser
energy less compression results and final energy gain is
~5 reduced. The problems caused by hot electrons and the
need to control same are discussed by ~. B. Spielman
et al., Physical Review Letters, Vol. ~fi, No. 13, p. 821
(30 March 1981). The measurement of a large return cur-
rent in a laser-produced positively charged plasma is
detailed by R. F~ Benjamin et al., Physical Review
Letters, Vol. 42~ No. 14, p. 890 (2 April 1979).
It is therefore an object of the present invention to
control the hot electrons of a laser produced plasma to
reduce pellet core penetration thereby.
!
It is another object of the present invention to con-
trol the hot electrons OI a laser produced plasma by the
generation of an intense transient magnetic field.
It is still another object of ~he present invention to
generate an intense transient magnetic field useful in
controlling the hot electrons of a laser produced plasma
and further useful in other areas such as in experimental
research concerned with the response of biological cells,
organisms and other items to magnetic stimuli.
In accordance with the present invention a first laser
produced plasma is initiated purposely to generate hot
electrons and a positively charged plasma which is pro-
vided a low impedance path to ground to create an intense
but very transient magnetic field via the high return cur-
rent mechanism of the positively charged plasma. In one
embodiment of the invention an object such as a biological
cell or the like is confined within or near the provided
low impedance path and is therefore subjected to the
intense transient magnetic field. In another embodiment
of the invention a fusion pellet is confined within or
near the provided low impedance path and is laser irradi-
ated shortly after the first laser produced plasma is ini-
tiated and during the period of the intense magnetic field
whereby the hot electrons of the fusion pellet plasma are
confined to extremely tight orbits and are controlled and
hindered from penetrating the core region of the fusion
pellet.
~ n advantage of the present invention is that hot
electrons of laser produced plasma are controlled by an
earlier laser produced event thus the laser equipment
needed and Eunctioning to produce the usion plasma can
also be employed to produce the earlier hot electron con-
trolling event.
Another a~vantage of the present invention is that an
extremely intense and transient magnetic field i5 produced
for organic and inorganic experimental purposes~
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Broadly, these objec-ts and advantages are at~ained by the
invention which provides for an intense transient magnetic field
producing apparatus for use with a high powered laser that com-
prises emitter means for emitting hot electrons and creating a
positively charged plasma when irradiated by the high powered
laser, a target means for subjection to an intense transient
magnetic fieldr and a grounding means connected to the emitter
means and positioned near the target means. The grounding means
provides a return current path to ground for the positively
charged plasma created by the emitter means whereby an intense
transient magnetic field is created near the target means when
the emitter means is irradiated by the high powered laser.
In a more specific manner, that inventive apparatus includes
a laser fusion system for the laser and the target means is a
laser-fusion pellet.
The invention also contemplates a method of subjecting a
plasma to an intense transient magne-tic field which comprises
khe step~ of providing a re-turn current pat.h, yene.r.at:i.ng an
lntense transient return current ~low ~lpon the r~tu:~n curr~nt
path, and creating a plasma near the return current path durlny
the intense transient return current flow thereupon whereby the
plasma is subjected to the magnetic field thereof.
In a urther embodiment, the invention contemplates a method
of subjecting an object to an intense -transient magnetic field
in a laser system which comprises the steps of providi.ng a grounded
return current path having an ungrounded end, securlng an emltter
on the ungrounded end of the grounded return current path with
the emltter fashloned so as to create a highly charged plasma
when irradiated by a laser, placing the object near to the groundecl
return current path, and laser irradiati.ny the emitter, the.reby
creating the hlghly charyed plasma and causlny an intense transient
return current flow on the grounded return current path whereby
the object is subjected to -the magne-tic Eleld thereof.
In the latter inventive method, the system can be a laser-
fuslon system, the object can be a laser-fuslon pelle-t that is
fashloned so as to create a plasma when lrradiated by a laser, and
the laser-fusion pellet is laser-irradiated during the intense
transient return current flow on the grounded return current pa-th,
thereby subjecting the plasma of the laser-fusion pellet to -the
magnetic field of the intense transien-t return current flow.
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Additional objects, advantages and novel features ofthe invention will be set foxth in part in the description
which follows, and in part will become apparent to those
skilled in the art upon examination oF the following or
may be learned by practice of the invention. The ob~ects
and advantages of the invention may be realized and
attained by means of the instrumentalities and combina-
tions particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated in
and form a part of the specification, illustrate an embod-
iment of the present invention and, together with the des-
crip~ion, serve to explain ~he principles of the inven-
tion. In the drawings:
Figure 1 i,llustrates a target surrounded by a coil
1~ attached to an emitter at one end and grounded at the
; other:
Figure 2 illustrates a target surrounded by a cylln~
drical cage attached to an emitter at one end and grounded
at the other; and
Figure 3 details a configuration which surrounds a
target with a coil in accord with embodiment of the inven-
tion shown in Fig. 1.
In accord with the present invention, a target 11 is
', secured within an intense transient magnetic field produc-
ing target assembly 13 connected to ground 15 at one end
'` and to an emitter 17 at the other, see Fig. 1. When
placed wi~hin an evacuated cavity and irradiated by an
intense laser beam pulse, the emitter 17 ls transformed
,~ into a highly charged positive plasma causing an intense
return current to flow between the ground 15 and the
emitter 17 through the ~arget assembly 13 thereby subject-
ing the target 11 to an intense txansient magnetic field.
The target assembly 13 formed as a helix or coil as
shown in Fig. 1 subjects the target 11 to longitudinal
magnetic field. Alternatively, the target assembly 13 may
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:
be formed as a cage to subject the targe~ 11 to an a~i-
muthal magnetic field, see Fig. 2~ It is of importance to
keep the target assembly 13 of low inductance and low
resistance in order to generate the desired intense tran-
sient magnetic field.
The emitter 17 is preferably a microballoon of glass,
plastic, metal, or any combination thereofO The emitter
17 is utilized to crea~e a positive plasma when irradiated
by a laser beam pulse of about lol5 watts/cm2 or
greater. With the hot electrons driven from the emitter
17 by the laser pulse, the high positive potential remain-
ing causes an intense but transient current to flow
through the target assembly 13 to ground 15, see Figs. 1
and 2. ~ suitable emitter is a glass microballoon of
approximately 500 micrometers diameter~
Studies such as detailed ~y R. F. ~enjamin et al~,
Physical Review Letters, Vol. 42, No. 14, p. 890 (2 April
1979) demonstrate that irradiated emitters can yenerate
plasma potentials in the order of 180 kilovolts lasting
for periods in the order of a nanosecond or more. Under
such conditions intense transient magnetic fields on the
order of a 100 or more kiloguass can be generated depend-
ing upon the resistance and reactance of the return cur-
rent path.
The target 11 is a small object on the order of
1 millimeter diameter. The target 11 may be organic,
inorganic, metallic, non-metallic, or otherwise. In one
application of the invention, the target 11 is a usion-
fuel containing pellet. These pellets are usually sub-
millimeter-diameter hollow spheres of glass or metal
(called microballoons) filled with a high-pressure DT gas
; and frequently coated with additional layers, or sur-
rounded by concentric shells of metal and/or plastic to
optimize the interaction of the target 11 with a laser
beam~
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When the present invention is used in laser fusion
experiments or power generation, a short duration o about
or less than a nanosecond before a main laser beam strikes
the ~arget 11, an auxiliary laser beam impinges on the
- 5 emitter 17. The above described resultant "return cur-
__
rent" crea~es a strong magnetic ~ield near the target 11
at the instant the target 11 is irradicated. The strong
magnetic ield traps the hot electrons from the target 11
and keeps them in small orbits preventing them from heat-
ing the fuel within the target 11 and causing their energyto be deposited in the region exterior to the fuel con-
taining core.
The target assembly 13 may be sized and located as
desired. For example, the target assembly 13 may be
exterior to the absorption region (i.e., the volume sur-
rounding the target 11 where the main laser beam irradia-
tion is absorbed). With the target assembly 13 fashioned
as a cylindrical cage (see Fig. 2) the Pree elec~rons from
the target 11 are trapped in small orbits as above des-
cribed. Alternatively, the target assembly 13 may be
located between the laser absorption region and the target
` 11 to magnetically shield t~e fuel of the target 11 from
; hot (or "energetic") electrons. Other options exist such
as fashioning the target assembly 13 as a coil or helix to
create a longitudinal magnetic field, see Fig. 1.
The actual physical fabrication of the subject inven-
tion is readily accomplished by those skilled in the art,
see Fig~ 3. A round glass rod 19 preferrably about 4 cm
' in length and 2 mm in diameter is pulled to a tip 21 which
`~ 30 is ground to 125 m diameter. The thick end 23 of the rod
19 is glued or otherwise secured to a metal base 25 while
the tip 21 is likewise glued to the target 11 which may be
a 1 mm aluminum ball or like sized DT fuel pellet The
target assembly 13 being a 255 m diameter copper wire is
soldered to the metal base 25 and coiled around the target
.
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11. The emitter 17 being preferrably a 500 m glass
microballoon is then glued to the target end 27 of the
target assembly 13. To provide proper operation, the
metal base 25 is connected ~o ground 15.
The present invention requires a ~ligh power laser
facility. Such facilities are available at the Los Alamos
National Laboratory and at numerous other laboratories,
institutions and universities. An intense laser beam of
about or greater than lol5 watts/cm2 irradiance is
preferrable to generate the electromagnetic field neces-
sary to accelerate electrons ~o velocities that enable
them to escape the plasma and thereby create a positive
potential at the plasma~ A carbon-dioxide laser is pre-
ferred because it generates energetic electrons more effi-
ciently than other commonly available lasers.
The foregoing description of a preferred embodiment ofthe invention has been presented for purposes oE illustra~
tion and description. It is not intended to be exhaustive
or to limit the invention to the precise form disclosed,
and obviously many modifications and variations are possi-
ble in light of the above teaching. The embodiment was
chosen and described in order to best explain the princi-
ples of the invention and its practical application to
thereby enable others skilled in the art to best utilize
the invention in various embodiments and with various
modifications as are suited to the particular use contem
plated. It is intended that the scope of the invention be
defined by the c1aims appended hereto.
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