Note: Descriptions are shown in the official language in which they were submitted.
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FINAL BEAM TRANSPORT SYSTEM
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent Application
No.
61/437,177, filed on January 28, 2011, the disclosure of which is hereby
incorporated by
reference in its entirety.
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The United States Government has rights in this invention pursuant to
Contract No.
DE-AC52-07NA27344 between the United States Department of Energy and Lawrence
Livermore National Security, LLC, for the operation of Lawrence Livermore
National
Security.
BACKGROUND OF THE INVENTION
[0003] Projections by the Energy Information Agency and current
Intergovernmental Panel
on Climate Change (IPCC) scenarios expect worldwide electric power demand to
double
from its current level of about 2 terawatts electrical power (TWe) to 4TWe by
2030, and
could reach 8-10 TWe by 2100. They also expect that for the next 30 to 50
years, the bulk of
the demand of electricity production will be provided by fossil fuels,
typically coal and
natural gas. Coal supplies 41% of the world's electric energy today, and is
expected to supply
45% by 2030. In addition, the most recent report from the IPCC has placed the
likelihood
that man-made sources of CO2 emissions into the atmosphere are having a
significant effect
on the climate of planet earth at 90%. "Business as usual" baseline scenarios
show that CO2
emissions could be almost two and a half times the current level by 2050. More
than ever
before, new technologies and alternative sources of energy are essential to
meet the
increasing energy demand in both the developed and the developing worlds,
while attempting
to stabilize and reduce the concentration of CO2 in the atmosphere and
mitigate the
concomitant climate change.
[0004] Nuclear energy, a non-carbon emitting energy source, has been a key
component of
the world's energy production since the 1950's, and currently accounts for
about 16% of the
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world's electricity production, a fraction that could "in principle" be
increased. Several
factors, however, make its long-term sustainability difficult. These concerns
include the risk
of proliferation of nuclear materials and technologies resulting from the
nuclear fuel cycle;
the generation of long-lived radioactive nuclear waste requiring burial in
deep geological
repositories; the current reliance on the once through, open nuclear fuel
cycle; and the
availability of low cost, low carbon footprint uranium ore. In the United
States alone, nuclear
reactors have already generated more than 55,000 metric tons (MT) of spent
nuclear fuel
(SNF). In the near future, we will have enough spent nuclear fuel to fill the
Yucca Mountain
geological waste repository to its legislated limit of 70,000 MT.
[0005] Fusion is an attractive energy option for future power generation, with
two main
approaches to fusion power plants now being developed. In a first approach,
Inertial
Confinement Fusion (ICF) uses lasers, heavy ion beams, or pulsed power to
rapidly compress
capsules containing a mixture of deuterium (D) and tritium (T). As the capsule
radius
decreases and the DT gas density and temperature increase, DT fusion reactions
are initiated
in a small spot in the center of the compressed capsule. These DT fusion
reactions generate
both alpha particles and 14.1 MeV neutrons. A fusion burn front propagates
from the spot,
generating significant energy gain. A second approach, Magnetic fusion energy
(MFE) uses
powerful magnetic fields to confine a DT plasma and to generate the conditions
required to
sustain a burning plasma and generate energy gain.
[0006] Important technology for ICF is being developed primarily at the
National Ignition
Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), assignee of
this
invention, in Liveimore, California. There, a laser-based inertial confinement
fusion project
designed to achieve thermonuclear fusion ignition and burn utilizes laser
energies of 1 to 1.3
MJ. Fusion yields of the order of 10 to 20 MJ are expected. Fusion yields in
excess of 200
MJ are expected to be required in central hot spot fusion geometry if fusion
technology, by
itself, were to be used for cost effective power generation. Thus, significant
technical
challenges remain to achieve an economy powered by pure inertial confinement
fusion
energy.
SUMMARY OF THE INVENTION
[0007] According to embodiments of the present invention, methods and systems
related to
inertial confinement fusion are provided. More particularly a final optics
beam transport
system is provided that meets the top level requirements appropriate for a
Laser Inertial
Fusion Engine (LIFE) system. The optics enable fast pointing and transport of
351 nm light
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through dual neutron pinholes and focusing of the beam on target (e.g., a
target tracking
system employed with embodiments described herein will be capable of making
shot pointing
corrections in the last 301..ts before ignition). The optical system described
herein enables a
target tracking system and coaligned diagnostic beam. The final optics have
been engineered
to be robust to neutron damage and target shock pressure waves while providing
minimal loss
to the 351 nm laser beam. A method of replacing the final optics is also
described.
Embodiments of the present invention are also applicable to other optical
systems in a high
radiation environment.
[0008] According to an embodiment of the present invention, a method of
replacing an
optical element positioned in a high radiation environment is provided. The
method includes
halting operations of a beamline, pulling a cable to transfer the optical
element through a
radiation wall, and exchanging the optical element with a replacement optical
element. The
method also includes pulling the cable to transfer the replacement optical
element through the
radiation wall, positioning the replacement optical element adjacent the first
end face of the
telescope, and seating the replacement optical element on the first end face
of the telescope.
The method further includes seating the replacement optical element on
kinematic elements,
verifying an optical alignment of the replacement optical element, and
resuming operations of
the beamline.
[0009] According to another embodiment of the present invention, an optical
system is
provided. The optical system includes a chamber having a first end and a
second end and an
optic mount mounted to the first end of the vacuum chamber. The optic mount
has a
mounting surface. The optical system also includes a Fresnel optic mounted to
the mounting
surface and a cable attached to the optic mount. The optical system further
includes a second
optical element mounted to the second end of the vacuum chamber.
[0010] According to a particular embodiment of the present invention, a system
is
provided. The system includes a laser system operable to provide a laser beam
along an
optical path and a fusion chamber coupled to the optical path. The system also
includes a
neutron pinhole disposed along the optical path between the laser system and
the fusion
chamber and a neutron attenuation region disposed along the optical path
between the laser
system and the fusion chamber.
[0011] According to an embodiment of the present invention, a thin Fresnel
optic is used as
the final optic. The final optic (which may be fabricated in fused silica) is
mounted in a
frame that is sealed to a transport telescope containing a neutron pinhole
(e.g., a large cement
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structure connected to the building) via a gasket (e.g., an 0-ring seal). In
an embodiment, the
aperture of the final optic is approximately 0.6 x 43 x 43 cm3 with an
external pressure of 21
torr (2800 Pa) and an internal pressure of ¨0.5 mtorr. In this embodiment,
approximately 116
pounds of force is present on the surface of the optic.
[0012] Embodiments of the present invention provide replaceable optics in an
accessible
manner without use of electronics, motors, hydraulics, or the like, which are
unable to
withstand a high radiation environment with an acceptable lifetime.
[0013] According to an embodiment of the present invention, a final optics
beam transport
system is provided that meets the top level requirements associated with high
radiation
environments found, for example, in LIFE. The optics allow slow pointing and
transport of
the 351 nm light through optically transparent neutron shielding (also
referred to as neutron
pinholes, which can be implemented in a dual pinhole configuration) and focus
the beam on
target. The final optics have been engineered to be robust to neutron damage
and target
shock pressure wave while providing reduced or minimal loss to the 351 nm
laser beam. A
method of replacing these optics is provided by embodiments of the present
invention.
[0014] Numerous benefits are achieved by way of the present invention over
conventional
techniques. For example, embodiments of the present invention provide methods
and
systems that enable the replacement of optics in a region that is shielded
from a neutron
source by a shield wall. In some embodiments, the final optic used to focus
laser light to a
target provides for both focusing of light as well as a vacuum barrier and/or
a tritium barrier.
These and other embodiments of the invention along with many of its advantages
and
features are described in more detail in conjunction with the text below and
attached figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a simplified schematic diagram illustrating elements of a
final beam
transport system according to an embodiment of the present invention;
[0016] FIG. 2 is a simplified schematic diagram illustrating a final beam
transport system
according to an embodiment of the present invention;
[0017] FIG. 3A is a schematic diagram illustrating a final beam transport
system including
two cascaded neutron pinholes according to an embodiment of the present
invention;
[0018] FIG. 3B is a schematic diagram illustrating a final beam transport
system including
a single neutron pinhole according to an embodiment of the present invention;
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[0019] FIG. 3C is a simplified schematic diagram illustrating elements of
neutron pinhole
telescopes according to an embodiment of the present invention;
[0020] FIG. 4A is a simplified plot of transmission in fused silica optics as
a function of
wavelength for a set of annealing conditions;
[0021] FIG. 4B is a simplified plot of the absorption in fused silica optics
as a function of
temperature;
[0022] FIG. 5 is a simplified graph illustrating a shock pressure wavefolin
incident on the
final optic according to an embodiment of the present invention;
[0023] FIG. 6A is a contour plot illustrating induced stress in the final
optic from the target
ignition shock;
[0024] FIG. 6B is a contour plot illustrating the maximum displacement of the
final optic
from target ignition shock;
[0025] FIG. 7A is a simplified schematic diagram of a final optic changeout
system
according to an embodiment of the present invention;
[0026] FIG. 7B is a simplified schematic diagram of an optical pass-thru for
final optic
replacement including a labyrinth neutronics barrier in a shield wall
according to an
embodiment of the present invention;
[0027] Fig. 8A is a simplified schematic diagram illustrating a system for
mechanical
mounting repeatability and vacuum capability according to an embodiment of the
present
invention;
[0028] FIG. 8B is a simplified schematic diagram illustrating a system
including
independent removability of window modules from any pair in the system
according to an
embodiment of the present invention;
[0029] FIG. 8C is a simplified flowchart illustrating a method of exchanging a
final optic in
a high radiation environment according to an embodiment of the present
invention;
[0030] FIG. 9A is a simplified schematic diagram illustrating a laser bay
labyrinth
maintenance entrance to area between shield walls according to an embodiment
of the present
invention;
[0031] FIG. 9B is a simplified schematic diagram illustrating a laser bay
labyrinth and
neutron pinhole architecture according to an alternative embodiment of the
present invention;
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[0032] FIG. 10 is a diagram illustrating the evolution of the environment near
target
chamber center as a function of time according to an embodiment of the present
invention;
[0033] FIG. 11A is a simplified graph illustrating laser transmission as a
function of
distance from the laser entrance hole due to inverse Bremstrahlung absorption
according to
an embodiment of the present invention; and
[0034] FIG. 11B is a simplified graph illustrating saturation of the SRS
signal in lead vapor
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0035] Embodiments of the present invention relate to fusion reaction
chambers.
Embodiments of the present invention are applicable to energy systems
including , but are not
limited to, a Laser Inertial-confinement Fusion Energy (LIFE) engine, hybrid
fusion-fission
systems such as a hybrid fusion-fission LIFE system, a generation IV reactor,
an integral fast
reactor, magnetic confinement fusion energy (MFE) systems, accelerator driven
systems and
others. In some embodiments, the energy system is a hybrid version of the LIFE
engine, a
hybrid fusion-fission LIFE system, such as described in International Patent
Application No.
PCT/US2008/011335, filed September 30, 2008, titled "Control of a Laser
Inertial
Confinement Fusion-Fission Power Plant", the disclosure of which is hereby
incorporated by
reference in its entirety for all purposes.
[0036] Embodiments of the present invention provide for protection of system
elements
from neutron fluence, which can potentially limit the lifetime of the optics.
One of the optics
at high risk is the final optic, which withstands all of the issues described
in Table 1 in
addition to the laser energy. The final optic is directly exposed to the gases
from the target
chamber (primarily xenon, but with target admixture of helium, hydrogen,
deuterium, tritium,
lead, carbon) and target shrapnel. For some commercial power plants utilizing
LIFE designs,
the baseline output power is 1950 MW. The ions and x-rays are absorbed by the
xenon gas in
the target chamber, leaving 1560 MW of 14 MeV neutrons from the fusion
reaction which
yield an average exposure of 1.5x1017 n/m2-sec at the final optic location. In
addition, there is
a pressure wave producing ¨ 0.53 torr of pressure at the final optic location
in addition to the
baseline pressure of 21 ton. Finally, the optic is positioned in an
environment coupled to the
vibrations associated with the gas expansion from ignition and liquid lithium
flow in the
target chamber blanket. This is somewhat mitigated in some LIFE designs by
mechanically
decoupling the first wall and blanket from the vacuum chamber that is
connected to the
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optical pipe assembly. The beamline apertures in the blanket also act to
attenuate the gas
shock incident on the final optic. The final optic is designed to survive the
residual threat and
efficiently transmit and focus the 351 nm laser light at ¨3 J/cm2 (noiinal to
the beam).
Target
Quantity* Nature of the Threat How it is
Handled
Emission
Atomic displacement Thermal annealing
and
/second
Neutrons= 2 x 1013 n/cm2-s
damage & nuclear design optic to be
transmutation tolerant
Charged ¨10% = 280 MW in Ion displacement, Stop ions in
sputtering, surface
particles ¨60keV Maxwellian chamber gas
heating / ablation
¨12% = 340 MW Surface heating & Attenuated
(stopped) in
X-rayschamber gas and beam
= 0.5 J/cm2 15Hz ablation
tubes
Thermal annealing and
Gamma- Breakage of chemical
<1% design optic to be
rays bonds
tolerant
Over-pressure of ¨4 kPaMechanical design of
Gas Mechanical stress to final
for 5-10 ms (20-40 kPa-optics and/or counter gas
shocks optic
s) flows
Table 1. Final optic threats
5 [0037] The final beam transport system includes the optics utilized to
transport the beam
from the exit of the frequency converter to the target chamber center. The
final optic system
is robust, serviceable, delivers the laser through optically transparent
neutron shielding (also
referred to as neutron pinholes since laser light is able to propagate through
the pinholes
without substantial optical losses), and survives multiple threats from the
target chamber.
10 Referring to FIG. 2, this final beam transport system includes optic M10
and all following
optics. In addition to the laser fluence at 351 nm, L11, LG1 and FL1 are
exposed to neutron
irradiation, and FL1, the final optic, is exposed to additional mechanical
shock and target
shrapnel from target ignition. The final optics transport shown schematically
in FIG. 2,
differs from the NIF architecture, which utilizes a wedged focusing lens and
debris shield
installed in close proximity to the frequency converter. To protect the laser
system and
operations personnel from neutron irradiation, devices called neutron pinholes
are used. The
neutron pinhole is a small (¨ lcm) hole in three meter thick concrete shield
walls which allow
light to pass, but absorbs most of the neutrons escaping the target chamber.
If this pinhole is
situated at the focal location of a Galilean relay telescope, the aperture of
this pinhole can be
minimized (theoretically to the same size as pinholes in the le) beamline)
thereby minimizing
transmitted neutrons while fully transmitting the laser light.
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[0038] FIG. 3A is a schematic diagram illustrating a final beam transport
system including
two cascaded neutron pinholes according to an embodiment of the present
invention.
Although the system can be referred to a "two neutron pinhole" system, it will
be understood
that the system utilizes two sets of neutron pinholes. Systems designed as
illustrated in FIG.
3A experience a radiation dose that is attenuated to 0.04 rem/year utilizing
two cascaded
neutron pinholes. It should be noted that the final optic 326/320 not only
focuses, but
deflects the beam from the axis of the second neutron pinhole relay telescope
(including
focus lens 314 and Fresnel lens (type 2a) 316 and matching focus lens 315 /
Fresnel lens 318
to the target chamber center 386. Since the final optic (i.e., Fresnel lens
(type 2b) 326 and
matching final optic (i.e., Fresnel lens (type 1 b) 320 deflects the laser
beam, it only acts as a
scattering source for neutrons, thereby preventing ballistic neutrons from
passing through the
neutron pinhole at location. The transmitted spectrum of neutrons from the
pinhole 330 will
be a roughly collimated beam of neutrons that have scattered from the
surrounding shield
materials and blanket after some collimation by the pinhole structure. As
shown in FIG. 3A,
the axis of the second neutron pinhole at location 332 is again deflected from
axis of the first
neutron pinhole at location 330, which prevents ballistic neutrons from the
second pinhole
from passing through the first one. Using this technique, the neutron dosage
can be
attenuated to levels such that human occupation of the laser bay is possible.
[0039] In an embodiment, the laser bays 310A and 310B include 2.2 m wide x
1.35 m high
x 10.4 m long lw lasers/amplifiers. These laser bays are able to produce laser
beams with
435 mm square beam dimensions suitable for fusion applications. Additionally,
in some
embodiments, the inner cone 324 is characterized by an angle of 26.9 and the
outer cone 322
is characterized by an angle of 47.25 , but these particular angles are not
required by the
present invention. As an example, in other embodiments, the cone angles are 30
and 50 .
[0040] The optical design of the final transport optical system meets many
requirements
simultaneously, including: the ability to point and center to incoming targets
at the target
chamber center, efficient transport of the 351 nm light to target chamber
center, and focus of
the energy into the Laser Entrance Hole (LEH) of the target hohlraum. To
achieve these
ends, the mirrors M10 and M1 1 shown in FIG. I are used to maintain centering
on the final
transport optics and slow pointing to the target. The lenses L9, L10, and L11
in addition to
transporting the beam through the first neutron pinhole, also serve to null
out the chromatic
dispersion induced by the Fresnel final optic, which has the opposite sign
relative to
traditional convex lenses. The grating LG1 compensates for the temporal skew
induced by
the deflection (diffraction) of the Fresnel final optic and also serves to
provide the deflection
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required between neutron pinhole 1 and 2. Thus fully compensated both
chromatically and
temporally, the Fresnel optic can focus the 351 nm drive laser beam into the
LEH of the
target.
[0041] Embodiments of the present invention utilize one of several optical
elements as the
final optic including: a grazing incidence metal mirror (GIMM), an elliptic
mirror, a thin
Fresnel optic, or the like. In embodiments utilizing a GIMM or parabolic
mirror, an
additional vacuum window is included in the design upstream (e.g.,
immediately) of the final
optic before the neutron spatial filter. This optic serves two purposes among
others: to
guarantee vacuum at the telescope focus so that the laser light can be
transmitted, and to
serve as a tritium barrier. The Fresnel optic illustrated in FIG. 1 acts as
both the final focusing
optic and as the vacuum barrier. By making this final optic thin, the neutron
induced
absorption can be reduced to level of a few percent.
[0042] Although the angle between the optical axes of the two relay telescopes
associated
with the neutron pinholes is angled at an angle of about 60 , this is not
required by the
present invention and other embodiments utilized different angles between
telescopes. In
some embodiments, the first relay telescope is oriented in a horizontal plane
and the second
relay telescope is oriented in a vertical plane, with a right angle between
the two optical axes.
Other orientations are included within the scope of the present invention in
addition to those
illustrated. One of ordinary skill in the art would recognize many variations,
modifications,
and alternatives.
[0043] FIG. 3B is a schematic diagram illustrating a final beam transport
system including
a single neutron pinhole according to an embodiment of the present invention.
Referring to
FIG. 3B, laser sources 350A through 350N are provided in a first region 351.
Light from the
laser sources 350A through 350N is directed toward a shield wall 352, for
example, a all 3 m
in thickness. A set of neutron pinholes 353A through 353N are provided in the
shield wall
352 to enable the laser radiation to pass through the shield wall after
focusing using a set of
optical system (e.g., a set of N relay telescopes). Thus, when this system is
referred to as a
"single neutron pinhole" system, this can be understood as utilizing a single
set of neutron
pinholes rather than two sets of neutron pinholes.
[0044] Light passing through the set of neutron pinholes 353A through 353N
reflects off
parabolic mirrors 360 in the illustrated embodiment to impinge on Fresnel
optics 362 and
364. In some embodiments, the distance between Fresnel optics 326 and 364 is
sufficient to
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enable parabolic mirrors 360 to be positioned under ledge 361. After focusing
by Fresnel
optic 364, light is focused onto target 386.
[0045] Neutrons generated at the target 386 propagate out in all directions
including cone
368, passing through the space between walls 366 and 365. Neutrons to the left
of cone 368
are reflected or absorbed by wall 366. In an embodiment, wall 363 and ledge
361 define the
angular spread of cone 368. Although the neutrons impinge on Fresnel optic
364, wall 365
prevents neutrons from impinging on Fresnel optic 362. Because the neutrons
are contained
between wall 363 and ledge 361, only a single set of neutron pinholes is
needed to reduce the
neutron density in region 351 to acceptable levels.
[0046] FIG. 3C is a simplified schematic diagram illustrating elements of
neutron pinhole
telescopes according to an embodiment of the present invention. As illustrated
in FIG. 3C, a
first telescope 370 focuses light through the secondary shield wall 372 to
pass through a
second neutron pinhole 374. The light is refracted through Fresnel optic 376A,
which forms
an element of a second relay telescope 370. The second relay telescope 380
focuses light
through the primary shield wall (not shown) to pass through a first neutron
pinhole at location
382A. As is evident by the figure, multiple, parallel light paths are provided
by embodiments
of the present invention, providing multiple neutron pinholes passing through
the primary and
secondary shield walls as illustrated by neutron pinhole 374B, light from
which is collected
by Fresnel optic 376B.
[0047] Light passing through the first neutron pinhole at location 382 is
incident on Fresnel
optic 384A, which collects and focuses the light onto the target 386. Because
both grating
structures present in Fresnel optics 376A and 384A receive light from a point
source and
focus light to a corresponding point source, the manufacturing of these
Fresnel optics is
simplified, enabling a high quality manufacturing process to be utilized. In
order to
manufacture these gratings, a point source is utilized to define the grating
structures since
light passing through the gratings originates and terminates as a point
source. As illustrated,
the gratings are receiving divergent light and producing convergence of the
received light.
Thus, grating exposure can be accomplished using point sources. Various cone
angles can be
utilized according to embodiments of the present invention, for example, an
angle of 26.9
for the inner cone between the target 386 and Fresnel optic 384A and an angle
of 47.2 for the
outer cone between the target 386 and Fresnel optic 384B.
[0048] According to some embodiments, the manufacturing process is improved in
comparison to other architectures since point sources can be utilized in the
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process. As an example, Fresnel optics manufactured for use in embodiments as
illustrated in
FIG. 3C have reduced aberrations in comparison with Fresnel optics in which a
divergent
beam is collimated.
[0049] The inventors note that the neutron-induced absorption in fused silica
saturates at
fairly modest neutron irradiation levels, and this absorption can be partially
annealed by
raising the temperature of the substrate as illustrated in FIG. 4A. In an
embodiment, a 5.3
mm thick fused silica substrate is utilized for the Fresnel optic, which is
sufficient to serve as
the vacuum barrier between the target chamber at 21 ton and the relay
telescope at
approximately 0.5 mtorr.
[0050] The inventors have determined that if an optic of sufficient thickness
(e.g., a 5.3 mm
thick optic) is maintained at ¨580 C, the absorption loss is reduced to ¨
0.5%. As illustrated
in FIG. 4B, the absorption of fused silica optics varies as a function of
temperature. The
heating can be accomplished through use of beam heating, an external heater
producing ¨3.4
MW, or a combination thereof. In embodiments in which no heater is used, the
inventors
have detennined that beam heating alone will raise the temperature of the
optic to ¨518 C,
with an associated transmission loss of 3.5%, which is suitable for some
applications.
According to an embodiment of the present invention, a 5.3 mm thick fused
silica Fresnel
optic is utilized for the final optic, although embodiments of the present
invention are not
limited to this particular thickness. Other thicknesses can also be utilized.
[0051] FIG. 4A is a graph illustrating corrected transmission percentage as a
function of
wavelength for a final optic according to an embodiment of the present
invention. FIG. 4B is
a graph illustrating laser absorption versus temperature for a 5.3 mm thick
fused silica optic.
[0052] Referring to FIG. 4A, the annealing processing of neutron damaged
silica
demonstrates a large change in 351 nm transmission as a result of the
annealing process.
[0053] In addition to the neutron threat, a shock wave generated by the target
ignition will
be incident on the final optic. FIG. 5 is a simplified graph illustrating a
shock pressure
waveform incident on the final optic according to an embodiment of the present
invention.
FIG. 6A is a contour plot illustrating induced stress in the final optic from
the target ignition
shock. FIG. 6B is a contour plot illustrating the maximum displacement of the
final optic
from target ignition shock. As illustrated in FIGS. 6A and 6B, there is about
2 um of
displacement in the final optic and about 40,000 Pascals of stress, which is
acceptable for the
designs described herein.
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100541 This optic can be mounted in a frame that can be sealed using a gasket
seal to a
transport telescope containing a neutron pinhole, which is a large cement
structure connected
to the building. In an embodiment, the aperture of the fused silica optic is
approximately 0.53
x 43 x 49.65 cm3 (43cm aperture at angle of 30 ) with an external pressure of
21 ton (2800
Pa) and an internal pressure of 0.5 mtorr, which results in 134 pounds of
force on the surface
of the optic. An additional 0.5 ton (70 Pa) is incident on the final optic
during the ¨135 jis
shock pulse as shown in FIG. 5. To understand the mechanical effect of this
impulse on the
5.3 mm thick optic, a model was built in quarter symmetry using a Shells model
(a finite
element gridding technique useful for thin substrates). For boundary
conditions, the contact
points to the gasket were modeled as knife edge rollers (supported normal to
the optic only).
The first modal frequency is 131 Hz. The induced stress and displacement of
the optic due to
the impulse are shown in FIG. 6. The maximum effective surface stress is 40600
Pa. The
maximum displacement is 2.62x10-6 m (2.62 m). Both the maximum displacement
and
maximum effective surface stress occur at approximately 6 ms into the
analysis. These
results indicate the final optic survival is not threatened by the shockwave,
and the maximum
surface displacement should not have a significant impact on the laser focal
spot. It should
be noted that mounting of the final optic can be designed to avoid resonance
at the modal
frequency or induced vibration from the building due to the previous shot
and/or support
equipment fluid flow (e.g., blankets, cooling, or the like). Engineering of
passive damping
mechanisms for the vibration can be performed based on the spectrum for this
final optic
including effects based on the chamber environment and the mechanical mounting
hardware
design.
[0055] As illustrated in FIG. 6A, the global maximum effective surface stress
is 4.06 x 104
with a global minimum of zero. As illustrated in FIG. 6B, the global maximum
displacement
of the final optic is 2.62 x 10-6 m and the global minimum displacement is -
2.47 x 10-6.
These values are not intended to limit embodiments of the present invention
but to provide
examples of the stress and displacement encountered in various embodiments of
the present
invention.
[0056] Embodiments of the present invention provide methods and systems for
replacement of the final optic (as well as other optics between the two
neutron pinholes) in a
radiation hot environment. To first order, no electronics are able to survive
in this
environment and would have a low MTTF. The replacement hardware will have a
very large
MTTF, since failure of these components would require plant shut-down
(affecting plant
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availability) to enable access to the hardware in the high radiation area
around the target
chamber.
[0057] FIG. 7A is a simplified schematic diagram of a final optic changeout
system
according to an embodiment of the present invention. The system illustrated in
FIG. 7A
provides a dual optic replacement capability and simple mechanical replacement
via a cable
drive 720 into the high radiation area. FIG. 7B is a simplified schematic
diagram of an
optical pass-thru for final optic replacement including a labyrinth neutronics
barrier in a
shield wall according to an embodiment of the present invention. Some
embodiments of the
present invention are enabled by the geometry of the Fresnel optic, for
example, 40 cm or 50
on a side, but only 5 mm thick and the associated low weight. The thin nature
of the Fresnel
optic 705 additionally enables removal through a thin labyrinth 730 as
illustrated in FIG. 7B.
[0058] As illustrated in FIG. 7A, a system for replacement of the thin Fresnel
optic 705 is
provided that does not utilize any hydraulic or motorized devices in the high
radiation area.
As illustrated in FIGS. 7A and 7B, this system and method uses cables with
pulleys or rollers
to guide damaged Fresnel lenses out of the high radiation environment through
curved slits in
the shield wall that serve as neutron labyrinths but allow exchange of the
final optic. A close-
up of one of these labyrinths is shown in FIG. 7B with dimensions that are
used in an
exemplary embodiment suitable for neutronics modeling.
[0059] Referring to FIG. 7B, optical assemblies pass through a labyrinth to
prevent the
neutrons from passing through the wall associated with the passage of the
optical assemblies.
As illustrated in FIG. 1, the neutron pinholes provided for passage of the
laser beams are
oriented at an angle with respect to each other to prevent neutrons passing
through the
innermost pinhole as undeflected neutrons. The optical assemblies are able to
pass through
the labyrinth, which blocks neutrons as a function of the shape of the
labyrinth. Referring to
FIG. 7B, in some embodiments, the labyrinth has a width of 15 cm and a radius
of curvature
of ¨150 cm, providing a distance between entrance and exit ports of 300 cm for
a 300 cm
thick wall.
[0060] Although not illustrated in FIG. 7A for purposes of clarity,
embodiments of the
present invention utilize two cable systems (one for each Fresnel optic 705)
with a cable
attached at the top and bottom of each optic (i.e., 4 cables total running
through the two
labyrinthine slits in the wall). In other embodiments, other implementations
can be utilized.
As illustrated in FIG. 7A, arrow 702A illustrates the movement of the left-
hand optic during
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replacement and arrow 702B illustrates the movement of the right-hand optic
during
replacement.
[0061] Although the labyrinth illustrated in FIG. 7B is a continuous curved
structure, this is
not required by embodiments of the present invention. In other embodiments, a
zig-zag
labyrinth as shown in FIG. 7C is utilized.
[0062] Since, in some embodiments, there is no adjustment capability in the
final optics,
the mounting hardware enables precision kinematic replacement. This is
achieved in the
illustrated embodiment by creating a telescope end face 805 as shown in FIG.
8A, where
ferromagnetic steel balls 825 (e.g., Nd-based magnets or other high strength
to mass ratio
magnets such as neodymium iron boron-based magnets, samarium cobalt-based
magnets, or
other similar magnets) are mounted into the surface to provide kinematic
registration points
827 for the Fresnel optic module, also shown in FIG. 8A. In other embodiments,
the
kinematic mounts are reversed, with the magnets and kinematic registration
points provided
on the opposing elements (i.e., magnets mounted on the LRU and registration
points on the
telescope end face). As illustrated in FIG. 8A, the Nd-based high power
magnets (which may
be replaced with other suitable high strength to mass ratio magnets), the
Fresnel optic, and
vacuum gasket 830, are all mounted together on the final optic frame 807 that
is a line
replaceable unit. The precision frame can be fabricated from a rigid material
such as stainless
steel with cable attachments 840. To enable independent removal of window
modules, two
pairs of cable drives are provided as shown in the front view illustrated in
FIG. 8B.
[0063] Referring to FIG. 8A, the end of the telescope includes a steel flange
including
kinematic nodules 825 on the end that are also made out of steel. The use of
steel enables the
elements illustrated in FIG. 8A to possess a lifetime similar to other chamber
elements.
Elements that are replaceable are mounted to the steel flange, for example,
the final optic 815
(e.g., the fused silica Fresnel lens, which can be mounted offline to provide
micro-alignment
capabilities), the gasket 830 for creating a vacuum seal at the surface, the
Nd-based magnets
827, the attachments 830 for cabling, which is used to move the assembly into
and out of the
system during replacement and repair operations, and the like. In some
embodiments, two
assemblies are provided side-by-side and the left hand one would changed out
to the left and
the right hand one would get changed out to the right using two independent
cabling systems.
As illustrated in FIG. 8B, independent cable pairs 852 and 854 enable allow
window modules
on both sides to be independently removable. In some implementations, the
gasket is
optional since some embodiments do not utilize a vacuum environment for
portions of the
system. In these embodiments, the optics can be mounted, but not sealed to a
neutron
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pinhole, with no pressure differential present across the optic. One of
ordinary skill in the art
would recognize many variations, modifications, and alternatives.
100641 FIG. 8C is a simplified flowchart illustrating a method of exchanging a
final optic in
a high radiation environment according to an embodiment of the present
invention. The
method 800 includes halting operations (810), optionally venting the telescope
to chamber
pressure (812), and optionally adding additional Xe gas to "burp" the lens
from the telescope
end face (814). The method also includes pulling the cable to retrieve the
final optic through
radiation wall (816), and exchanging the final optic using robotics in the
region between
neutron pinhole #1 and neutron pinhole #2 (818). The method further includes
pulling the
cable to position the replacement final optic in front of telescope end face
(820), using the
magnets to pulls the final optic into kinematic position (822), optionally
pulling a vacuum to
seat the final optic on kinematics (824), verifying the alignment and
repointing the beam as
necessary (826), and resuming operations (828). Although kinematics are
utilized in the
illustrated embodiment, this is not required by embodiments of the present
invention and
other alignment techniques are included within the scope of the present
invention.
100651 It should be appreciated that the specific steps illustrated in FIG. 8C
provide a
particular method of exchanging a final optic in a high radiation environment
according to an
embodiment of the present invention. Other sequences of steps may also be
performed
according to alternative embodiments. For example, alternative embodiments of
the present
invention may perform the steps outlined above in a different order. Moreover,
the individual
steps illustrated in FIG. 8C may include multiple sub-steps that may be
performed in various
sequences as appropriate to the individual step. Furthermore, additional steps
may be added
or removed depending on the particular applications. One of ordinary skill in
the art would
recognize many variations, modifications, and alternatives.
100661 Although a particular method of sealing optics is provided in relation
to FIG. 8C,
embodiments of the present invention are not limited to these approaches. In
other
embodiments, a push-pull seal is utilized as a valve, similar to a canister's
seal. Thus,
embodiments of the present invention provide for seal creation at a distance
by venting one
side of a valve and then changing out the optic. By actuating a lever, a seal
can be created as
the optical mount is urged against a flange. In order to release the seal, the
lever is actuated
in an opposing direction to enable the optical mount to move away from the
flange in a
manner analogous to a canister lid. One of ordinary skill in the art would
recognize many
variations, modifications, and alternatives.
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[0067] The method illustrated in FIG. 8C allows the passive method of magnetic
kinematic
mounting to maintain alignment across multiple servicing operations. All
expendable
components can be readily removed from the high radiation environment without
shutting
down the power plant for direct maintenance. All required pneumatic vacuum
valves,
motorized cable drive, and robotic optic exchange have been limited to the
area between
neutron pinhole #1 and #2. This area and access are schematically shown in
FIG. 9A. An
airlock door 920 which provides both vacuum and tritium barrier is located in
an entrance of
labyrinth 940, which keeps the laser bay radiation levels safe for personnel.
Robotic service
vehicles 930 can enter supplying new materials (optics and/or hardware) to
replace
components. These same vehicles can be used to carry used components in
shielded
containers to radiation hazardous waste locations in the plant for recycling
and/or disposal.
As illustrated in FIG. 9A, after the final optic is removed through the
labyrinth into the lower
radiation area 911, a vehicle is able to remove the optic through an
interlock.
[0068] Embodiments of the present invention operate such that the pressure of
this
environment is low (-21 torr), which will prevent all but the lightest of
particulates from
remaining suspended in the chamber gas and thereby promote cleanliness. Gas
purge nozzles
can be located in the region of the final optic with their ultimate purpose
being both to
provide counter flow pressure to offset the "puffs" of chamber gas from target
ignition and
also to provide a low pressure "air knife" to clean the final optic as it is
being replaced and
maintain that cleanliness during operations. Control of this purge flow rate
can be done using
pneumatic valves located in low radiation areas outside of the primary shield
wall. During
maintenance operations this purge pressure can be briefly increased to enable
gas cleaning of
the final optic to meet requirements.
[0069] In addition to the systems to provide for long lived and replaceable
final optics,
embodiments of the present invention address concerns for the optics located
between the
neutron These optics (L11 and LG1 as illustrated in FIG. 9) and their
associated hardware are
in a radiation environment that compounds the threat to the optic and limits
access and
maintenance capabilities. Initial neutronics calculations show the neutron
pinhole #2
(pinhole 925 in the primary shield wall 926) attenuating the 1.5 x 1017 n/m2
sec incident
dosage significantly. These neutrons appear to be highly collimated, which
will enable
relatively simple neutron dumps to be used to limit the neutron flux in the
area between the
neutron pinholes. Motorized vehicles 930 and actuators with electronics will
be allowed into
this area for servicing and some components may be allowed permanent
occupation in this
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area. Servicing of optics already in this area is much more straightforward
and can be
accomplished with handling methods commensurate with standard cleanliness
protocols.
[0070] FIG. 9B is a simplified schematic diagram illustrating a laser bay
labyrinth and
neutron pinhole architecture according to an alternative embodiment of the
present invention.
As illustrated in FIG. 9B, an architecture is provided that provides neutron
shielding using a
single set of neutron pinholes (e.g., the set of four neutron pinholes 950
shown in FIG. 9B).
[0071] Laser/amplifiers are provided in the laser bay 955, which is at
atmospheric pressure
and utilizes an air environment in the illustrated implementation. Light from
the
lasers/amplifiers is directed, using optics, to pass through the shield wall
952 including the
single set of neutron pinholes 950. Utilizing other optics as illustrated, the
laser beams, after
passing through the set of neutron pinholes, are directed in a zig-zag manner
around walls
956 and 958 to impinge on the target chamber 960. In the illustrated
implementation, the
target chamber 960 is at 21 torr, with a mixture of xenon and tritium,
produced as a
consequence of the fusion reactions. The labyrinth area 962 is at
substantially the same
atmospheric conditions as the target chamber, 21 ton with a mixture of Xe and
T. Neutrons
produced in the target chamber 960 that are propagating toward the labyrinth
area 962 are
blocked by walls 958 and 956 and are, therefore, not able to reach the neutron
pinholes in
substantial densities.
[0072] In order to replace optics, including the final optic 970, a cable/rail
guided system
980 is provided to enable removal and replacement of optics as described more
fully
throughout the present specification. After their useful life, optics are
removed using the
cable/rail guide system 980 as they are routed along shield wall 982 and
extracted through an
airlock door 984, which also serves as a tritium barrier. After extraction
through the airlock
door 984, robotic optics replacement vehicles 990 can be used to remove spent
optics and
deliver new optics. The environment for the robotic optics replacement
vehicles can be
atmospheric pressure, for example, air.
[0073] Referring once again to FIG. 1, the final beam transport system
provides a
mechanism to direct the 351 nm laser light to the target chamber center. In
contrast with
some other fusion technology systems (e.g., NIF), the chamber is not at hard
vacuum. In
some embodiments, the chamber is intentionally filled with a protection
mechanism, such as
xenon gas at 4 g/cm3, to protect the chamber walls from ions and x-rays.
Target ignition at
15 Hz adds components of the target (hydrogen, deuterium, tritium, helium,
carbon, lead, and
the like) to this gas mixture as well since the vacuum system doe not
typically replace all of
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the gas before the next shot. Therefore, a detailed analysis of the beam
propagation through
this complex gas mix has been performed to provide information related to the
beam
dynamics (e.g., filamentation or scattering due to nonlinear processes). The
analysis begins
by in one implementation by propagating the 2TW, 15 ns, laser beam at 0.351 tm
through
tens of cm of Xe/Pb plasma near target chamber center and meters of gas
starting at the final
optic. The generation and evolution of this target chamber environment is
illustrated in FIG.
10.
[0074] Referring to FIG. 10, prior to the laser shot, the gas is at a non-
ionized state with a
5% lead mixture at ¨ 0.5 eV. As the foot of the laser pulse begins to heat the
target, a plasma
ball farms that grows to an extent of 25 cm during the laser pulse. After ¨ 1
lis, the plasma
has radiatively cooled to a hot mix of neutral gas, which continues to radiate
until the next
target shot.
[0075] The interactions of the laser with the gas and the expanding plasma are
given in
Table 2, where the interactions are characterized by type and an assessment of
the effect on
the beam is given. Most of the effects are well understood and will depend on
the actual
target gas mix and temperature environment. It should be noted that the
primary loss
mechanisms for the beam appears to be the ionized gas in the plasma ball
surrounding the
target. This transmission loss as a function of distance from the target is
shown in FIG. 11A,
and induces a negligible loss of 0.5% at 351 nm. In an extreme case in which
the entire
chamber were to remain ionized, the loss would only increase to 1.5% for the
61.tg/cm3 case.
The second loss mechanism of interest is Stimulated Raman Scattering from the
lead gas
from the target hohlraum. Electronic Stimulated Raman Scattering (scattering
from bound
electrons) has been extensively studied with dye laser in heat pipes (alkali
vapors) in the past.
Conversion efficiency > 60 % was observed for Pb vapors around 1 ton. The
inventors have
determined that the intensity - length product for a LIFE beamlet is so large
that the relevant
gain exponent will reach 10X threshold (G ¨ 30) after the 1 ns of the laser
impulsion. This
will result in full saturation of the SRS medium.
[0076] Referring to FIG. 11B, lmJ of SRS @0.02 Ton corresponds to ¨ 1 photon
per
atom. Therefore, if all of the available lead atoms that are in the laser
illumination volume in
the target area are excited, this corresponds to ¨ 20 kJ for the entire LIFE
laser system (all
beams), which would be equivalent to doubling the energy of the first "picket"
on the laser
pulse shape. The SRS loss corresponds to a beamline loss of 0.83%. Based on
these
values, the apparent loss to the incoming beams is very reasonable ¨1.33 ¨
2.33% for all
affects at the 6 p,g/cm3 case discussed herein. It should be noted that the
previous analysis
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does not include the last centimeter of propagation (near the hohlraum LEH)
where the beams
begin to overlap, which is addressed by energetics analysis.
Interaction Assessment
Laser- Inverse Bremstrahlung (photons less than 50 cm of beam path
is ionized
plasma absorbed by free electrons) and calculated I. B.
absorption is 0.5%
(free @ 6 [ig/cc Xe and 1.5 % @ 8
g/cm3
electrons) Xe
Stimulated Brillouin scattering Gsbs <1 for a beamlet due to
low
(photons scattered by ion-acoustic density/low intensity
waves)
Stimulated Raman scattering (photon Gsrs < 1 for a beamlet due to low
scattered by "free" electron plasma density/low intensity
waves)
Ponderomotive filamentation G << 1
(nonlinear plasma refraction)
Laser- Raman scattering from bound not a problem for Xe/Xe; Pb
debris
gas/vapor electrons requires doubling the first
picket energy
(bound to saturate the medium (20 kJ)
electrons) B-integral (index of refraction B-integral <0.1 radian for a
beamlet; no
nonlinear with intensity; Kerr effect) filamentation
Refraction through a density gradient a 40 jig/cm3 cold Xe jet leads
to a 301.im
(beam deflection) / turbulence mispointing at 5m; turbulence
likely not
an issue
Break down in Xe gas along the path breakdown threshold in Xe gas is
to TCC reached ¨ 50 cm from TCC at
peak
power (11ns), while plasma ball radius
is <25 cm, but density too low for
cascading/self-focusing
interaction with Xe droplets; Pb2 unlikely in 0.5 eV Xe/Pb gas
dimers (Rayleigh scattering; break
down; absorption)
Table 2. Laser focal interactions with target gas
[0077] It is also understood that the examples and embodiments described
herein are for
illustrative purposes only and that various modifications or changes in light
thereof will be
suggested to persons skilled in the art and are to be included within the
spirit and purview of
this application and scope of the appended claims.
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