Inertial confinement fusion using lasers rapidly progressed in the late 1970s and early 1980s from being able to deliver only a few joules
of laser energy (per pulse) to being able to deliver tens of kilojoules
to a target. At this point, incredibly large scientific devices were
needed for experimentation. Here, a view of the 10 beam LLNL Nova laser, shown shortly after the laser's completion in 1984. Around the time of the construction of its predecessor, the Shiva laser, laser fusion had entered the realm of "big science".
Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium. Typical fuel pellets are about the size of a pinhead and contain around 10 milligrams of fuel.
To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons, almost all ICF devices as of 2015
have used lasers. The heated outer layer explodes outward, producing a
reaction force against the remainder of the target, accelerating it
inwards, compressing the target. This process is designed to create shock waves
that travel inward through the target. A sufficiently powerful set of
shock waves can compress and heat the fuel at the center so much that
fusion reactions occur.
ICF is one of two major branches of fusion energy research, the other being magnetic confinement fusion.
When it was first proposed in the early 1970s, ICF appeared to be a
practical approach to power production and the field flourished.
Experiments during the 1970s and '80s demonstrated that the efficiency
of these devices was much lower than expected, and reaching ignition
would not be easy. Throughout the 1980s and '90s, many experiments were
conducted in order to understand the complex interaction of
high-intensity laser light and plasma. These led to the design of newer
machines, much larger, that would finally reach ignition energies.
The largest operational ICF experiment is the National Ignition Facility
(NIF) in the US, designed using the decades-long experience of earlier
experiments. Like those earlier experiments, however, NIF has failed to
reach ignition and is, as of 2015, generating about 1⁄3 of the required energy levels.
Description
Basic fusion
Indirect drive laser ICF uses a hohlraum
which is irradiated with laser beam cones from either side on its inner
surface to bathe a fusion microcapsule inside with smooth high
intensity X-rays. The highest energy X-rays can be seen leaking through
the hohlraum, represented here in orange/red.
Fusion reactions combine lighter atoms, such as hydrogen, together to form larger ones. Generally the reactions take place at such high temperatures that the atoms have been ionized, their electrons stripped off by the heat; thus, fusion is typically described in terms of "nuclei" instead of "atoms".
Nuclei are positively charged, and thus repel each other due to the electrostatic force. Overcoming this repulsion costs a considerable amount of energy, which is known as the Coulomb barrier or fusion barrier energy.
Generally, less energy will be needed to cause lighter nuclei to fuse,
as they have less charge and thus a lower barrier energy, and when they
do fuse, more energy will be released. As the mass of the nuclei
increase, there is a point where the reaction no longer gives off net
energy—the energy needed to overcome the energy barrier is greater than
the energy released in the resulting fusion reaction.
The best fuel from an energy perspective is a one-to-one mix of deuterium and tritium; both are heavy isotopes
of hydrogen. The D-T (deuterium & tritium) mix has a low barrier
because of its high ratio of neutrons to protons. The presence of
neutral neutrons in the nuclei helps pull them together via the nuclear force,
while the presence of positively charged protons pushes the nuclei
apart via electrostatic force. Tritium has one of the highest ratios of
neutrons to protons of any stable or moderately unstable nuclide—two
neutrons and one proton. Adding protons or removing neutrons increases
the energy barrier.
A mix of D-T at standard conditions
does not undergo fusion; the nuclei must be forced together before the
nuclear force can pull them together into stable collections. Even in
the hot, dense center of the sun, the average proton will exist for
billions of years before it fuses.
For practical fusion power systems, the rate must be dramatically
increased by heating the fuel to tens of millions of degrees, and/or
compressing it to immense pressures. The temperature and pressure
required for any particular fuel to fuse is known as the Lawson criterion. These conditions have been known since the 1950s when the first H-bombs
were built. To meet the Lawson Criterion is extremely difficult on
Earth, which explains why fusion research has taken many years to reach
the current high state of technical prowess.
ICF mechanism of action
In a hydrogen bomb, the fusion fuel is compressed and heated with a separate fission bomb (see Teller-Ulam design).
A variety of mechanisms transfers the energy of the fission "primary"
explosion into the fusion fuel. A primary mechanism is that the flash of
x-rays given off by the primary is trapped within the engineered case
of the bomb, causing the volume between the case and the bomb to fill
with an x-ray "gas". These x-rays evenly illuminate the outside of the
fusion section, the "secondary", rapidly heating it until it explodes
outward. This outward blowoff causes the rest of the secondary to be
compressed inward until it reaches the temperature and density where
fusion reactions begin.
The requirement of a fission bomb makes the method impractical
for power generation. Not only would the triggers be prohibitively
expensive to produce, but there is a minimum size that such a bomb can
be built, defined roughly by the critical mass of the plutonium
fuel used. Generally it seems difficult to build nuclear devices
smaller than about 1 kiloton in yield, and the fusion secondary would
add to this. This makes it a difficult engineering problem to extract
power from the resulting explosions; the Project PACER studies solutions to the engineering issues, but also demonstrated the cost was simply not economically feasible.
One of the PACER participants, John Nuckolls,
began to explore what happened to the size of the primary required to
start the fusion reaction as the size of the secondary was scaled down.
He discovered that as the secondary reaches the miligram size, the
amount of energy needed to spark it fell into the megajoule range. This
was far below what was needed for a bomb, where the primary was in the
terajoule range, the equivalent of about 6 ounces of TNT.
This would not be economically feasible, such a device would cost
more than the value of the electricity it produced. However, there were
any number of other devices that might be able to repeatedly deliver
this sort of energy level. This led to the idea of using a device that
would "beam" the energy at the fusion fuel, ensuring mechanical
separation. By the mid-1960s, it appeared that the laser would develop to the point where the required energy levels would be available.
Generally ICF systems use a single laser, the driver,
whose beam is split up into a number of beams which are subsequently
individually amplified by a trillion times or more. These are sent into
the reaction chamber (called a target chamber) by a number of mirrors,
positioned in order to illuminate the target evenly over its whole
surface. The heat applied by the driver causes the outer layer of the
target to explode, just as the outer layers of an H-bomb's fuel cylinder
do when illuminated by the X-rays of the fission device.
The material exploding off the surface causes the remaining
material on the inside to be driven inwards with great force, eventually
collapsing into a tiny near-spherical ball. In modern ICF devices the
density of the resulting fuel mixture is as much as one-hundred times
the density of lead, around 1000 g/cm3. This density is not high enough to create any useful rate of fusion on its own. However, during the collapse of the fuel, shock waves
also form and travel into the center of the fuel at high speed. When
they meet their counterparts moving in from the other sides of the fuel
in the center, the density of that spot is raised much further.
Given the correct conditions, the fusion rate in the region
highly compressed by the shock wave can give off significant amounts of
highly energetic alpha particles.
Due to the high density of the surrounding fuel, they move only a short
distance before being "thermalised", losing their energy to the fuel as
heat. This additional energy will cause additional fusion reactions in
the heated fuel, giving off more high-energy particles. This process
spreads outward from the centre, leading to a kind of self-sustaining
burn known as ignition.
Schematic
of the stages of inertial confinement fusion using lasers. The blue
arrows represent radiation; orange is blowoff; purple is inwardly
transported thermal energy.
- Laser beams or laser-produced X-rays rapidly heat the surface of the fusion target, forming a surrounding plasma envelope.
- Fuel is compressed by the rocket-like blowoff of the hot surface material.
- During the final part of the capsule implosion, the fuel core reaches 20 times the density of lead and ignites at 100,000,000 ˚C.
- Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.
Issues with successful achievement
The
primary problems with increasing ICF performance since the early
experiments in the 1970s have been of energy delivery to the target,
controlling symmetry of the imploding fuel, preventing premature heating
of the fuel (before maximum density is achieved), preventing premature
mixing of hot and cool fuel by hydrodynamic instabilities and the formation of a 'tight' shockwave convergence at the compressed fuel center.
In order to focus the shock wave on the center of the target, the target must be made with extremely high precision and sphericity
with aberrations of no more than a few micrometres over its surface
(inner and outer). Likewise the aiming of the laser beams must be
extremely precise and the beams must arrive at the same time at all
points on the target. Beam timing is a relatively simple issue though
and is solved by using delay lines in the beams' optical path to achieve picosecond levels of timing accuracy. The
other major problem plaguing the achievement of high symmetry and high
temperatures/densities of the imploding target are so called "beam-beam"
imbalance and beam anisotropy. These problems are, respectively, where
the energy delivered by one beam may be higher or lower than other beams
impinging on the target and of "hot spots" within a beam diameter
hitting a target which induces uneven compression on the target surface,
thereby forming Rayleigh-Taylor instabilities in the fuel, prematurely mixing it and reducing heating efficacy at the time of maximum compression. The Richtmyer-Meshkov instability is also formed during the process due to shock waves being formed.
An
Inertial confinement fusion target, which was a foam filled cylindrical
target with machined perturbations, being compressed by the Nova Laser.
This shot was done in 1995. The image shows the compression of the
target, as well as the growth of the Rayleigh-Taylor instabilities.
All of these problems have been substantially mitigated to varying
degrees in the past two decades of research by using various beam
smoothing techniques and beam energy diagnostics to balance beam to beam
energy; however, RT instability remains a major issue. Target design
has also improved tremendously over the years. Modern cryogenic
hydrogen ice targets tend to freeze a thin layer of deuterium just on
the inside of a plastic sphere while irradiating it with a low power IR laser to smooth its inner surface while monitoring it with a microscope equipped camera, thereby allowing the layer to be closely monitored ensuring its "smoothness".
Cryogenic targets filled with a deuterium tritium (D-T) mixture are
"self-smoothing" due to the small amount of heat created by the decay of
the radioactive tritium isotope. This is often referred to as "beta-layering".
Mockup of a gold plated National Ignition Facility (NIF) hohlraum.
An inertial confinement fusion
fuel microcapsule (sometimes called a "microballoon") of the size to be
used on the NIF which can be filled with either deuterium and tritium
gas or DT ice. The capsule can be either inserted in a hohlraum (as
above) and imploded in the indirect drive mode or irradiated directly with laser energy in the direct drive
configuration. Microcapsules used on previous laser systems were
significantly smaller owing to the less powerful irradiation earlier
lasers were capable of delivering to the target.
Certain targets are surrounded by a small metal cylinder which is
irradiated by the laser beams instead of the target itself, an approach
known as "indirect drive". In this approach the lasers are focused on the inner side of the cylinder, heating it to a superhot plasma which radiates mostly in X-rays.
The X-rays from this plasma are then absorbed by the target surface,
imploding it in the same way as if it had been hit with the lasers
directly. The absorption of thermal x-rays by the target is more
efficient than the direct absorption of laser light, however these hohlraums
or "burning chambers" also take up considerable energy to heat on their
own thus significantly reducing the overall efficiency of
laser-to-target energy transfer. They are thus a debated feature even
today; the equally numerous "direct-drive" design does not use them. Most often, indirect drive hohlraum targets are used to simulate thermonuclear weapons tests due to the fact that the fusion fuel in them is also imploded mainly by X-ray radiation.
A variety of ICF drivers are being explored. Lasers have improved
dramatically since the 1970s, scaling up in energy and power from a few
joules and kilowatts to megajoules (see NIF laser) and hundreds of terawatts, using mostly frequency doubled or tripled light from neodymium glass amplifiers.
Heavy ion beams are particularly interesting for commercial
generation, as they are easy to create, control, and focus. On the
downside, it is very difficult to achieve the very high energy densities
required to implode a target efficiently, and most ion-beam systems
require the use of a hohlraum surrounding the target to smooth out the
irradiation, reducing the overall efficiency of the coupling of the ion beam's energy to that of the imploding target further.
History
First conception
In the US
Inertial
confinement fusion history can be traced back to the "Atoms For Peace"
conference held in 1957 in Geneva. This was a large, international UN
sponsored conference between the superpowers of the US and Russia. Among
the many topics covered during the event, some thought was given to
using a hydrogen bomb to heat a water-filled underground cavern. The
resulting steam would then be used to power conventional generators, and
thereby provide electrical power.
This meeting led to the Operation Plowshare efforts, given this name in 1961. Three primary concepts were studied as part of Plowshare; energy generation under Project PACER,
the use of large nuclear explosions for excavation, and as a sort of
nuclear fracking for the natural gas industry. PACER was directly tested
in December 1961 when the 3 kt Project Gnome
device was emplaced in bedded salt in New Mexico. In spite of all
theorizing and attempts to stop it, radioactive steam was released from
the drill shaft, some distance from the test site. Further studies as
part of PACER led to a number of engineered cavities replacing natural
ones, but through this period the entire Plowshare efforts turned from
bad to worse, especially after the failure of 1962's Sedan which released huge quantities of fallout.
PACER nevertheless continued to receive some funding until 1975, when a
3rd party study demonstrated that the cost of electricity from PACER
would be the equivalent to conventional nuclear plants with fuel costs
over ten times as great as they were.
Another outcome of the "Atoms For Peace" conference was to prompt John Nuckolls
to start considering what happens on the fusion side of the bomb. When A
fission bomb explodes, it releases X-Rays, which implodes the fusion
side. This "secondary," was scaled down to very small size. His earliest
work concerned the study of how small a fusion bomb could be made while
still having a large "gain"
to provide net energy output. This work suggested that at very small
sizes, on the order of milligrams, very little energy would be needed to
ignite it, much less than a fission "primary".
He proposed building, in effect, tiny all-fusion explosives using a
tiny drop of D-T fuel suspended in the center of a metal shell, today
known as a hohlraum. The shell provided the same effect as the bomb
casing in an H-bomb, trapping x-rays inside so they irradiate the fuel.
The main difference is that the x-rays would not be supplied by a
primary within the shell, but some sort of external device that heated
the shell from the outside until it was glowing in the x-ray region (see
thermal radiation). The power would be delivered by a then-unidentified pulsed power source he referred to using bomb terminology, the "primary".
The main advantage to this scheme is the efficiency of the fusion
process at high densities. According to the Lawson criterion, the
amount of energy needed to heat the D-T fuel to break-even conditions at
ambient pressure is perhaps 100 times greater than the energy needed to
compress it to a pressure that would deliver the same rate of fusion.
So, in theory, the ICF approach would be dramatically more efficient in
terms of gain.
This can be understood by considering the energy losses in a
conventional scenario where the fuel is slowly heated, as in the case of
magnetic fusion energy;
the rate of energy loss to the environment is based on the temperature
difference between the fuel and its surroundings, which continues to
increase as the fuel is heated. In the ICF case, the entire hohlraum is
filled with high-temperature radiation, limiting losses.
In Germany
Around the same time (in 1956) a meeting was organized at the Max Planck Institute in Germany by the fusion pioneer Carl Friedrich von Weizsäcker. At this meeting Friedwardt Winterberg proposed the non-fission ignition of a thermonuclear micro-explosion by a convergent shock wave driven with high explosives.
Further reference to Winterberg's work in Germany on nuclear micro
explosions (mininukes) is contained in a declassified report of the
former East German Stasi (Staatsicherheitsdienst).
In 1964 Winterberg proposed that ignition could be achieved by an
intense beam of microparticles accelerated to a velocity of 1000 km/s. And in 1968, he proposed to use intense electron and ion beams, generated by Marx generators, for the same purpose.
The advantage of this proposal is that the generation of charged
particle beams is not only less expensive than the generation of laser
beams but also can entrap the charged fusion reaction products due to
the strong self-magnetic beam field, drastically reducing the
compression requirements for beam ignited cylindrical targets.
In the USSR
In 1967 research fellow Gurgen Askaryan published article with proposition to use focused laser beam in fusion lithium deuteride or deuterium.
Early research
Through the late 1950s, Nuckolls and collaborators at the Lawrence Livermore National Laboratory
(LLNL) ran a number of computer simulations of the ICF concept. In
early 1960 this produced a full simulation of the implosion of 1 mg of
D-T fuel inside a dense shell. The simulation suggested that a 5 MJ
power input to the hohlraum would produce 50 MJ of fusion output, a gain
of 10. At the time the laser had not yet been invented, and a wide
variety of possible drivers were considered, including pulsed power
machines, charged particle accelerators, plasma guns, and hypervelocity
pellet guns.
Through the year two key theoretical advances were made. New
simulations considered the timing of the energy delivered in the pulse,
known as "pulse shaping", leading to better implosion. Additionally, the
shell was made much larger and thinner, forming a thin shell as opposed
to an almost solid ball. These two changes dramatically increased the
efficiency of the implosion, and thereby greatly lowered the energy
required to compress it. Using these improvements, it was calculated
that a driver of about 1 MJ would be needed, a five-fold improvement. Over the next two years several other theoretical advancements were proposed, notably Ray Kidder's development of an implosion system without a hohlraum, the so-called "direct drive" approach, and Stirling Colgate and Ron Zabawski's work on very small systems with as little as 1 μg of D-T fuel.
The introduction of the laser in 1960 at Hughes Research Laboratories in California appeared to present a perfect driver mechanism. Starting in 1962, Livermore's director John S. Foster, Jr.
and Edward Teller began a small-scale laser study effort directed
toward the ICF approach. Even at this early stage the suitability of the
ICF system for weapons research was well understood, and the primary
reason for its ability to gain funding. Over the next decade, LLNL made several small experimental devices for basic laser-plasma interaction studies.
Development begins
In 1967 Kip Siegel started KMS Industries using the proceeds of the sale of his share of an earlier company, Conductron, a pioneer in holography. In the early 1970s he formed KMS Fusion to begin development of a laser-based ICF system.
This development led to considerable opposition from the weapons labs,
including LLNL, who put forth a variety of reasons that KMS should not
be allowed to develop ICF in public. This opposition was funnelled
through the Atomic Energy Commission,
who demanded funding for their own efforts. Adding to the background
noise were rumours of an aggressive Soviet ICF program, new
higher-powered CO2 and glass lasers, the electron beam driver concept, and the 1970s energy crisis which added impetus to many energy projects.
In 1972 Nuckolls wrote an influential public paper in Nature
introducing ICF and suggesting that testbed systems could be made to
generate fusion with drivers in the kJ range, and high-gain systems with
MJ drivers.
In spite of limited resources and numerous business problems, KMS
Fusion successfully demonstrated fusion from the ICF process on 1 May
1974.
However, this success was followed not long after by Siegel's death,
and the end of KMS fusion about a year later, having run the company on
Siegel's life insurance policy. By this point several weapons labs and universities had started their own programs, notably the solid-state lasers (Nd:glass lasers) at LLNL and the University of Rochester, and krypton fluoride excimer lasers systems at Los Alamos and the Naval Research Laboratory.
Although KMS's success led to a major development effort, the
advances that followed were, and still are, hampered by the seemingly
intractable problems that characterize fusion research in general.
High-energy ICF
High-energy
ICF experiments (multi-hundred joules per shot and greater experiments)
began in earnest in the early-1970s, when lasers of the required energy
and power were first designed. This was some time after the successful
design of magnetic confinement fusion systems, and around the time of
the particularly successful tokamak design that was introduced in the early '70s. Nevertheless, high funding for fusion research stimulated by the multiple energy crises
during the mid to late 1970s produced rapid gains in performance, and
inertial designs were soon reaching the same sort of "below break-even"
conditions of the best magnetic systems.
LLNL was, in particular, very well funded and started a major laser fusion development program. Their Janus laser
started operation in 1974, and validated the approach of using Nd:glass
lasers to generate very high power devices. Focusing problems were
explored in the Long path laser and Cyclops laser, which led to the larger Argus laser.
None of these were intended to be practical ICF devices, but each one
advanced the state of the art to the point where there was some
confidence the basic approach was valid. At the time it was believed
that making a much larger device of the Cyclops type could both compress
and heat the ICF targets, leading to ignition in the "short term". This
was a misconception based on extrapolation of the fusion yields seen
from experiments utilizing the so-called "exploding pusher" type of fuel
capsules. During the period spanning the years of the late '70s and
early '80s the estimates for laser energy on target needed to achieve
ignition doubled almost yearly as the various plasma instabilities and
laser-plasma energy coupling loss modes were gradually understood. The
realization that the simple exploding pusher target designs and mere few
kilojoule (kJ) laser irradiation intensities would never scale to high
gain fusion yields led to the effort to increase laser energies to the
100 kJ level in the UV and to the production of advanced ablator and
cryogenic DT ice target designs.
Shiva and Nova
One of the earliest serious and large scale attempts at an ICF driver design was the Shiva laser,
a 20-beam neodymium doped glass laser system built at the (LLNL) that
started operation in 1978. Shiva was a "proof of concept" design
intended to demonstrate compression of fusion fuel capsules to many
times the liquid density of hydrogen. In this, Shiva succeeded and
compressed its pellets to 100 times the liquid density of deuterium.
However, due to the laser's strong coupling with hot electrons,
premature heating of the dense plasma (ions) was problematic and fusion
yields were low. This failure by Shiva to efficiently heat the
compressed plasma pointed to the use of optical frequency multipliers
as a solution which would frequency triple the infrared light from the
laser into the ultraviolet at 351 nm. Newly discovered schemes to
efficiently frequency triple high intensity laser light discovered at
the Laboratory for Laser Energetics in 1980 enabled this method of target irradiation to be experimented with in the 24 beam OMEGA laser and the NOVETTE laser, which was followed by the Nova laser design with 10 times the energy of Shiva, the first design with the specific goal of reaching ignition conditions.
Nova also failed in its goal of achieving ignition, this time due
to severe variation in laser intensity in its beams (and differences in
intensity between beams) caused by filamentation which resulted in
large non-uniformity in irradiation smoothness at the target and
asymmetric implosion. The techniques pioneered earlier could not address
these new issues. But again this failure led to a much greater
understanding of the process of implosion, and the way forward again
seemed clear, namely the increase in uniformity of irradiation, the
reduction of hot-spots in the laser beams through beam smoothing
techniques to reduce Rayleigh–Taylor instabilities imprinting on the
target and increased laser energy on target by at least an order of
magnitude. Funding for fusion research was severely constrained in the
80's, but Nova nevertheless successfully gathered enough information for
a next generation machine.
National Ignition Facility
The resulting design, now known as the National Ignition Facility,
started construction at LLNL in 1997. NIF's main objective will be to
operate as the flagship experimental device of the so-called nuclear stewardship program, supporting LLNLs traditional bomb-making role. Completed in March 2009,
NIF has now conducted experiments using all 192 beams, including
experiments that set new records for power delivery by a laser.
The first credible attempts at ignition were initially scheduled for 2010, but ignition was not achieved as of September 30, 2012.
As of October 7, 2013, the facility is understood to have achieved an
important milestone towards commercialization of fusion, namely, for the
first time a fuel capsule gave off more energy than was applied to it. This is still a long way from satisfying the Lawson criterion, but is a major step forward.
Fast ignition
A more recent development is the concept of "fast ignition,"
which may offer a way to directly heat the high density fuel after
compression, thus decoupling the heating and compression phases of the
implosion. In this approach the target is first compressed "normally"
using a driver laser system, and then when the implosion reaches maximum
density (at the stagnation point or "bang time"), a second ultra-short
pulse ultra-high power petawatt
(PW) laser delivers a single pulse focused on one side of the core,
dramatically heating it and hopefully starting fusion ignition. The two
types of fast ignition are the "plasma bore-through" method and the
"cone-in-shell" method. In the first method the petawatt laser is simply
expected to bore straight through the outer plasma of an imploding
capsule and to impinge on and heat the dense core, whereas in the
cone-in-shell method, the capsule is mounted on the end of a small
high-z (high atomic number)
cone such that the tip of the cone projects into the core of the
capsule. In this second method, when the capsule is imploded, the
petawatt has a clear view straight to the high density core and does not
have to waste energy boring through a 'corona' plasma; however, the
presence of the cone affects the implosion process in significant ways
that are not fully understood. Several projects are currently underway
to explore the fast ignition approach, including upgrades to the OMEGA laser at the University of Rochester, the GEKKO XII device in Japan, and an entirely new £500 million facility, known as HiPER, proposed for construction in the European Union.
If successful, the fast ignition approach could dramatically lower the
total amount of energy needed to be delivered to the target; whereas NIF
uses UV beams of 2 MJ, HiPER's driver is 200 kJ and heater 70 kJ, yet
the predicted fusion gains are nevertheless even higher than on NIF.
Other projects
Laser Mégajoule,
the French project, has seen its first experimental line achieved in
2002, and its first target shots were finally conducted in 2014. The machine was roughly 75% complete as of 2016.
Using a different approach entirely is the z-pinch device. Z-pinch
uses massive amounts of electric current which is switched into a
cylinder comprising extremely fine wires. The wires vaporize to form an
electrically conductive, high current plasma; the resulting
circumferential magnetic field squeezes the plasma cylinder, imploding
it and thereby generating a high-power x-ray pulse that can be used to
drive the implosion of a fuel capsule. Challenges to this approach
include relatively low drive temperatures, resulting in slow implosion
velocities and potentially large instability growth, and preheat caused
by high-energy x-rays.
Most recently, Winterberg has proposed the ignition of a
deuterium microexplosion, with a gigavolt super-Marx generator, which is
a Marx generator driven by up to 100 ordinary Marx generators.
As an energy source
Practical
power plants built using ICF have been studied since the late 1970s
when ICF experiments were beginning to ramp up to higher powers; they
are known as inertial fusion energy, or IFE plants. These
devices would deliver a successive stream of targets to the reaction
chamber, several a second typically, and capture the resulting heat and
neutron radiation from their implosion and fusion to drive a
conventional steam turbine.
Technical challenges
IFE
faces continued technical challenges in reaching the conditions needed
for ignition. But even if these were all to be solved, there are a
significant number of practical problems that seem just as difficult to
overcome. Laser-driven systems were initially believed to be able to
generate commercially useful amounts of energy. However, as estimates of
the energy required to reach ignition grew dramatically during the
1970s and '80s, these hopes were abandoned. Given the low efficiency of
the laser amplification process (about 1 to 1.5%), and the losses in
generation (steam-driven turbine systems are typically about 35%
efficient), fusion gains would have to be on the order of 350 just to
energetically break even. These sorts of gains appeared to be impossible
to generate, and ICF work turned primarily to weapons research.
With the recent introduction of fast ignition and similar
approaches, things have changed dramatically. In this approach gains of
100 are predicted in the first experimental device, HiPER. Given a gain
of about 100 and a laser efficiency of about 1%, HiPER produces about
the same amount of fusion energy as electrical energy was needed
to create it. It also appears that an order of magnitude improvement in
laser efficiency may be possible through the use of newer designs that
replace the flash lamps with laser diodes
that are tuned to produce most of their energy in a frequency range
that is strongly absorbed. Initial experimental devices offer
efficiencies of about 10%, and it is suggested that 20% is a real
possibility with some additional development.
With "classical" devices like NIF about 330 MJ of electrical
power are used to produce the driver beams, producing an expected yield
of about 20 MJ, with the maximum credible yield of 45 MJ. Using the same
sorts of numbers in a reactor combining fast ignition with newer lasers
would offer dramatically improved performance. HiPER requires about
270 kJ of laser energy, so assuming a first-generation diode laser
driver at 10% the reactor would require about 3 MJ of electrical power.
This is expected to produce about 30 MJ of fusion power.
Even a very poor conversion to electrical energy appears to offer
real-world power output, and incremental improvements in yield and laser
efficiency appear to be able to offer a commercially useful output.
Practical problems
ICF
systems face some of the same secondary power extraction problems as
magnetic systems in generating useful power from their reactions. One of
the primary concerns is how to successfully remove heat from the
reaction chamber without interfering with the targets and driver beams.
Another serious concern is that the huge number of neutrons
released in the fusion reactions react with the plant, causing them to
become intensely radioactive themselves, as well as mechanically
weakening metals. Fusion plants built of conventional metals like steel would have a fairly short lifetime and the core containment vessels will have to be replaced frequently.
One current concept in dealing with both of these problems, as shown in the HYLIFE-II baseline design, is to use a "waterfall" of FLiBe, a molten mix of fluoride salts of lithium and beryllium, which both protect the chamber from neutrons and carry away heat. The FLiBe is then passed into a heat exchanger where it heats water for use in the turbines.
The tritium produced by fissioning lithium nuclei can also be extracted
in order to close the power plant's thermonuclear fuel cycle, a
necessity for perpetual operation because tritium does not exist in
quantity naturally and must be manufactured. Another concept, Sombrero, uses a reaction chamber built of Carbon-fiber-reinforced polymer which has a very low neutron cross section.
Cooling is provided by a molten ceramic, chosen because of its ability
to stop the neutrons from traveling any further, while at the same time
being an efficient heat transfer agent.
An
inertial confinement fusion implosion in Nova, creating "micro sun"
conditions of tremendously high density and temperature rivalling even
those found at the core of our Sun.
Economic viability
Even
if these technical advances solve the considerable problems in IFE,
another factor working against IFE is the cost of the fuel. Even as
Nuckolls was developing his earliest detailed calculations on the idea,
co-workers pointed this out: if an IFE machine produces 50 MJ of fusion
energy, one might expect that a shot could produce perhaps 10 MJ of
power for export. Converted to better known units, this is the
equivalent of 2.8 kWh of electrical power. Wholesale rates for
electrical power on the grid were about 0.3 cents/kWh at the time, which
meant the monetary value of the shot was perhaps one cent. In the
intervening 50 years the price of power has remained about even with the
rate of inflation, and the rate in 2012 in Ontario, Canada was about 2.8 cents/kWh.
Thus, in order for an IFE plant to be economically viable, fuel
shots would have to cost considerably less than ten cents in year 2012
dollars. At the time this objection was first noted, Nuckolls suggested
using liquid droplets sprayed into the hohlraum from an eye-dropper-like
apparatus.
Given the ever-increasing demands for higher uniformity of the targets,
this approach does not appear practical, as even the inner ablator and
fuel itself currently costs several orders of magnitude more than this.
Moreover, Nuckolls' solution had the fuel dropped into a fixed hohlraum
that would be re-used in a continual cycle, but at current energy levels
the hohlraum is destroyed with every shot.
Direct-drive systems avoid the use of a hohlraum and thereby may
be less expensive in fuel terms. However, these systems still require an
ablator, and the accuracy and geometrical considerations are even more
important. They are also far less developed than the indirect-drive
systems, and face considerably more technical problems in terms of
implosion physics. Currently there is no strong consensus whether a
direct-drive system would actually be less expensive to operate.
Projected development
The
various phases of such a project are the following, the sequence of
inertial confinement fusion development follows much the same outline:
- Burning demonstration
- Reproducible achievement of some fusion energy release (not necessarily a Q factor of >1).
- High gain demonstration
- Experimental demonstration of the feasibility of a reactor with a sufficient energy gain.
- Industrial demonstration
- Validation of the various technical options, and of the whole data needed to define a commercial reactor.
- Commercial demonstration
- Demonstration of the reactor ability to work over a long period, while respecting all the requirements for safety, liability and cost.
At the moment, according to the available data,
inertial confinement fusion experiments have not gone beyond the first
phase, although Nova and others have repeatedly demonstrated operation
within this realm. In the short term a number of new systems are
expected to reach the second stage.
For a true industrial demonstration, further work is required. In
particular, the laser systems need to be able to run at high operating
frequencies, perhaps one to ten times a second. Most of the laser
systems mentioned in this article have trouble operating even as much as
once a day. Parts of the HiPER budget are dedicated to research in this
direction as well. Because they convert electricity into laser light
with much higher efficiency, diode lasers also run cooler, which in turn
allows them to be operated at much higher frequencies. HiPER is
currently studying devices that operate at 1 MJ at 1 Hz, or alternately
100 kJ at 10 Hz.
R&D continued on inertial fusion energy in the European Union
and in Japan. The High Power laser Energy Research (HiPER) facility is a
proposed experimental fusion device undergoing preliminary design for
possible construction in the European Union
to continue the development of laser-driven inertial confinement
approach. HiPER is the first experiment designed specifically to study
the fast ignition approach to generating nuclear fusion. Using
much smaller lasers than conventional designs, yet produces fusion power
outputs of about the same magnitude would offer a much higher Q
with a reduction in construction costs of about ten times. Theoretical
research since the design of HiPER in the early 2000s has cast doubt on
fast ignition but a new approach known as shock ignition has been proposed to address some of these problems. Japan developed the KOYO-F fusion reactor design and laser inertial fusion test (LIFT) experimental reactor. In April 2017, Bloomberg News reported that Mike Cassidy, former Google vice-president and director of Project Loon with Google[x], started a clean energy startup, Apollo Fusion, to develop a hybrid fusion-fission reactor technology.
Nuclear weapons program
The
very hot and dense conditions encountered during an Inertial
Confinement Fusion experiment are similar to those created in a
thermonuclear weapon, and have applications to the nuclear weapons
program. ICF experiments might be used, for example, to help determine
how warhead performance will degrade as it ages, or as part of a program
of designing new weapons. Retaining knowledge and corporate expertise
in the nuclear weapons program is another motivation for pursuing ICF.
Funding for the NIF in the United States is sourced from the 'Nuclear
Weapons Stockpile Stewardship' program, and the goals of the program are
oriented accordingly. It has been argued that some aspects of ICF research may violate the Comprehensive Test Ban Treaty or the Nuclear Non-Proliferation Treaty. In the long term, despite the formidable technical hurdles, ICF research might potentially lead to the creation of a "pure fusion weapon".
Neutron source
Inertial confinement fusion has the potential to produce orders of magnitude more neutrons than spallation.
Neutrons are capable of locating hydrogen atoms in molecules, resolving
atomic thermal motion and studying collective excitations of photons
more effectively than X-rays. Neutron scattering studies of molecular structures could resolve problems associated with protein folding, diffusion through membranes, proton transfer mechanisms, dynamics of molecular motors, etc. by modulating thermal neutrons into beams of slow neutrons. In combination with fissionable materials, neutrons produced by ICF can potentially be used in Hybrid Nuclear Fusion designs to produce electric power.
