Technology

Principles of Fusion Energy Generation
The fusion reaction between deuterium and tritium releases huge amounts of energy. Fusion of a fraction of a gram of material (about 1/30th of a gram) releases the amount of energy contained in two barrels of oil. This is one essential aspect of an ideal energy source. There is no possibility of more fusion energy being released than planned, because only the desired amount of fuel is injected and subjected to the demanding technical processes that will bring the fuel to the very special conditions that will initiate the reaction. These special conditions, and the processes to bring the fuel to them, are described in summary form below.

  Principles of Fusion

Fusion energy is based on combining the nuclei of light elements to form nuclei of heavier elements. This releases energy proportional to the difference between the sum of the mass-energies of the fusing nuclei and mass-energies of the product particles, according to E=mc2.  In the current instance, our fuel (the fusing nuclei) will be the two heavy isotopes of hydrogen.  We will produce ordinary helium as our product isotope along with an energetic neutron, as shown in Figure T-1.      

Figure T-1.  Reactants and products in the Fusion reaction

The isotopes of hydrogen have specific names, unlike the isotopes of other elements, namely deuterium and tritium.  Deuterium(2H) is naturally present in all water and thus seawater is our primary source of fuel.  Tritium(3H), the other component of fuel in a fusion power source, is of very low abundance in nature.  This is in consequence of tritium being an unstable isotope with a relatively short half-life, 12.3 years. Tritium to start-up the first of our fusion systems will come from stores extracted from fission power plants, where it serves no useful purpose and is unwanted. Containment of tritium is virtually the sole radiological safety issue for fusion power. The difficulty of achieving zero release of tritium in fission power plants comes from having water both in contact with the core and to drive steam turbines. Fusion does not have this challenge, and zero release is a practical goal.

Although an external source of tritium is needed to start our operations, we will
produce it for long-term operations via a feature of the D-T reaction. Like all D-T
fusion systems, we will use the neutron from the fusion reaction to produce tritium from neutron-lithium reactions. Lithium is consumed in the D-T fuel cycle. As
discussed in the last section (below), the lithium needed to start-up the first fusion system will come from conventional, land-based sources. However, the oceans contain large quantities of lithium, and FPC’s overall system includes extraction of lithium from seawater to produce the energy the world needs. Thus resources for our two long-term fuel needs for deuterium and lithium are found in the oceans. We will extract our fuel in processes that are sensitive environmentally, and these resources are enough to last millions of years.
 
The FPC system has a unique potential to breed substantially more tritium than it
burns. This is an important asset to the start-up of the additional HIF power sites
needed around the world for two reasons. First, because it uses the more plentiful
lithium isotope (7Li) as well as 6Li (7.5% of the total), it reduces the net amount of
lithium that will ultimately be consumed over time in the fusion fuel cycle.
Second, the excess tritium will supply the startup needs of successive fusion plants,
avoiding a potential bottleneck due to limited tritium from non-fusion sources.
Most of the excess tritium will be sold for this purpose, but some may be securely
stored and allowed to decay to 3He, a valuable substance with extraordinary
physical properties as well as being a fusion fuel. 
 
The fusion reaction between deuterium and tritium releases huge amounts of
energy.  Fusion of a fraction of a gram of material (about 1/30th of a gram) releases
the amount of energy contained in two barrels of oil. This is one essential aspect of
an ideal energy source. There is no possibility of more fusion energy being
released than planned, because only the desired amount of fuel is injected and
subjected to the demanding technical processes that will bring the fuel to the very
special conditions that will initiate the reaction. These special conditions, and the
processes to bring the fuel to them, are described in summary form below.


  Conditions Required for Fusion

The essential challenge of fusion is that the fuel nuclei are electrically charged with
the same “sign”. The electrostatic (Coulombic) force pushes the nuclei apart,
opposing their coming close enough together for the nuclear force to take over and
cause fusion to occur. To get past this “Coulomb barrier”, the nuclei must be on a
collision course at very high speed.  This means the fuel must be raised to extreme temperatures.

Temperatures on the order of 100 million degrees are needed to fuse the lightest
elements––regardless of the technology used to confine matter at such
temperatures. But temperature is not the only challenge for fusion power; there
must be enough fuel and it must be confined in some volume—somehow—long
enough to react. The simplest process to imagine is to suddenly heat a mass of fuel
so that it reacts before it flies apart. Here, the only means for confinement is
“inertia”, giving this genre of fusion power its name, “inertial confinement fusion
(ICF).” This defines another key requirement for ICF; the energy to initiate the
reactions must be input in a very short period of time. 
 
Efficient ICF (yielding more energy from fusion than was put in to initiate the
reactions) involves yet another challenge: fuel density. The time available to input
the ignition energy is set by the small dimensions of the pellets containing the
small amount of fuel that will be “burned” in each pulse (about 40% of the total
fuel in each pellet). However, at “normal” density, the reaction would proceed too
slowly to occur before the fuel flies apart at the speed of sound at 100 million oC.
The physics of the solution lies in the increase of the reaction rate with the square
of the fuel density: density of “target” fuel nuclei times density of “projectile” fuel
nuclei.
 
The fuel density needed for the reaction to be rapid enough to occur in the short
time available means the fuel must be compressed far beyond the density of the
fuel in the normal solid state. For hydrogen isotopes, before the ignition sequence
begins the fuel is at cryogenic temperatures, because hydrogen freezes at –259 oC.
In the energetics of the ensuing process, however, freezing the fuel during its
preparation immediately pales into insignificance. 
 
One of the most exciting facts of the status of ICF development is that the ability to
compress the fuel to the density that FPC’s system needs has been within the state
of the art for some time. This requires, however, the ability to accomplish the two-
step ignition process known as “fast ignition”, which, as discussed in the next section, FPC’s igniter system (“driver”) design is poised to do. 
 
For the fusion burn to yield many times the input energy (“high gain”), the input
pulse is tailored to the fuel dynamics so that only a portion of the high-density fuel
is heated all the way to ignition temperature. This initially burning mass needs to
be only big enough to absorb the energy carried by the energetic !-particles
(helium nuclei) produced by fusion. When this condition is met, the fuel self-heats
to temperatures much higher (by 8 to 10 times) than 100 million oC to greatly boost the reaction rate. The same energy redeposition process spreads (propagates) the
high temperature to the rest of the fuel mass, achieving efficient burn within the
instant of time allowed by inertia. In this sudden process is an aspect that is
beautiful for safety: in the brief instant of the reactions, all the fuel that can burn
does burn. After the intended reactions, there is absolutely zero chance of any
additional heating.
 
The history of the R&D effort on ICF has been a steady march toward achieving
the necessary fuel compression. For the brief introduction here, the most important
point is that the fuel compression needed for FPC’s heavy ion fusion power system
has been demonstrated in experimental laboratories: a density of 100 grams per
cubic centimeter (about ten times the density of lead). These demonstrations are
backed up by strong correlation to mathematical physics simulations. The great
importance of compression to 100 g/cc being already within the state of the art is
made clear in the next section, which is devoted to the heavy ion accelerator driver.
 
The repeatability and reliability of the demonstrations of 100 g/cc underscores the
high degree of understanding that has been achieved over the decades of ICF
research. The world’s first ICF program began at the Lawrence Livermore
National Laboratory, ten years after the basic physical processes were
demonstrated in 1952—albeit with two features that needed radical improvement
to make clean fusion power possible: 1. The demonstration was an extremely large
explosion, and B. The source of the ignition energy was a fission explosion.  ICF
research began in 1962, very soon after the invention of the “giant pulse” or “Q-
switched” laser. The laser’s “coherent” light provided the ability to focus to the
needed small dimensions for the small fuel pellets, and the laser’s short pulses
could meet the short time requirement for the energy input.
 
By the mid-1970s, realization had set in that lasers would meet their inherent
limitations short of the energy needed to drive fusion power plants. The
fundamental reason is that lasers by definition require a lasing medium. Heating of
the laser medium by the laser energy while it passes through the medium in
extremely powerful pulses ultimately is too intense for the medium to survive.
Figure T-2 shows the consequence of this crucial issue schematically by the
relationship of the energy multiplication (vertical axis) to the input energy
(horizontal axis). 


Figure T-2. Ignition and burn parameter space

The National Ignition Facility (NIF) operated by the Lawrence Livermore National
Laboratory (LLNL) is the culmination of 50 years of laser development to meet the
requirements for ignition established in the mid-1960s. As foreseen in the 1970s,
no laser is on the drawing board to provide more robust ignition pulses.
Fortunately, contemporaneously with the realization of the limitations of lasers, a
technology able to meet all the needs for practical power by ICF was identified.
This technology uses the particle accelerators developed for the area of basic
research known as high-energy physics.  The small size of fusion pellets needs the
ions to stop quickly, as heavy ions do, giving the name heavy ion fusion (HIF) to
the approach to ICF based on this accelerator technology. 
 

  Heavy Ion Accelerator Driver

Accelerator technology, under development for most of a century, has been
essential to mankind’s deepening understanding of nature. By the early 1970's, the storied development of “atom smashers” had achieved the ability to accelerate
electrons, protons and heavy ions to very high energy – much higher than the
energy per “particle” required in the accelerator drivers needed to initiate fusion.
(See Figure T-3.)  It is important for practical fusion power, as planned by FPC,
that accelerator technology is a mature technology.  Once the desired ion source is
identified and the desired particle energy is specified, engineers can design and
build a machine to supply the needed number of particles (ionized heavy ions in
HIF) in the desired (very short) time window.  In our case, this requires a linear
accelerator that is several miles long.
 
Linear accelerators are a known technology, similar to that used in hundreds of
accelerator facilities around the world. The short stopping distance of heavy ions
(i.e., comparable to dimension of the fuel pellet) allows each ion to be high-energy,
many billions of “electron-Volts, or eV.” (Note the eV units on the vertical axis in
Figure T-3. M is for mega or millions, G for giga or billions, etc.)  
 
Since Power = Volts x Amps, raising the particle energy translates to reducing the
particle current (beam current).  While the technology to attain high energy per
particle was well established by 1975, the big discovery of that year was that
rearrangement of the known accelerator hardware could provide the robust ignition
beams needed to get into the high gain regime that is shown in Figure T-2. The
same spirit of innovation and creativity that led to the steady advances shown in
Figure T-3 would achieve the total beam current needed for the beam to have
Power = Energy x Current ~ 1 PW (a petaWatt is one million billion Watts); this is
the canonical instantaneous power requirement for igniting inertial confinement
fusion. 
 
Heavy ion accelerator drivers have another special advantage for reliable ignition.
This is the ability to achieve the two-step process called “fast ignition”. More
efficient than using the same process that compresses the fuel to ignite a central
“hot spot”, in fast ignition a separate, very sharp pulse (high peak-power and less
than 1/10 the duration of the compression process) is used to ignite only the
desired mass of fuel after it has been compressed.  The “fast ignited” fuel sets off
the rest of the fuel much like a blasting cap sets off a stick of dynamite. The great
importance of this feature of FPC’s driver (also a feature of the Russian design) is
that the required fuel compression has been within the state of the art for some
years already, as noted previously.



Figure T-3. The Livingston curve encapsulates the history of innovation of particle accelerators,
which has been crucial in achieving new discoveries by the steadily increased capabilities.  The
capabilities needed for HIF accelerator-drivers have been present since 1975

Completing the exciting dimensions of the discovery of HIF in 1975 were the
everyday features of accelerators: high repetition rate, high efficiency, and
durability. The innovations in our driver system capitalize on those put forward by
accelerator scientists at, and soon after, the conception of HIF in 1975. The front-
end of our accelerator driver will consist of an array of ion sources. These feed
high-voltage DC accelerator sections, which increase the speed of the ions up to
that needed by RF accelerators. Both ion source and high-voltage DC accelerators
are industrialized technologies, used in applications ranging from ion implantation
in semi-conductors, station keeping on space satellites, sterilization of medical
instruments, and industrial treatment of polymers. Thus, the ion source and low
energy accelerator can be built from known and proven components.
 
Since the heavy ion sources, DC “pre-accelerators”, and RF linear accelerators are
known technology, what remains is the detailed design for the beam
“manipulations” that get the ions to the target as a short duration pulse. Early HIF
driver designs, by groups at major accelerator laboratories worldwide, devised
systems to manipulate the accelerated beams into the necessary short pulses.
Intensive analysis at international HIF workshops, held annually, confirmed the
workability of these concepts. 
 
These systems would produce the beams called for by the fusion target experts, but
sober reflection on the magnitude of the task of igniting fusion (calling for peak
power ~ 1PW and many MJ—several kW-hr or the energy in few sticks of
dynamite) with the high reliability called for by power plants indicated the
desirability of even more intense beams. In response, accelerator system designs to
produce more robust beams were developed in Russia and by Arcata Systems in
the USA. Arcata’s concept offers the most intense beams. Both improved designs
use “fast ignition.” Arcata’s system also avoids use of storage rings. Although
highly reliable in accelerator systems for high-energy physics research, and
important to the founding of HIF, international assessments found storage rings to
be problematic in the HIF context. FPC’s design approach is to systematically
remove all problematic issues.
 
Arcata’s IP now is owned by the Fusion Power Foundation, and the Fusion Power
Corporation is their exclusive licensee. 


  Reaction Chambers

The space in which the fusion reactions take place is called a reaction chamber. 
Three factors influence its design.  First, the chamber needs to hold a good vacuum
to enable the heavy ions from the accelerator system to reach the fuel pellet and to
provide a secure containment vessel for the capture of the tritium that is generated after the reaction takes place.  Second, the chamber must be able to withstand the
pressure generated by the fusion reaction.  And third, the reaction chamber must
contain a liquid that can be heated to a high temperature as part of the energy
extraction process.  All of this has been designed on paper, and unique hardware
has been demonstrated, but an integrated fusion system has, of course, never been
built.  Reaction chambers for ICF experiments have been built at numerous
installations worldwide, the largest of which is part of the National Ignition
Facility at the Lawrence Livermore National Laboratory. 
 
There is a fourth factor that must also be considered in the design of the reaction
chamber. As stated earlier, the neutrons produced by the fusion reaction carry 80%
of the reaction’s energy. The energy must be captured as thermal energy, for
downstream conversion to electricity and other energy products, and the neutrons
must be prevented from degrading the structural properties of the chamber
materials. FPC’s chamber concept accomplishes all the required missions, and
much more. The numerous advantages of the chamber’s configuration include a unique combination of long chamber life and the high temperatures in working fluid that are needed for efficient energy conversion. Ultimately, the set of advantages results in very large economic benefits.
(Also see chamber summary .)
 
 Figure T-4.1  StarPower Reaction Chamber ready for ignition

A cutaway view of the interior of FPC’s fusion chamber is shown conceptually in
Figure T-4.1. To capture the neutrons’ energy, the fuel pellet is located in the
center of an expendable sphere of lithium. The emitted neutrons interact with this
lithium to produce heat and tritium. Additional lithium between the sphere and the
chamber wall completes the capture of the heat and the neutrons. Lithium injectors
create rain with a patterned density, e.g., most dense in proximity to the sphere. Lithium also is injected tangentially to flow along the walls. This performs
multiple functions for efficient handling of the heat and chamber protection. 
 


Figure T-4.2 Microseconds after ignition

Nanoseconds after the fusion reactions take place, the lithium sphere has vaporized and an expanding sphere of lithium plasma (ionized gas) is now present.  As this plasma expands, it cools by radiation (photon flow), expansion, and by interaction with lithium droplets and sprays throughout the chamber. (Figure T-4.2)

Vaporization of the lithium droplets strongly damps the pressure of the expanding
plasma. By mixing with enough mass of lithium in the form of droplets not far
above lithium’s melting point, the vapor condenses and enters the heat exchangers,
at each end of the cylindrical chamber. There, the heat is transferred to secondary
working fluids to be used in various energy conversion processes. The largest
energy products will be hydrogen for liquid fuels and electricity generation by
conventional steam turbines, as illustrated in Figure T-5. 


Figure T-4.3  Chamber ready for the next reaction 

While the hot lithium vapor is condensing at the ends of the chamber, and before
the ion beams enter to ignite the next fusion reaction, the vacuum is being restored
by the “sorption pump” action of fresh lithium droplets, as shown in Figure T-4.3. 

The vapor pressure of lithium at 100 oC above its melting point is very low, which
gives the lithium rain an enormous pumping speed. New fuel pellets, enclosed in
spheres of lithium, are fed into the chamber every 1-3 seconds, and the supply of
heat to the energy conversion system is effectively continuous.



Figure T-5  Schematic of the Heat Exchange Process, illustrating use for electricity generation


  Production of Synthetic Fuels                                                                                

We generally think that fusion power plants will be sources of electricity.  FPC's
StarPower System (SPS) will also produce electricity, but electricity will not be the
only product, and may not be the primary product.  The availability of very high
temperature heat permits the consideration of another process of energy conversion
and that is the direct production of hydrogen from the thermal disassociation of
water. The known, and well researched, Sulfur–Iodine process and the high
temperature electrolysis process both produce hydrogen at about 50% efficiency.
 
Hydrogen is the key component in the production of synthetic fuels and can be
produced from SPS systems at a cost comparable to the cost of production from
reforming of natural gas.  The hydrogen can be used directly as a fuel source or it
can be combined with carbon to form various hydrocarbons including gasoline,
kerosene (jet fuel), or diesel oil.  Since these latter fuels support the existing
transportation infrastructure, we intend to focus our hydrogen output on production
of these synthetic fuel products. 
 
The source of carbon can be biomass, or coal, or the CO2 in the atmosphere. 
Evaluation of the technology for extracting CO2 from the atmosphere by the Los
Alamos National Laboratory concluded that this technology is now mature enough
that it is viable as a source of CO2.  Since we have to dispose of waste heat in any
case, why not use the waste heat to drive a cooling tower that processes a large
volume of air and extracts the CO2 from it?  Carbon from an atmospheric source is
slightly more expensive than carbon from coal, but the use of atmospheric carbon means that the liquid fuel we produce would become carbon neutral – the carbon
comes from the atmosphere and returns to the atmosphere again when the fuel is
consumed.
 
The processes for conversion of hydrogen and CO2 to form synfuels is a mature
industrial process that has been used by Exxon-Mobil in the past and is currently
being used by Sasol to produce liquid fuels in plants in South Africa and Arabia. 
Until now, the constraint for the production of synthetic fuels has been the
availability of an inexpensive source of hydrogen.  The heat from an SPS will
remedy this shortcoming and make the synthesis of liquid fuel possible at a cost
less than the current cost of the same fuel from a crude oil feedstock.  
 

  Development of Direct Conversion Technology                                                 

Fusion power promises to be much more efficient than conventional electrical
power sources.  A fission power plant converts about one third of the energy in the
fuel to useful power sent to the customer as electricity.  This is because the Carnot
(ideal) efficiency is necessarily low since the energy conversion process for fission
is constrained by a low ratio of the input and output temperatures. The ambient
temperature of the environment outside the plant sets the output temperature, but
the input temperature in fission plants is constrained by the limits imposed on the
materials of the pressure vessel. The efficiency of energy conversion in other
fusion systems encounters similar temperature constraints. Their chamber designs
perform similar roles, which lead to similar temperature constraints on their
materials, which need to operate slightly above the temperature of the steam that
will drive the turbines. 
 
In contrast, FPC’s chamber system produces very high temperature heat at the
same time that the chamber operates at uniquely low temperatures. For initial
operations, we base our revenue estimates on the conservative use of conventional
electricity generation using steam turbine systems. However, the high temperatures
phases that the lithium passes through in each fusion power sequence can be
exploited to raise the efficiency by implementing advanced energy conversion
techniques.
 
The earliest of the advanced techniques to be implemented will be high
temperature gas turbines, as illustrated in Figure T-5. By transferring the heat at
the highest temperature, near the condensation temperature of lithium, to a gas such as helium, the efficiency would reach the higher levels that are a well-known
advantage of combined-cycle power plants fired by natural gas. 
 
Exciting prospects for further efficiency increases in the future involve the direct
conversion that becomes a possibility because of the existence of a plasma state
during the first phase of the lithium’s cycle. (Figure T-6) Because the
temperature of this plasma will be above 20,000 oC, 50 percent or more of the heat
can be converted “off the top” before the lithium temperature decreases (by further
expansion and mixing with lithium rain) to be compatible with the steel tubes of
heat exchangers. Considering the ideal efficiency of a thermodynamic process
starting at 20,000 oC, overall efficiencies as high as 90 percent are seen to be
possible. This underscores the fact that releasing fusion energy will not be the end
of the story, but the beginning of a continuous improvement process, as has been
the history with steam power, internal combustion, and even batteries. Most
remarkably, these prospects for developing extraordinarily high efficiencies, which
depend on having extraordinarily high temperatures, are made possible by the
design that keeps the temperature of the structural materials of FPC’s chamber at
extraordinarily low levels.

Figure T-6  Schematic view of a reaction chamber

 

  Materials and Engineering                                                                                      

Hundreds of accelerators exist in the world.  All are designed using ordinary
materials that are readily available.  The materials needed for our Heavy Ion
Fusion accelerator will be no different than for these other machines.  The
accelerator will be longer than Stanford’s original SLAC accelerator (built in the1960s) or Germany’s UNILAC (built in the 1970s), but it will continue to use the
latest versions of the same technology. The precision tunneling employed in the
construction of large accelerators will be applied to the design and construction of
the first SPS, as well as all of its successors. 
 
The ion source system is a unique element in the FPC design.  It will use heavy
isotopes rather than electrons or protons as used in most accelerators.  But this
technology is also mature.  Heavy ion accelerators operate at numerous
laboratories in the US, Europe, and Asia.  Our system will be different in that we
will use a multiplicity of single-isotope ion sources.  This requires a supply of the
specific isotopes of the elements we plan to use – xenon, tin, lead, and mercury –
elements chosen first to achieve the requirements for fusion ignition and second for
their abundance of isotopes within the mass range that we want to use.  Most
likely, preparation of these single isotope supplies will be special for our project.
 
The materials and technologies used to construct the reaction chambers are
common to many high-stress, high-temperature applications, such as in refineries,
metal processing industries, and rocket engines. Because our chamber operates at
temperatures lower than any other power plant based on heat, and because the
hazards of radiation are very much lower (in kind and amount) than in fission
power plants, we anticipate some relaxation of the expensive “N-stamp” required
for fission systems. We expect to benefit, however, from the excellent steels and
processes that have been developed for fission systems, including those for liquid
metal heat exchangers. 

No special or 'magic' materials are needed. FPC's chamber does not face the problem posed by fusion’s high-energy neutrons for most of the other fusion chamber concepts. The protection of the system from the fusion neutrons is based on our unique use of the lithium needed to generate the tritium fuel. Our system for
handling lithium benefits from experience that dates from early in the space
program and from liquid metal fission systems, as well as from fusion energy
development. 
 
Lithium was considered a rare and hazardous metal until recently.  The use of
lithium in batteries for electronic equipments, and even electric automobiles, has
changed much of that.  Safe handling and packaging processes have been
developed and the metal is now in many household goods, from cameras to
computers.  Its supply has expanded as well and it is now produced in quantities
needed to supply the start-up of several fusion power plants per year. For long-term operations, and for the build out of the fusion energy system worldwide, we
will extract lithium from seawater.
 
Modern engineering design is highly dependent on computational technology in
roles from computer assisted drafting to detailed component modeling and
manufacture.  This project is no exception.  We will design and model all aspects
of the system in a computer prior to construction.  Besides assuring that all
components fit together, this will build the end-to-end model to simulate operation
of the system and define flow of data for precise and safe system control.



Copyright (C) 2011.  All Rights Reserved
ć
Hal Helsley,
Jan 24, 2013, 4:16 PM
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