Nuclear physicist at the Idaho National Laboratory sets up an experiment using an electronic neutron generator.
Neutron generators are neutron source devices which contain compact linear particle accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions take place in these devices by accelerating either deuterium, tritium, or a mixture of these two isotopes into a metal hydride
target which also contains deuterium, tritium or a mixture of these
isotopes. Fusion of deuterium atoms (D + D) results in the formation of a
helium-3 ion and a neutron with a kinetic energy of approximately 2.5 MeV. Fusion of a deuterium and a tritium atom (D + T) results in the formation of a helium-4
ion and a neutron with a kinetic energy of approximately 14.1 MeV.
Neutron generators have applications in medicine, security, and
materials analysis.
The basic concept was first developed by Ernest Rutherford's team in the Cavendish Laboratory in the early 1930s. Using a linear accelerator driven by a Cockcroft–Walton generator, Mark Oliphant
led an experiment that fired deuterium ions into a deuterium-infused
metal foil and noticed that a small number of these particles gave off alpha particles.
This was the first demonstration of nuclear fusion, as well as the
first discovery of Helium-3 and tritium, created in these reactions. The
introduction of new power sources has continually shrunk the size of
these machines, from Oliphant's that filled the corner of the lab, to
modern machines that are highly portable. Thousands of such small,
relatively inexpensive systems have been built over the past five
decades.
While neutron generators do produce fusion reactions, the number
of accelerated ions that cause these reactions is very low. It can be
easily demonstrated that the energy released by these reactions is many
times lower than the energy needed to accelerate the ions, so there is
no possibility of these machines being used to produce net fusion power. A related concept, colliding beam fusion, attempts to address this issue using two accelerators firing at each other.
Neutristor in its simplest form as tested by the inventor at Sandia National Laboratories
Neutron generator theory and operation
Small neutron generators using the deuterium (D, hydrogen-2, 2H) tritium (T, hydrogen-3, 3H)
fusion reactions are the most common accelerator based (as opposed to
radioactive isotopes) neutron sources. In these systems, neutrons are
produced by creating ions of deuterium, tritium, or deuterium and
tritium and accelerating these into a hydride target loaded with
deuterium, or deuterium and tritium. The DT reaction is used more than
the DD reaction because the yield of the DT reaction is 50–100 times
higher than that of the DD reaction.
D + T → n + 4He En = 14.1 MeV
D + D → n + 3He En = 2.5 MeV
Neutrons produced by DD and DT reactions are emitted somewhat anisotropically
from the target, slightly biased in the forward (in the axis of the ion
beam) direction. The anisotropy of the neutron emission from DD and DT
reactions arises from the fact the reactions are isotropic in the center of momentum coordinate system (COM) but this isotropy is lost in the transformation from the COM coordinate system to the laboratory frame of reference. In both frames of reference, the He nuclei recoil in the opposite direction to the emitted neutron consistent with the law of conservation of momentum.
The gas pressure in the ion source region of the neutron tubes generally ranges between 0.1–0.01 mm Hg. The mean free path
of electrons must be shorter than the discharge space to achieve
ionization (lower limit for pressure) while the pressure must be kept
low enough to avoid formation of discharges at the high extraction
voltages applied between the electrodes. The pressure in the
accelerating region, however, has to be much lower, as the mean free
path of electrons must be longer to prevent formation of a discharge
between the high voltage electrodes.
The ion accelerator usually consists of several electrodes with cylindrical symmetry, acting as an einzel lens.
The ion beam can thus be focused to a small point at the target. The
accelerators typically require power supplies of 100–500 kV. They
usually have several stages, with voltage between the stages not
exceeding 200 kV to prevent field emission.
In comparison with radionuclide neutron sources, neutron tubes can produce much higher neutron fluxes and consistent (monochromatic) neutron energy spectra can be obtained. The neutron production rate can also be controlled.
Sealed neutron tubes
The
central part of a neutron generator is the particle accelerator itself,
sometimes called a neutron tube. Neutron tubes have several components
including an ion source, ion optic elements, and a beam target; all of
these are enclosed within a vacuum-tight enclosure. High voltage
insulation between the ion optical elements of the tube is provided by
glass and/or ceramic insulators. The neutron tube is, in turn, enclosed
in a metal housing, the accelerator head, which is filled with a
dielectric medium to insulate the high voltage elements of the tube from
the operating area. The accelerator and ion source high voltages are
provided by external power supplies. The control console allows the
operator to adjust the operating parameters of the neutron tube. The
power supplies and control equipment are normally located within 3–10
metres (10–30 ft) of the accelerator head in laboratory instruments, but
may be several kilometers away in well logging instruments.
In comparison with their predecessors, sealed neutron tubes do not require vacuum pumps
and gas sources for operation. They are therefore more mobile and
compact, while also durable and reliable. For example, sealed neutron
tubes have replaced radioactive modulated neutron initiators, in supplying a pulse of neutrons to the imploding core of modern nuclear weapons.
Examples of neutron tube ideas date as far back as the 1930s,
pre-nuclear weapons era, by German scientists filing a 1938 German
patent (March 1938, patent #261,156) and obtaining a United States
Patent (July 1941, USP #2,251,190); examples of present state of the art
are given by developments such as the Neutristor, a mostly solid state device, resembling a computer chip, invented at Sandia National Laboratories in Albuquerque NM. Typical sealed designs are used in a pulsed mode and can be operated at different output levels, depending on the life from the ion source and loaded targets.
Neutristor in an inexpensive vacuum sealed package ready for testing
A good ion source should provide a strong ion beam
without consuming much of the gas. For hydrogen isotopes, production of
atomic ions is favored over molecular ions, as atomic ions have higher
neutron yield on collision. The ions generated in the ion source are
then extracted by an electric field into the accelerator region, and
accelerated towards the target. The gas consumption is chiefly caused by
the pressure difference between the ion generating and ion accelerating
spaces that has to be maintained. Ion currents of 10 mA at gas
consumptions of 40 cm3/hour are achievable.
For a sealed neutron tube, the ideal ion source should use low
gas pressure, give high ion current with large proportion of atomic
ions, have low gas clean-up, use low power, have high reliability and
high lifetime, its construction has to be simple and robust and its
maintenance requirements have to be low.
Gas can be efficiently stored in a replenisher, an electrically
heated coil of zirconium wire. Its temperature determines the rate of
absorption/desorption of hydrogen by the metal, which regulates the
pressure in the enclosure.
Cold cathode (Penning)
The Penning source is a low gas pressure, cold cathode
ion source which utilizes crossed electric and magnetic fields. The ion
source anode is at a positive potential, either dc or pulsed, with
respect to the source cathode. The ion source voltage is normally
between 2 and 7 kilovolts. A magnetic field, oriented parallel to the
source axis, is produced by a permanent magnet. A plasma
is formed along the axis of the anode which traps electrons which, in
turn, ionize gas in the source. The ions are extracted through the exit
cathode. Under normal operation, the ion species produced by the Penning
source are over 90% molecular ions. This disadvantage is however
compensated for by the other advantages of the system.
One of the cathodes is a cup made of soft iron,
enclosing most of the discharge space. The bottom of the cup has a hole
through which most of the generated ions are ejected by the magnetic
field into the acceleration space. The soft iron shields the
acceleration space from the magnetic field, to prevent a breakdown.
Ions emerging from the exit cathode are accelerated through the
potential difference between the exit cathode and the accelerator
electrode. The schematic indicates that the exit cathode is at ground
potential and the target is at high (negative) potential. This is the
case in many sealed tube neutron generators. However, in cases when it
is desired to deliver the maximum flux to a sample, it is desirable to
operate the neutron tube with the target grounded and the source
floating at high (positive) potential. The accelerator voltage is
normally between 80 and 180 kilovolts.
The accelerating electrode has the shape of a long hollow cylinder. The ion beam has a slightly diverging angle (about 0.1 radian).
The electrode shape and distance from target can be chosen so the
entire target surface is bombarded with ions. Acceleration voltages of
up to 200 kV are achievable.
The ions pass through the accelerating electrode and strike the
target. When ions strike the target, 2–3 electrons per ion are produced
by secondary emission. In order to prevent these secondary electrons
from being accelerated back into the ion source, the accelerator
electrode is biased negative with respect to the target. This voltage,
called the suppressor voltage, must be at least 500 volts and may be as
high as a few kilovolts. Loss of suppressor voltage will result in
damage, possibly catastrophic, to the neutron tube.
Some neutron tubes incorporate an intermediate electrode, called
the focus or extractor electrode, to control the size of the beam spot
on the target. The gas pressure in the source is regulated by heating
or cooling the gas reservoir element.
Radio frequency (RF)
Ions
can be created by electrons formed in high-frequency electromagnetic
field. The discharge is formed in a tube located between electrodes, or
inside a coil. Over 90% proportion of atomic ions is achievable.
Targets
The targets used in neutron generators are thin films of metal such as titanium, scandium, or zirconium which are deposited onto a silver, copper or molybdenum substrate. Titanium, scandium, and zirconium form stable chemical compounds called metal hydrides when combined with hydrogen or its isotopes. These metal hydrides are made up of two hydrogen (deuterium or tritium)
atoms per metal atom and allow the target to have extremely high
densities of hydrogen. This is important to maximize the neutron yield
of the neutron tube. The gas reservoir element also uses metal hydrides,
e.g. uranium hydride, as the active material.
Titanium is preferred to zirconium as it can withstand higher
temperatures (200 °C), and gives higher neutron yield as it captures deuterons
better than zirconium. The maximum temperature allowed for the target,
above which hydrogen isotopes undergo desorption and escape the
material, limits the ion current per surface unit of the target;
slightly divergent beams are therefore used. A 1 microampere ion beam
accelerated at 200 kV to a titanium-tritium target can generate up to 108 neutrons per second. The neutron yield is mostly determined by the accelerating voltage and the ion current level.
An example of a tritium target in use is a 0.2 mm thick silver
disc with a 1 micrometer layer of titanium deposited on its surface; the
titanium is then saturated with tritium.
Metals with sufficiently low hydrogen diffusion can be turned
into deuterium targets by bombardment of deuterons until the metal is
saturated. Gold targets under such condition show four times higher
efficiency than titanium. Even better results can be achieved with
targets made of a thin film of a high-absorption high-diffusivity metal
(e.g. titanium) on a substrate with low hydrogen diffusivity (e.g.
silver), as the hydrogen is then concentrated on the top layer and can
not diffuse away into the bulk of the material. Using a
deuterium-tritium gas mixture, self-replenishing D-T targets can be
made. The neutron yield of such targets is lower than of
tritium-saturated targets in deuteron beams, but their advantage is much
longer lifetime and constant level of neutron production.
Self-replenishing targets are also tolerant to high-temperature bake-out of the tubes, as their saturation with hydrogen isotopes is performed after the bakeout and tube sealing.
High voltage power supplies
One approach for generating the high voltage fields needed to accelerate ions in a neutron tube is to use a pyroelectric crystal. In April 2005 researchers at UCLA demonstrated the use of a thermally cycled pyroelectric crystal to generate high electric fields in a neutron generator application. In February 2006 researchers at Rensselaer Polytechnic Institute
demonstrated the use of two oppositely poled crystals for this
application. Using these low-tech power supplies it is possible to
generate a sufficiently high electric field
gradient across an accelerating gap to accelerate deuterium ions into a
deuterated target to produce the D + D fusion reaction. These devices
are similar in their operating principle to conventional sealed-tube
neutron generators which typically use Cockcroft–Walton
type high voltage power supplies. The novelty of this approach is in
the simplicity of the high voltage source. Unfortunately, the
relatively low accelerating current that pyroelectric crystals can
generate, together with the modest pulsing frequencies that can be
achieved (a few cycles per minute) limits their near-term application in
comparison with today's commercial products (see below). Also see pyroelectric fusion.
Other technologies
In
addition to the conventional neutron generator design described above
several other approaches exist to use electrical systems for producing
neutrons.
Another type of innovative neutron generator is the inertial electrostatic confinement
fusion device. This neutron generator avoids using a solid target
which will be sputter eroded causing metalization of insulating
surfaces. Depletion of the reactant gas within the solid target is also
avoided. Far greater operational lifetime is achieved. Originally called
a fusor, it was invented by Philo Farnsworth, the inventor of electronic television.
Applications
Neutron generators find application in semiconductor production
industry. They also have use cases in the enrichment of depleted
uranium, acceleration of breeder reactors, and activation and excitement
of experimental thorium reactors.
In material analysis neutron activation analysis is used to determine concentration of different elements in mixed materials such as minerals or ores.
The history of nuclear fusion began early in the 20th century
as an inquiry into how stars powered themselves and expanded to
incorporate a broad inquiry into the nature of matter and energy, as
potential applications expanded to include warfare, energy production
and rocket propulsion.
Early research
Various
authors have also put forth ways to organize all the fusion approaches
that have been tested over the past 70+ years. This flow chart above
groups the approaches into color coded families, these are: the Pinch
Family (orange), The Mirror Family (red), Cusp Systems (violet),
Tokamaks & Stellarators (Green), Plasma Structures (gray), Inertial
Electrostatic Confinement (dark yellow), Inertial Confinement Fusion
(ICF, blue), Plasma Jet Magneto Inertial Fusion (PJMIF, dark pink).
In 1920 British physicist Francis William Aston discovered that the mass equivalent of four hydrogen atoms is heavier than the mass of one helium atom (He-4),
which implied that net energy can be released by combining hydrogen
atoms to form helium. This provided the first hints of a mechanism by
which stars could produce energy. Throughout the 1920s, Arthur Stanley Eddington became a major proponent of the proton–proton chain reaction (PP reaction) as the primary system running the Sun. Quantum tunneling was discovered by Friedrich Hund in 1929, and shortly afterwards Robert Atkinson and Fritz Houtermans used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei.
Henry Norris Russell observed that the relationship in the Hertzsprung–Russell diagram
suggested that a star's heat came from a hot core rather than from the
entire star. Eddington used this to calculate that the core would have
to be about 40 million K. This became a matter of debate because the
value is much higher than astronomical observations that suggested about
one-third to one-half that value. George Gamow introduced the mathematical basis for quantum tunnelling in 1928. In 1929 Atkinson and Houtermans
provided the first estimates of the stellar fusion rate. They showed
that fusion can occur at lower energies than previously believed,
backing Eddington's calculations.
Neutrons from fusion were first detected in 1933. The experiment involved the acceleration of protons towards a target at energies of up to 600,000 electron volts.
The first man-made device to achieve ignition was the detonation of this fusion device, codenamed Ivy Mike.Early photo of plasma inside a pinch machine (Imperial College 1950/1951)
The first successful man-made fusion device was the boosted fission weapon tested in 1951 in the Greenhouse Item test. The first true fusion weapon was 1952's Ivy Mike, and the first practical example was 1954's Castle Bravo.
In these devices, the energy released by a fission explosion compresses
and heats the fuel, starting a fusion reaction. Fusion releases neutrons. These neutrons
hit the surrounding fission fuel, causing the atoms to split apart much
faster than normal fission processes. This increased the effectiveness
of bombs: normal fission weapons blow themselves apart before all their
fuel is used; fusion/fission weapons do not waste their fuel.
Stellarator
In 1949 expatriate German Ronald Richter proposed the Huemul Project in Argentina, announcing positive results in 1951. These turned out to be fake, but prompted others' interest. Lyman Spitzer
began considering ways to solve problems involved in confining a hot
plasma, and, unaware of the z-pinch efforts, he created the stellarator.
Spitzer applied to the US Atomic Energy Commission for funding to build a test device.
During this period, James L. Tuck,
who had worked with the UK teams on z-pinch, had been introducing the
stellarator concept to his coworkers at LANL. When he heard of Spitzer's
pitch, he applied to build a pinch machine of his own, the Perhapsatron.
Spitzer's idea won funding and he began work under Project Matterhorn. His work led to the creation of Princeton Plasma Physics Laboratory
(PPPL). Tuck returned to LANL and arranged local funding to build his
machine. By this time it was clear that the pinch machines were
afflicted by instability, stalling progress. In 1953, Tuck and others
suggested solutions that led to a second series of pinch machines, such
as the ZETA and Sceptre devices.
Spitzer's first machine, 'A' worked, but his next one, 'B', suffered from instabilities and plasma leakage.
In 1954 AEC chair Lewis Strauss foresaw electricity as "too cheap to meter". Strauss was likely referring to fusion power, part of the secret Project Sherwood—but
his statement was interpreted as referring to fission. The AEC had
issued more realistic testimony regarding fission to Congress months
before, projecting that "costs can be brought down... [to]... about the
same as the cost of electricity from conventional sources..."
In the mid-1950s the theoretical tools used to calculate the
performance of fusion machines were not predicting their actual
behavior. Machines invariably leaked plasma at rates far higher than
predicted. In 1954, Edward Teller
gathered fusion researchers at the Princeton Gun Club. He pointed out
the problems and suggested that any system that confined plasma within
concave fields was doomed due to what became known as interchange instability.
Attendees remember him saying in effect that the fields were like
rubber bands, and they would attempt to snap back to a straight
configuration whenever the power was increased, ejecting the plasma. He
suggested that the only way to predictably confine plasma would be to
use convex fields: a "cusp" configuration.
When the meeting concluded, most researchers turned out papers
explaining why Teller's concerns did not apply to their devices. Pinch
machines did not use magnetic fields in this way, while the mirror and
stellarator claques proposed various solutions. This was soon followed
by Martin David Kruskal and Martin Schwarzschild's paper discussing pinch machines, however, which demonstrated those devices' instabilities were inherent.
ZETA
The largest "classic" pinch device was the ZETA, which started operation in the UK in 1957. Its name is a take-off on small experimental fission reactors that often had "zero energy" in their name, such as ZEEP.
In early 1958, John Cockcroft
announced that fusion had been achieved in the ZETA, an announcement
that made headlines around the world. He dismissed US physicists'
concerns. US experiments soon produced similar neutrons, although
temperature measurements suggested these could not be from fusion. The
ZETA neutrons were later demonstrated to be from different versions of
the instability processes that had plagued earlier machines. Cockcroft
was forced to retract his fusion claims, tainting the entire field for
years. ZETA ended in 1968.
Scylla
The first experiment to achieve controlled thermonuclear fusion was accomplished using Scylla I at LANL in 1958.
Scylla I was a θ-pinch machine, with a cylinder full of deuterium.
Electric current shot down the sides of the cylinder. The current made
magnetic fields that pinched the plasma, raising temperatures to 15 million degrees Celsius, for long enough that atoms fused and produced neutrons.
The Sherwood program sponsored a series of Scylla machines at Los
Alamos. The program began with 5 researchers and $100,000 in US funding
in January 1952. By 1965, a total of $21 million had been spent. The θ-pinch approach was abandoned after calculations showed it could not scale up to produce a reactor.
Tokamak
In 1950–1951 in the Soviet Union, Igor Tamm and Andrei Sakharov first discussed a tokamak-like approach. Experimental research on those designs began in 1956 at the MoscowKurchatov Institute by a group of Soviet scientists led by Lev Artsimovich.
The tokamak essentially combined a low-power pinch device with a
low-power stellarator. The notion was to combine the fields in such a
way that the particles orbited within the reactor a particular number of
times, today known as the "safety factor".
The combination of these fields dramatically improved confinement times
and densities, resulting in huge improvements over existing devices.[1]
In 1951 Ivy Mike, part of Operation Ivy, became the first detonation of a thermonuclear weapon, yielding 10.4 megatons of TNT using liquid deuterium. Cousins and Ware built a toroidal pinch
device in England and demonstrated that the plasma in pinch devices is
inherently unstable. In 1953 The Soviet Union tested its RDS-6S test,
(codenamed "Joe 4" in the US) demonstrated a fission/fusion/fission ("Layercake") design that yielded 600 kilotons. Igor Kurchatov spoke at Harwell on pinch devices, revealing that the USSR was working on fusion.
Seeking to generate electricity, Japan, France and Sweden all start fusion research programs
In 1956 the Soviet Union began publishing articles on plasma
physics, leading the US and UK to follow over the next several years.
The Sceptre III
z-pinch plasma column remained stable for 300 to 400 microseconds, a
dramatic improvement on previous efforts. The team calculated that the
plasma had an electrical resistivity around 100 times that of copper,
and was able to carry 200 kA of current for 500 microseconds.
Spitzer published a key plasma physics text at Princeton in 1963. He took the ideal gas laws and adapted them to an ionized plasma, developing many of the fundamental equations used to model a plasma.
Laser fusion was suggested in 1962 by scientists at LLNL. Initially, lasers had little power. Laser fusion (inertial confinement fusion) research began as early as 1965.
At the 1964 World's Fair, the public was given its first fusion demonstration. The device was a Theta-pinch from General Electric. This was similar to the Scylla machine developed earlier at Los Alamos.
By the mid-1960s progress had stalled across the world. All of
the major designs were losing plasma at unsustainable rates. The 12-beam
"4 pi laser" attempt at inertial confinement fusion developed at LLNL
targeted a gas-filled target chamber of about 20 centimeters in
diameter.
The magnetic mirror was first published in 1967 by Richard F. Post and many others at LLNL.
The mirror consisted of two large magnets arranged so they had strong
fields within them, and a weaker, but connected, field between them.
Plasma introduced in the area between the two magnets would "bounce
back" from the stronger fields in the middle.
A.D. Sakharov's group constructed the first tokamaks. The most successful were the T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, producing the first quasistationary fusion reaction.
When this was announced, the international community was skeptical. A
British team was invited to see T-3, and confirmed the Soviet claims. A
burst of activity followed as many planned devices were abandoned and
tokamaks were introduced in their place—the C model stellarator, then
under construction after many redesigns, was quickly converted to the
Symmetrical Tokamak.
In his work with vacuum tubes, Philo Farnsworth
observed that electric charge accumulated in the tube. In 1962,
Farnsworth patented a design using a positive inner cage to concentrate
plasma and fuse protons. During this time, Robert L. Hirsch joined Farnsworth Television labs and began work on what became the Farnsworth-Hirsch Fusor. This effect became known as the Multipactor effect. Hirsch patented the design in 1966 and published it in 1967.
Plasma temperatures of approximately 40 million degrees Celsius and 109 deuteron-deuteron fusion reactions per discharge were achieved at LANL with Scylla IV.
In 1968 the Soviets announced results from the T-3 tokamak,
claiming temperatures an order of magnitude higher than any other
device. A UK team, nicknamed "The Culham Five", confirmed the results.
The results led many other teams, including the Princeton group, which
converted their stellarator to a tokamak.
1970s
Shiva laser, 1977, the largest ICF laser system built in the seventies
The Tandem Mirror Experiment (TMX) in 1979
Princeton's conversion of the Model C stellarator
to a tokamak produced results matching the Soviets. With an apparent
solution to the magnetic bottle problem in-hand, plans begin for a
larger machine to test scaling and methods to heat the plasma.
In 1972, John Nuckolls outlined the idea of fusion ignition,
a fusion chain reaction. Hot helium made during fusion reheats the fuel
and starts more reactions. Nuckolls's paper started a major development
effort. LLNL built laser systems including Argus, Cyclops, Janus, the neodymium-doped glass (Nd:glass) laser Long Path, Shiva laser, and the 10 beam Nova in 1984. Nova would ultimately produce 120 kilojoules of infrared light during a nanosecond pulse.
The "advanced tokamak" concept emerged, which included
non-circular plasma, internal diverters and limiters, superconducting
magnets, and operation in the so-called "H-mode" island of increased
stability. Two other designs became prominent; the compact tokamak sited the magnets on the inside of the vacuum chamber, and the spherical tokamak with as small a cross section as possible.
In 1974 J.B. Taylor
re-visited ZETA and noticed that after an experimental run ended, the
plasma entered a short period of stability. This led to the reversed field pinch concept. On May 1, 1974, the KMS fusion company (founded by Kip Siegel) achieved the world's first laser induced fusion in a deuterium-tritium pellet. Neutron-sensitive nuclear emulsion detectors, developed by 1961 Nobel Prize winner Robert Hofstadter, were used to provide evidence of this discovery.
Progress in power and energy levels attainable by inertial confinement lasers has increased dramatically since the early 1970s.
The Princeton Large Torus
(PLT), the follow-on to the Symmetrical Tokamak, surpassed the best
Soviet machines and set temperature records that were above what was
needed for a commercial reactor. Soon after it received funding with the
target of breakeven.
In the mid-1970s, Project PACER, carried out at LANL explored the possibility of exploding small hydrogen bombs (fusion bombs) inside an underground cavity.
As an energy source, the system was the only system that could work
using the technology of the time. It required a large, continuous supply
of nuclear bombs, however, with questionable economics.
In 1976, the two beam Argus laser became operational at LLNL. In 1977, the 20 beam Shiva laser
there was completed, capable of delivering 10.2 kilojoules of infrared
energy on target. At a price of $25 million and a size approaching that
of a football field, Shiva was the first megalaser.
The DOE selected a Princeton design Tokamak Fusion Test Reactor (TFTR) and the challenge of running on deuterium-tritium fuel.
The 20 beam Shiva laser
at LLNL became capable of delivering 10.2 kilojoules of infrared energy
on target. Costing $25 million and nearly covering a football field,
Shiva was the first "megalaser" at LLNL.
1980s
Magnetic
mirrors suffered from end losses, requiring high power, complex
magnetic designs, such as the baseball coil pictured here.
The Novette target chamber (metal sphere with diagnostic devices protruding radially), which was reused from the Shiva project and two newly built laser chains visible in background.
Inertial confinement fusion implosion on the Nova laser during the 1980s was a key driver of fusion development.
In the German/US HIBALL study,
Garching used the high repetition rate of the RF driver to serve four
reactor chambers using liquid lithium inside the chamber cavity. In 1982
high-confinement mode (H-mode) was discovered in tokamaks.
Magnetic mirror
The US funded a magnetic mirror program in the late 1970s and early 1980s. This program resulted in a series of magnetic mirror devices including: 2X,[ Baseball I, Baseball II, the Tandem Mirror Experiment and upgrade, the Mirror Fusion Test Facility, and MFTF-B. These machines were built and tested at LLNL from the late 1960s to the mid 1980s. The final machine, MFTF cost 372 million dollars and was, at that time, the most expensive project in LLNL history. It opened on February 21, 1986, and immediately closed, allegedly to balance the federal budget.
Laser
Laser fusion progress: in 1983, the NOVETTE laser was completed. The following December, the ten-beam NOVA laser was finished. Five years later, NOVA produced 120 kilojoules of infrared light during a nanosecond pulse.
Research focused on either fast delivery or beam smoothness. Both
focused on increasing energy uniformity. One early problem was that the
light in the infrared wavelength lost energy before hitting the fuel. Breakthroughs were made at LLE at University of Rochester. Rochester scientists used frequency-tripling crystals to transform infrared laser beams into ultraviolet beams.
Chirping
In 1985, Donna Strickland and Gérard Mourou
invented a method to amplify laser pulses by "chirping". This changed a
single wavelength into a full spectrum. The system amplified the beam
at each wavelength and then reversed the beam into one color. Chirp
pulsed amplification became instrumental for NIF and the Omega EP
system.
LANL constructed a series of laser facilities. They included Gemini (a two beam system), Helios (eight beams), Antares (24 beams) and Aurora (96 beams). The program ended in the early nineties with a cost on the order of one billion dollars.
In 1987, Akira Hasegawa
noticed that in a dipolar magnetic field, fluctuations tended to
compress the plasma without energy loss. This effect was noticed in data
taken by Voyager 2, when it encountered Uranus. This observation became the basis for a fusion approach known as the levitated dipole.
In tokamaks, the Tore Supra was under construction from 1983 to 1988 in Cadarache, France. Its superconducting magnets permitted it to generate a strong permanent toroidal magnetic field. First plasma came in 1988.
In 1983, JET achieved first plasma. In 1985, the Japanese tokamak, JT-60 produced its first plasmas. In 1988, the T-15 a Soviet tokamak was completed, the first to use (helium-cooled) superconducting magnets.
In 1998, the T-15 Soviet tokamak with superconducting helium-cooled coils was completed.
Spherical tokamak
In 1984, Martin Peng proposed
an alternate arrangement of magnet coils that would greatly reduce the
aspect ratio while avoiding the erosion issues of the compact tokamak: a
spherical tokamak.
Instead of wiring each magnet coil separately, he proposed using a
single large conductor in the center, and wiring the magnets as
half-rings off of this conductor. What was once a series of individual
rings passing through the hole in the center of the reactor was reduced
to a single post, allowing for aspect ratios as low as 1.2.
The ST concept appeared to represent an enormous advance in tokamak
design. The proposal came during a period when US fusion research
budgets were dramatically smaller. ORNL was provided with funds to
develop a suitable central column built out of a high-strength copper
alloy called "Glidcop". However, they were unable to secure funding to
build a demonstration machine.
Failing at ORNL, Peng began a worldwide effort to interest other
teams in the concept and get a test machine built. One approach would be
to convert a spheromak. Peng's advocacy caught the interest of Derek Robinson, of the United Kingdom Atomic Energy Authority. Robinson gathered a team and secured on the order of 100,000 pounds to build an experimental machine, the Small Tight Aspect Ratio Tokamak,
or START. Parts of the machine were recycled from earlier projects,
while others were loaned from other labs, including a 40 keV neutral
beam injector from ORNL. Construction began in 1990 and operation
started in January 1991. It achieved a record beta (plasma pressure compared to magnetic field pressure) of 40% using a neutral beam injector
In 1991 JET's Preliminary Tritium Experiment achieved the world's first controlled release of fusion power.
In 1992, Physics Today published Robert McCory's outline of the current state of ICF, advocating for a national ignition facility. This was followed by a review article from John Lindl in 1995,
making the same point. During this time various ICF subsystems were
developed, including target manufacturing, cryogenic handling systems,
new laser designs (notably the NIKE laser at NRL) and improved diagnostics including time of flight analyzers and Thomson scattering. This work was done at the NOVA laser system, General Atomics, Laser Mégajoule and the GEKKO XII
system in Japan. Through this work and lobbying by groups like the
fusion power associates and John Sethian at NRL, Congress authorized
funding for the NIF project in the late nineties.
In 1992 the United States and the former republics of the Soviet Union stopped testing nuclear weapons.
In 1993 TFTR at PPPL experimented with 50% deuterium, 50% tritium, eventually reaching 10 megawatts.
In the early nineties, theory and experimental work regarding fusors and polywells was published. In response, Todd Rider at MIT developed general models of these devices,
arguing that all plasma systems at thermodynamic equilibrium were
fundamentally limited. In 1995, William Nevins published a criticism arguing that the particles inside fusors and polywells would acquire angular momentum, causing the dense core to degrade.
In 1995, the University of Wisconsin–Madison built a large fusor, known as HOMER. Dr George H. Miley at Illinois built a small fusor that produced neutrons using deuterium and discovered the "star mode" of fusor operation. At this time in Europe, an IEC device was developed as a commercial neutron source by Daimler-Chrysler and NSD Fusion.
The next year, Tore Supra reached a record plasma duration of two
minutes with a current of almost 1 M amperes driven non-inductively by
2.3 MW of lower hybrid frequency waves (i.e. 280 MJ of injected and extracted energy), enabled by actively cooled plasma-facing components.
The upgraded Z-machine opened to the public in August 1998. The key attributes were its 18 million ampere current and a discharge time of less than 100 nanoseconds. This generated a magnetic pulse inside a large oil tank, which struck a liner (an array of tungsten wires). Firing the Z-machine became a way to test high energy, high temperature (2 billion degrees) conditions. In 1996.
In 1997, JET reached 16.1 MW (65% of heat to plasma), sustaining over 10 MW for over 0.5 sec. As of 2020 this remained the record output level. Four megawatts of alpha particle self-heating was achieved.
ITER was officially announced as part of a seven-party consortium
(six countries and the EU). ITER was designed to produce ten times more
fusion power than the input power. ITER was sited in Cadarache. The US withdrew from the project in 1999.
JT-60 produced a reversed shear plasma with the equivalent fusion amplification factor of 1.25 - as of 2021 this remained the world record.
In the late nineties, a team at Columbia University and MIT developed the levitated dipole, a fusion device that consisted of a superconducting electromagnet, floating in a saucer shaped vacuum chamber. Plasma swirled around this donut and fused along the center axis.
Starting in 1999, a growing number of amateurs have been able to fuse atoms using homemade fusors, shown here.The Mega Ampere Spherical Tokamak became operational in the UK in 1999
"Fast ignition"
appeared in the late nineties, as part of a push by LLE to build the
Omega EP system, which finished in 2008. Fast ignition showed dramatic
power savings and moved ICF into the race for energy production. The HiPER experimental facility became dedicated to fast ignition.
In 2001 the United States, China and Republic of Korea joined ITER while Canada withdrew.
The next year, China's EAST test reactor was completed. This was the first tokamak to use superconducting magnets to generate both toroidal and poloidal fields.
In the early 2000s, LANL researchers claimed that an oscillating plasma could reach local thermodynamic equilibrium. This prompted the POPS and Penning trap designs.
In 2005 NIF fired its first bundle of eight beams, achieving the most powerful laser pulse to date - 152.8 kJ (infrared).
MIT researchers became interested in fusors for space propulsion, using fusors with multiple inner cages. Greg Piefer founded Phoenix Nuclear Labs and developed the fusor into a neutron source for medical isotope production. Robert Bussard began speaking openly about the polywell in 2006.
In March 2009, NIF became operational.
In the early 2000s privately backed fusion companies launched to develop commercial fusion power. Tri Alpha Energy, founded in 1998, began by exploring a field-reversed configuration approach. In 2002, Canadian company General Fusion began proof-of-concept experiments based on a hybrid magneto-inertial approach called Magnetized Target Fusion. Investors included Jeff Bezos (General Fusion) and Paul Allen (Tri Alpha Energy). Toward the end of the decade, Tokamak Energy started exploring spherical tokamak devices using reconnection.
2010s
The preamplifiers of the National Ignition Facility. In 2012, the NIF achieved a 500-terawatt shot.The Wendelstein7X under constructionExample
of a stellarator design: A coil system (blue) surrounds plasma
(yellow). A magnetic field line is highlighted in green on the yellow
plasma surface.
Private and public research accelerated in the 2010s.
Private projects
In 2017, General Fusion developed its plasma injector technology and Tri Alpha Energy constructed and operated its C-2U device. In August 2014, Phoenix Nuclear Labs announced the sale of a high-yield neutron generator that could sustain 5×1011deuterium fusion reactions per second over a 24-hour period.
In October 2014, Lockheed Martin's Skunk Works announced the development of a high beta fusion reactor, the Compact Fusion Reactor.
Although the original concept was to build a 20-ton, container-sized
unit, the team conceded in 2018 that the minimum scale would be 2,000
tons.
In 2017, Helion Energy's fifth-generation plasma machine went into operation, seeking to achieve plasma density of 20 T and fusion temperatures. ST40 generated "first plasma".
In 2018, Eni announced a $50 million investment in Commonwealth Fusion Systems, to attempt to commercialize ARC technology using a test reactor (SPARC) in collaboration with MIT.The reactor planned to employ yttrium barium copper oxide
(YBCO) high-temperature superconducting magnet technology. Commonwealth
Fusion Systems in 2021 tested successfully a 20 T magnet making it the
strongest high-temperature superconducting magnet in the world.
Following the 20 T magnet CFS raised $1.8 billion from private
investors.
General Fusion began developing a 70% scale demo system. In 2018, TAE Technologies' reactor reached nearly 20 M°C.
Government and academic projects
In
2010, NIF researchers conducted a series of "tuning" shots to determine
the optimal target design and laser parameters for high-energy ignition
experiments with fusion fuel. Net energy gain was achieved in August 2013.
In April 2014, LLNL ended the Laser Inertial Fusion Energy (LIFE) program and directed their efforts towards NIF.
A 2012 paper demonstrated that a dense plasma focus had achieved temperatures of 1.8 billion degrees Celsius, sufficient for boron fusion, and that fusion reactions were occurring primarily within the contained plasmoid, necessary for net power.
In August 2014, MIT announced a tokamak it named the ARC fusion reactor, using rare-earth barium-copper oxide
(REBCO) superconducting tapes to construct high-magnetic field coils
that it claimed produced comparable magnetic field strength in a smaller
configuration than other designs.
In October 2015, researchers at the Max Planck Institute of Plasma Physics completed building the largest stellarator to date, the Wendelstein 7-X. In December they produced the first helium plasma, and in February 2016 produced hydrogen plasma.
In 2015, with plasma discharges lasting up to 30 minutes, Wendelstein
7-X attempted to demonstrate the essential stellarator attribute:
continuous operation of a high-temperature plasma.
In 2014 EAST achieved a record confinement time of 30 seconds for plasma in the high-confinement mode (H-mode), thanks to improved heat dispersal. This was an order of magnitude improvement vs other reactors.
In 2017 the reactor achieved a stable 101.2-second steady-state high
confinement plasma, setting a world record in long-pulse H-mode
operation.
In 2018 MIT scientists formulated a theoretical means to remove
the excess heat from compact nuclear fusion reactors via larger and
longer divertors.
In 2019 the United Kingdom announced a planned £200-million
(US$248-million) investment to produce a design for a fusion facility
named the Spherical Tokamak for Energy Production (STEP), by the early 2040s.
2020s
Plot of NIF results from 2012 to 2021 shows a dramatic breakthrough in August of 2021 in the performance of ICF on NIF
In 2020, Chevron Corporation
announced an investment in start-up Zap Energy, co-founded by British
entrepreneur and investor, Benj Conway, together with physicists Brian
Nelson and Uri Shumlak from University of Washington. In 2021 the company raised $27.5 million in Series B funding led by Addition.
In 2021, the US DOE launched the INFUSE program, a public-private knowledge sharing initiative involving a PPPL, MIT Plasma Science and Fusion Center and Commonwealth Fusion Systems partnership, together with partnerships with TAE Technologies, Princeton Fusion Systems, and Tokamak Energy.
In 2021, DOE's Fusion Energy Sciences Advisory Committee approved a
strategic plan to guide fusion energy and plasma physics research that included a working power plant by 2040, similar to Canadian, Chinese, and U.K. efforts.
In January 2021, SuperOx announced the commercialization of a new
superconducting wire, with more than 700 A/mm2 current capability.
TAE Technologies announced that its Norman device had sustained a
temperature of about 60 million degrees C for 30 milliseconds, 8 and 10
times higher, respectively, than the company's previous devices. The
duration was claimed to be limited by the power supply rather than the
device.
In August 2021, the National Ignition Facility recorded a
record-breaking 1.3 megajoules of energy created from fusion. This was
certified as the first example of a burning plasma, where fusion
reactions were used to create secondary fusion reactions in the spring
of 2022.
In December 2020, the Chinese experimental nuclear fusion reactor HL-2M achieved its first plasma discharge. In May 2021, Experimental Advanced Superconducting Tokamak
(EAST) announced a new world record for superheated plasma, sustaining a
temperature of 120 M°C for 101 seconds and a peak of 160 M°C for 20
seconds. In December 2021 EAST set a new world record for high temperature (70 M°C) plasma of 1,056 seconds.
In February 2022, JET sustained 11 MW and a Q value of 0.33 for
over 5 seconds, outputting 59.7 megajoules, using a mix of deuterium and
tritium for fuel. In March 2022 it was announced that Tokamak Energy achieved a record plasma temperature of 100 million kelvins, inside a commercial compact tokamak.
In December 2022, the NIF achieved the first scientific breakeven controlled fusion experiment, with an energy gain of 1.5.
A single-domain antibody (sdAb), also known as a NANOBODY®, is an antibody fragment consisting of a single monomericvariable antibody domain. Like a whole antibody, it is able to bind selectively to a specific antigen. With a molecular weight of only 12–15 kDa, single-domain antibodies are much smaller than common antibodies (150–160 kDa) which are composed of two heavy protein chains and two light chains, and even smaller than Fab fragments (~50 kDa, one light chain and half a heavy chain) and single-chain variable fragments (~25 kDa, two variable domains, one from a light and one from a heavy chain).
The first single-domain antibodies were engineered from heavy-chain antibodies found in camelids; these are called VHH fragments. Cartilaginous fishes also have heavy-chain antibodies (IgNAR, 'immunoglobulin new antigen receptor'), from which single-domain antibodies called VNAR fragments can be obtained. An alternative approach is to split the dimeric variable domains from common immunoglobulin G
(IgG) from humans or mice into monomers. Although most research into
single-domain antibodies is currently based on heavy chain variable
domains, NANOBODIES® derived from light chains have also been shown to
bind specifically to target epitopes.
Camelid NANOBODIES® have been shown to be just as specific as
antibodies, and in some cases they are more robust. They are easily
isolated using the same phage panning procedure used for antibodies,
allowing them to be cultured in vitro in large concentrations. The
smaller size and single domain make these antibodies easier to transform
into bacterial cells for bulk production, making them ideal for
research purposes.
A single-domain antibody is a peptide chain of about 110 amino acids long, comprising one variable domain (VH)
of a heavy-chain antibody, or of a common IgG. These peptides have
similar affinity to antigens as whole antibodies, but are more
heat-resistant and stable towards detergents and high concentrations of urea. Those derived from camelid and fish antibodies are less lipophilic and more soluble in water, owing to their complementarity-determining region 3
(CDR3), which forms an extended loop (coloured orange in the ribbon
diagram above) covering the lipophilic site that normally binds to a
light chain.
In contrast to common antibodies, two out of six single-domain
antibodies survived a temperature of 90 °C (194 °F) without losing their
ability to bind antigens in a 1999 study. Stability towards gastric acid and proteases depends on the amino acid sequence. Some species have been shown to be active in the intestine after oral application, but their low absorption from the gut impedes the development of systemically active orally administered single-domain antibodies.
The complex of a single domain antibody and a protein antigen reveals a buried binding site. (left) The nurse shark VNAR single domain in complex with lysozyme (PDB 1T6V). (right) The humanized HyHEL-10 Fv in complex with lysozyme (PDB 2EIZ)
The comparatively low molecular mass leads to a better permeability in tissues, and to a short plasma half-life since they are eliminated renally. Unlike whole antibodies, they do not show complement system triggered cytotoxicity because they lack an Fc region.
Camelid and fish derived sdAbs are able to bind to hidden antigens that
are not accessible to whole antibodies, for example to the active sites
of enzymes. This property has been shown to result from their extended CDR3 loop, which is able to penetrate such buried sites.
Production
A
shark (left) and a camelid (middle) heavy-chain antibody in comparison
to a common antibody (right). Heavy chains are shown in a darker shade,
light chains in a lighter shade. VH and VL are the variable domains.
From heavy-chain antibodies
A single-domain antibody can be obtained by immunization of dromedaries, camels, llamas, alpacas or sharks with the desired antigen and subsequent isolation of the mRNA coding for the variable region (VNAR and VHH) of heavy-chain antibodies. Large phage displayed VNAR and VHH single domain libraries were established from nurse sharks and dromedary camels. Screening techniques like phage display and ribosome display help to identify the clones binding the antigen. The single domain antibodies including VNARs can be humanized for clinical applications.
From conventional antibodies
Alternatively, single-domain antibodies can be made from common murine, rabbit or human IgG with four chains.
The process is similar, comprising gene libraries from immunized or
naïve donors and display techniques for identification of the most
specific antigens. A problem with this approach is that the binding
region of common IgG consists of two domains (VH and VL), which tend to dimerize
or aggregate because of their lipophilicity. Monomerization is usually
accomplished by replacing lipophilic by hydrophilic amino acids, but
often results in a loss of affinity to the antigen. If affinity can be retained, the single-domain antibodies can likewise be produced in E. coli, S. cerevisiae or other organisms.
From human single-domain antibodies
Humans
occasionally produce single domain antibodies by the random creation of
a stop codon in the light chain. Human single-domain antibodies
targeting various tumor antigens including mesothelin, GPC2 and GPC3 were isolated by phage display. The HN3 human single-domain antibodies have been used to create immunotoxins and chimeric antigen receptor (CAR) T cells
for treating liver cancer. Blocking the Wnt binding domain of GPC3 by
the HN3 human single-domain antibody inhibits Wnt activation in liver
cancer cells.
Potential applications
Single-domain
antibodies allow a broad range of applications in biotechnical as well
as therapeutic use due to their small size, simple production and high
affinity.
Biotechnological and diagnostic
The fusion of a fluorescent protein to a NANOBODY® generates a so-called chromobody.
Chromobodies can be used to recognize and trace targets in different
compartments of living cells. They can therefore increase the
possibilities of live cell microscopy and will enable novel functional
studies. The coupling of an anti-GFP
NANOBODY® to a monovalent matrix, called GFP-nanotrap, allows the
isolation of GFP-fusion proteins and their interacting partners for
further biochemical analyses. Single molecule localization with super-resolution imaging techniques requires the specific delivery of fluorophores
into close proximity with a target protein. Due to their large size the
use of antibodies coupled to organic dyes can often lead to a
misleading signal owing to the distance between the fluorophore and the
target protein. The fusion of organic dyes to anti-GFP Nanobodies
targeting GFP-tagged proteins allows nanometer spatial resolution and
minimal linkage error because of the small size and high affinity. The size dividend of NANOBODIES® also benefits the correlative light-electron microscopy
study. Without any permeabilization agent, the cytoplasm of the
chemically fixed cells are readily accessible to the fluorophore tagged
NANOBODIES®. Their small size also allows them to penetrate deeper into
volumetric samples than regular antibodies. High ultrastructural quality
is preserved in the tissue that is imaged by fluorescence microscope
and then electron microscope. This is especially useful for the
neuroscience research that requires both molecular labeling and electron
microscopic imaging.
In diagnostic biosensor
applications NANOBODIES® may be used prospectively as a tool. Due to
their small size, they can be coupled more densely on biosensor
surfaces. In addition to their advantage in targeting less accessible
epitopes, their conformational stability also leads to higher resistance
to surface regeneration conditions. After immobilizing single-domain
antibodies on sensor surfaces sensing human prostate-specific antigen
(hPSA) were tested. The NANOBODIES® outperformed the classical
antibodies in detecting clinical significant concentrations of hPSA.
To increase the crystallization probability of a target molecule, NANOBODIES® can be used as crystallization chaperones.
As auxiliary proteins, they can reduce the conformational heterogeneity
by binding and stabilizing just a subset of conformational states. They
also can mask surfaces interfering with the crystallization while
extending regions that form crystal contacts.
Therapeutic
Nanobodies for photothermal therapy. NANOBODIES®, which are able to bind tumor antigens like HER2, are coupled to branched gold nanoparticles that absorb light energy and create heat in order to kill cancer cells.
Single-domain antibodies have been tested as a new therapeutic tool against multiple targets. In mice infected with influenza A virus subtype H5N1, NANOBODIES® directed against hemaglutinin suppressed replication of the H5N1 virus in vivo and reduced morbidity and mortality. Nanobodies targeting the cell receptor binding domain of the virulence factors toxin A and toxin B of Clostridium difficile were shown to neutralize cytopathic effects in fibroblastsin vitro. NANOBODY® conjugates recognizing antigen presenting cells have been successfully used for tumor detection or targeted antigen delivery to generate strong immune response.
Orally available single-domain antibodies against E. coli-induced diarrhoea in piglets have been developed and successfully tested. Other diseases of the gastrointestinal tract, such as inflammatory bowel disease and colon cancer, are also possible targets for orally available single-domain antibodies.
Detergent-stable species targeting a surface protein of Malassezia furfur have been engineered for use in anti-dandruff shampoos.
As an approach for photothermal therapy NANOBODIES® binding to the HER2
antigen, which is overexpressed in breast and ovarian cancer cells,
were conjugated to branched gold nanoparticles (see figure). Tumor cells
were destroyed photothermally using a laser in a test environment.
Ablynx expects that their NANOBODIES® might cross the blood–brain barrier
and permeate into large solid tumours more easily than whole
antibodies, which would allow for the development of drugs against brain cancers.
NANOBODIES® that tightly bind to the RBD domain of the spike protein of betacoronaviruses (including SARS-CoV-2 which causes COVID-19) and blocks interactions of spike with the cell receptor ACE2, has been recently identified
Application of various single domain antibodies (NANOBODIES®) for
the prevention and treatment of infection by various highly pathogenic
human coronaviruses (HPhCoVs) has been reported. The prospects, potency
and challenges of deploying NANOBODIES® to bind and neutralize
SARS-CoV-2 and akin have been recently highlighted.
One of the most common causes of nagana – Trypanosoma brucei brucei – can be targeted by sdAbs. Stijlemans et al. 2004 succeeded in inducing effective sdAbs from rabbit and Camelus dromedarius by displaying a variable surface glycoprotein
antigen to the vertebrates' immune systems using a phage. In the
future, these therapies will surpass natural antibodies by reaching
locations currently unreachable due to natural antibodies' larger size.
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