快猫短视频

A hollow stainless steel sphere about the size of a football sits on a
laboratory bench. From inside the globe, a purple glow radiates through a small
glass window. George Miley peers in and glimpses a tiny luminescent ball hanging
in the centre and spires of light that seem to radiate from it. 鈥淚t鈥檚 a
beautiful sight,鈥� Miley sighs.

Miley is professor of nuclear, electrical and computer engineering at the
University of Illinois in Urbana-Champaign and his sphere is a fusion machine.
Unlike other fusion machines, this one is small enough to sit on a desktop, it
can be switched on and off at will and it produces virtually no radioactive
waste.

Molecular microscope

The sphere is different in other ways too. It鈥檚 primary purpose is not make
energy but to generate neutrons. Billions of them, every second. Neutrons are
subatomic particles with no electric charge that are extraordinarily useful.
快猫短视频s use them for materials analysis鈥攖hey can help to identify most
common elements in seconds. By contrast, chemical analysis can take hours.
Neutrons can also help to work out the structure of new molecules and crystals.
Beams of these particles can even be used for cancer treatment.

The trouble is that neutrons are notoriously difficult to make. Nothing less
than a nuclear reactor or a high powered particle accelerator will do the job.
This means that neutron analysis can only take place with the help of a handful
of specialised laboratories. Until now.

Miley is about to begin selling his spheres. The University of Illinois has
licensed the technology to Daimler-Benz Aerospace, which in return is helping to
finance his research. Next year, the spheres will go on sale for as little as
$60 000鈥攁 tiny fraction of the cost of the nuclear reactors or
particle accelerators that are now needed to produce neutron beams. Likely users
include mining companies, who will be able to spot impurities in an ore as it is
being mined, specialist metal smelters who will be able to monitor the
composition and quality of their alloys in real time, and airport security staff
who will use the neutron beams to spot bombs in suitcases as they pass by.

Miley wants to go even further. The holy grail of fusion research is to
create a source of cheap, clean energy. Although his spheres currently use up
much more energy than they produce, he says they have the potential to generate
more in future. And unlike any other type of fusion research, he hopes to fund
his work with the profits from the desktop neutron generators. 鈥淏y marketing
inexpensive neutron generators, we hope we can finance the work that might
transform these devices into cost-effective power generators,鈥� he says.

Fusion spheres were first conceived more than 40 years ago by Philo
Farnsworth, an inventor who developed much of the technology behind early
televisions. But Farnsworth was never able to reap the financial benefits of his
inventions, and in the late 1950s he started work for the defence and
electronics company ITT based in Fort Wayne, Indiana. There he set out to create
the ultimate energy generator by fusing nuclei together. What makes Farnsworth鈥檚
idea different is the way he chose to initiate and control the fusion
reaction.

The conventional approach is to take deuterium鈥攁 harmless, stable
isotope of hydrogen that has a neutron as well as a proton in its
nucleus鈥攁nd heat it to many millions of degrees. This strips away the
electrons from individual atoms, leaving a soup or 鈥減lasma鈥� of positively
charged nuclei and negatively charged electrons. At these temperatures, a small
proportion of the nuclei have enough energy to fuse with each other when they
collide, forming a highly energetic neutron and nuclei of hydrogen, tritium and
helium-3 in the process.

Sometimes researchers use a mixture of deuterium and another isotope of
hydrogen, tritium, which has two neutrons and a proton in its nucleus. This
reaction results in a far higher number neutrons but tritium is highly
radioactive and difficult to handle. In either case, the big problem is how to
keep the plasma contained. It is much too hot for any ordinary container to
withstand, so it is held in place by a magnetic field.

Collision course

Farnsworth chose a different route. Instead of heating deuterium gas, he used
an electric field to accelerate individual ions to the energy at which they
would fuse, in the same way that electrons are accelerated towards the screen in
a television tube. He argued that this was far more efficient than heating an
entire volume of deuterium, particularly when only a small portion of the gas
would reach the energies required for fusion. He then aimed several beams of
deuterium nuclei towards the centre of a sphere where he hoped they would
collide and fuse. Of course, the beams have to be precisely aligned for any
chance of fusion to occur, and even then only some of the nuclei actually
collide.

But this is not the only chance they have to fuse. Farnsworth calculated that
the build-up of positive ions near the centre of the sphere would attract
negative electrons. This shell of negative charge would then trap the positively
charged ions at the centre, multiplying the chances that they would collide and
fuse. To distinguish it from the traditional magnetic confinement technique,
Farnsworth鈥檚 method of fusion is known as inertial electrostatic confinement
(IEC).

When Farnsworth retired in 1967, four years before his death, he and a newly
minted physics PhD from the University of Illinois named Robert Hirsch seemed to
have proved the notion workable. Measuring the energy released in the
conventional way鈥攁s the rate at which a fusion reaction liberates
neutrons鈥擧irsch鈥檚 final machine, which was fuelled by a deuterium-tritium
mixture, delivered more than 10 billion (10¹⁰) neutrons per second, a generous
number even by today鈥檚 standards. Unable to raise enough money to continue the
work, Hirsch joined the US Atomic Energy Commission in 1968 and eventually
became the director of its fusion program. He now works as a power technology
consultant in Washington DC.

But before Hirsch gave up IEC fusion research, he had developed an entirely
new way to accelerate his ions鈥攖his time using pure deuterium in place of
the deuterium-tritium mixture. In his original experiments Farnsworth used
accelerators around his sphere to fire in beams of nuclei. Hirsch replaced this
arrangement with a spherical grid roughly the size of a tennis ball, made of
wire 1 millimetre thick.

To begin the fusion process, Hirsch admitted a small amount of deuterium gas
into an evacuated sphere. Next, he set up an electric potential of 60 000 volts
between the grid and the outer sphere, setting up an electric field that is
strong enough to ionise the deuterium gas. The field then draws the positive
ions towards the grid. Some of the ions collide with the wires of the grid and
play no further part in the process. But others pass between the wires and enter
the region within it. There, a significant proportion collide in the centre of
the sphere and fuse, producing neutrons and energy.

When Hirsch left fusion research, he donated one of his experimental spheres
to his alma mater, the University of Illinois. Two decades later, Miley returned
to the concept and developed the rudimentary desktop fusion device that now sits
on his bench. The neutrons stream out of the sphere in all directions, because
they carry no electrical charge, and so cannot be directed by electric or
magnetic fields. 鈥淣eutrons go anywhere they want to. That鈥檚 why increasing the
yield and efficiency is so important,鈥� says Miley. His initial yield was 10⁶
but has now reached 10⁹ neutrons per second.

The design of the spherical grid is crucial to the success of Miley鈥檚 fusion
machine, and he has spent several years perfecting the design. The wires combine
to create an electric potential shaped like the surface of a sphere.
鈥淥riginally, we thought we had to make this sphere as smooth as possible,鈥� says
Miley. This meant a tightly wound grid with small gaps between the wires. 鈥淏ut
eventually we found a better design.鈥�

The grid Miley now uses is like the lines of latitude and longitude on a
globe and has large holes. This produces dimples in the basically spherical
electric field. 鈥淚t鈥檚 more like the surface of a golf ball,鈥� says Miley. Only
those deuterium ions that are heading for the exact centre of each dimple are
accelerated towards the middle of the sphere. This leads to the loss of many of
the deuterium ions, but turns out to improve the machine鈥檚 performance overall.
The reason for this is that the grid promotes the formation of beams of ions
that avoid hitting the wires, preventing corrosion. This is a key advantage for
commercial applications.

鈥淣ow we have all sorts of calculations and modelling to explain why this
particular arrangement of wires aligns the ions into near-perfect beams,鈥� Miley
says, 鈥渂ut at the time we just stumbled onto it.鈥� The form of the grid is the
key breakthrough that makes Miley鈥檚 sphere work. 鈥淚t is this that makes the
design so forgiving. You no longer need the precise alignment that Farnsworth
struggled with,鈥� he says.

His spheres already produce neutrons more cheaply and safely than existing
methods. One of the most common of these is nuclear fission in which atoms of
heavy metals are split apart, liberating neutrons inside a nuclear reactor. But
nuclear reactors are expensive, complex and potentially dangerous machines.
What鈥檚 more, they produce highly radioactive waste, which must be disposed of
safely. By contrast, Miley鈥檚 machine produces less radioactive waste in a year
than is contained in a single luminous cinema 鈥淓xit鈥� sign.

Another approach uses a particle accelerator to shoot ions of deuterium into
a metal target impregnated with tritium. But this is an expensive option too.
Because tritium is a radioactive gas, it requires special handling facilities
and licences. Particle accelerators are complex machines, and the target must be
rotated and cooled to prevent overheating and damage. Yet another technique uses
high-energy protons to chip neutrons out of heavy metal atoms. Known as
spallation, this technique is perhaps the most expensive of all: its advantage
is that it can produce intense neutron beams of up to 10¹⁶ particles per
second.

Reactors and accelerators are big machines, so samples to be analysed usually
have to be taken to the neutron source rather than the other way round. One way
to provide a more portable neutron source is to use a reactor to manufacture
californium, a radioactive element that produces neutrons when it decays. But
this solution has its limitations: californium produces no more than 10⁷
neutrons per second and requires special handling facilities for the gas.

Safety first

鈥淭hen there鈥檚 the big problem,鈥� notes physicist Richard Nebel of the Los
Alamos National Laboratory in New Mexico. 鈥淲hen you鈥檙e done using a radioactive
material, you can鈥檛 turn it off.鈥� Californium continues to decay whether the
neutrons it produces are needed or not. Within two years, it must be replaced
with a fresh batch. Nebel is part of a team at Los Alamos that is developing
spherical neutron generators. In addition to spherical grid machines like
Miley鈥檚, they are considering another method that works on a different
principle. In place of a grid, they use a pulsating field tuned to the resonant
frequency of the plasma to give deuterium nuclei the energy they need to
fuse.

Being safe and easy to use is what gives machines like Miley鈥檚 an edge over
today鈥檚 neutron sources, says John Sved, an engineer with Daimler-Benz
Aerospace. 鈥淢any of our potential customers are concerned about
liability鈥攖he `Chernobyl effect鈥�, if you will,鈥� Sved says. These companies
do not want to own nuclear reactors or handle radioactive gases, because of the
risk of contamination. 鈥淚f there were a fire or an accident in their plant and
these isotopes were set free, they would face a contamination problem.鈥�

With Miley鈥檚 desktop neutron generators they avoid these risks. They are
fuelled by harmless deuterium, and the only waste is helium-3 gas, a whiff of
hydrogen and negligible traces of radioactive tritium. 鈥淎 small IEC neutron
generator could run for decades without creating enough radioactive waste to
exceed minimum regulated levels,鈥� Sved says. 鈥淭he machine could be completely
consumed in a fire and there would be virtually no concern about escaping
radiation.鈥� And to allay fears about even these small amounts of radiation,
Daimler-Benz plans to remove the tritium from the spheres safely each time they
are recharged with fresh deuterium.

And there should be no shortage of buyers, Sved predicts. When neutrons
collide with atomic nuclei, they generate gamma rays with an energy signature
unique to that material. 鈥淚f you wanted to check airport luggage for bombs, you
could bombard the area in question with neutrons. If a 10-megaelectronvolt gamma
ray comes back, that鈥檚 a signature of nitrogen鈥攁 primary constituent in
virtually every form of high explosive,鈥� explains Nebel. The
technique鈥攌nown as neutron activation analysis鈥攊s often faster,
neater and more thorough than messy chemical assays and other conventional forms
of analysis. The problem until now has been finding a safe, portable source of
neutrons.

Miley believes that the first customers for his neutron spheres will be
manufacturers of high-quality alloys in which traces of impurities have a huge
effect on the properties of the metal. 鈥淲ith a neutron source they could measure
the composition of the metal as it was made,鈥� he says. Mining companies would be
another potential buyer. A neutron source would allow them to measure the
impurities in an ore as it is being mined, or the proportion of minerals
remaining in mine waste and whether it is worth reprocessing. To protect workers
who might be around the machine for extended periods of time from the neutron
bombardment, the machines would need to be screened with a material such as
boron that captures neutrons. 鈥淏ut this is straightforward,鈥� says Miley. He and
his team use a simple shield around the sphere to protect them.

The practical applications envisaged for these machines require a yield of
between 10⁷ and 10¹⁰ neutrons per second. Miley believes his spheres will fall
well within this range, but he hopes to produce higher rates in future. 鈥淭here鈥檚
a ladder of applications,鈥� Sved notes. 鈥淭he larger the volume of neutrons you
produce, the faster the analyses you can perform; the faster the analysis, the
more uses neutron scanning can be put to鈥攁s long as the devices are
cost-effective and of manageable size.鈥�

Blasting tumours

At very high yields of 10¹⁶ neutrons per second medical applications become
possible. Researchers in Japan and America have developed a cancer treatment in
which patients are first given a boron isotope that lodges in their tumour.
Bombarding the boron isotope with a beam of neutrons produces energetic ions of
helium, which in turn destroy the tumour. While this treatment is only possible
today at the small number of clinics where these high yields of neutrons are
available, Miley鈥檚 machines could bring it to many more hospitals.

But boosting the neutron yield to these levels will not be easy. The spheres
will have to be larger, and the wire grid will have to cope with thousands of
amps rather than the small currents it handles today. Since fusion releases
energy, the grid and the sphere will have to be cooled, probably by pumping
water through cooling tubes. The vessels themselves could be protected by lining
them with heat-resistant cladding developed by Daimler-Benz Aerospace to
protect spacecraft re-entering the Earth鈥檚 atmosphere.

Then there is the 鈥渃onfinement problem鈥�. In Farnsworth鈥檚 device, nuclei that
failed to fuse were quickly lost. Ideally, the energetic deuterium nuclei should
continue to circulate, retaining their energy, until they collide and fuse. This
is the problem that magnetic confinement fusion researchers have struggled to
solve. Miley argues that his method is more efficient than magnetic confinement,
which has to heat a large mass of plasma to give just a small proportion of the
nuclei sufficient energy to fuse. 鈥淎 temperature of about 22 掳C is really a
distribution of velocities with a mean energy of about 0.02 electronvolts,鈥� says
Miley. To give a significant number of nuclei an energy of 10 kiloelectronvolts
needed for fusion requires a temperature of millions of degrees.

Miley鈥檚 design follows the more efficient strategy of accelerating each
deuterium ion individually to this energy, and gives each ion at least 10
chances to fuse before it strikes the grid or picks up an electron and drifts
away as a neutral atom. But this is still not enough even to reach higher
yields, says Miley. 鈥淭o economically deliver 10¹⁰ neutrons per second or more
we need to confine the ions long enough to give each perhaps a thousand chances
to bounce back inside the grid and fuse,鈥� he says.

That would also be major step towards the goal of making a fusion energy
generator. 鈥淭o make economical fusion energy, we鈥檇 have to give each ion not
thousands, but tens of thousands of chances to fuse if we鈥檙e going to reach the
energy breakeven point. The question is, how many times can we bounce an ion
back and forth before it gets lost?鈥�

Only future experiments with large spheres will tell. For the moment, the
physicists can do little more than speculate. But one thing is for sure: with
fusion power as with television, someone else will reap the benefits of
Farnsworth鈥檚 pioneering work. 鈥淏ut wouldn鈥檛 it be the perfect vindication if his
ideas solved the world鈥檚 energy problems?鈥� Nebel muses. 鈥淚f that could happen,
I鈥檇 like to think that somewhere he鈥檇 have a smile on his face.鈥�