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| The Other Route to Nuclear Fusion |
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The Other Route to Nuclear Fusion
The intensity increase planned for the SIS accelerator will open the door to
an unexplored realm in plasma research. It will, for example, enable
researchers to investigate matter under extreme conditions, at pressures of
several megabar and temperatures up to several hundred thousand Kelvin.
Present results already suggest that inertial confinement fusion (ICF) could
represent a technically feasible alternative to nuclear fusion in tokamaks
and stellarators. In pursuit of this alternate approach, GSI is participating
in a European study group, which has been formed to assess the technical
feasibility of such a heavy ion ignition facility and develop a coherent set of
parameters for its design.
Not only are swift, heavy ions efficient energy carriers - their interaction properties also make
them ideal candidates for creating high energy densities in matter. This is because their high
atomic numbers result in very effective energy deposition, and their range of penetration in
matter can be precisely selected by controlling the energy of the ion beam. Because of these
properties, heavy ions are finding use in a new approach to the problem of how to create very
dense plasmas. In addition, they also allow researchers to explore the properties of matter
under extreme temperatures and pressures on a laboratory scale under reproducible conditions.
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A beam of neon ions at an energy of 300 MeV per nucleon penetrates into a glass object. The
energy is very uniformly deposited along the particle path and causes uniform volume heating.
Only near the end of the ion path, at the so-called Bragg maximum, does increased energy
deposition produce greater heating of the target material. (Above: fast streak camera.
Below: Plot of detected light intensities along the path, in relative units.)
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The investigation of heavy ion interactions with ionized matter has received new impetus with
the development and expansion of GSI's accelerator facilities. Heavy ion beams are now
available in a wide range of energies from 45 keV per nucleon to more than 1 GeV per
nucleon. As a result, it is possible to vary the penetration range of the ions in matter by
selecting the appropriate energy. For example, a beam of neon ions (Ne10+) at an energy of 300
MeV per nucleon can penetrate nearly 50 mm into a glass object.
The light
generated along the interaction path serves as a measure of the deposited energy, and can be
photographed with a fast streak camera. The energy is deposited very uniformly over a wide
range of the ion path, and causes uniform heating of the target volume. At the end of the path,
however, locally greater heating occurs as a result of increased energy deposition. This is the
so-called Bragg maximum.
Enhanced understanding of ion interaction processes
The goal of investigations up to now has been to enhance our understanding of the interactions
of ions with hot, ionized matter. The energy loss of ions in a completely ionized plasma, and
the effect of the ionized matter on the charge distribution of ions traversing the plasma, have
been measured for a wide range of energies.
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A hydrogen discharge is ignited as part of an investigation into the interactions between
uranium ions and a plasma. Using this setup, it was possible, for the first time, to
demonstrate the increased energy loss of heavy ions when traversing a plasma.
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The diagram shows the theoretical curve of temperatures attained in a solid-state target as
a function of delivered power density. The regions of particular interest for investigations
in plasma physics have been identified by arrows. The power depositions required for a
fusion accelerator are two to three orders of magnitude greater than those attainable in
the SIS. However, such values may become technically attainable with the advent of the next
generation of high-current ion accelerators.
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These measurements have shown
that both the charge distribution and the energy loss are highly dependent on the degree of
ionization, the temperature, and the plasma density. Moreover, the energy losses were found to
be considerably higher in the plasma than in the cold gas. This effect is most pronounced at low
ion energies, where the observed energy loss in the plasma is 40 times greater than in the
corresponding cold hydrogen gas. This is the result of more effective energy transfer to free
electrons in the plasma, and of the higher charge state of the projectile ions, which occurs in
the plasma.
Within the framework of an intensity upgrade program, it is planned to increase the particle
fluxes for very heavy ions accelerated in the SIS by more than two orders of magnitude over
the next few years (see next Section).
This will allow an increase in the power density generated
in solid matter from about 0.1 TW/g (terawatt per gram) to values around 10 TW/g.
Temperatures of 20 to 30 eV - corresponding to approximately 200,000 to 350,000 K
(Kelvin) - will thus be attainable. This development, in conjunction with the high particle
densities of plasmas generated in a solid target, will open the door to as yet unexplored areas
of plasma physics.
For example, it will become possible to investigate the
equation of state of "normal" matter - as contrasted with nuclear matter - at
extremely high pressures of several megabar (Mbar) and extremely high temperatures. In this
way, it should be possible, for example, to create metallic hydrogen. At temperatures above 10
eV, we can expect to observe hydrodynamic phenomena in the hot, dense plasmas. In the
region above 30 eV radiation phenomena should become observable. In other words, we
should expect to observe black-body radiation in this realm. The SIS will thus be entering
previously uncharted territory in the area of plasma research in the coming years.
The new investigations made possible in this way are not only of interest for basic physics
research. They also include important preliminary explorations in pursuit of the long-term
technological goal of creating a fusion reactor using inertial confinement fusion (ICF). At
present, power densities attainable in the SIS still fall short of that necessary for ignition of a
fusion target by two to three orders of magnitude. However, it is hoped to attain the required
specifications with the advent of the next generation of high-current ion accelerators.
Until now, discussions concerning drivers for an ICF reactor have focused mainly on high-
power lasers and short-pulse, light-ion accelerators. But the most recent research has shown
that heavy ion accelerators offer by far the best approach. This is especially true if the reactor
is intended for energy production in a commercial context. Key advantages of heavy ion
accelerators are high repetition rates for the production of intense ion pulses and highly
efficient conversion of electric power into ion beam energy. Additional benefits result from the
characteristics of heavy ion beams: unlike laser beams, ion beams penetrate into the volume of
the target material, allowing effective heating of the bulk material. And in contrast to light-ion
beams from pulsed power accelerators, intense heavy ion beams are easy to focus.
The objective of fusion research is to derive energy in a controlled manner from thermonuclear
fusion processes. Among many possible fusion reactions, the easiest to exploit for energy
production is the fusion of the hydrogen isotopes deuterium and tritium. This is because fusion
with a relatively high reaction cross-section occurs at temperatures as low as 50 million
degrees with this mixture. To achieve this type of reaction, the fusion fuel, which is composed
of deuterium and tritium (D and T), must be heated to this high temperature and kept closely
confined long enough to allow a substantial fraction of the particles to react with each other,
thereby releasing more energy than is consumed in the heating process.
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In the indirect heating process, the fusion target is bombarded on two sides by intense
heavy ion radiation. In the converter material, the energy of this heavy ion radiation is
converted into black-body radiation - i.e., into soft X-rays. This radiation causes the
shell of the target to vaporize explosively. The imploding components subsequently compress
the fusion fuel at the center of the hollow sphere. The resulting spherically symmetrical
implosion generates the high temperatures and densities required for initiation of the
fusion process.
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Let us look at how inertial confinement fusion works. The DT mixture is enclosed in the inner
wall material of a hollow sphere several millimeters in diameter. This sphere is exposed to an
intense beam. Heated abruptly, the material constituting the wall of the sphere evaporates
explosively, and the fusion fuel is accelerated inward by the recoil.
The spherical
symmetry of the implosion causes the DT fuel to be highly compressed at the center and to
attain the temperature required to ignite nuclear fusion. The high level of plasma compression
is briefly maintained by the plasma's own mass inertia: additional magnetic fields are not
required. It is only necessary to control the dynamics of the process so as to generate a central
region of very high temperature that will ignite the fusion reactions.
This "ignited" region of the plasma then spreads out as the heating process becomes self-
sustaining until it ultimately encompasses a large portion of the available fuel. This is mainly
due to the fact that helium nuclei produced by fusion and, to a lesser extent, the neutrons
emitted during the fusion reaction are chiefly responsible for the spread of this region. The
compression of the DT mixture must attain densities exceeding 200g/cm3; in other words, the
mixture must be more than a thousand times denser than liquid DT. Initiation of such a process
in a fusion target necessitates a beam pulse possessing an energy of several megajoules (MJ).
The shell of the fusion target can be heated directly or indirectly, i.e. either by direct irradiation
with an intense pulse of heavy ions, or by first heating a converter target which subsequently
passes on the converted radiation energy to the fuel mixture in its cavity. Both approaches
require a thorough understanding of the basic mechanisms involved when heavy ions interact
with hot, ionized matter.
Beam pulses of more than 1,000,000,000,000,000 ions
A fusion accelerator must meet extremely challenging requirements. Heating the outer shell or
the converter region of the fusion target to a temperature of 300 eV requires depositing an
energy of 5 to 10 MJ there - within a mere 10 nanoseconds (ns). At a kinetic energy of 10
GeV - an energy level corresponding to 50 MeV per nucleon for bismuth ions - this requires
radiation pulses with an intensity of more than 1015 ions. Given the advanced state of
accelerator physics and the emergence of new methods of generating, transporting, and
focusing such intense ion pulses on a target, there is reason to believe that a dedicated heavy
ion driver may be within the range of the next generation of acceleration technology.
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Developing and optimizing new linear accelerator designs is of special importance in
generating high intensities. The photo shows a modern RFQ structure built at the University
of Frankfurt, which is now to be modified to make it suitable for the acceleration of
high-intensity particle beams.
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Accelerator physics thus faces a tremendous challenge: to investigate the potential and
perspectives of high-intensity accelerators for heavy ion driven inertial confinement fusion.
In
pursuit of this long-term goal, GSI—together with other leading laboratories in accelerator and
plasma physics research - has established a European study group, whose aim is to assess the
feasability of such a heavy ion ignition facility and to develop a coherent set of parameters for
its technical design.
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