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.) |
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. |
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. |
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. |
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.