Tutorial on Heavy-Ion Fusion Energy
This tutorial provides basic information about fusion physics and on-going research to generate electricity from controlled fusion.
Overview of
fusion
Approaches to controlled
fusion
Types of
inertial-fusion drivers
Types of heavy-ion
accelerators
Typical accelerator
requirements
Choice of ion
species
Fusion-chamber
conceptual design
Cost of fusion
energy
Glossary of HIF
terms
Overview of fusion
What is fusion?
Fusion is the process that powers the sun and other stars. It
is the reaction in which two light atoms, such as atoms
of hydrogen, combine or fuse to form a heavier
atom, such as an atom of helium. In the process, some
of the mass of the hydrogen is converted into energy.
Hydrogen atoms repel each other due to the electrical charge
of their core or nucleus. For fusion to occur,
the atoms of hydrogen must be heated to extremely high
temperatures (millions of degrees C) so they have enough
thermal energy to overcome this repulsion, and then they must
be held together or confined long enough for fusion to
occur. The sun and stars are held together by gravity, but
this method only works when the amount of fuel is much larger
than the earth. Two alternative methods are being
studied to produce controlled fusion on earth. With
magnetic confinement fusion, strong magnetic fields
hold the electrically charged or ionized atoms
together as they are heated. With inertial
confinement fusion (ICF), the method discussed here, a
tiny pellet of frozen hydrogen is compressed and heated so
quickly that fusion occurs before the atoms can fly apart, so
the reaction is confined, in effect, by the inertia of
the fuel.
Why is fusion power
attractive?
Controlled fusion has the potential of becoming an important energy source because the fuel is widely available and because the reaction is relatively clean.
The easiest fusion reaction to produce is combining two forms orisotopes of hydrogen, deuterium (also called heavy hydrogen) and a heavier isotope tritium, to make helium and a neutron. Deuterium is found abundantly in ordinary water, and tritium can be produced by combining the fusion neutron with the light metal lithium. There is enough deuterium in the oceans to provide an effectively unlimited supply of energy.
Compared with the main sources of electrical energy used now, fusion is a clean source of energy. Unlike the burning of fossil fuels, like coal and petroleum, fusion produces no "greenhouse" gases and therefore does not contribute to global warming. Also, even though fusion is a nuclear process, it produces no long-lived radioactive waste. A properly designed fusion power plant would therefore be safe, and design studies indicate that electricity from such a plant would cost about the same as today.
Approaches to controlled fusion
- The largest US fusion energy research program uses strong magnetic fields to confine the fuel and is, therefore, called magnetic fusion energy (MFE). The thermonuclear fuel is in the form of a very hot, ionized gas or plasma. Most of the MFE effort is currently being spent on the tokamak approach, in which the reactor is doughnut shaped and surrounded by strong magnets. For more information on MFE, see the Princeton Plasma Physics Lab page.
-
- Inertial fusion energy (IFE) relies upon the fuel's resistance to movement or inertia to hold it together as is it being heated and undergoing fusion. In IFE, energetic laser or charged-particle beams are used to heat a small (~1 cm) inertial fusion target for about 10 nanoseconds (10-8 sec). The fusion target consists of a metal shell or Hohlraum containing a spherical shell of frozen thermonuclear fuel. The heated Hohlraum emits intense X-rays that compress the fuel capsule to thousands of times its initial density and heat it, near the center, to thermonuclear temperatures. The resulting fusion reaction, which occurs in less than a nanosecond, should produce about 100 times more energy than is supplied by the beams.
For either method of controlled fusion, the surplus energy is
carried away by radiation and hot neutrons. In current
conceptual designs, this energy would be absorbed in
lithium-containing blanket with three principal
functions: protecting the reactor wall, breeding new
tritium fuel, and heating the working fluid of an otherwise
conventional electrical power plant. The fusion
reaction in an MFE plant would either be continuous or occur
in pulses of a few seconds each, while in an IFE power plant,
typically five to ten fusion targets would be detonated per
second. A sketch of a heavy-ion power plant is shown in
the figure.
Types of inertial fusion drivers
Commercial energy production imposes additional requirements on an inertial-fusion driver. Besides producing the required power density, a commercial driver must also have an adequate repetition rate and be efficient and reliable. These added requirements are best satisfied by ion accelerators. The highest efficiencies achieved by lasers are around 15%, with 6-10% being more typical, while an induction accelerators can achieve an efficiency above 30%. Krypton-fluoride (KrF) lasers and diode-pumped solid-state lasers (DPSSLs) are the only types of high-power laser able to operate at the needed repetition rate, and both presently have technical problems, such as cooling of the lasing medium and protection of the final optics from radiation and debris. Also, the current high cost of DPSSLs would make a power plant based on that technology impractical. Accelerators have been shown to work reliably at high repetition rates for ten years or more, giving them a better demonstrated reliability then large laser systems. For these reasons, committees chartered by the US Department of Energy and by Congress have identified ion accelerators as the most promising drivers for inertial-fusion power plants.
Ion drivers for fusion are expected to share the same basic technology of existing accelerators. As discussed elsewhere, US researchers are developing designs using induction accelerators, while European and Japanese groups favor radio-frequency accelerators, like those used in high-energy physics. In either case, however, the unusual demands for very large instantaneous beam power and a small (~3 mm) focal spot require a substantial revision of conventional designs. A low transverse beam temperature or emittance is essential for a small focal spot, so low-temperature injectors with a high current density are being developed. To manage the large space charge of the ions, US conceptual designs accelerate many beams in parallel, and Europeans plan to accumulate charge gradually in a complicated series of storage rings. Both approaches also require the beam duration to be severely reduced from its initial value, by about three orders of magnitude for induction machines and six for rf accelerators. Much of the present research is directed toward meeting these stringent requirements.
A schematic diagram of a generic induction accelerator designed to produce 100 kA of cesium ions at 4 GeV is shown in below. To achieve 100 kA, it uses several methods: multiple beams, beam combining, acceleration, and longitudinal bunching. The accelerator systems and beam manipulations found in typical heavy-ion driver designs are represented by boxes. Typical values of ion kinetic energy, beam current, and pulse length at various points in the accelerator are shown below the figure.
Types of heavy-ion accelerators
Radio-Frequency Accelerators
How do rf accelerators work?A radio-frequency (rf) accelerator works by creating a rapidly oscillating electric field that interacts with and accelerates short bunches of charged particles. Rf power is fed into a resonant cavity, setting up standing-wave electric fields across one or more gaps in a conducting pipe around the beam path. The electric field across each gap changes direction at the frequency of the rf source. The charged-particles bunches are timed so that they traverse each gap while the electric field points in the proper direction to accelerate the particles. This class of accelerator can produce large fields (up to about 100 MeV/m) but is limited to modest currents, typically less than 200 mA.
The current in an rf accelerators is presently limited to values that are far below driver requirements. To obtain the required current, conceptual designs developed in Europe and in Japan first stack beams in storage rings, then bunch the beams in compression rings, and finally extract many beams simultaneously for heating the target. The following sketch shows a typical layout.
Advantages and disadvantages of rf accelerators
Advantages:
- Rf accelerator technology is well established.
- Emittance dominates transverse beam dynamics during acceleration and storage, rather than space charge.
- Obtaining the required ion energy is straightforward due the high acceleration gradients in rf structures.
- The rf power is relatively expensive.
- The low current in rf accelerators requires a long residence time in the accelerator to accumulate the required total charge. Consequently, beams must be accumulated in storage rings.
- The long residence time in rings necessitates a high vacuum.
- The Liouville theorem asserts that phase-space density cannot be increased, so stacking without dilution is difficult.
Induction Accelerators
How do induction accelerators work?In an induction accelerator, a changing magnetic field produces the electric field that accelerates beam particles. A pulsed voltage causes a magnetic field to build around a ferromagnetic ring, called a "core." The change in magnetic flux around the core induces an electric field along its axis, according to Faraday's law. The voltage pulse is timed so that the field is present when beam particles pass through the core. Induction accelerator can handle very large currents (up to 10 kA) but generate much lower voltage than typical rf accelerators.
What would an induction driver look like?
Since induction cells can simultaneously accelerate and compress multiple high-current beams, driver designs can be simpler that those fusing rf accelerators. Two configurations have been proposed. The linear layout (top) is the main approach presently being studied by the VNL, while the recirculating layout (bottom), being investigated at the University of Maryland, promises reduced cost.
Why is the US developing induction accelerators?
HIF research in the US is focused on induction accelerators for several reasons:- Induction modules can produce pulses with programmed voltage variation, allowing pulse shaping and compression along with acceleration.
- Storage rings are not needed, since beams can be compressed in length as they are accelerated.
- Safety factors for beam quality or brightness are larger than in typical rf designs.
- Cost is lower due to the relative simplicity of the total system.
- The development path is favorable because the critical issues occur at low energies.
Typical accelerator requirements
Typical Parameters
Focal spot radius 3 mm
Ion range 0.1 g/cm2 (1 mm in typical materials)
Pulse duration 10 ns
Peak power 400 TW
Ion Energy 3-10 GeV
Current on target 40 kA (total)
Ion mass 200 amu
Additional Constraints
The target chamber and final focus requirements add more constraints:- Target yield requires the repetition rate to be about 5 Hz for an attractive power plant of ~ 1 GWe.
- Thermal and mechanical stresses require the standoff from the target to the wall to be about 5 m.
- Chromatic aberration of optical system requires the energy variation to be less than 0.3%.
- Standoff and spot size require the transverse beam temperature to be less than 1 keV.
The main scientific and engineering challenge is to develop a working driver at a reasonable cost. A beam must be accelerated, directed, and focused to a small spot. This requires that the quality of the beam be maintained as summarized above.
Choice of ion species
Fusion-chamber conceptual design
At present, we believe that an IFE reactor chamber will have a 3 - 5 m radius. As shown below in a schematic of the HYLIFE-II reactor concept, fuel pellets will be "shot" into the chamber at a rate 5-10 per second with each one being irradiated by intense energy pulses produced by heavy ion beams (or possible future laser or light ion beams). The energy released by the fuel (mainly in the form of high energy neutrons and x-rays) will be absorbed by a special fluid blanket whose heat is eventually transferred to a relatively conventional steam generator, where it is converted into electricity. The fluid is continually recirculated back through the reactor chamber. In most IFE reactor designs, the fluid contains the element lithium which will "breed" tritium when struck by neutrons from the D-T fusion reactions. The fluid blanket also serves the function of protecting the reactor walls from irradiation and resultant nuclear activation.
Reactor lifetime and activity
An attractive feature of fusion versus fission plants is the smaller amount of radioactive byproducts produced during the plant lifetime. Although the fusion reactions produce high-energy neutrons which have the capability to contaminate parts of the reactor, there are several IFE reactor designs which prevent these neutrons from reaching and activating the reactor walls. The recirculating fluid inside the reactor both absorbs the neutrons and breeds additional tritium fuel. The fluid will be pumped through small ports in the reactor walls and take the imparted energy to a generator.
Modularity
A power plant must be able to produce continual power for decades without unwanted long-term interruptions. The inherent modularity of a multiple-laser driver or heavy-ion accelerator driver is attractive in terms of the ease of making repairs. Furthermore, it is also feasible for an IFE power plant to have multiple reactor chambers, so that if one should have to be shut down, the driver would still ignite fuel pellets in the other chambers.
Cost of fusion energy
For comments or questions contact WMSharp@lbl.gov or DPGrote@lbl.gov. Work described here was supported by the Office of Fusion Energy at the U.S. Department of Energy under contracts DE-AC03-76SF00098 and W-7405-ENG-48. This document was last revised June, 2001.