Heavy ion fusion

Heavy ion fusion is a fusion energy concept that uses a stream of high-energy ions from a particle accelerator to rapidly heat and compress a small pellet of fusion fuel. It is a subclass of the larger inertial confinement fusion (ICF) approach, replacing the more typical laser systems with an accelerator.

Accelerators have the potential to be much more efficient in terms of delivering energy to the fuel pellet; typical laser-based "drivers" have overall efficiency on the order of 1%, while heavy-ion systems aim for 30% or more. Additionally, they can produce pulses of energy many times a second, while existing high-energy laser systems require lengthy cooling periods between "shots". These advantages would be useful in a commercial setting, as they would greatly lower the cost of operation, and somewhat lower the cost of building the plant compared to a laser system.

The basic concept of replacing photon beams with particle beams had been suggested on occasion before 1970, using either electrons or protons. Fundamental limits on the beam focusing and stopping distances using pulsed power acceleration of electrons or protons led to the concept of using heavy ions, whose higher atomic number causes each ion to stop more rapidly in fusion target material, which allows them convey thousands of times electrons or protons with the same stopping range.

The high energy of each ion means a given beam power level will be achieved with less current. Conventional accelerator beams focus to small spots. For example, CERN's Large Hadron Collider started out to hit beams head-on with 250nm focal spot radius, and in routine operation has achieved much better.

The high energy ions need to be in a low charge state to handle space charge forces. Singly ionized ions are strongly favored. The ions have substantial rigidity and do not scatter easily in modest vacuum, allowing the high quality beams focus to small radius spots in inertial fusion reaxtion chambers.

A major meeting in 1976 led to the rapid uptake of the concept through the late 1970s and early 1980s. In the late 1970s, heavy ion fusion (HIF) was described as "the conservative approach" to a working fusion reactor. In the early 1970s, definitive studies showed that inerital fusion chambers can avoid the damage of chamber materials by high energy neutrons from the deuterium-tritium reaction.

Lawrence Livermore National Laboratory (LLNL) confirmed with the 8-yr HYLIFE study (1977-85) that inertial fusion chambers can, because of the neutron protection from injected lithium, be built with "common steels." HYLIFE's highly desirable conclusion required that each pellet yield the energy equivalent of a ton of high explosives, 4 GJ, or more.

Results from the NOVA laser at LLNL in the late 1980s and early 1990s indicated that a laser would not be able to ignite such a large energy release. Livermore conceived an innovative pattern of jets to create the thick liquid wall to work with smaller fusion yields and higher repetition rates, upending HYLIFE's clean design by substituting FLIBE for pure lithium in the 1992-94 HYLIFE-II design [citation].

Since that time, in spite of continued interest, no large-scale experimental device using the approach has been built. It has the disadvantage that accelerators with the required energies can only be built in a large size, on the order of kilometres, which makes it difficult to test with low-cost systems. In contrast, even small lasers can reach the desired conditions, which is why they remain the focus of the ICF approach.

Confidence in ion acclerator system design has historically seen multiple occasions where large, new machines use innovations to increase performance parameters by factors of ten. For this reason, large new machines can be built based on the precedents that reduced new ideas to practice in industrial as well as research accelerator systems.