In inertial fusion energy (IFE) the burning nuclear fusion reaction is ignited by illuminating and compressing a target – a pellet that contains deuterium and tritium – by the use of intense laser or ion beams. A power reactor would operate by igniting several such pellets per second.
A favorable feature of inertial fusion is that the components (target factory, driver, fusion chamber) can be isolated from each other. In addition, the driver can be modular, thereby enabling a staged development. As is the case for magnetic fusion energy, progress in inertial fusion has been remarkable.
The scientific basis of inertial fusion has progressed to the point where the driver and pellet requirements to achieve ignition are known to high confidence and are within reach. Experimental diagnostics are capable of probing details of physical properties under extreme conditions. Knowledge of the laser-plasma interaction and implosion hydrodynamics has progressed from a rudimentary empirical level, with much uncertainty, to a current state in which there is good agreement between theory and experiment. Significant advances in computational power and technique, in concert with experiments, have led to good predictive capability.
At the same time, laser driver technology has progressed from a few joules to megajoules, with sufficiently good beam control and pulse characteristics to implode ignition pellets. Likewise, advances in technology to fabricate complex targets with nearly sufficient surface smoothness to satisfy the program requirements have been remarkable. The United States is clearly the world leader in such research.
There is a high level of confidence that ignition-level performance will be achieved on the National Ignition Facility (NIF), now under construction at Lawrence Livermore National Laboratory. But the challenges that must be overcome to achieve a practical IFE reactor are at least as large as those before MFE. A practical fusion reactor would require large extrapolations of performance in numerous areas, including driver technology, pellet fabrication costs, and reactor wall technology.
A variety of drivers are being explored (several types of lasers, heavy ion beams, and z-pinch pulse power), as well as different means of coupling driver energy to the fuel pellet (direct and indirect drive). Some considerations favor heavy ion beams as the driver technology for IFE. Because lasers have lower driver efficiency than an ion beam driver, a reactor employing a laser driver would require higher pellet gains than a reactor that uses an ion beam driver. Moreover, the inherent high-repetition rate, high efficiency and high-current capacity of an induction linac, plus the reactor-compatible nature of the final focusing element, favor an ion-beam driver. On the other hand, because an ion beam driver would probably employ indirect drive, a reactor using an ion beam driver which would require the fabrication of more complicated targets than a reactor using a laser for direct drive.
Given the immature state of the technology, it is not appropriate at this time to select only one driver technology for continued exploration. This is particularly the case since laser technology is the mainstay of the defense application and IFE should seek to obtain leverage from the large defense effort that relies on lasers. As noted above, the achievement of IFE requires very significant technical progress in a wide variety of areas – in driver technology, in target design, in target fabrication, in chamber design – and the solutions must lend themselves to cost-effective application in a practical reactor. Reactor studies can be useful in this connection by serving to reveal the practical barriers to the use of the technology for power generation – including the cost constraints within which such a system must operate. Such studies should continue to be used as guides in establishing the direction and balance of research efforts, as well as to establish goals that constitute thresholds for further investment.