Colin Bruce is a physicist and science writer living in Oxford. He is an expert in mathematical paradoxes and a lover of mysteries.
There is a cheap, practicable way to generate energy from fusion. An input pulse of at least 20 MJ X-rays to the capsule is possible (two orders of magnitude greater than the National Ignition Facility achieved): this is known from both theory and experiment to be ample for fusion ignition.
The method is similar to the NIF: a fuel capsule implodes within a hohlraum. However the hohlraum is heated not by lasers, but by the impact of fast pellets. A related idea was proposed before powerful lasers existed, and charged microspheres have been fired at ultravelocity from modified particle accelerators since the 1960s. However, a key enabling technology did not exist at that time. It is now possible, using COTS (commercial off the shelf) available equipment, to track and adjust the position and orientation of many thousands of pellets individually as they fly down a long beamline. This is a multipotent enabler.
• Fibers can be used as pellets, with better charge/mass ratio.
• Pellets catch up together during flight through a long vacuum pipe, so an accelerator of modest power can provide a very high peak input. A train of pellets launched over ~1 millisecond can arrive within a span of nanoseconds or less, a peak multiplication factor of 1 million. This compares with 5,000 for the latest heavy ion accelerator.
• The pellets are discharged by stages as they travel, so mutual repulsion at convergence is eliminated. Successively smaller course corrections fine-tune their trajectories with increasing precision: they arrive in a precisely tailored spatio-temporal pattern.
• Only pinhole access to the detonation site is required. Detonation can take place completely surrounded by flowing lithium, which captures all neutrons and energy produced from DT fusion, breeding tritium to close the fuel cycle.
What has already been done
Charged microspheres have often been fired from modified particle accelerators at speeds ~100 km/sec to test spacecraft meteor shields. A speed only a few times higher is sufficient to ignite fusion via hohlraum. A suitable pellet type is long thin fibres, such as carbon fibres, which can take a much higher charge/mass ratio without breaking apart than microspheres of equivalent mass.
Fast Electronic Switches, CCD/CMOS cameras and laser strobe
The specific enabling technologies are
1. Power MOSFET solid state switches, which drive both the accelerator and the course adjustment electrodes;
2. Fast ADCs which detect pellet position at a very high reporting rate in ballistic flight;
3. CCD/CMOS camera chips with on-board processing, which in conjunction with laser strobe illumination determine pellet position with high accuracy within the accelerator.
One Proposed Design
A practical design for a power station uses an 800 meter vertical shaft. The top portion contains closely aligned linear accelerators 350 meter long. These fire pellets downward toward a chamber at the base of the shaft. Disposable lithium pipes containing the hohlraum-fuel capsule combo are dropped down the shaft to enter the 5 meter diameter chamber, which contains a lithium waterfall. As the hohlraum approaches the chamber centre, the accelerators fire streams of pellets which impact it there, generating Xrays which drive implosion of fuel capsule containing 11 mg DT. Effectively all neutrons and energy produced are absorbed by the surrounding lithium, which is circulated through heat exchangers by electromagnetic pumps to extract the thermal energy.
The design consumes only lithium and deuterium, breeding sufficient tritium to close the fuel cycle. Tritium inventory is kept minimal by continuous extraction from the lithium melt. No other radioactives are produced. A secondary liquid metal circulation loop transports heat energy to the surface, where it is used to boil water to drive steam turbines. Each detonation produces 700 MJ net electric output of which 200 MJ is returned to drive the accelerator, so 10 detonations per second produce 5 GW. The accelerator system has a capital cost of ~$30 per joule of pulse kinetic energy delivered, $3 billion for 100 MJ as required. This is equivalent to the cost of 2.5 year’s coal supply for a 5 GW power station.
The cost of the power station itself (including steam turbines, generators,
cooling towers, etc.) is similar to that of a coal-fired unit.
Thus even on quite pessimistic assumptions, the system will be costcompetitive with coal without subsidy.