­Burning Plasmas

There have been several attempts to solve the problems associated with achieving terrestrial nuclear fusion from thermal ignition, without success. However, the development of high frequency, high-powered lasers might now make side-on ignition of solid density fuel, which has been experimentally proven for more than 30 years, a viable proposition. Eric Payne reports.

Considered by many to be the Holy Grail of sustainable energy generation, nuclear fusion is remarkably difficult to achieve. This should come as no surprise. The temperatures required to overcome the resistance between the atoms in a conventional thermal ignition reaction, where the reactive elements are heated inside magnetically contained plasmas (partially ionised gas), are in the region 100 million degrees centigrade – considerably hotter than Earth’s sun. But concerns about climate change, the limitations of fossil fuels and the world’s ever-increasing power demand mean that interest and investment are at an all time high.

The aim is to achieve ‘ignition’, which is the point at which a nuclear fusion reaction becomes self-sustaining. A variety of ignition methods are presently being pursued: fast ignition, block ignition and spark ignition by means of spherical laser compression and magnetic confinement. Two of the most advanced experiments are: ITER, based in the south of France for the magnetic option; and NIF, based on lasers at Lawrence Livermore Laboratory in the US. The difficulties involved in both projects are legion. ITER involves an attempt to replicate the ultrahigh temperature and high pressure conditions found at the heart of a star by containing a plasma suspended within a toroidal superconducting magnetic field more than 100,000 times stronger than the Earth’s own. NIF involves firing 192 high-powered lasers at a fuel source the size of a peppercorn for the purpose of heating and compression. A fusion reaction occurs when two light atomic nuclei (deuterium and tritium currently being the favoured elements) fuse together to create a single nucleus with a mass smaller than the sum of the elements’ original masses. The difference in mass is released as energy, in accordance with Einstein’s mass-energy equivalence formula E=mc², while the hydrogen isotopes combine to produce a helium nucleus and a neutron. The new helium nuclei are harmless, but the neutrons induce radioactivity, which is still a cause for concern among environmentalists who oppose the idea.

The method of ‘side-on’ ignition proposed by Professor Heinrich Hora of the University of New South Wales, Australia, offers the promise of nuclear fusion without nuclear waste or the associated neutron radiation problem. The method involves firing picosecond frequency, terawatt power laser pulses at a solid density fuel comprised of hydrogen and boron isotope 11 (HB11) with ignition resulting from ultrahigh energy acceleration of plasma blocks. The radioactivity generated by the secondary reaction is experimentally negligible, less than the amount of radioactivity produced by burning coal.

From calculation to experimentation

The proposed approach of Professor Hora and his team stands in direct contrast to the approaches of ITER in Europe and NIF in the US. Both ITER and NIF are attempting to achieve fusion via thermal ignition of a solid density fuel, while Professor Hora’s method involves non-thermal ignition of a fusion flame by means of the high-efficiency conversion of optical laser energy into the motion of plasma blocks. This is only possible now thanks to the development of lasers capable of emitting picosecond pulses of more than one terawatt power, thereby enabling them to produce ultrahigh accelerations above 1020 cm/s2.

The theory of non-thermal direct energy conversion has been well known in the realm of theoretic physics since 1969 as a means for explaining the very high energy of emitted ions. Indeed, the ultrahigh acceleration of plasmas by high-efficiency conversion of picosecond-terawatt lasers was confirmed by calculation in the 1970s, long before it was possible to manufacture lasers with the required frequency or power output. “We were seeing that we could create accelerations like the surface of black holes in the cosmos and it was fantastic,” Professor Hora recalls. “Still, we knew it would be very complicated to convince the scientific community to pursue this avenue of research.

Thermal ignition of fusion is interesting for studying nuclear explosions, while the nonlinear-force process is a basically non-thermal process typical for lasers. The difficulties involved in using thermal processes for controlled energy generation were formulated in 1952 by Edward Teller and are related to problems with the stabilisation of ‘complex systems’ detailed by Lord May of Oxford.”

Teller indicated that those problems could be avoided if all of the particles are reacting fast enough. But without sufficiently capable lasers, the compression of deuterium-tritium (heavy and superheavy hydrogen) was adopted as the favoured method for achieving controlled fusion. “Sophisticated compression can be avoided by using the non-thermal scheme with efficient laser energy conversion,” Professor Hora explains. But the difficulty involved in providing experimental confirmation of those remarkable atomic-level accelerations remained. “Accelerations are measured in centimetres per second squared. Gravitational acceleration on Earth is around 1,000 centimetres per second squared. I published the first calculations with my students 30 years ago, with 1017-times higher accelerations by laser pulses of far higher power than then available. In 1996, when these laser pulses were available, thanks to the work of Gerard Mourou and others, Roland Sauerbrey developed an extremely clever experiment and measured the predicted acceleration by using Doppler Shift.”

The practical verification that those ultrahigh accelerations – measured by Sauerbrey and repeated by Foldes et al., in full agreement with the theoretical predictions 20 years before by Hora – can be used for laser driven fusion following the computations for side-on ignition of a fusion flame by Chu and Bobin. After updating of their hydrodynamic computations, Professor Hora says that a larger scale computation project is now underway, with the aim of mounting a new experimental campaign now that laser technology has developed to the point where it is capable of producing the kinds of extremely short pulses of very high power to provide the ultrahigh plasma accelerations. The scheme itself now looks like a good candidate to be explored in greater detail.

No waste nuclear fusion

Another important way in which Professor Hora’s proposed methodology for producing ultrahigh energy accelerations in plasmas by means of picosecond-terawatt laser pulses deviates from most fusion research experiments is with respect to his favoured choice of fuel. “Deuterium-tritium (DT) may be a first solution, but handling tritium and the associated neutrons is still a radiation problem,” he remarks. “ITER is limited to DT with magnetic confienement of the tokamak-type design. One alternative containment device is the stellarator, which has been promoted by a strong group from Japan. Indeed, the most advanced stellarator-type design is now being built by the Max Planck Institute in Germany and is due for completion in 2015. The best ITER-type experiment was actually 12 years ago at Culham in England, based on a neutral beam operation. I was always telling them to go ahead with beam design, but I had reservations that the tokomak-type design has high currents, wall erosion and instabilities that are not yet under control. Insiders were aware of this the entire time, but the issues were not solved.”

The ITER design for the deuterium-tritium experiment is complicated further by the need to develop new materials capable of long-term high-temperature (in the 500 degrees Centigrade range) and radioactivity containment. The ‘inners’ should be able to cope with massive thermal shock effects – rapid transition through the thermal stress cycle – without suffering from embrittlement or the possibility of cracking. Also, very high-energy neutrons will irradiate the reactor core, but should not cause transmutation products in the reactor walls. It is the combination of coping with both extreme heat and radiation that makes the material design so complex.

“Containment in the picosecond laser pulse duration range is simplified because the expansion is small enough to disperse the fusion flame via solid-state fuel or pre-compression,” Professor Hora notes. “There are none of the extreme wall erosion problems associated with ITER. What is excluded for ITER is the use of the better fusion fuel hydrogen-boron11, which is free from the problems with neutron damage. This has been known since the earliest computations 40 years ago that a fusion burn using hydrogen and HB11 would produce no primary neutrons. Secondary processes make a little bit of nitrogen but the Lawrence Radiation Laboratory in Livermore precisely calculated 30 years ago that the side products of this reaction are so small that the total radioactivity per energy unit produced is less than burning coal. Still, until a few years ago, it was thought that the high compression and thermal ignition methods were 100,000 times more difficult for hydrogen-boron11 than for deuterium-tritium fusion. Everybody knew this and all sorts of other approaches were resulting in the same difficulties.

“When we made our recent computations with the new scheme, however, I was surprised to learn that the side-on, non-linear force method of producing the fusion flame is only 10 times more difficult and therefore should be feasible for HB11. The final solutions for power stations are to use laser pulses above the presently available petawatt (1015 W) up to exawatt (1018 W) lasers, as just developed for particle accelerators.”

The next stage

Further development obviously hinges upon its ability to attract the interest of experts in the field. The European Union recently earmarked around one billion euros for investment in three giant laser research projects to be sited in Romania, Hungary and the Czech Republic. These include the Extreme Light Infrastructure (ELI) and International Zeta-Exawatt Science Technology (IZEST) projects, which Professor Hora hopes can develop in a favourable direction. “There are a number of centres with excellent sub-picosecond laser pulses with up to petawatt and higher power capacity which can be used immediately to explore the next steps of the project,” he affirms. “After that, special attention is demanded because the various measurements of the ultrahigh accelerations need some co-ordination. The ideal HB11 fusion reaction with nearly no waste has been well known for dozens of years but it is now, for the first time, a solution at hand, as verified by some rather serious computations. Indeed, the development of lasers with up to exawatt power pulses is well on the way.”

“The ability to ignite a fusion flame in an uncompressed solid density plasma would indeed be a kind of dream scenario because one would not have to solve the complicated compression tasks associated with NIF (laser compression),” Professor Hora attests. “The usual method for achieving controlled fusion involves replicating the temperatures and pressures that create the hydrodynamic processes associated with ignition in stars. But, as Edward Teller noted, every thermal system is comprised of very small units that operate individually, which makes it very difficult to keep all of those units moving in one direction. The mechanism of picosecond (and shorter) laser pulses provides a process whereby these complex systems definitely can be overcome. I have been involved in this area of theoretical physics for more than 30 years and, fortunately, there are now some very sophisticated and complicated experiments that have confirmed this is the case. We are still at the beginning of this process but I am confident that we can make further progress.”

Not to say that Professor Hora is critical of the development of fusion power projects that do not follow his favoured methodology; far from it. He is a keen advocate of a varied approach to achieving ignition. “The progress of NIF must go ahead as a highly developed first solution,” he notes. “The new method will not be a ‘free lunch’. But the basically non-thermal high-efficiency method for the conversion of laser energy (into a fusion reaction) offers the possibility to avoid the complicated compression process, and other simplifications may reduce the size of the project [still further].” To that end, it is worth noting that Steve Haan of Lawrence Livermore Labs in California, where NIF is sited, said in an interview with the Royal Society of Chemistry in 2010 that “this has the potential to be the best route to fusion energy”, which is sure to attract greater interest, and impetus for investment.