One of the world’s leading specialists in laser fusion, the Australian physicist Prof. Heinrich Hora, has proposed a new type of nuclear reactor which promises to provide highly-efficient, radioactivity-free generation of electric power, with virtually unlimited reserves of fuel. The design uses ultra-high-power, ultra-short-pulsed lasers to trigger fusion reactions between nuclei of hydrogen and boron. Hora believes that a prototype of his reactor could be running within the decade.
In the previous installments of this series, Jonathan Tennenbaum introduced readers to the new reactor concept and its fascinating scientific and technological background.
It is fitting to conclude this series with an interview Tennenbaum conducted in March this year with Heinrich Hora.
Jonathan Tennenbaum: The first experimental realization of fusion energy happened 70 years ago, with the explosion of the first hydrogen bomb. Why have we not yet learned how to use the energy of fusion in a controlled way, to produce energy for mankind? Today we have these gigantic experimental devices, the National Ignition Facility (NIF) laser fusion facility in the United States, and the International Torus Experimental Reactor (ITER) magnetic confinement fusion device under construction in France. ITER has an official price tag of $20 billion, but many expect it to rise to $50 billion by 2035 when ITER begins full-scale experiments with deuterium-tritium fuel. In 2040 it might be possible to start designing a prototype reactor. If that is supposed to be the right approach, then the practical generation of energy by fusion reactions is far away, indeed.
What makes you think it can be done much faster?
Heinrich Hora: Most experiments for fusion are based on the assumption, that one needs temperatures of several hundred million degrees Celsius. The question is, how can you get around having to use these high temperatures?
The equation for a laser acting on a target contains pressure, given by the density and temperature, but there is a second term for the ponderomotive force produced by the electric and magnetic fields of the laser. (Today with lasers) you can produce very high fields for extremely short times, of millionths of a millionth of a second, and one can produce petawatt (a million billion watts) or multiples of a petawatt of power. From this one can produce forces from the laser fields. One can generate higher pressures than the pressures from thermal mechanisms (i.e. heating). This is an essential difference to everything that was done before. As a matter of fact the idea to use lasers for fusion [has existed] since the early 1960s, but where the laser energy would go into the thermal energy of a spherical compressed plasma. This was still via temperature.
Now, the latest results – which were not measured by us but which we understand because we have been involved in all kinds of detailed research – is that now ignition can be done with extreme laser pulses. That sends a new message.
Tennenbaum: And this happens without heat?
Hora: Indeed. (The pulse) produces the ignition, and from then on (heat) develops in the hydrogen-boron fuel at moderate density and produces reactions, which then have to be confined by ultra-high magnetic fields. These fields are now available. In Japan, Fujioka has produced kilo-tesla and higher fields (using laser pulses). These fields are more than 100 times those used in the systems operating without lasers.
Tennenbaum: Such as the ITER?
Hora: Yes. A new situation.
Tennenbaum: How has it become possible to produce laser pulses of such huge powers, that you can succeed with a non-thermal ignition of the hydrogen-boron fuel?
Hora: This has a fascinating history. The scheme of so-called chirped pulse amplification (CPA) of laser pulses was discovered in 1985 by Gérard Mourou and Donna Strickland. After that, the laser intensities went up like a rocket.
Tennenbaum: It reminds me of the famous Moore’s law in microelectronics.
Hora: It was more dramatic.
Tennenbaum: Up to now when fusion is discussed, people always think of the hydrogen isotopes deuterium and tritium. And as far as I know practically all the experimental reactors use DT fuel. Tritium is radioactive and DT reactions produce a lot of neutrons. Your proposal, on the other hand, would use boron together with hydrogen as a “clean” nuclear fuel. Why was this not considered before?
Hora: The reaction of deuterium and tritium is the easiest and fastest way to fusion, even for the ITER experiment with magnetic field confinement. [It was thought to be] the only way to realize fusion.
The hydrogen-boron reaction is well known, but it is about five orders of magnitude more difficult when going through the usual procedure of compression and heating. The reaction rate is so low that this reaction was usually neglected.
But using lasers with chirped pulse amplification, for which the Nobel Prize was awarded, experiments have now been made which gave a billion times higher hydrogen boron reactions, than the very frustrating low values [obtained earlier].
You get directly three alpha particles with no radioactive ash. And the alpha particles are no environmental problem at all. Carrying the energy of the nuclear reaction, they can be slowed down by electric fields so that the energy can be directly converted into electric power, without heat exchangers and turbines.
It is simple to make a spherical (reaction) chamber, charge it to a high voltage and have the alpha particles run into this high voltage and change their energy into electricity.
Ninety percent of the nuclear energy would be converted directly into electricity.
Tennenbaum: But how would the fusion reactions be produced in the reactor?
Hora: For five or six years we did calculations, to [realize hydrogen-boron fusion] the same way as in all the other (laser fusion) experiments: to make a spherical compression, heating with extreme laser pulses. And it turns out that one needs not a petawatt but an exawatt – a thousand thousand times higher powers, and this is too far away. And then came the simple idea, to make it not spherically but to trap the reaction in a cylindrical geometry. And just at this time to produce the highest magnetic fields using another laser, kilo-tesla fields. These fields are enough, for a short time, to trap this cylindrical volume with the reaction.
Tennenbaum: How is the reaction triggered?
Hora: By one laser pulse rather than 192 laser pulses [as is done in the NIF]. We need only one pulse, and all the complicated apparatus is not necessary; only a single pulse which is extremely short and of extremely high power. A petawatt is about as much power as all power stations on the Earth, however only for a time of one-millionth of a millionth of a second. And this has been developed over the years since the discovery by Donna Strickland and Gérald Mourou.
Tennenbaum: You have emphasized the role of accelerated plasma blocks in the ignition process. Can you explain how that works?
Hora: This is a very interesting question. We can refer to computations by Jean Louis Bobin in Paris and C. K. Chu from Columbia University in New York. If you have a plane geometry [as in the end-on irradiation of a cylindrical target], how short must laser pulses be in order to start the ignition? I was involved very early in pulse theory and numerical calculations. Computations resulted in the following: for ignition in this plane geometry, the energy input has to be in an extremely short pulse, in the range of millionths of millionths of a second. And in this plane – not spherical – geometry it ignites a self-sustained reaction which indeed then does produce high temperatures, but by itself, in the fuel. The interaction of the laser field and the plasma generates, so-to-say a piece of neutralized plasma block, getting energy from the light, and moving into the fusion fuel, igniting the fusion reactions.
Tennenbaum: How fast is this plasma block moving?
Hora: About 1,000 km per second. It is interesting that these are also the velocities given in the Google article for the processes in H-bombs. [The Wikipedia article “Thermonuclear Weapon” gives the implosion velocity of the fusion secondary of the first hydrogen bomb, the Ivy Mike device, as around 400 km per second – JT.] It is disclosed there, and one can compare the numbers. And it is interesting, that the plasma block generation [in the hydrogen-boron reactor] is obviously of a similar kind, but in a fully controlled way, for a power station. Nothing can explode, nothing can meltdown like in a uranium [fission] reactor. This is safe and controlled in an easy and inexpensive way.
Tennenbaum: How big would such a power plant have to be? Could the plant be made relatively small? Or would it have to be as large as a present-day nuclear power plant?
Hora: No, instead of the gigawatts power stations it could go down to 100 megawatts, perhaps 50 or even smaller. We have a whole design, for which we have patents granted in the US, Japan, and China.
Tennenbaum: And the laser?
Hora: These types of lasers are fortunately just now available from companies, you can buy them, but indeed then they need to be specifically developed to be much cheaper and cost not $50 million but much less through standardized production. This is all possible.
Tennenbaum: In other words, the technical parameters needed for the lasers are essentially already in the commercial sphere?
Hora: Exactly on the same level. One leading company is in France but the main leaders are at the University of Austin, Texas. They can generate one pulse per minute. Whereas in Livermore [with NIF] they can make two pulses per day.
Tennenbaum: But for the reactor would you need a much higher pulse rate?
Hora: A higher pulse frequency going down to one per second or two seconds, five seconds. This can be optimized according to what is desired.
Tennenbaum: Can you say anything about the economics?
Hora: Provided that the scientific efforts on this track do not encounter unknown difficulties, then these reactors can be on the market in eight years altogether.
Tennenbaum: In eight years?
Hora: To have a prototype available from which you can then produce for the market. There are mountains of boron, we could power mankind for thousands of years.
Tennenbaum: What would be the order of magnitude of the investment required to build a prototype hydrogen-boron power plant?
Hora: A prototype would cost 100 million or so. That is so little money, that it is suspicious! But it will be in this order of magnitude. But then there are many components which can be made much cheaper in volume and so on. One can definitely say that one kilowatt-hour will cost a fifth to a tenth of the present lowest price.
Part 5: Lighting the nuclear fusion fire
Tennenbaum: Over the last 10 years you have published many scientific papers about hydrogen-boron fusion and I can see that you have a very impressive list of co-authors, from the US, China, Germany, France, Israel, the Czech Republic, and they include the Nobel Prize winner Gérard Mourou. How strong is the interest and support of the scientific community for your idea, now? Or are some people saying, this is crazy?
Hora: Well, I mentioned the measurements in Prague. Before that, a similar experiment with hydrogen-boron was performed at the École Polytechnique in Paris. The first good experiment producing hydrogen-boron reactions was done by Belyaev and his colleagues in 2005 near Moscow. Christine Labaune made, with her experiment in Paris, a big step forward and then came the results from Prague. We could then ask a number of former students and established physicists to follow up the computations which we published in papers with a large number of people from around the world, to confirm our progress step-by-step.
Tennenbaum: So you have a kind of international community around you which would be pulled into the project or which is available to answer various questions and to participate in this effort?
Hora: Yes, definitely.
Tennenbaum: What is the next step, from your standpoint?
Hora: Knowing what the difficulties would be if we would go to the government and say, please give us the money to do the research – we would need a year of discussions, and people who are not fully qualified would have the power to say, this is nonsense. The other way is to find investors and to use the existing, high-level laboratories around the world and give them tasks in an outsourcing way. We will have to pay for this. We have a number of very well-known advisors, to optimize this outsourced research. That is how we will try to proceed. Hopefully, it will go ahead. For the last three years, we have had good news. In recent weeks we have had good publicity in the media. I hope that despite all the other world problems, we will really get support from investors, and outsource experiments. The recent results from Prague make me even more optimistic than before. [See “High-current stream of energetic α particles from laser-driven proton-boron fusion” by Lorenzo Giuffrida et al., Physical Review E 101 (2020) 013204, specifically the last paragraph of Section 1 and Figure 9]
(More information on the proposed hydrogen-boron reactor, including scientific references, can be found on the website. A good summary with an extensive bibliography is also provided by the 2018 paper “Extreme laser pulses for non-thermal fusion ignition of hydrogen–boron for clean and low-cost energy” by H. Hora et al. published in the journal Laser and Particle Beams, Volume 36, Issue 3, September 2018, available on the internet; and “Pressure of picosecond CPA laser pulses substitute ultrahigh thermal pressures to ignite fusion,” by H. Hora et al, High Energy Density Physics 35, 2020, Article 100739).
Jonathan Tennenbaum received his PhD in mathematics from the University of California in 1973 at age 22. Also a physicist, linguist and pianist, he’s a former editor of FUSION magazine. He lives in Berlin and travels frequently to Asia and elsewhere, consulting on economics, science and technology.