Diodes for Fusion Power Plants
The Ferdinand-Braun-Institut is accelerating the development of a key component
As part of the “Fusion 2040 – Research on the Way to a Fusion Power Plant” programme, the German government is investing around 1.2 billion euros over the next five years into the development of this climate-neutral, intrinsically safe and almost unlimited energy source. The first 16 funded projects have already started work. The Ferdinand-Braun-Institut is playing an important role in advancing the development of a key component.
Paul Crump has to go back to the beginning to explain his research. “There are various approaches to harnessing fusion as an energy source on Earth,” says the head of the High-Power Diode Lasers Lab at the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH) in Adlershof. Alongside magnetic fusion, which has long been researched, laser-ignited inertial fusion is making huge strides. The historic breakthrough came on 5 December 2022 when researchers at the Lawrence Livermore National Laboratory in California were first successful in igniting fusion plasma using lasers at their National Ignition Facility (NIF) – a feat which has since been repeated. More recently, a fusion carried out there released four times the amount of laser energy supplied.
In essence, this involves the fusion of the hydrogen isotopes deuterium and tritium. When fused, they form helium, which weighs less than the two nuclei; the difference is released in the form of extremely high-energy neutrons. One kilogramme of fusion fuel contains as much energy as 55,000 barrels of heating oil or 18,600 tonnes of coal, which is equivalent to a five-kilometre-long freight train. It is not just from a logistical point of view that fusion power plants are interesting; like renewable energies, they could decouple energy supply from the carbon cycle, with the necessary hydrogen being available in almost unlimited quantities. Although fusion produces weakly radioactive materials, these would be recyclable after a few decades of decay. There is a safety advantage, but this also happens to make fusion complicated – and this is the area where Crump is focusing his research on the performance, efficiency, beam quality, and reliability of high-power diode lasers.
Atomic nuclei repel each other and can only overcome the underlying Coulomb barrier (the potential that a positively charged particle must overcome in order to enter the atomic nucleus) with a great deal of kinetic energy. To achieve this, high-energy laser pulses lasting a nanosecond heat the plasma to around 150 million degrees. This accelerates the atomic nuclei within it to such an extent that they approach each other at less than 2.5 femtometres during collisions. It is at this distance – roughly 20 millionths of a human hair – that nuclear force comes into play, causing the nuclei to fuse as it is so much stronger than Coulomb force. In the sun, this has been happening for 4.6 billion years at a ‘cool’ 15 million degrees Celsius as the pressure there is immense. By contrast, on Earth, fusion stops immediately once the pressure and temperature drop or the fuel supply stops. The Coulomb barrier guarantees this safety, but requires very complex technology, of which high-performance diode lasers are a key component.
“The pilot plant in California has proven that the underlying physics works. But it’s still a long way from continuous power plant operation and was never designed for energy production,” explains Crump, who has spent many years researching and working in the United States and the UK. New approaches are needed for a fusion power plant. That is why Germany is investing 1.2 billion euros in the ‘Fusion 2040’ funding programme – to develop these plants, build an innovation ecosystem, and lay the foundation for future supply chains. In one of the 16 funding projects launched to date, the FBH is working together with ams-OSRAM, Jenoptik, Laserline, TRUMPF (as coordinator) and the Fraunhofer-Institut für Lasertechnik ILT in Aachen on diode lasers, which are vital for plasma ignition in such power plants. These are the only components which enable the necessary efficiency and frequency of the high-energy laser pulses. Unlike the experimental facility which is capable of a maximum of one ignition per day, the future power plant must ignite up to 20 fuel pellets per second. Each pulse must focus three to four megajoules of energy on the pellet for a few nanoseconds. 300 to 400 parallel laser beams must reach the required energy level to generate such high-energy pulses. They are pumped by passing through doped crystal plates which are stimulated by the light from diode lasers. The optical intensity and thus the photon density increase exponentially. The pumped beams are then combined, converted to the correct wavelength in the X-ray range, and directed as individual, highly-focused megajoule ignition pulses onto the fuel pellets injected into the fusion chamber.
In California, this pumping is done with flash lamps powered by huge energy storage devices which have to be left to cool down for hours after each pulse. For future power plants, these will have to be replaced by diode lasers to achieve the necessary optical energy for the system. It is thus most convenient that Crump’s team at FBH is working with many other researchers to systematically increase the performance, quality, and reliability of comparatively ‘cold’ semiconductor lasers. The researchers understand that their ideas must be feasible for fully automated mass production in the future. Every detail is being tweaked, from the performance of the individual diode laser bars to their smart integration into cooled diode stacks, which must be incorporated into highly efficient pump modules with optimised electronic and optical interfaces in a very confined space. This highly ambitious project is very much in line with Crump’s thinking. “Fusion is the most exciting high-power application imaginable. As a team, we are delighted to be involved and to have the chance to contribute our expertise in diode lasers.”
The opportunities here are enormous. Manufacturing the diodes for a single power plant would utilise all of the manufacturing capacity currently available worldwide for years to come. Even if the cost of diodes was able to be reduced to less than a penny per watt, it would open up a new multi-billion market for photonics. But in order to achieve this, the performance, efficiency, and lifespan of semiconductor lasers must be significantly increased. “This is a very big and extremely exciting challenge, for which the FBH has built up the relevant know-how,” explains Crump. Current diode lasers achieve output powers of 400 to 500 watts per 1cm laser bar. Research into highly efficient kilowatt laser bars is being advanced in his lab. This is an important approach to reducing the cost per watt to the required level for fusion power plants. His team is already preparing for the next big step in the DioHELIOS project by “focusing on multi-kilowatt bars”. Technological approaches from LiDAR and VCSEL development could be used for this purpose, where high energy densities are also required, but with much shorter laser pulses.
Researchers are able to integrate greater numbers of laser diodes into chips by using tunnel junction technology and smart solutions for chip design, heat dissipation, and the guidance and stabilisation of emitted light wavelengths, including optical gratings. By combining these approaches, the LiDAR team at FBH has recently developed the first single emitters with more than 400 watts of optical output power in nanosecond pulses – something which previously required bars that were at least 20 times larger. “If we succeed in implementing them with sufficiently high efficiency, then this could be a very interesting solution for fusion. Outputs of up to 15kW per bar would be feasible by combining these emitters at the bar level, resulting in transformative cost reductions,” explains the researcher. But a lot of detailed work remains to be done before then and this statement applies to fusion research as a whole. There are still many technological hurdles to overcome before this promising energy source can be harnessed. But the course has been set. Time to cast off. “We are very enthusiastic about it all,” says Crump with infectious optimism.
Peter Trechow for Adlershof Journal