Rob Coppinger probes the difficulties of super laser operation and the technology that will overcome the challenges
Probing the heart of a nuclear weapon explosion or igniting an artificial sun to power the world – these are the activities that super lasers are energising.
With crystals approaching 800kg in weight, hundreds of lasers employed and frequencies not just doubled but tripled, the power required – measured in petawatts – for these laser systems present researchers with significant technical challenges. For fusion power, for example, the level of use of these very powerful systems will be great compared to today’s standards. As Gooch & Housego technology vice president Dr Andrew Robertson explains, laboratories’ super lasers ‘are fired once a day’, referring to the US Lawrence Livermore National Laboratory and France’s Laser Megajoule facility, while ‘for power generation, for fusion, you need to be firing at 2-10Hz, two to 10 times a second.’
Gooch & Housego is involved in two of the US government’s fusion power efforts: the National Ignition Facility (NIF), which is a fusion physics test facility; and the Laser Inertial Fusion for Energy (LIFE) project. Inertial fusion involves firing fuel pellets into a chamber; a laser fires at the pellet, causing fusion. The resultant release of energy is then used for power. The near-term goal for NIF is to get ignition and generate more power than they put in, with the fuel burning on its own. ‘Lawrence Livermore is scheduling that for the end of the year. Everyone is looking at Lawrence Livermore to deliver this ignition,’ says Robertson. If successful, the plan is for a commercial reactor prototype by 2020.
One major technical challenge to creating lasers powerful enough is the waveplates. Made of quartz, these are for manipulating the polarisation state of the light beam; hundreds and hundreds of these waveplates are used in the beamline for major laser systems such as Lawrence Livermore’s NIF. But for the next-generation systems needed for fusion power, they need larger quartz waveplates. Robertson says: ‘You can’t grow quartz to the size they need so we’re developing novel, adhesive-free, optical joining techniques to bond smaller plates together in order to form larger aperture optics, while maintaining mechanical integrity and high laser-damage thresholds.’ One solution being investigated is silicate bonding.
Another challenge for the super lasers is the frequency doubling and tripling. ‘We’ve done well over a thousand frequency doublers and triplers for that site [Lawrence Livermore] and several others. We specialise in growing the frequency doubling, frequency tripling and polarisation rotator crystals,’ says Robertson. ‘Most of lasers start at 1µm and they are frequency doubled and tripled to be more efficiently absorbed by the targets they are hitting.’
Grown in solution by Gooch & Housego at its Cleveland facility, the crystals can be as heavy as 750kg. ‘These are the big crystals,’ explains Robertson. Such large crystals are cut into many squares that have a diagonal dimension of 60cm. They are then polished and coated, using sol-gel. ‘Because of the large size of the optics, spin- or dip-coating techniques are used’, says Robertson.
Gooch & Housego is also involved in the laser that sits at the start of the fusion-inducing super laser. ‘The beginning of the super laser is normally a low-power fibre laser with a narrow bandwidth to allow efficient doubling and tripling later on,’ says Robertson. Laser fusion systems have a fundamental wavelength of 1053nm: ‘We manufacture fibre couplers, optical branching components for the fibre lasers at our Torquay facility,’ says Robertson.
But the lasers today will need to be replaced with systems with better cooling systems if they are going to achieve the 10Hz operation. The Rutherford Appleton Laboratory’s DiPOLE project’s concept has been proposed as an efficient, very high repetition-rate laser that is cooled by gas. ‘We’re polishing the windows for that [DiPOLE laser],’ says Robertson. Another key technology is expected to be ceramics for lasers because ceramics have excellent thermal conductivity properties.
‘We set up complete pump engines and the multi-hundred terawatt up to the petawatt range,’ explains Lastronics’ general manager Dr Thomas Töpfer. Lastronics builds equipment for super lasers, building the tools for the scientific community to explore potential applications. ‘In 2009 I realised there was a growing demand; many research institutions wanted to build petawatt-type lasers,’ he explains. Ten years ago this sort of technology was viewed as something for demonstrators – feasibility lasers – but now there is a market for high-power laser diode solid state technology. Researchers want this ultra-high peak power, in the tera- to petawatt range. ‘We set up Lastronics to build laser diode-based pump modules and take those pump modules and integrate them with laser diode driver chillers, a control unit, into a pump engine.’ To reach the power levels required, the approach is to set up a chain of amplifiers.
This market has slowly developed over the last 10 years and Töpfer estimates that it is ‘perhaps €8-10 million a year’. But there is no single application that Töpfer is seeing drive the market demand. He admits it has a very limited number of customers. The major customers are government and university laboratories in the UK, France, Germany, the US, Canada and Japan. Töpfer sees other governments investing in the technology too, ‘maybe Koreans and Chinese in the future.’ Töpfer explained that scientists can study particle physics with these sorts of tools, the petawatt lasers, and that his company is focused on building these tools. ‘With today’s solid state diode technology you can drive the diodes much harder and you can integrate the systems much more,’ says Töpfer. Laser diode stacks have been actively cooled. Töpfer explains that these ‘were a headache’ but now they have ‘passively cooled elements,’ that are very reliable with a ‘very long’ lifetime.’
That was one of the drivers that helped us make the products. But the diodes still remain the biggest contributors to price.’ To tackle this, his company views the laser systems as consisting of a series of blocks. Through this sort of design, Lastronics aims to bring down the systems’ costs. Lastronics’ approach is to have certain buildings blocks, standard blocks that are combined for the beam delivery system. Using these standard blocks, Lastronics aims to keep the system cost to a ‘certain level,’ according to Töpfer, so his team can ‘tailor the design to the needs of the institution.’
Looking to the future, it’s not only commercial fusion power systems Töpfer expects to want these super lasers: ‘Eight to 10 years down the road we’ll hear something from the medical community. We see the medical field providing an incentive to generate particle beams, and they aim to use those for treatment of certain diseases.’
Gentec-EO sales and marketing vice president Claude Lachance agrees: ‘It’s not just fusion anymore. We started in the fusion business about 20 years ago.’ His company provides technology for the measurement of the super lasers. For the fusion projects his company started to serve, the nature of these huge systems means a great many detectors for measurement are needed. ‘Fusion projects have many beams that are focused at the end; we are talking 200 lasers, and so 200 lasers mean 200 detectors,’ explains Lachance. ‘The detector is needed to detect the level of energy, so the detector is critical. Calorimeter is the name of the product.’ The biggest challenge for these calorimeters is the absorption, absorbing the energy from the laser to detect its power and ensuring that energy is slowly released on to the sensor on the back of the product. Failure means damage of the product and then the beam could damage other optics in the system beyond the detector. ‘It is glass in the front of the product – and if you damage the glass that is done, the damage is done.’ Because of this danger, this absorption glass volume is ‘very special,’ according to Lachance. ‘We are always searching for advanced materials for the absorption material; it’s a very special glass, it is a key development for the super laser market.’ Another challenge for developing the detector is the lack of any super lasers for testing the detector and its absorption glass during their development. ‘At the kilojoule level, to be on the safe side, there is no laser we can try so all the design was based on engineering calculation,’ explains Lachance. For the no-fusion super laser projects, Gentec-EO is dealing with universities. ‘There are now more and more projects that aren’t fusion and are at the university level. That is the trend now.’
Although more and more universities are requiring detectors for these super lasers the government-funded projects, such as fusion research, represent a large part of the market. The market for measuring lasers is in the millions of dollars. As such, selling the detector is not enough. ‘You have to recalibrate the device, servicing the measurement system; it’s a service we offer after selling the product.’ And that service will last the lifetime of the measurement system. ‘They have to work for 30 years,’ adds Lachance.
The joke about fusion power is that it is the energy source of the future, and always will be. But for super lasers, their future is no joke and they are now emerging as a tool for an increasingly wide range of applications for the research community and the many projects represent a rich vein of revenue for the companies ready to offer long-term services. Super lasers could mean super profits.