Fusion energy may very well be the solution to our current energy crisis. There is much research taking place in a very competitive environment, and the results are eagerly anticipated. A PRACE project in Portugal has studied fast ignition, which could pave the way for more practical and efficient use of lasers to generate fusion energy. The project involved simulating fast ignition with realistic target properties. The results provide information about the energy deposited in the target which, unlike previous studies, demonstrates a higher probability of achieving a successful configuration for fast ignition with ultra-intense lasers.
The PRACE project, “Predictive full-scale fast ignition with PW plasma amplified laser pulses”, modelled a full-scale interaction of an ignition laser with compressed fuel for inertial fusion for the first time. Inertial fusion energy refers to fusion energy with the use of lasers. The project was headed by Luís Silva, a professor in The Department of Physics and head of the Lasers and Plasma Group at the Instituto Superior Técnico (IST) in Lisbon.
“When an intense laser interacts with matter, the matter is transformed into plasma. Our group studies how lasers interact with plasma. The group members are physicists, but they represent expertise from a wide range of methodologies, from theory to computational methods”, Silva says.
The PRACE project is a cooperative project between approximately 10 post-doctorate and doctorate researchers at IST in Portugal, UCLA in the US, and the Rutherford Appleton Laboratory in the UK.
Figure Caption: Electron heat flux (violet–green–red) driven by the interaction between the ignition laser (orange) and a compressed fast ignition target. The kinetic interaction in the low-density region is modelled with a full-PIC algorithm, whereas the resistive transport in the high-density plasma is modelled with a MHD algorithm.
Possible breakthrough in the near future
In the US scientists are on the verge of demonstrating nuclear fusion with lasers at the National Ignition Facility in Livermore. The main aim is to get more energy out of the process than what is put in.
“Fast ignition is potentially a way of making fusion energy with lasers a practical and efficient energy source. Fast ignition and conventional ignition can be compared to different kinds of car engines. A diesel engine ignites only after full compression of the fuel. Conventional ignition with lasers works in the same way. Gasoline engines work differently: a spark is used to ignite the fuel that is only partially compressed. Fast ignition works on the same principle. We compress the fuel a little and then we shoot an intense laser into the slightly compressed target. This intense laser generates a beam of electrons that can then ignite the fuel in the target, acting like a spark in a car engine”, Silva explains.
The centre of the target is highly compressed and very dense. As a result the laser can not propagate in that region; the laser interacts with the outer regions of the target and accelerates the electrons there. These accelerated electrons penetrate the target, heating up the core and creating the hot spot that ignites the target. This releases the energy in the target core.
“We were positively surprised when we performed simulations. We had previously done smaller scale simulations with smaller targets and in those cases, the beam of electrons was not optimal. There were electrons all over the place, but when we simulated a bigger target, with a realistic size, the electrons went straighter and on a much narrower path. The scale difference and the effects at the target boundary accounted for the result. This means that fast ignition actually works better in targets of realistic size”, Silva points out.
Magnetic field (blue–red) out of the simulation plane from the interaction between an intense laser and a compressed target. The fast particle transport along the target surface leads to the onset of shear instabilities.
Silva explains that the research will mainly be applied in producing energy with lasers. “Our study shows that very high energy fluxes can be generated by the laser and that these energy fluxes can be deposited in the core of the fusion target. The breakthrough is that we are not modelling an idealized situation, but a real fast ignition scenario. Our results show that the interaction of intense laser pulses with plasma targets can generate electrostatic shocks capable of accelerating ions to high-energies with low-energy spread. These ion beams can be used in the near future in medical applications, such as ion cancer therapy.”
Accelerated ions for proton therapy scientific discoveries often occur when scientists chance upon things they were not looking for. When the laser was interacting with the target, the researchers realised that strong shockwaves were generated and that these shockwaves were accelerating ions effectively. There is a constant demand for ions with high energies. They are used in so-called “proton therapy” to target tumours that are deep inside tissue.
“The standard technology used in this treatment is very expensive, so scientists are trying to find out cheaper techniques using lasers. By optimizing the acceleration of ions, it is possible to produce beams with the energy needed for this kind of treatment. We have now found an alternate way to accelerate ions to energies relevant to therapy. This is one of the topics that we will be studying further with medical experts. Sometimes spin-offs of the main research are also very important”, Silva says. “And this seems to be the case here.”
OSIRIS framework and Jugene supercomputer
The project used 30 million CPU hours on the Jugene Blue Gene/P supercomputer between November 2010 and October 2011.
“The difference between our group and others is that we have a unique numerical code, OSIRIS, that can take advantage of the tremendous peace resources for studying laser-plasma interactions at ultra-high intensities. We also have the Jugene supercomputer, the largest machine in Europe, at our disposal. We simulated the evolution of hundreds of millions of particles and therefore we needed a lot of computing time. The size of the problems, as measured by the number of particles simulated and the dimensions of the simulation domain, made this project challenging. We needed a supercomputer to completely understand the collective dynamics of such a large particle system”, Silva explains.
In addition to the practical applications of the research, it has much more to offer physicists. For example, it also enables the study of generating shockwaves in plasma, which are associated with cosmic rays, the most energetic particles in the universe.
“These shockwaves are high amplitude nonlinear waves generated when a perturbation moves at supersonic velocities in a given medium. For instance, when an airplane moves faster than the speed of sound, it generates a shockwaveas evidenced by the corresponding blast. Shockwaves in plasmas can now be simulated and the acceleration of particles in these waves can be explored”, Silva says. The group’s plans for the future include the exploration of these shockwaves and the acceleration of cosmic rays and expanding simulation models even more.
“The possibility to use European supercomputers through PRACE is invaluable for small countries such as Portugal since we don’t have our own Tier-0 supercomputers. If we didn’t have European support, it would not be possible for us to do this kind of research with cutting-edge computational resources. The European cooperation is highly appreciated by our group”, Silva emphasizes.
Text: Päivi Brink, Otavamedia Communication (Finland)
The work presented here is a collaboration between IST, UCLA, and RAL. Special thanks to all the members participating in the project. The key participants in each institution were: at IST, Prof. Luís Silva (PI), Prof. Ricardo Fonseca (Chief Developer of Osiris), Dr. Frederico Fiúz, at UCLA, Prof. Warren Mori, and at RAL, Dr. Raoul Trines. Some of the research pursued in this project is financed by the European Research Council through the Advanced Grant ACCELERATES (PI: Luís Silva).