Nuclear fusion — the physical process that powers the Sun — has massive implications for generating energy on Earth, yet not all of the fundamental physics needed for its practical realisation is well understood. Attempts to replicate fusion on Earth have been ongoing for over half a century, but doing so in a controlled fashion has proven more difficult than originally thought.
In physics and engineering there are many phenomena that are familiar on macroscopic scales, such as the transfer of heat in a coal power plant, or conversion of kinetic energy from a waterfall into hydroelectricity, but as things scale down further and further scientists encounter exotic phenomena that are not universally known, if at all. When researchers begin to run into problems they have never encountered before, new theoretical research is needed. This is especially true in fusion physics, where enormous temperatures reduce atoms to their constituent charged particles, form a plasma, and then interact via electromagnetic forces in unusual ways.
Recent research in plasma physics led by Dr John Bissell from the Tipping Points project could help point physicists and engineers working on nuclear fusion projects, such as the National Ignition Facility (NIF) in the US, in the right direction. According to Bissell and colleagues from Imperial College London and the University of Oxford, researchers need to be aware of a range of phenomena that their models currently neglect if they are to better understand fusion plasmas. In particular, some answers may lie in resolving plasma instabilities, not least for Inertial Confinement Fusion (ICF) experiments like that at NIF, where plasma inside a gold can — or hohlraum — is heated by high energy lasers.
For starters, the basic theory behind how electrons transport heat in laser-fusion experiments is not always considered. Despite some progress made, NIF still has much work to do if it is to reach its goal of achieving ‘ignition’ (a self-sustained fusion reaction, or burn), and one of the reasons for this may be not fully accounting for how electrons transfer heat in the first place. In the absence of magnetic fields, ‘conductive’ or ‘diffusive’ heat flow is normally well understood: electrons collide with particles in the plasma, and as they spread through it they take energy with them. However, if a magnetic field is present, charged electrons can be ‘deflected’, complicating the process of heat flow.
This is important because concentration of heat is essential for igniting a fusion burn, and the intimate relationship between thermal energy and pressure means that localised ‘hot spots’ form interfering with fuel compression that is needed for fusion to take place. If ignition can be achieved it will bring nuclear fusion closer to becoming a reliable and powerful source of energy, perhaps capable of resolving some of the world’s energy problems, but that may only be the beginning. The dream of fusion is locked in with phenomena that in extreme physical conditions, such as those at NIF, can unexpectedly interfere with experimental design.
According to Bissell, it is often assumed that for a magnetic field to be present in a plasma, it must be imposed externally, but researchers sometimes ignore the fact that plasmas tend to generate their own magnetic fields, with consequences for the heat-flow required to make atomic nuclei fuse together. If researchers are neglecting the magnetic field generated by the plasma itself, then they may not be able to see some of the root causes of their problems.
Indeed, while ICF physicists recognise that magnetic fields do exist inside of the hohlraum, the effects of the field on laser-plasma interactions are not necessarily acknowledged.
“When they started working on NIF they assumed that they were talking about experiments where they could do very simple modelling and I think they’re beginning to realise that things
like electron transport, magnetic field generation, all these kinds of more exotic laser plasma phenomena, actually might be really key”, said Bissell.
In order to understand what researchers may be missing you need to look closely at how lasers heat up plasmas in the first place. Laser light itself is an electromagnetic wave that oscillates (or swings) back and forth in a constant rhythm. As the laser light passes through the plasma, it causes electrons to accelerate up and down like boats bobbing in a harbour, giving them energy. The electrons then collide with ions, which scatters their motion into thermal energy, a process known as inverse bremsstrahlung (IB) heating.
IB heating is important for understanding the laser heating of plasmas in fusion experiments because it can distort the distribution of energies in the plasma. Bissell and colleagues note in their study that IB heated plasmas tend to have more slow electrons and fewer fast electrons, relative to other kinds of plasmas, and because fast electrons are responsible for most of the thermal energy transfer, heat flow in IB plasmas can be strongly suppressed by up to 80-90%. Given that IB is a key mechanism for heating inertial confinement fusion plasmas, this effect
could be significant: not only for heat-flow itself, but also for related phenomena, such as the generation of magnetic fields.
If the plasma exhibits temperature gradients (spatial variations in the temperature) and density gradients (spatial variations in the density) then these features can interact to generate a magnetic field. As noted earlier, this is important because heat-flow deflected by the magnetic field can further concentrate thermal energy, enhance the temperature gradient, and lead to even more field being generated via positive feedback, or instability. This is called ‘field generating thermal instability’ and although Bissell says that it isn’t really clear what’s determining the actual field, “…it could be contributing to field generation in ICF hohlraums, and if it’s doing that it could be important later on for drive uniformity” (compression of the fuel). “Instabilities are amongst some of the main problems with doing fusion”, he said. Another issue with ICF is in the techniques they’re using to model the distribution of electrons in the plasma.
Normally in a plasma the distribution of electron speeds follow what is known as a ‘Gaussian’ or `bell curve’, but the problem is that because the lasers fired at the plasma are of such high
intensity, it’s not Gaussian at all. When a plasma is heated by IB it generates more slow-moving electrons and fewer fast heat carrying electrons.
On the other hand, even relatively small numbers of very fast-moving electrons can be important, because they can transfer thermal energy without diffusing in the normal way. Much of the computational modelling at NIF fails to account for these effects, and for good reason: cost. The numerical calculations necessary for simulating inertial confinement fusion experiments are extremely expensive.
But Bissell thinks there’s a way of getting around some of the problems by including IB in the underlying physical description. “That’s something that can be implemented relatively
easily. And it could be important, especially at early times when IB has really dominant effects”, he said. But he admits that “while certain phenomena may be key at early stages in ICF experiments, they may not be as important later on”.
Nevertheless, physicists and engineers working on ICF projects probably would do best to listen to their theoretical colleagues, especially since ignition is proving far more elusive than anticipated. “They need to start getting more theorists onto this kind of area, and pay attention to complicated phenomena that require much more theoretical and experimental investigation”, said Bissell.
The study this article is based on is available for free online: http://dro.dur.ac.uk/10512/
For more information or questions about the study contact Dr John Bissell at the Institute of Hazard, Risk and Resilience, Durham University. Email: firstname.lastname@example.org
Photos Credit: Lawrence Livermore Laboratory https://lasers.llnl.gov/