The Interaction of Transport and Reconnection in High Temperature Plasmas

Magnetised plasmas are extremely important. They protect us from the harmful solar wind; they are employed in a range of plasma processing and coating technologies, and they offer the possibility of a relatively clean, abundant energy source with no greenhouse gas emissions. They are rich in physics, as is evidenced by the range of phenomena from dramatic eruptions in the Sun and aggressive storms in the tail of the magnetosphere, to the gentle, beautiful swaying of the Northern Lights. One phenomenon that has challenged plasma physicists for decades is reconnection: a process that cuts across many applications of the field. In reconnection, magnetic field lines break and then reconnect to form a different magnetic topology. This can happen very fast in events such as solar flares, magnetospheric substorms or, here on earth, tokamak plasma eruptions. Tokamaks have advantages over space and inter-planetary plasmas: they are accessible, parameters are controllable, and they are well-diagnosed. In this project we shall use new diagnostic tools that are not available anywhere else in the world to probe the physics of reconnection events in MAST tokamak plasmas. We are particularly interested in the interaction between reconnection and transport processes (ie, the processes that determine the distribution of density and temperature through the plasma). Tokamak plasmas are sensitive to this interaction because of something called the bootstrap current: an electrical current that depends on the plasma pressure distribution. This bootstrap current is a major element of an instability called the neoclassical tearing mode (NTM), which drives reconnection in tokamaks. It is this instability that we shall use to probe the interaction between reconnection and transport processes, employing a new, world-leading Thomson Scattering diagnostic on the MAST tokamak.The NTM is an important issue for ITER as the reconnection process associated with it causes corrugations of the magnetic flux surfaces around so-called magnetic islands. These corrugations degrade the plasma confinement and, on ITER, will limit the fusion power. A control system has been developed to drive current in the vicnity of the NTM, cancelling the effect of the bootstrap current and reducing the amplitude of the corrugations. If the amplitude falls below a threshold, then the islands self-heal, good confinement is recovered, and the fusion power in ITER would rise. Let us refer to this as the THRESHOLD amplitude. The design of the control system requires knowledge of the threshold amplitude on ITER. This is uncertain, but there are two main contending theories, neither of which can be ruled out. We shall test one of them, as follows. Heat travels rapidly along magnetic field lines. This means that the temperature is approximately constant on the corrugated magnetic flux surfaces. This, it turns out, is what is reponsible for driving the NTM. However, if the corrugations are of sufficiently low amplitude, then the diffusion of heat across the flux surfaces competes with the transport along the magnetic field lines, and the corrugated flux surfaces are then not isothermal; the drive is suppressed, and the NTM is stable. Let us call the amplitude when this happens the CRITICAL amplitude. This clearly describes how a threshold for instability might arise, but is that threshold the one that is observed? That is, does this critical amplitude match the threshold amplitude? This is the fundamental question that we shall address with advanced computational modelling and the most detailed measurements of the temperature distribution on the MAST tokamak to date. The new Thomson Scattering system on MAST, which is the best in the world, can measure the critical amplitude down to 1cm, which is the approximate threshold amplitude on MAST (measured from the magnetic fluctuations). If the threshold and critical amplitudes agree, we confirm the theory; if not, we eliminate it.

We first address the questions Who will benefit from this research? and How will they benefit? . The academic beneficiaries are the general plasma science community and, in particular, the fusion energy community, who will all benefit from the scientific discoveries we anticipate. The ITER Organisation will benefit from our improved understanding of the threshold to neoclassical tearing modes (NTMs), providing valuable, quantitative input to the design of control systems, and for optimising operational scenarios to achieve maximum performance without triggering NTMs. The UK's tokamak facility, MAST, also experiences NTMs, which can limit the plasma pressure that can be achieved. We will provide the MAST team with a better understanding of how to avoid NTMs, increasing the capabilities of the device. Finally, arguably the most important and urgent issue for ITER is the control of large plasma eruptions, called ELMs, which are expected to cause excessive erosion of materials if uncontrolled. The favoured control method, which on ITER could cost 100'sM to install, is to use coils to to drive reconnection at the plasma edge, degrading the confinement there to reduce the edge plasma pressure gradient and remove the free energy driving the instability. This completely suppresses ELMs on one tokamak in the World, DIII-D in the US, but only mitigates them on other tokamaks. The mechanisms are not understood, and this study that uses the NTM to explore the effect of reconnection on transport processes is expected to shed light on the issue. In the longer term, the most efficient fusion reactors (i.e. those that maximise the ratio of plasma thermal energy to confining magnetic field energy) will need to avoid or control NTMs. Thus, our research will feed into the design of fusion reactors, ultimately benefitting all mankind through plentiful, economic, relatively clean, safe fusion power production with no greenhouse gas emissions. We have a number of strategies in place to ensure that our research makes the high impact we expect. First, we plan a number of conferences, both fusion-specific and general plasma ones, that will disseminate our results to the academic community. We will also publish the results in plasma physics journals and would, in addition, expect at least 2 papers that warrant publication in high quality, general physics journals (e.g. Phys Rev Lett). Frequent visits to Culham will ensure the UK Government fusion programme is able to take full benefit from our research. The physics research for ITER is organised by 7 international teams under the International Tokamak Physics Activity. The Co-I, Howard Wilson, leads one of these teams (tasked with solving the problem of plasma eruptions on ITER) and reports directly to the ITER Organistion (IO). He also knows the person leading the team responsible for plasma instabilities, including NTMs (A Sen). This influential position will mean the work will find its way directly to the IO at the highest level. The general public will be kept informed of the importance of fusion energy research, including the ITER project, through our series of public lectures which will include an exciting demonstration of a toroidal plasma discharge: ideal for enthusing A-level students. The PI and Co-I have an excellent track record of giving lectures: both public lectures and to schools and school teacher conferences (e.g. at the National Science Learning Centre at York). The immediate benefit to the UK economy is difficult to gauge. ITER will place tenders for components, which will include the NTM and ELM control systems. Working with the Culham Fusion & Industry team, we will help to ensure that UK industries have access to the knowledge we gain, helping to increase their competitiveness in future calls for tenders. Finally, we will be training two PDRAs and a PhD student in a range of technical and communication skills that will contribute to enhancing the UK skill base.