The Interaction of Transport and Reconnection in High Temperature Plasmas

Lead Research Organisation: University of York
Department Name: Physics


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.

Planned Impact

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.


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Description The research funded through this grant sought to understand the interplay between magnetic reconnection and heat and particle transport in magnetised plasmas. In particular it focussed on studying the evolution of so-called Neoclassical Tearing Modes, an important plasma instability that as well as having great physics interest can have serious consequences for the efficiency and operation of future fusion energy reactors. During the course of the award we have (as reported in peer reviewed publications) :
(i) Developed a real-time control system that allows very specialised laser scattering measurements to identify the evolution of small scale (~1cm ) structures in hot tokamak plasmas with microsecond resolution
(ii) Demonstrated how certain models of NTM evolution provide a good explanation of our experimental results on the MAST tokamak
(iii) Demonstrated on MAST new techniques for controlling the NTM instability
(iv) Shown how our analysis techniques can be extended to other diagnostic systems by applying our analysis techniques to the KSTAR tokamak in South Korea and the DIII-D tokamak in the US
(v) Made progress on developing a fully kinetic computational model of NTM behaviour
Exploitation Route Our experimental data has added to international databases that are used to guide the design of future fusion devices such as ITER. The analysis techniques we have developed are currently been used by researchers on the KSTAR tokamak in South Korea and the DIII-D tokamak in the US.
Sectors Energy

Description The output of this award has been an increased understanding of the structure and evolution of neoclassical tearing modes (NTMs), an important plasma instability that can limit the power output from future fusion reactors. The results from this project have been utilised in international scaling databases that help guide the design of systems on ITER. In particular, the output from this grant has contributed to the International Tokamak Physics Agreement MHD group tasks to understand the evolution of NTMs.
First Year Of Impact 2011
Sector Energy
Impact Types Policy & public services

Description EFDA Fellowship
Amount £100,000 (GBP)
Organisation European Fusion Development Agreement (EFDA) 
Sector Charity/Non Profit
Country Germany
Start 04/2012 
End 03/2014
Title ITPA NTM scaling database 
Description The International Tokamak Physics Activity database on NTM scaling is used to guide the design of future fusion devices and NTM control systems. 
Type Of Material Database/Collection of data 
Year Produced 2012 
Provided To Others? Yes  
Impact The ITPA NTM scaling database is an important tool in understanding and predicting the behaviour of NTMs in future fusion devices. Our work has provided significant input to tokamak aspect ratio effects (the data we collected was on a low aspect ratio, spherical tokamak, MAST). 
Description CCFE 
Organisation Culham Centre for Fusion Energy
Country United Kingdom 
Sector Academic/University 
PI Contribution Much of the experimental work undertaken as part of this project was hosted on the MAST tokamak facility at CCFE. In addition to regular visits for scientific and planning meetings by the PI and Co-I of the project, funded University of York postdocs as well as PhD students associated with the project spent extended time (>6 months) based at CCFE. Contributions from the University of York team include further developments of diagnostic hardware (in particular the development of a real time laser triggering system), experimental scenario development and planning, leadership of experiments and associated data analysis. in addition the University of York team were able to participate remotely in experiments from the "Tokamak Remote Control Room" at the York Plasma Institute, demonstrating the use of this technology in fusion research.
Collaborator Contribution CCFE made available up to 10% of the available time on the MAST facility to help support the experimental aspects of this project. In addition they hosted York visits and secondments, and provided technical and scientific support through the contributions of various CCFE staff.
Impact Peer reviewed publications describing the principal experimental results are listed elsewhere. Some of the hardware and triggering systems developed as part of this project have been used to support other aspects of the MAST research programme.
Start Year 2010
Description General Atomics 
Organisation General Atomics
Country United States 
Sector Private 
PI Contribution Members of the University of York team have spent extended periods (> 1 month) working with colleagues from the DIII-D tokamak in San Diego on data analysis techniques to analyse noisy data.
Collaborator Contribution Hosting extended visit, offering access to DIII-D data and providing scientific guidance on some areas.
Impact Conference presentations have already been made on the outcomes of this collaboration and peer reviewed publication(s) are in preparation.
Start Year 2013
Description NFRI 
Organisation National Fusion Research Institute
Country Korea, Republic of 
Sector Academic/University 
PI Contribution University of York academics were given facility access on the KSTAR superconducting tokamak. Two days of experimental run-time were provided to make measurements of NTMs in a low error field tokamak with a world leading electron temperature diagnostic.
Collaborator Contribution Providing access to the KSTAR facility and the support of associated scientists and engineers.
Impact Peer reviewed publications are listed elsewhere. The
Start Year 2011
Description POSTECH University 
Organisation Pohang University of Science and Technology
Country Korea, Republic of 
Sector Academic/University 
PI Contribution Members of the University of York team spent extended periods (>1 month) at the fusion group of POSTECH University to work on analysis of electron cyclotron emission imaging data. Techniques have been developed at York to recover signals of interest in very low signal to noise scenarios.
Collaborator Contribution POSTECH university staff have hosted University of York team members and have jointly worked on the analysis of diagnostic data.
Impact Peer reviewed publications are listed elsewhere. Further academic papers are in preparation dealing with issues of handling "noisy" data.
Start Year 2011
Description Schools talks 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact Demonstration equipment funded through this project has been used at a series of schools outreach presentation covering the whole of the UK. Typically these presentations have averaged one per month and have targeted a range of school ages.

There are many examples from changing the perception of scientists in the eyes of younger children through to individual young people selecting physics as an undergraduate degree option.
Year(s) Of Engagement Activity 2011,2012,2013,2014
Description YPI Opening 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Demonstrations funded through this project were a feature of the York plasma Institute Opening Day. Notable attendees included the Government Chief Scientific Advisor.

Year(s) Of Engagement Activity 2012