Dissertation: “Density gradients in spherical tokamak plasmas”Eindhoven University of Technology, 10 April 2003
The work described in this dissertation is part of the worldwide program that has the aim to develop nuclear fusion as energy source. Fusion - the process in which two light nuclei fuse, releasing vast amounts of energy - is very attractive as a source of energy: it is a clean and safe process, and it does not produce the greenhouse gas CO2. The fuel resources are inexhaustible and widely available. Fifty years of research has proven the scientific feasibility of controlled nuclear fusion. The first fusion reactor, ITER, in which the produced power will be approximately ten times bigger than the power input, will be built in a worldwide collaboration.
ITER is a ‘tokamak’, a reactor in which the fuel is contained in a torus-shaped tank. The fuel is hydrogenic gas, which must be heated to 150 million degrees Celsius. Thus the gas ionises and becomes a plasma. The plasma is kept in place by means of magnetic fields.
The ITER plasma - and that of other so-called conventional tokamaks - has a shape that can be compared to the inner tube of a car tire. However, it is also possible to design the tokamak in such a way that the plasma has the shape of a cored apple. This ‘spherical tokamak’ can result in a more compact construction of the fusion reactor. This is a rather recent concept and needs to be compared thoroughly with the conventional tokamak.
The first spherical tokamak in which plasmas of millions of degrees were produced was START (in Culham, England). The promising results obtained on this experiment, have led to the development of the Mega-Ampere Spherical Tokamak MAST (also in Culham). This machine achieved its first plasma in December 1999 and has since demonstrated plasma currents of 1.35 mega-ampere during 0.7 seconds.
ITER follows the ‘standard scenario’ that has resulted from fusion research. This means that it will operate in the so-called ‘H-mode’ (high-confinement) regime, a state in which the plasma can be efficiently heated. Just like conventional tokamaks, MAST can also operate in H-mode.
This dissertation is the result of a study of several aspects of H-mode in MAST, and makes a comparison with the conventional tokamak. In comparison to the L - (‘low-confinement’) mode, the H-mode can achieve much higher plasma pressure using equal heating power. This is the result of the formation of a ‘transport barrier’ at the edge of the plasma. Associated with H-mode is the occurrence of edge instabilities, so-called ELMs (Edge Localised Modes). These instabilities produce pulsed outflows of heat and particles. These outflows are deposited in a short time on the wall of the reactor, which could lead to increased material wear. ELMs occur in all tokamaks, spherical as well as conventional. It is to be verified whether the power exhaust during an ELM will cause problems in the compact spherical tokamak.
The key question of this dissertation is: how does H-mode in MAST - as representative of spherical tokamaks - compare with that on conventional tokamaks. In this, two aspects are especially considered. First, the development of density and temperature gradients at the edge of the plasma, and secondly: the exhaust of particles during ELMs.
In order to be able to answer these questions two diagnostics have been developed. The first uses the scattering of a laser beam (Thomson scattering) to perform an accurate measurement of the density and temperature gradients. This measurement can be taken once per plasma discharge. This system was originally built with a resolution of 30-40 data points distributed over the diameter of the plasma. At a later stage, the system was upgraded to close to 300 data points in order to provide better resolution. This has resulted in a measurement of the density and temperature profile from the inside to the outside along the central midplane of the plasma. With this unique measuring capability, MAST is in a good position to study these quantities in H-mode. The second system uses the light that is emitted by neutral hydrogen at the edge of the plasma. This system, too, produces a measurement of the density gradient at the edge of the plasma, but in contrast to the scattering system which produces a single measurement, this system produces a time resolved measurement with a frequency of 1 kHz or higher. However, for calibration purposes this system requires the single measurement of the laser scattering diagnostic and only a measurement of the outside gradient is produced. The following results have been obtained using these diagnostics.
It has turned out that H-mode in MAST resembles that in a conventional tokamak to great extent. Of great importance is that the energy confinement time - a measure of the efficiency of the magnetic confinement of the plasma - follows the same scaling laws, making MAST, in this respect, very comparable to conventional tokamaks. However, there are also substantial differences:
- During H-mode in MAST a very pronounced density gradient is formed - potentially leading to the formation of the edge density ‘ear’ - but the increase in temperature is much smaller than that in conventional tokamaks.
- The plasma pressure - which is the product of density and temperature - at the edge of the plasma in MAST is small. Nonetheless, the energy confinement - being the result of the total plasma pressure - scales as in conventional tokamaks.
The development of the density ears can be well understood as the result of very low particle transport, which image is generally excepted to explain the formation of gradients in a conventional tokamak. The other observations, however, indicate that the exchange of particles and heat between the plasma and its surroundings in MAST differs from that in conventional tokamaks. This is likely due to the fact that the MAST plasma is surrounded by a large volume of neutral hydrogen, which can act as a continuous source of low-energetic neutrals. In a conventional tokamak this surrounding, neutral layer is much narrower and the role of the wall in the recycling process much more pronounced. However, the question remains to what degree the difference in plasma shape plays an important role in this.
Also concerning the outflow of plasma during ELMs interesting results were obtained. Of these the most significant is that the outflow is strongly biased to the outside of the plasma sphere. In other words, the column that cuts through the centre of the plasma and that could potentially be damaged by the effect of ELMs, only receives a very small portion of the total exhaust.
The fact that the power exhaust is strongly concentrated on the outside of the plasma is a positive result for the development of reactor concepts based on the spherical tokamak design. In this framework, the impact of the results concerning the edge gradients and neutrals is not yet entirely clear.