SAMI (Synthetic Aperture Microwave Imaging): Measuring tokamak plasma current using electron Bernstein wave emission

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This research project will apply aperture synthesis, a diagnostic technique used routinely in radio astronomy, to make the first time-resolved measurements of the current density in the tokamak plasma edge. This measurement is crucial for understanding violent eruptions known as ELMs which could be extremely damaging for ITER, the next generation fusion device. At EUR10Bn, ITER is one of the largest international science projects on Earth.Fusion involves making two positively-charged nuclei collide to produce a heavier nucleus, releasing energy in the process. This can only occur at temperatures of about 100 million degrees. The fundamental challenge to performing fusion is to confine the hot ionised gas (plasma) sufficiently well. The principle behind the leading candidate design for a fusion power plant (called a tokamak) is to use the fact that the charged particles of the plasma state respond to electromagnetic fields, which can be used to confine them away from the material walls of the device. If sufficient heating power is injected into a tokamak plasma, then it enters a high-confinement mode. In this mode, the thermal energy of the plasma increases by about a factor of two due to the creation of a highly insulating layer near the plasma edge, which is typically only a few centimetres thick, compared to the body of the plasma which can be a metre or so across. The pressure gradient in this edge layer is extremely high, so there is a vulnerability to instabilities. The plasma experiences a repetitive series of violent plasma eruptions called Edge Localised Modes, or ELMs, which expel large amounts of energy typically within about a hundred millionths of a second. These are an interesting scientific phenomenon on today's tokamaks but on ITER, where the ejected power in an ELM is expected to be an order of magnitude larger, they could cause serious damage if not controlled. There are ideas for how to control ELMs that work on existing tokamaks, but to extrapolate them reliably to ITER requires a more detailed understanding of the physics. In order to test and constrain theoretical models for ELMs, we need to be able to measure the current density and pressure gradient in the thin edge layer. While a number of tokamaks have a good measurement of the pressure gradient, the current density is much more challenging, and the role of the current density in ELMs remains unconfirmed experimentally.This project will develop a novel diagnostic technique to measure the edge current density on the MAST tokamak routinely (in the sense that in principle the process could be automated). Our diagnostic technique will also have good time resolution, being able to make several measurements of the edge current density through an ELM and address the intriguing question of how (or whether) the current density is flushed out of the plasma edge region within the ELM time-scale (ie about 100 microseconds).The physical basis for this diagnostic technique is the directional emission of electron Bernstein wave (EBW) radiation, which is an example of electron cyclotron emission (ECE). Bernstein waves are electrostatic plasma waves generated in the plasma core at frequencies typically around tens of gigahertz. Most of these outgoing waves are reflected back into the core from a cut-off layer, but waves travelling at a particular angle with respect to the equilibrium magnetic field undergo a mode conversion to an electromagnetic wave that enables them to travel to the plasma edge and to be observed. The EBW emission profile allows us to measure both the direction of the magnetic field, and the rate at which it is changing. Since we know the absolute value of the toroidal magnetic field (it varies inversely proportionally with the distance from the centre of the device), we can use the rate of change of direction of the field to calculate the current density.