Scrape Off Layer and Divertor Physics

Research in scrape-off-layer (SOL) and divertor physics is concerned with studies of the edge region of the tokamak plasma where the poloidal magnetic field is "diverted" such that the plasma comes into contact with a region specifically designed to withstand intense plasma-surface interactions.

We are involved in a range of experimental studies (on the MAST tokamak and the ULS linear divertor simulator) to investigate ways in which the steady-state and transient heat fluxes to plasma facing components (PFCs) in the divertor can be reduced to acceptable levels.

One way in which to reduce the flux of heat and particles on to PFCs is to operate the tokamak in a "detached" scenario, a regime characterised by strong pressure gradients along the magnetic field of the SOL and where volume recombination can dominate particle losses. On MAST, our studies are focussed on comparisons between spectroscopic imaging diagnostics (DIVCAM) and interpretive onion-skin-method (OSM) models of the SOL. DIVCAM (short for DIVertor CAMera) allows the spectral emission of a large region of the divertor to be imaged at two different wavelengths. DIVCAM results are compared with predicted emission profiles derived from the OSM plasma model coupled to the EIRENE Monte Carlo kinetic neutral transport code (see figure 1). Work is currently under way to further develop the OSM physics model, for example by including the effects of particle drifts which arise due to gradients and curvature of the magnetic field within a tokamak. We are also investigating the inclusion of molecular ion transport in EIRENE and to incorporate their presence into the plasma solution obtained by OSM, essential for a complete description of the importance of molecular processes in detachment.

Imaging spectroscopy in the MAST divertor.
Figure 1. Imaging spectroscopy in the MAST divertor: top left quadrant shows a schematic view of MAST upper divertor; top right quadrant shows raw data image (filtered to show Hydrogen Balmer alpha emission) of the upper divertor target plates. Lower two images show inverted image data (left) with modelled prediction of emission (right).

The effect of transient heat loads at the plasma edge is another important issue for future tokamak fusion reactors and we have recently started a project on MAST to study the impact of mitigation schemes for disruptions (using fast gas injection) and ELMs (using perturbations of the magnetic field at the plasma edge).

Finally, another area of research in divertor plasma physics we are leading is in developing strategies to control the retention of tritium within next step devices such as ITER. The main source of the in-vessel inventory of tritium in ITER is expected to be in the form of amorphous hydrocarbon films which are deposited on plasma facing components. The amount of tritium in these films, which grow as the result of the re-deposition of chemically sputtered carbon, is predicted to exceed the maximum allowable inventory within a short operating period. We are studying the effectiveness of "photonic cleaning" techniques for the in-situ removal or de-tritiation of such films: a technique which has already been demonstrated on the JET tokamak (see figure 2).

In-situ photonic cleaning of JET divertor tiles.
Figure 2. In-situ photonic cleaning of JET divertor tiles: a high power flashlamp, capable of delivering peak powers of up to 0.5GW/square metre, is positioned adjacent to the divertor tile surface using the JET remote handling robotic arm. The removal of tritiated films from the surface of plasma facing components using this technique is a promising technology for tritium inventory control on ITER.