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Density and Impurities


A fundamental aspect in tokamak plasmas concerns the interactions between the hot plasma and the cold material wall that surrounds it. This contact is essential to exhaust the power that is produced by the reactor, and also to exhaust the particles. However, plasma-surface interactions need to be controlled in order to improve the screening of impurities from the plasma core.
A divertor configuration, i.e. a magnetic field configuration in which the toroidally confined (plasma) region is separated from the outside world by a separatrix, is one possible solution envisaged to deal with this issue (the term 'divertor' can refer both to the magnetic field structure beyond the X-point and in contact with material surfaces and to the material structure intersecting the 'outgoing legs' of the magnetic separatrix surface). In such a configuration the magnetic field lines at the periphery of the plasma are "diverted" to a location separate from the main plasma. Thus, the remote location from the plasma heat source allows the plasma temperature adjacent to material surfaces to be reduced, hence reducing the sputtering yield of the incident plasma ions
An alternative to the divertor configuration is a limiter configuration in which the plasma's Last Closed Magnetic Surface (LCFS) is determined by the intersection of field lines by a material object.
The expected behaviour of the impurities in Ignitor has led to the choice to adopt a limiter configuration and not to include a divertor in the design of the machine, since it would have a weak role as a heat and particle sink for the density plasma regimes considered
[1], [6]. In fact, Ignitor is expected to work at high central density regimes (10^21 m -3), with higher neutral density and lower temperatures at the edge. The value of the edge density (n ~ (2-3) x 10^20 m^-3 [4]) has been evaluated from a model that assumes edge fuelling (gas puffing) and a simple edge transport model [5], which gives a very good fit with a wide range of limiter experiments. The edge temperature (T ~ 35-60 eV at the last closed flux surface are expected for the core radiated powers between 10 and 25 MW) has been derived by an energy balance between the total power, the radiated power and the power transported to the limiter, with the assumption of no poloidal temperature gradient in the scrap-off-layer (SOL), which is the outer plasma region characterized by open field lines that come in contact with a material surface and that in limiter configurations in located outside the LCFS.
The relatively high plasma edge density helps in confining impurities to the SOL, where the induced radiation contributes to distributing the thermal wall loading more uniformly on the first wall
[2]. Furthermore, the low edge temperatures, below the physical sputtering for high Z materials, reduce the level of impurity contamination coming from the Molybdenum tiles covering the first wall. As a result, radial pressure profiles tend to be more peaked at the centre with respect to the case when low Z elements are dominant.

1) B. Coppi et al, Nucl. Fusion 41, 1253 (2001)
2) F. Bombarda et al, Braz. J. Phys. 34, 1786 (2004)
4) F. Bombarda et al, MIT (RLE) Report PTP 00/05, Cambridge, MA, July 31, 2000
5) R. Zanino and C. Ferro, Contrib. Plasma Phys. 36, 260 (1996)
6) B. Coppi et al, Overview Paper OV/P-02, Proceedings of the 24 th IAEA Fusion Energy Conference, San Diego, US, 8-13/10/2012

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