Speaker
Description
An overview of the effects of low recycling lithium walls, plasma-limiting surfaces, and divertor targets on the scrape-off layer (SOL) and core confined plasmas is presented. The discussion will primarily reference the tokamak, although consequences for alternative magnetic confinement configurations (primarily stellarators and mirrors) will also be discussed. For the SOL plasma, reduction of recycling to minimal levels drastically lowers the collisionality. Trapped particle populations develop which are mirror confined, producing a confined plasma with finite pressure in the SOL. The disparity between electron and ion pitch angle scattering times leads to the development of an ambipolar or Pastukhov potential in the SOL, which can produce drifts and transport which are significant on the SOL “confinement time”. In addition to a reduction in main (and impurity) ion recycling, a clean lithium surface has the lowest secondary electron emission (SEE) coefficient known, with a SEE coefficient peaking at ~ 0.5, to reduce SOL cooling from electron recycling as well. Although a divertor for a reactor concept may be designed with accurately controlled, near-tangential magnetic field lines to prevent introduction of secondary electrons to the SOL, such precise magnetic control is not always available on medium-scale devices. The large reduction in secondary electron production, with an SEE coefficient of 0.1 – 0.2 at relevant energies, produced by a lithium surface relaxes magnetic field design requirements at the divertor target. For the core plasma, attention has focused on the ability of lithium to generate flat temperature profiles for both electron and ion populations. But low recycling lithium walls can also mitigate charge exchange losses in the core plasma, by strongly reducing the neutral population, even in small scale devices (like LTX-beta). The confinement properties of novel magnetic configurations (e.g. quasisymmetric stellarators, or magnetic mirrors) can thus be accurately assessed at much smaller scale than would otherwise be possible with high recycling walls, where charge exchange losses can reduce ion energy confinement, and mask any reduction in turbulent transport. Although results from LTX-beta will be cited here, the intent is to call attention to the broader advantages offered by the use of low recycling walls on a variety of novel, smaller scale, magnetic confinement configurations.
Work was supported by US DoE contract DE-AC02-09CH11466.