Developments in theoretical studies of divertor and pumped limiter systems

Limiters and divertors are used in magnetically confined fusion devices to control plasma/wall interactions. Ideally, they protect the device structure from damage by absorbing the bulk of the heat leaving the plasma and by minimizing the flow of impurities entering the main discharge. Pumps are often used together with limiters and divertors to remove impurities and control the hydrogen density. The energy confinement of the main discharge may also be influenced by the behavior of the plasma/wall interaction at limiters and in divertors. The first major push for development of these limiter/divertor models came from the INTOR project. One important result from this INTOR work was the numerical demonstration of the existence of a stable high-density, low-temperature plasma near the neutralizer plate of a divertor. Such a regime is caused by the multiple reionization of particles near the plate, with returning particles hitting the plate each time with only a fraction of the energy carried by a particle entering the scrape-off-layer. A flux of hot (few 100 eV), highly eroding ions is thus transformed into a larger flux of particles, at a temperature below the sputtering thresholds of the plate material. A high recycling plasma has been seen in D-III that tends to support these predictions