This paper presents a contribution to bifurcation phenomena in scrapeoff layer modelling. The bifurcations are caused by the non-monotonic temperature behaviour of the radiation characteristics of plasma impurities. Low-frequency relaxation oscillations observed in divertor plasmas of the ASDEX Upgrade tokamak are analyzed and computed using a simple model for the dynamics of the impurity density and the electron temperature. The corresponding model equations are a special case of a.general system of transport equations whose stability is analyzed. Bifurcations are calculated by solving autonomous and parametrically driven model equations.
2 Experimental
Contrib. Plasma Phys. 36 (1996) 4
Target plates in ASDEX Upgrade are made from carbon which is released by plasma action. This process is routinely recorded by a spectrometer with lines of sight parallel to the outer target at different distances. Sometimes oscillations of the carbon line emission with a period of 12 ms are observed. Fig. 1 shows the emission of GI1 at 514 nm of the four lowest spectrometer channels with elevations of 1.5, 3, 4.5 and 16 mm. Only in the first 3 channels oscillations occur with equal phase. Furthermore, they also appear in CIII emission.
Oscillations with the same frequency are also observed with Langmuir probes mounted in the targets (see Fig. 2). They occur only in a limited range of the electron j I I I I*