4. Nonlinear low-frequency oscillations at the detachment onset

As it was discussed in the first part of the present chapter, there is a significant difference between the detachment scenario in LHD without and with RMP. In the former case, one needs very fine tuning of the radiation level by gas puffing of fuel or impurity, and detachment onset may lead to a total radiation collapse of the discharge. Unlikely, with RMP after the transition to the detached state, the plasma density can be increased further, with corresponding growth of the radiated power and without a significant deterioration of the discharge performance. Finally, a radiation collapse occurs roughly at the same plasma density as without RMP.

Experimental Studies of and Theoretical Models for Detachment in Helical Fusion Devices DOI: http://dx.doi.org/10.5772/intechopen.87130

Interestingly, that by the detachment onset with RMP, there is some density range where nonlinear oscillations of high amplitude and relatively low frequency of 100 Hz have been observed in diverse plasma parameters such as the radiation intensity and ion saturation current to the divertor target plates. Also in tokamaks, similar large-scale self-sustained periodic oscillations in various plasma characteristics have been seen, by approaching to critical conditions, see, for example, [37, 32]. Several models [38–40] have been proposed previously to explain these phenomena.

Here, we consider two new mechanisms. The first one is relevant for LHD, with plasma of relatively low density in its divertor legs. In such a situation, by approaching to the critical density, the plasma detaches practically directly from the periphery of the edge stochastic layer. This reminds the phenomenon of radial detachment in limiter tokamaks such as TFTR and TEXTOR. The threshold conditions for radial detachment have been analyzed in [41].

The second mechanism is pertinent for the case of tokamaks with divertors like JET, ASDEX-U, DII-D, operating before detachment in the regime of strong plasma recycling on the target plates [7].
