1. Introduction

Handling of power loads onto divertor target plates is one of the most critical problems by the realization of a nuclear fusion reactor. The divertor configuration foreseen for ITER (International Thermonuclear Experimental Reactor) has been designed on the basis of present knowledge on physics and is currently available through most advanced engineering technology. It is presumed to handle a total heat load up to 100 MW, which is, however, expected to be reached during the DT (Deuterium-Tritium) plasma phase [1]. In devices beyond ITER, for example, DEMO (Demonstration Power Station), even much higher heat fluxes into the divertor volume are expected. Therefore, up to 90% of power, coming out of the confinement region, has to be removed before the plasma contacts the divertor plates to guarantee a long enough lifetime of the targets [2]. One of the most attractive ways to reach this is the realization of the state where the plasma is detached from the plates, and the energy is mostly dissipated through the radiation from impure particles in the whole divertor volume. The understanding of the detachment mechanisms and searching

for possibilities to reliably control the strongly radiating divertor plasma, being simultaneously compatible with the confinement requirements for the plasma core, is one of the most important issues for fusion studies.

Although axis-symmetric tokamaks are presently the most advanced concept for the realization of the magnetic fusion, studies of the detachment in helical devices [3, 4] are also of high interest and importance for the reactor design. Because of inherently nonaxisymmetric magnetic configurations, the magnetic field topology in heliotrons has unique features, in particular, the existence of magnetic islands and stochasticity of field lines. The magnetic field in helical systems is completely generated by currents in external coils. Therefore, the field topology and its effects on the plasma transport and, in particular, on the plasma detachment conditions and characteristics can be studied by varying the magnetic structure in a wide range. Moreover, such investigations are also useful for tokamak devices, where recently resonant magnetic perturbations (RMP) have been introduced to mitigate excessive divertor power load [5, 6]. Due to RMP, the magnetic field in tokamaks exhibits similar structure as in helical devices, that is, with the presence of magnetic islands and stochastic field lines. Thus, the understanding of detachment features in heliotrons is, therefore, of general interest for magnetic fusion program.

The structure of the present chapter is as follows. In next section, we briefly review the features and main differences in the detachment phenomena in tokamaks and helical devices. Experimental observations on the detached divertor plasmas in LHD without and with the application of RMP are presented. The RMP generates a broad magnetic island embedded in the intrinsic edge stochastic layer, which significantly influences features such as the impurity radiation, divertor foot prints, and detachment stability. The impacts on the core plasma transport characteristics during the detached discharge phase are also analyzed here. In Section 3, an interpretation of the detachment phenomena and features of detached plasmas based on the edge plasma energy balance is presented. In Section 4, different mechanisms of nonlinear oscillations during the detachment onset both in helical devices and in tokamaks are discussed. Conclusions are summarized in Section 5.
