2. Experimental observations on detachment in the heliotron LHD and comparison with tokamaks

#### 2.1 Main characteristics of divertor plasmas

By rising the plasma density in tokamaks, the plasma in the scrape-off layer (SOL) and divertor goes through several qualitatively different "regimes" [7]. At a low density level, neutral particles, appearing by the recombination of electrons and ions on the divertor target plates, escape freely into the confined plasma volume. Here, these so-called recycling neutrals are ionized and charged species generated diffuse across the magnetic field back into the SOL. Such a particle convection effectively transports heat coming from the plasma core, and the temperatures of the plasma components vary weakly along the magnetic field in the SOL. This regime is referred as either the sheath-limited one or as that of a weak recycling.

With the increasing plasma density, the fraction of recycling neutrals ionized in the vicinity of the targets is growing up. Therefore, beyond the recycling zone, the plasma convection becomes relatively weaker. As a result, a significant parallel temperature gradient develops in the main part of the SOL and the energy toward the targets is transported predominantly by the heat conduction. This regime is called as the conduction limited or a high recycling one. On the one hand, due to the strong temperature dependence of the parallel heat conductivity, T<sup>2</sup>:<sup>5</sup> , the temperature in

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

the SOL changes weakly with parameters such as the plasma density at the separatrix, ns, being comparable with the density in the confined plasma, and the heat flux from the core. On the other hand, the plasma density near the divertor targets, nt, and the plasma flux to the targets, Γt, rise rapidly with ns, as nt ns <sup>3</sup> and Γ<sup>t</sup> ns <sup>2</sup>, respectively [8]. As a result, the divertor plasma can be brought into a state of a very high density of 1020–<sup>21</sup> m<sup>3</sup> and a temperature below 5 eV. Under these conditions, the impurity radiation plays an important role in the divertor power balance.

Contrarily to tokamaks in helical devices, such as LHD and W7-AS, it has been found that the SOL and divertor plasma characteristics do not show such strong nonlinear variation with ns, even if this is already close to the threshold at the detachment onset, nt ns <sup>1</sup><sup>1</sup>:<sup>5</sup> [9, 10]. It has been interpreted as a result of the momentum (pressure) loss in the stochastic field line region [11, 12] or in the island divertor structure [10]. In these regions, parallel plasma particle flows, along flux tubes of very different connection lengths or even streaming in opposite directions [13], are strongly interconnected through the cross-field momentum transfer. Therefore, the divertor plasma density remains relatively low, of 10<sup>19</sup> m<sup>3</sup> , and the temperature relatively high, of 10 eV, till the detachment transition [9]. (Numerical simulations for W7-X [14] predict, however, a high recycling regime, due to the larger spatial separation of the counter-streaming flows in the larger island.)

Nonetheless, experimental observations demonstrate that in the LHD impurity, radiation plays an important role for the divertor plasma cooling, detachment onset, and stability conditions. Here, however, the main radiation source is not located in the divertor legs but in the stochastic layer. Figure 1 shows the tomographic reconstruction of carbon impurity emission in the edge stochastic layer of LHD just before the detachment transition, as well as the magnetic field line connection length (LC) [15]. Although, the emission of low charge state, CII (C1+), is distributed along divertor leg and the very periphery of the stochastic layer, that from the higher charge state, CIV (C3+), being the main radiating species [16], comes from the stochastic layer only.

In the LHD, studies on the divertor detachment are performed by seeding impurities deliberately [17–19] and by applying RMP from special coils [20–22]. In this chapter, we focus on the detachment with the RMP application, which is a unique feature of the LHD.
