Equilibrium effects on the structure of island divertor and its impact on the divertor heat flux distribution in Wendelstein 7-X

Abstract

To support the planning of high-performance long-pulse plasma operation for W7-X, the role of equilibrium effects for the structure of island divertor and its profound impacts on the divertor heat flux distribution have been investigated. Finite-beta full-field 3D equilibria for the four most important configurations of W7-X (“standard”, “high-mirror”, “high-iota” and “low-iota”) were obtained with the HINT code. Equilibrium calculations show a strong dependence of the edge stochastization on the magnetic configurations. Parameterization to estimate equilibrium effects on edge stochastization and plasma shape was attempted for individual functions. The results indicate that a more stochastic boundary can be attained with increasing plasma beta, except for the low-iota configuration. According to fully diffusive heat-load estimations, it turns out that equilibrium effects are beneficial for spreading the heat flux on the divertor target. The interactions of the beta effects with effects driven by the toroidal plasma current are also analyzed for the standard configuration. IR camera measurements confirm the predictions concerning global heat load distributions.

Invariant manifold growth formula in cylindrical coordinates and its application for magnetically confined fusion

“…The plasma edge is not suitable to be characterized by a single closed surface when the 3D effect is strong, for which it is proposed to use the notion of invariant manifolds of outmost saddle cycle(s). In fact, it has been observed in some existing simulation [19][20][21][22] and experiment results. On EAST, the helical current filaments induced by the lower hybrid wave heating impacted the plasma edge topology and caused an evident splitting of strike points in experiments [5].…”

Modeling of plasma beta effects on the island divertor transport in the standard configuration of W7-X

“…Not only this, the changes of the impurity transport induced by the magnetic topology modification also contribute to the increases of the impurity radiation power. An increase in the plasma beta leads to an enhancement of edge stochastization [12], facilitating the deeper penetration of impurities in the magnetic island region. Figure 15(b) depicts the recycling flux as a function of the total radiation power in different beta strength plasmas.…”

“…Here, the toroidal plasma current is quite different from the Pfirsch-Schlüter current driven by the pressure gradient in beta effects. Similar to a previous work [12], a toroidal net plasma current amplitude of 5 kA with a profile of I ∝ (1 − s) 2 is assumed in the following qualitative study. Figure 20 shows the simulated connection length distribution of field lines with positive toroidal currents in a central beta of 5% plasma at the bean shape cross-section.…”

“…It can be foreseen that the plasma beta will be even higher in the next campaign (OP2) benefitting from device upgrades. At these levels, MHD currents in the plasma may be strong enough to modify the vacuum magnetic field, thus resulting in the magnetic topology changes [8][9][10][11][12]. Previous studies have also showed that the divertor plasma operation is highly dependent on the magnetic topology [13][14][15][16][17][18][19][20][21][22][23][24].…”

Towards advanced divertor configurations on the J-TEXT tokamak

“…For the island divertor configuration, longer field-line connection lengths at the SOL (in the magnitude of 10 2 -10 3 m, much longer than the electron mean free path) could be obtained, which benefits heat load spreading on the divertor target [62]. In order to investigate the effects of the edge magnetic topology on the divertor heat load distribution, a 2D infrared thermography camera system viewing the HFS divertor target from the LFS mid-plane window with a high spectral resolution of 0.5 mm was established on J-TEXT.…”