The Wendelstein 7-X (W7-X) optimized stellarator fusion experiment, which went into operation in 2015, has been operating since 2017 with an un-cooled modular graphite divertor. This allowed first divertor physics studies to be performed at pulse energies up to 80 MJ, as opposed to 4 MJ in the first operation phase, where five inboard limiters were installed instead of a divertor. This, and a number of other upgrades to the device capabilities, allowed extension into regimes of higher plasma density, heating power, and performance overall, e.g. setting a new stellarator world record triple product. The paper focuses on the first physics studies of how the island divertor works. The plasma heat loads arrive to a very high degree on the divertor plates, with only minor heat loads seen on other components, in particular baffle structures built in to aid neutral compression. The strike line shapes and locations change significantly from one magnetic configuration to another, in very much the same way that codes had predicted they would. Strike-line widths are as large as 10 cm, and the wetted areas also large, up to about 1.5 m 2 , which bodes well for future operation phases. Peak local heat loads onto the divertor were in general benign and project below the 10 MW/m 2 limit of the future water-cooled divertor when operated with 10 MW of heating power, with the exception of low-density attached operation in the high-iota Submitted to Nuclear Fusion configuration. The most notable result was the complete (in all 10 divertor units) heat-flux detachment obtained at highdensity operation in hydrogen.

Diverted plasmas in the stellarator W7-X are investigated using a hopping Poloidal Correlation Reflectometer and a set of Langmuir probes to investigate the plasma edge and the scrape of layer at two different toroidal positions and poloidal cross sections, respectively. The properties of the scrape off layer are studied as function of two magnetic configurations and as function of plasma parameters. The experimentally determined radial position of the inversion point of the radial electric field is compared with calculations. For certain magnetic configurations a remnant island in the scrape of layer is observed and studied as function of the plasma current. Different methods to determine the radial electric field are applied which allow to estimate the phase velocity of the turbulence. The measured phase velocity is small and shows some evidence for the existence of resistive ballooning modes in the scrape of layer. Spectrum decomposition of coherence spectra is applied for a radial localization of modes and to determine their properties. An estimate of the radial correlation length of the broad band turbulence in the vicinity of the last closed flux surface is calculated from the decrease of broad band turbulence amplitude as obtained from the coherence spectra. The obtained radial correlation length is found to be λ r ≈ 15 mm

Recent experiments from the Experimental Advanced Superconducting Tokamak (EAST) show that lower hybrid waves (LHWs) can profoundly change the magnetic topology by inducing helical current filaments flowing along magnetic field lines in the scrape-off layer. Here, it is investigated for the first time how these magnetic perturbations caused by LHWs affect the edge plasma transport utilizing the three-dimensional Monte Carlo code EMC3-EIRENE, both in double-null and single-null configurations. The 3D magnetic topology structure is reflected in the plasma properties, due to much stronger parallel field transport compared with cross field diffusion. Good qualitative agreements between simulation results and experimental data from various edge diagnostics demonstrate that the EMC3-EIRENE code now is capable of taking into account the LHW-induced magnetic perturbation fields with both physical and geometrical effects being considered. Combined with experimental observations, the simulation results strongly support that total current amplitude of LHWinduced filaments increases with an increase in LHW input power. It can further deepen the penetration depth of the additional transport channel by extending the stochastic edge layer, and influence the ratio of heat (or particle) flux between split striated and original strike line on divertor targets. The 3D simulation results also indicate that the additional plasma transport channel induced by LHWs can significantly cause the redistribution of heat load between inner and outer divertor targets, which could not be found by the field line tracing method in previous works.

No abstract

Wendelstein 7-X (W7-X) was operated successfully with the first divertor plasma in the operation phase 1.2a (OP1.2a). A new combined probe head, developed and installed on the multiple-purpose manipulator, is able to measure the edge plasma profiles (𝑇 𝑒 , 𝑛 𝑒 , 𝜙 𝑓 , 𝑀 ∥ ), variation of magnetic field, poloidal and radial turbulence structures. The scrape-off layer (SOL) plasma parameters in two magnetic configurations (standard and high mirror) are in good agreement with the magnetic island structure and the field line connection length calculated by the field line tracer. In both the standard and high mirror configurations, the radial turbulent heat flux and particle flux have strong dependence on the local magnetic topology, revealing two distinct transport patterns: a broadband turbulence dominant region in the outer SOL and a low frequency dominant region in the inner SOL. In the standard divertor configuration, the broadband turbulence with a frequency range of 40-120 kHz is located near the island center along the probe path, leading to outward transport. These broadband fluctuations propagate with a velocity of 2.3-4.4 km/s poloidally along the ion diamagnetic drift direction in the plasma frame, with 𝑘 𝜃 𝜌 𝑠 close to 0.1. The large radial transport induced by the broadband turbulence is accompanied by a steep electron density gradient. The low frequency (5-30 kHz) dominant transport exhibits obvious intermittent structure. Some statistical techniques are applied to the characterization of the intermittent transport.