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.

An important goal of stellarator optimization is to achieve good confinement of energetic particles such as, in the case of a reactor, alphas created by Deuterium-Tritium (D-T) fusion. In this work, a fixed-boundary stellarator equilibrium was re-optimized for energetic particle confinement via a two-step process: first, by minimizing deviations from quasiaxisymmetry (QA) on a single flux surface near the mid-radius, and secondly by maintaining this improved quasi-axisymmetry while minimizing the analytical quantity ΓC , which represents the angle between magnetic flux surfaces and contours of J ||, the second adiabatic invariant. This was performed multiple times, resulting in a group of equilibria with significantly reduced energetic particle losses, as evaluated by Monte Carlo simulations of alpha particles in scaled-up versions of the equilibria. This is the first time that energetic particle losses in a QA stellarator have successfully been reduced by optimizing ΓC. The relationship between energetic particle losses and metrics such as QA error (E qa) and ΓC in this set of equilibria were examined via statistical methods and a nearly linear relationship between volume-averaged ΓC and prompt particle losses was found.

Experiments were performed during the operational phase (OP) 1.2a of W7-X to verify predictions of potential overload conditions corresponding to certain high-power long-pulse OP2 scenarios. Heat flux measurements were obtained in a series of magnetic configurations designed to mimic the magnetic topology evolution caused by net toroidal current and beta, which is not directly accessible in OP1.2. The measured heat fluxes are compared to field line diffusion calculations used to design the scraper element, which provides one potential solution to the overload issue by intercepting heat flux that would otherwise be incident on low-rated divertor edges. The experimental flux patterns are qualitatively reproduced by the simulations in position and magnitude for ad-hoc cross-field diffusivities near the value used to design the scraper element. However, some important differences including an anomalous shift towards the pumping gap and low-rated components is observed.

Gyrocenter following simulations of fusion born alpha particles in a stellarator reactor are preformed using the BEAMS3D code. The Wendelstein 7-X high mirror configuration is scaled in geometry and magnetic field to reactor relevant parameters. A 2 × 10 20 m−3 density plasma with 20 keV core temperatures is assumed and fusion birth rates calculated for various fusion products assuming a 50/50 deuterium-tritium mixture. It is found that energetic He4 ions comprise the vast majority of the energetic particle inventory. Slowing down simulations of the He4 population suggest plasma heating consistent with scaled energy confinement times for a stellarator reactor. Losses for this configuration appear large suggesting optimization beyond the scope of the W7-X device is key to a future fusion reactor. These first simulations are designed to demonstrate the capability of the BEAMS3D code to provide fusion alpha birth and heating profiles for stellarator reactor designs.

In this paper we describe the development, testing, and characterization of three low-emission rate AmLi neutron sources. The sources are used to calibrate the nuclear recoil response of the LUX-ZEPLIN (LZ) dark matter experiment. The sources' neutron emission rate was measured using 3He proportional tubes. The sources' gamma emissions were characterized using a high-purity germanium (HPGe) detector. Source-validated Geant4 Monte Carlo simulations allowed to calibrate the Ge and neutron detectors' responses.