Properties of boron layers deposited during boronisations in W7-X

Boronisation was first used for wall conditioning in W7-X during the OP 1.2b operational period, which was characterized by the use of the fine-grain graphite Test Divertor Unit (TDU) and inertial cooling only. After this period, deposited layers were observed on all inner surfaces. Deposited layers were analyzed on 21 inner wall tiles using ion beam analysis methods, the deposited layers consisted mostly of boron with additional carbon and oxygen. During the operational period OP 2.1 with an actively water cooled divertor made of carbon fiber reinforced carbon, different materials were exposed during two individual boronisations using the multi-purpose manipulator. Deposited boronisation layers on the samples were analyzed using nuclear reaction analysis. The deposited layer thicknesses showed some variation depending on substrate material and surface roughness, but a systematic dependence on material and/or roughness was not observed. Under the typical boronisation condi-tions at W7-X, one A × h (Ampere times hour) of boronisation results in a boronisation layer with a thickness of about 30 ± 15 × 1015 B-atoms/cm2 (about 3 ± 1.5 nm) at the position of the multi-purpose manipulator. The oxygen gettering capacity of the layers is up to 0.5 – 0.9O/B

Overview of the first Wendelstein 7-X long pulse campaign with fully water-cooled plasma facing components

After a long device enhancement phase, scientific operation resumed in 2022. The main new device components are the water cooling of all plasma facing components and the new water-cooled high heat flux divertor units. Water cooling allowed for the first long-pulse operation campaign. A maximum discharge length of 8 min was achieved with a total heating energy of 1.3 GJ. Safe divertor operation was demonstrated in attached and detached mode. Stable detachment is readily achieved in some magnetic configurations but requires impurity seeding in configurations with small magnetic pitch angle within the edge islands. Progress was made in the characterization of transport mechanisms across edge magnetic islands: Measurement of the potential distribution and flow pattern reveals that the islands are associated with a strong poloidal drift, which leads to rapid convection of energy and particles from the last closed flux surface into the scrape-off layer. Using the upgraded plasma heating systems, advanced heating scenarios were developed, which provide improved energy confinement comparable to the scenario, in which the record triple product for stellarators was achieved in the previous operation campaign. However, a magnetic configuration-dependent critical heating power limit of the electron cyclotron resonance heating was observed. Exceeding the respective power limit leads to a degradation of the confinement

Overview of the first Wendelstein 7-X long pulse campaign with fully water-cooled plasma facing components

after a long device enhancement phase, scientific operation resumed in 2022. The main new device components are the water cooling of all plasma facing components and the new water-cooled high heat flux divertor units. Water cooling allowed for the first long-pulse operation campaign. A maximum discharge length of 8 min was achieved with a total heating energy of 1.3 GJ. Safe divertor operation was demonstrated in attached and detached mode. Stable detachment is readily achieved in some magnetic configurations but requires impurity seeding in configurations with small magnetic pitch angle within the edge islands. Progress was made in the characterization of transport mechanisms across edge magnetic islands: Measurement of the potential distribution and flow pattern reveals that the islands are associated with a strong poloidal drift, which leads to rapid convection of energy and particles from the last closed flux surface into the scrape-off layer. Using the upgraded plasma heating systems, advanced heating scenarios were developed, which provide improved energy confinement comparable to the scenario, in which the record triple product for stellarators was achieved in the previous operation campaign. However, a magnetic configuration-dependent critical heating power limit of the electron cyclotron resonance heating was observed. Exceeding the respective power limit leads to a degradation of the confinement

Measurements of radial neutral density profiles from Balmer-α emission in Wendelstein 7-X

Radial neutral density profiles are estimated from measurements of passive Hα emission in the Wendelstein 7-X stellarator. To parametrize the generally three-dimensional distribution with a low number of degrees of freedom, the neutral density is reduced to a flux surface quantity. Accounting for emission from excitation and recombination processes, neutral density profiles are derived independently for each of the available lines of sight. Density profiles obtained from the different viewing geometries are found to vary within one order of magnitude. Toroidally oriented lines of sight predict systematically lower neutral densities when compared to poloidally oriented ones. This discrepancy is attributed to the simplifications inherent in the imposed model and significant differences in integration volumes across the viewing geometries. In line with expectations, obtained neutral densities are found to decrease with increasing plasma density. Key restrictions of the model include the reduction of the neutral density to a flux surface quantity, uncertainties in the plasma profiles and instrument function, and line integration effects outside the last closed flux surface