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•Hydrogen neutral beams were injected into W7-X by its NBI system.•The maximum combined heating power is estimated at 3.1 MW.•The extracted power of the NBI system can completely be ...accounted for by calorimetry.
The neutral beam injection (NBI) system at Wendelstein 7-X (W7-X) was operated for the first time in 2018. Detailed calorimetric measurements were carried out to accurately determine the energy flow within the NBI system and the injected power into the plasma. Of the electric energy put into the system (89± 6) % can be accounted for. The time-averaged injected NBI power is assessed to be Pinj ≈ (3.1 ± 0.8) MW for hydrogen injection with two positive ion sources at the maximum acceleration voltage of 54 kV. This corresponds to a conversion efficiency of the initially generated ion beam power to the injected neutral beam power of about 33%. The ionization in a plasma with an ion and electron temperature of Ti ≈ Te ≈ 1.3 keV and an electron density of ne ≈ 4 ·1019 m−3 is calorimetrically determined to be about 90%. The injected power is validated against beam emission spectroscopy applying Fast-Ion Dα analysis (FIDASIM). The inferred injected power leads to an energy confinement time of around τE ≈ 150 ms for pure NBI heating plasma experiments at W7-X with these heating and plasma parameters.
•Selected areas in W7-Xwere equipped with tungsten-coated graphite tiles on the heat shield and with pure metal tiles in the baffle areas.•In OP2.1, high plasma performance could be achieved with the ...installed tungsten PFCs, tungsten accumulation did not occur.•It was demonstrated that the geometrical changes made to the modified baffle tiles resulted in the predicted reductions of heat loads.•In preparation for the upcoming campaigns, more and more graphite tiles from the heat shield and baffle areas will be replaced by tungsten-coated tiles.
The transition to reactor-relevant materials for the plasma facing components (PFCs) is an important and necessary step to provide a proof of principle that the stellarator concept can meet the requirements of a future fusion reactor by demonstrating high performance steady-state operation. As a first step to gain experience with tungsten as plasma-facing material in the Wendelstein 7-X (W7-X) stellarator, graphite tiles coated with an approximately 10 µm MedC tungsten layer (NILPRP Bucharest) were installed to complete the ECRH beam dump area in two of the five plasma vessel modules over an area of approximately one square meter each. In addition, tungsten baffle tiles are installed (40 tiles in total) with either bulk tungsten as part of NBI shine-through target or with a tungsten heavy alloy (W95-Ni3.5-Cu1.5) to replace the graphite tiles that were previously thermally overloaded. Based on an advanced diffusive field line tracing method and EMC3-Eirene simulations, the overloaded baffle tiles were redesigned by making the tiles thinner (i.e. moving the plasma-facing surface (PFS) away from the hot plasma region) and by reducing the local angle of incidence through toroidal displacement of the watershed. Significant erosion of the tungsten tiles can only be expected if sputtering by impurity ions such as carbon or oxygen ions contributes. However, the resulting central concentration of tungsten and the corresponding radiation losses are expected to be marginal. The expected deposition of carbon on the tungsten surfaces in the baffle regions mitigates further the possible tungsten enrichment in the core plasma. In OP2.1, no adverse effects of the installed tungsten PFCs on the plasma performance were observed during normal plasma operation. With the design changes made in the baffle area, the predicted heat load reductions could be experimentally confirmed.
Under construction for the stellarator project Wendestein 7-X is a neutral beam heating system based on RF driven positive ion sources. It is planned to start operation with 2 sources capable of ...injecting 5 MW of heating power in deuterium. This paper gives the current status and future plans of the construction of the injector boxes and subsequent installation in the experimental hall. The fruitful collaboration with the National Center for Nuclear Research in Swierk, Poland is also detailed. Lastly, results from an initial study on fast ions in Wendelstein 7-X will be given.
Abstract
Simulations exploring neutral beam operation in Wendelstein 7-X (W7-X) at reduced magnetic field are performed using a newly implemented gyro orbit model in the BEAMS3D code. Operation at ...field strengths below the nominal 2.5 T are seen as a path to explore both high beta plasmas and as a means to access magnetic configurations not possible at 2.5 T. As the field strength becomes smaller, the gyro radius for 55 keV fast protons grows from
∼
1
c
m
at 2.5 T to
∼
5
c
m
at 0.75 T in a device with minor radius
∼
50
c
m
bringing into question the applicability of the gyro center approximation. To address this a gyro orbit model was implemented in the BEAMS3D code. Agreement is found between the gyro center and gyro orbit models in a circular cross section tokamak equilibrium at high field. A set of W7-X equilibria are assessed with fixed density and temperature profiles but decreasing magnetic field strength (increasing plasma beta). Neutral beam deposition is found to be mostly unaffected with changes in the core of the plasma associated with the Shafranov-shift. In general good agreement is found between gyro orbit and gyro center simulations at 2.5 T. Both models indicate increasing losses with decreasing magnetic field strength with the gyro orbit losses being higher at all field strengths. Gyro orbit simulations to the first wall of W7-X show a change in loss pattern with decreasing magnetic field strength. A preliminary assessment of losses to fast ion loss detectors are made.
Abstract A spontaneous reduction in anomalous particle transport in the plasma core is seen experimentally in reproducible, purely neutral beam heated plasma phases at Wendelstein 7-X (W7-X). Heating ...and fueling the plasma exclusively with the neutral beam injection (NBI) system for several seconds leads to continuously peaking plasma density profiles with strong gradients inside mid minor radius. A significant acceleration of the density peaking occurs after a certain onset time and is examined with a detailed particle transport analysis in several discharges. By invoking the particle continuity equation, the total experimental radial electron flux is deduced from the time evolution of the electron density profile and the radially resolved particle sources. Subtracting the modeled neoclassical particle flux contribution gives the anomalous particle flux. Exploiting the evolving plasma conditions, anomalous diffusion and convection coefficients are computed from the flux variation with density and density gradients. In several discharges a significant and consistent change of the anomalous transport coefficients is seen when crossing a specific normalized density gradient length.
The stellarator W7-X will be equipped with two Neutral-Beam-Injector (NBI) boxes for balanced injection. Each NBI box has 2 tangential and 2 radial source positions. For the experimental start-up ...phase each NBI box will be only equipped with 2 ion sources. For the selection of the initial 2 NBI source positions per box three physical aspects were examined (transmission and duct power deposition, shine through and heating efficiency).
Using hydrogen injection the heating power to the plasma under typically planned conditions should be 1.3
MW for the tangential sources and 1.1
MW for the radial sources (deuterium: 2
MW for the tangential sources, 1.8
MW for the radial sources). The tangential source positions all have similar heating efficiencies. One of them suffers from the lowest duct transmission (highest power-load to the duct). The same source hits a component with a low power-load capability. The W7-X inner wall design will be modified in order to enhance the maximum power-load capability of that component. For the radial source positions there is no clear physics advantage of one position over the other. Taking all aspects into consideration the decision was made to use one tangential source and one radial source per box during the experimental start-up phase.
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