Abstract
During the first test divertor campaign of the stellarator experiment Wendelstein 7-X (Pedersen
et al
2022
Nucl. Fusion
62
042022), OP1.2b, 13 neutral gas pressure gauges collected data in ...different locations in the plasma vessel, enabling a detailed analysis of the neutral gas pressures, the compression ratios and the particle exhaust rates via the turbomolecular pumps in the different magnetic field configurations. In Wendelstein 7-X, the edge magnetic islands are intersected by the divertor target plates and used to create a plasma-wall interface. As the number and position of the magnetic islands varies in different magnetic field configurations, the position of the strike line on the target plates and thus the neutral gas pressure in the subdivertor differs between the configurations. Neutral gas pressures on the order of few 10
−4
mbar were measured in the subdivertor region. The highest neutral gas pressure of
1.75
×
10
−
3
mbar was obtained in the so-called high iota configuration featuring four edge magnetic islands per cross section. The neutral particle flux through the pumping gaps into the subdivertor volume was provided by EMC3-EIRENE simulations and allowed to analyze the relation between the particle flux entering the subdivertor and the pressure distribution in the subdivertor. Finite element simulations in ANSYS provide a detailed picture of the pressure distribution in the subdivertor volume and agree with the neutral gas pressure measurements in the subdivertor in the standard configuration featuring an island chain of 5 edge magnetic islands. Surprisingly high neutral gas pressures that were not predicted by the simulation were measured in the subdivertor region away from the main strike line for discharges in the most used magnetic configuration, the standard configuration. While the pressure ratio between the two sections of the subdivertor volume, the low and high iota section is 0.06 in high iota configuration, a ratio of 2–5 was obtained in the other configurations, indicating significant particle loads and exhaust rates on the high iota section of the subdivertor in magnetic configurations with the main strike line on the low iota divertor targets.
Abstract In this work, the Coherence Imaging Spectroscopy technique is exploited for active charge exchange radiation measurements to infer high spatial resolution 2D ion temperature ( T i ) maps in ...the core region of Wendelstein 7-X plasmas. A synthetic model of the diagnostic is developed and used for the optimization of the hardware components for the expected ion temperatures ( T i ∼ 2 keV) prioritizing T i measurements while also considering the ion velocity flow resolution. The experimental set-up is shown and the diagnostic calibration procedure for T i measurements is introduced. A combination of both simulations and experimental calibrations enable high fidelity system group delay ( ∂ ϕ ∂ λ ) characterization in the whole visible spectral range. Finally, the signal processing techniques applied to the diagnostic signal are introduced and first measurements of 2D T i maps are presented and validated against standard Charge eXchange Recombination Spectroscopy T i profiles, finding excellent agreement.
We report on the first experimental verification of theoretically predicted multiple bundles of counter-streaming plasma flows in the island-divertor scrape-off layer (SOL) of the stellarator ...Wendelstein 7-X. In the standard toroidal field direction (counter-clockwise when looking from the top) experiments, the overall structure of the SOL flows, such as flow directions, the number of flow bundles and the magnitude of the flow velocities, are consistent with numerical predictions obtained with EMC3-EIRENE. However, the modelling does not predict changes of the flow patterns with reversal of the magnetic field direction, which are experimentally observed. This indicates that additional relevant physics, such as particle drifts, will need to be incorporated into the numerical model to better describe the whole stellarator scrape-off layer behaviour.
Measurement of ion dynamics is of great importance to the study of plasma exhaust and impurity particle flows in the scrape-off-layer (SOL) and divertor in magnetic confinement experiments. Doppler ...coherence imaging spectroscopy (CIS) is a passive optical diagnostic that produces 2D images of line-integrated ion flow velocity. Doppler CIS flow measurements of neutral deuterium and impurities were conducted for the first time in the divertor of the medium-sized tokamak experiment ASDEX Upgrade. A detailed sightline, emission and magnetic field analysis was undertaken to identify location and direction of flows. Under the assumption of toroidal axisymmetry, they revealed mainly parallel impurity flows in the proximity of the X-point and target plates on the order of 20 km s−1. Two Doppler CIS systems are currently set up for the optimized stellarator Wendelstein 7-X (W7-X). The main task of this diagnostic will be the measurement of impurity ion flows in one of the island divertors of W7-X. EMC3-EIRENE simulations have been carried out to estimate the flow behaviour in the W7-X SOL.
Abstract
Investigations of particle parallel flow velocities have been carried out for the scrape-off layer (SOL) of the Wendelstein 7-X (W7-X) stellarator, in order to gain insights on the SOL ...transport properties during attached and detached plasma scenarios. The experimental evidence is based on the coherence imaging spectroscopy (CIS) diagnostic, able to measure 2D impurity emission intensity and flow velocity. The impurity monitored by CIS is C
2+
, characterized by a line-emission intensity observed to be linearly proportional to the total plasma radiated power in both attached and detached plasmas. The related C
2+
velocity shows a strong dependence on the line-averaged electron density while remaining insensitive to the input power. During attached plasmas, the velocity increases with increasing line-averaged density. The tendency reverses in the transition to and during detachment, in which the velocity decreases by at least a factor of 2. The sharp drop in velocity, together with a rise in line-emission intensity, is reliably correlated to the detachment transition and can therefore be used as one of its signatures. The impurity flow velocity appears to be well coupled with the main ions’ one, thus implying the dominant role of impurity-main ion friction in the parallelimpurity transport dynamics. In view of this SOL impurity transport regime, the CIS measurement results are here interpreted with the help of EMC3-Eirene simulations, and their major trends are already explainable with a simple 1D fluid model.
•W7-X achieved several of its project goals during the first operation phases.•Boronisation reduced the impurity content strongly and allowed high-density operation.•Stationary full detachment with ...strongly reduced target heat flux was demonstrated.•The next operation phase will feature water cooling of in-vessel components.•W7-X is a key project to verify the physics models for stellarator reactor.
The Wendelstein 7-X project is aimed at demonstrating that an optimised stellarator is an attractive candidate for a fusion reactor. This requires the achievement of a number of technical and physics goals. Several of these goals have already been achieved in the first three experimental campaigns. We shall exemplify this by a number of results. One important goal is the demonstration of quasi-steady-state operation at high plasma density and temperature for half an hour, which encompasses the physical and technical requirements of stable operation with density and impurity control, cw heating, water-cooled targets, particle exhaust, and an appropriate control and data acquisition system for long-pulse operation. In the previous experimental campaigns, with uncooled targets, stationary discharges for up to 25 s with 5 MW heating power and up to 100 s with 2 MW heating power were achieved. For the next operational phase, to start in 2022, water-cooled targets are being installed, water cooling will be provided for all first-wall components, and a number of further upgrades to plasma heating, vacuum and fuelling systems as well as diagnostics will become available. With this equipment, the further goals of the project will be tackled, including the stepwise extension of discharge intervals to the multi-minute range. The experimental results so far have confirmed the neoclassical theory underlying several of the optimisation goals. At the same time, they showed that an optimisation is also required with respect to anomalous transport. In the future operational phases we aim at building a solid theoretical, experimental and technical foundation for the design of a next-step stellarator device.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Summarizes the set of edge, divertor and scrape-off layer diagnostics of W7‐X during operation phase with uncooled divertor.•Indicates all performed upgrades respectively optimizations with respect ...to first operation phase of W7‐X.•Gives a guideline what has to be considered if developing, fabricating and implementing a diagnostic for W7‐X.
Wendelstein 7-X (W7‐X) is the world’s largest superconducting nuclear fusion experiment of the optimized stellarator type. In the first Operation Phase (OP1.1) helium and hydrogen plasmas were studied in limiter configuration. The heating energy was limited to 4 MJ and the main purpose of that campaign was the integral commissioning of the machine and diagnostics, which was achieved very successfully. Already from the beginning a comprehensive set of diagnostics was available to study the plasma. On the path towards high-power, high-performance plasmas, W7‐X will be stepwise upgraded from an inertially cooled (OP1.2, limited to 80 MJ) to an actively cooled island divertor (OP2, 10 MW steady-state plasma operation). The machine is prepared for OP1.2 with 10 inertially cooled divertor units, and the experimental campaign has started recently.
The paper describes a subset of diagnostics which will be available for OP1.2 to study the plasma edge, divertor and scrape-off layer physics including those already available for OP1.1, plus modifications, upgrades and new systems. The focus of this summary will be on technical and engineering aspects, like feasibility and assembly but also on reliability, thermal loads and shielding against magnetic fields.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The island divertor concept is an innovative and promising idea to handle heat and particle exhaust in stellarators. At the Wendelstein 7-X (W7-X) stellarator, this divertor concept plays a central ...role in the device mission to demonstrate reactor relevant plasma confinement for steady-state time scales of up to 30 minutes in the high-performance campaign (OP2) starting in 2022. During the recently concluded first campaign with the inertially cooled island divertor, a large step in the experimental qualification of this divertor concept has been made. In discharges heated with electron cylotron resonance heating of 5−6 MW, central densities in the range of 0.7−1.2 × 1020 m−3 have been reached in combination with full divertor heat flux detachment. Also, significant neutral gas pressures and neutral compression ratios were shown for the first time in combination with reduced divertor particle flux. The divertor heat loads drop by an order of magnitude from >5 MW m−2 to below 0.5 MW m−2 with increasing density, and substantial compression of neutrals reaching neutral pressure in the sub-divertor volume of >6.0 × 10−4 mbar was seen. These elevated neutral pressure levels can be obtained and maintained with an up to 80% reduction of the particle fluxes onto the divertor target tiles. This discharge scenario was held stably detached for up to 28 seconds, which is equivalent to several hundred energy confinement times τE and longer than the time scales for current relaxation. No impurity accumulation was seen at constant Zeff ≈ 1.5 and the stored energy stayed constant at levels of Wdia > 600 kJ. The level of neutral pressure and compression reached in this scenario extrapolates well to the steady-state particle exhaust requirements for high-performance steady-state operation in OP2, in which the fully actively cooled high-heat-flux divertor will be available. An overview of this recently discovered divertor regime is given and the status ofthe physics understanding based on modeling of these regimes with the EMC3-EIRENE code is presented.
Abstract
Wendelstein 7-X (W7-X), the largest advanced stellarator, is built to demonstrate high power, high performance quasi-continuous operation. Therefore, in the recent campaign, experiments were ...performed to prepare for long pulse operation, addressing three critical issues: the development of stable detachment, control of the heat and particle exhaust, and the impact of leading edges on plasma performance. The heat and particle exhaust in W7-X is realized with the help of an island divertor, which utilizes large magnetic islands at the plasma boundary. This concept shows very efficient heat flux spreading and favourable scaling with input power. Experiments performed to overload leading edges showed that the island divertor yields good impurity screening. A highlight of the recent campaign was a robust detachment scenario, which allowed reducing power loads even by a factor of ten. At the same time, neutral pressures at the pumping gap entrance yielded the particle removal rate close to the values required for stable density control in steady-state operation.
Abstract
The optimized superconducting stellarator Wendelstein 7-X (W7-X) is equipped with an island divertor for energy control and efficient pumping. We investigated the performance of the island ...divertor in terms of gas exhaust. For this purpose we have installed 18 pressure gauges in the vacuum vessel. This allowed us to determine the exhaust efficiency, the leakage, the collection efficiency and the compression ratio of the island divertor. These quantities depended strongly on the magnetic configuration. The best performance was obtained in the high-iota configuration. The exhaust efficiency was 2.9%, significantly higher than in the standard configuration (0.44%), and the maximum neutral compression was about 80. The high-iota configuration appears particularly promising for long-pulse operation of W7-X.
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