Plasma Vessel Research Articles

Qualification of the laser powder bed fusion (L-PBF) manufacturing process for water-cooled protective components made from 316 L for plasma-exposed use in the Stellarator fusion device Wendelstein 7-X

A multi-stage qualification program of components, made by the additive manufacturing process (AM process) of laser powder bed fusion of metals (L-PBF) was performed.The aim of the qualification program was the test of the currently available L-PBF manufacturing options for practical usability at Wendelstein 7-X (W7-X) and the fabrication of water-cooled protective components made from 316 L stainless steel for W7-X diagnostics.The qualification program included a whole series of examinations and tests to provide information on the following characteristics: mechanical material properties, helium tightness, burst pressure, weldability, corrosion behavior, durability during thermal cycles, microwave durability, machinability, surface roughness, hydraulic pressure loss of cooling channels, outgassing behavior in vacuum, magnetic permeability, cobalt content, internal defects and geometric accuracy. The qualification program also resulted in recommendations for heat treatment parameters and a proposal for an indicator of process faults.After successful qualification, 13 L-PBF-components of four different component types with internal cooling channels were manufactured and installed in the plasma vessel of W7-X.

Performance of tungsten plasma facing components in the stellarator experiment W7-X: Recent results from the first OP2 campaign

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.

Contribution of electron density to plasma decomposition rate of methane

Hydrogen extraction from methane will be a common and useful technique in hydrogen production systems and fuel cycle systems of deuterium-tritium fusion reactors. Since tritium is a precious fuel, it needs to be extracted from impurity gases such as tritiated methane contained in the exhaust gas from the plasma vessel of a fusion reactor. An experimental work was conducted in this study to investigate the electron collision of methane decomposition reaction in helium plasma. A special attention was placed on the electron density distribution in a plasma chamber, and the dependence of the decomposition rate on the methane/helium ratio. The experimental results showed that methane decomposition rate and electron density tended to increase linearly with increasing RF power. This suggests that a strong dependence of electron collision on methane decomposition. Eventually, the dependence of methane decomposition rate on electron density, total pressure, and inlet methane concentration in a gas flow-type helium plasma reactor was successfully formulated. This study provides new sight for demonstrating the contribution of electron density in RF plasma assisted methane decomposition, and the summarized equation offers potential for reactor design on promising energy utilization.

Enhancements of the Fast Interlock System for Wendelstein 7-X Operational Phase OP 2.1

An extended modification phase of the superconducting fusion experiment Wendelstein 7- X was completed late 2021 after installing actively cooled divertor units and the integration or modification of new technical components and diagnostics. Currently, the commissioning of the W7-X experiment is underway. First technical plasma operation is scheduled for the end of September 2022, and scientific plasma operation will start at the end of November. The Fast Interlock System (FIS) is an important component of the Safety Instrumented System (SIS) of W7-X. The main task of the FIS is to protect the inner plasma vessel components from thermal overload from both plasma heating systems (e.g. neutral beam injection) and the plasma itself. There are strict limits for reaction times after detection of interlock relevant events, which have to be observed in order to prevent overload situations of the first wall. The requirements for the FIS were substantially modified and extended, in order to address the challenges due to the actively cooled divertor. This led to extensive revision of the technical implementation and the software of the FIS. In this paper, after a short introduction of FIS design and operation in OP1, the modification of FIS for OP2.1 operation is presented. Finally, the first results during the OP2.1 commissioning and operation of the FIS are discussed.

Overview over the neutral gas pressures in Wendelstein 7-X during divertor operation under boronized wall conditions

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 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.

Some Aspects of Neonatal Essential Fatty Acid Status are Altered by Linoleic Acid Supplementation of Women during Pregnancy

To study the effect of maternal linoleic acid [18:2(n-6), LA] supplementation during pregnancy on neonatal essential fatty acid status, pregnant women with relatively low plasma linoleic acid concentrations before 16 wk of gestation (n = 21) were supplied with foods rich in linoleic acid, resulting in an additional intake of 10 g/d of linoleic acid from the 20th week of gestation until delivery. One of the two control groups consisted of pregnant women with comparably low plasma linoleic acid concentrations at the start of the study (LL-control group, n = 22); the other consisted of women with habitually high plasma linoleic acid concentrations (HL-control group, n = 21). The neonatal essential fatty acid status was assessed by determining the fatty acid composition of phospholipids (PL) isolated from umbilical plasma and umbilical vessel walls. The maternal linoleic acid status in the LA-supplemented group increased to a level comparable to that of the HL-control group, but the neonatal linoleic acid status did not differ from that of either control group. Linoleic acid supplementation did result in slightly, but significantly, higher total amounts of (n-6) long-chain polyenes in umbilical plasma and vein vessel wall phospholipids compared with the LL-control group. This increase was associated with significantly lower total amounts of (n-3) long-chain polyenes. In the HL-control group, the concentration of (n-3) long-chain polyenes in umbilical plasma and vessel walls was significantly lower than in the LA-supplemented and the LL-control group. The results of this study indicate that maternal linoleic acid supplementation can affect some aspects of neonatal essential fatty acid status.

Refined electromagnetic analysis of W7-X thermal insulation during fast plasma current decay event

During the second operation phase of the most advanced stellarator Wendelstein 7-X (W7-X) in 2018, a fast plasma current decay event was observed with the time constant of ∼1 ms. The event is much more rapid than the design phase assumptions: 100 kA and 2.7 MA plasma current decay with the time constants of 140 and 50 ms for toroidal and diamagnetic currents, respectively. As a result, significantly higher eddy currents are expected to be induced in some of the W7-X components. The comprehensive campaign of electromagnetic (EM) analyses has been launched in order to identify the necessity of proper reinforcement of critical in-vessel diagnostics or lowering the plasma currents for future operation. One of the critical W7-X system found is the complex thermal insulation (TI).The TI is located in the cryostat to separate cryogenically cooled magnet system and warm vessels. The system consists of actively cooled thermal radiation panels and port tubes, multi-layer insulations, supports and clamps. The TI panels and tubes form the main frame of TI covering the plasma vessel (PV) and ports, respectively. Preliminary EM analysis with few panels and tubes indicates that the EM forces are increased by about 7 times due to the newly postulated faster plasma current decay event in comparison with original design loads. To obtain more accurate results, an elaborated EM model has been created, which includes the excitation coils for toroidal and diamagnetic plasma currents, superconducting coils, TI panels and tubes, the PV and ports.The paper introduces at first the TI structure, continues with the field and eddy current accuracy studies and the lessons learned for the modelling accuracy improvement in ANSYS®. Then the EM analysis results of different plasma current decay scenarios are presented and discussed. Finally, the EM forces for static and transient mechanical analyses are chosen and extracted for further mechanical analyses.

The ECRH-Power Upgrade at the Wendelstein 7-X Stellarator

The existing ECRH system at W7-X consists of 10 gyrotrons, with output power levels ranging from 0.6 MW up to 1.0 MW each at a frequency of 140 GHz, quasi-optical transmission lines and microwave launchers at the plasma vessel. Compared to other large fusion experiments, W7-X has a relatively low power-to-volume ratio. However high heating power is particularly necessary for achieving high plasma beta values, where the improved confinement of fast ions, one of the optimization criteria of W7-X, can be examined. It is therefore necessary to expand the ECRH systems in several consecutive steps. It is planned to increase the number of gyrotron positions from 10 to 12 and at the same time to evolve the gyrotron output power in several development steps from 1 MW to nominal 1.5 MW and, finally, up to 2 MW. At the same time, the transmission lines will also be upgraded for 2 MW operation. A special effort is also made to improve the reliability of the system by the fast control system.

Thermal loading test of a Wendelstein 7-X pumping gap panel

For the upcoming long pulse operation phase 2 "OP2" of Wendelstein 7-X (W7-X), new water cooled non planar stainless steel panels have been manufactured to protect the wall of the plasma vessel behind the divertor pumping gap. Such a panel is made of a machined ground plate with channels which are covered by likewise machined sheets. The latter are electron-beam welded to the ground plates. There are 60 panels of 7 different types which are designed to remove a stationary heat flux of up to 100 kW/m². The specified water cooling conditions are: 2.5 MPa inlet pressure, 30 °C inlet temperature, and 0.27 ℓ/s flow rate. A panel prototype has been manufactured to validate the design and manufacturing process. In order to verify the thermo-hydraulic calculations performed with ANSYS® CFX, a thermal loading test on the prototype was carried out in the SIRHEX test facility at KIT. The loaded surface of the prototype (about 0.11 m²) was black-finished, and thermocouples were installed at different positions. The prototype was placed in the vacuum tank and heated by an array of six infrared lamps (400 V, max. 16 kW per lamp). The deposited load of up to about 100 kW/m² was measured by calorimetry. The paper describes the loading experiment of the panel, and its results. A good agreement was found between calculations and test results, and thus the thermo-hydraulic panel design was well validated.

New horizontal and vertical field coils with optimised location for robust decentralized plasma position control in the IGNITOR tokamak

The paper deals with the improvement of the poloidal system of the IGNITOR tokamak, and the design and simulation of the robust decentralised control systems of the plasma’s vertical and horizontal position. The results were obtained on a linear model that is constructed from the circuit equations for the coils, plasma, and vacuum vessel (VV) currents, and from the force balance equations for the plasma. Two new coils that are located close to the VV have been introduced in the IGNITOR poloidal system; specifically, a Horizontal Field Coil (HFC) that is designated for the plasma vertical position control and a Vertical Field Coil (VFC) that is used for the horizontal position control. For the first time in the design of D-shaped tokamaks poloidal systems, the location of the HFC was optimised to achieve the maximum size of the plasma controllability region in the vertical direction and the location of the VFC was determined from the optimisation of the plasma position response to the voltage step function. Although the ideal position of the VFC with fast response time is on the equatorial plane, its location usually is occupied by the other devices such as ports, diagnostic system, and heating systems, so the separated two-section VFC is used. Careful calculation shows the performance is degraded by only 20 % in this layout. For the improved IGNITOR poloidal system, the new robust decentralised plasma position control systems were designed by means of two robust approaches: H∞-optimisation and Linear Matrix Inequalities (LMI). The results of the simulation of the control systems with two types of actuator models as the multiphase thyristor rectifier and the voltage inverter in Pulse- Width Modulation (PWM) mode for the new IGNITOR poloidal system are presented.

Improvement of Nanostructured Polythiophene Film Uniformity Using a Cruciform Electrode and Substrate Rotation in Atmospheric Pressure Plasma Polymerization.

In atmospheric pressure (AP) plasma polymerization, increasing the effective volume of the plasma medium by expanding the plasma-generating region within the plasma reactor is considered a simple method to create regular and uniform polymer films. Here, we propose a newly designed AP plasma reactor with a cruciform wire electrode that can expand the discharge volume. Based on the plasma vessel configuration, which consists of a wide tube and a substrate stand, two tungsten wires crossed at 90 degrees are used as a common powered electrode in consideration of two-dimensional spatial expansion. In the wire electrode, which is partially covered by a glass capillary, discharge occurs at the boundary where the capillary terminates, so that the discharge region is divided into fourths along the cruciform electrode and the discharge volume can successfully expand. It is confirmed that although a discharge imbalance in the four regions of the AP plasma reactor can adversely affect the uniformity of the polymerized, nanostructured polymer film, rotating the substrate using a turntable can significantly improve the film uniformity. With this AP plasma reactor, nanostructured polythiophene (PTh) films are synthesized and the morphology and chemical properties of the PTh nanostructure, as well as the PTh-film uniformity and electrical properties, are investigated in detail.

"Zyflex": Next generation plasma chamber for complex plasma research in space.

In this paper, we give a detailed description of a novel plasma chamber-the Zyflex chamber-that has been specifically designed for complex/dusty plasma research under reduced gravitational influence as realized during parabolic flight or aboard the International Space Station. The cylindrical, radio-frequency driven discharge device includes a variety of innovations that, for example, allow us to flexibly adjust plasma parameters and its volume via enhanced plasma generation control and a movable, multi-segmented electrode system. The new complex/dusty plasma research tool also supports, due to its overall increased size compared to former space based complex plasma experiments such as PKE-Nefedov or PK-3 Plus, much larger particle systems. Additionally, it can be operated at much lower neutral gas pressures, thus reducing the damping of particle motion considerably. Beyond the technical description and particle-in-cell simulation based characterization of the plasma vessel, we show sample results from experiments performed with this device in the laboratory as well as during parabolic flights, both of which clearly demonstrate the new quality of complex/dusty plasma research that becomes accessible with this new plasma device.

Completion of the production of the W7-X divertor target modules

The installation of an actively water cooled divertor in the stellarator Wendelstein 7-X (W-7X) is mandatory to achieve stationary power and particle exhaust for pulse lengths up to 30 min. The highly loaded divertor area is made of 100 target modules distributed in ten divertor units. The target modules have mechanical support frames with attachment systems to the plasma vessel, and manifolds to distribute water equally between the target elements. A target element is designed to remove a stationary heat flux up to 10 MW/m² and is made of a CuCrZr copper alloy heat sink armored with CFC NB31 tiles. The manufacturing process, assembly and quality assessment of the last 70 target modules has been successfully completed in the Integrated Technical Centre of IPP-Garching. Some parts such as the target elements and manifolds were delivered by industry. The quality was assessed as follows: visual inspections, measurement of the 3-D CFC surface, dynamic pressure tests, He leak testing under pressure at different temperature (20 °C, 160 °C) in vacuum oven, high heat flux testing. The production of the water cooled divertor is now completed, and the mounting operation of the target modules in the plasma vessel of W7-X has started.

Prediction of unusual plasma discharge by using Support Vector Machine

This paper proposes a method for predicting an unusual emission of visible light inside the plasma vessel by using a Support Vector Machine (SVM) because the unusual emission of visible light can be caused by unexpected heating on the vessel surface. This emission must be predicted to avoid unexpected situations in which it causes some damage to the vessel. The light reflected from the divertor tiles is used as the unusual emission light. This study aims to predict such unusual emission through pictures before the start of the unusual emission, regardless of the plasma physics. This study experimentally confirms that the unusual emission of visible light inside the plasma vessel can be predicted with an accuracy rate of 96.4%, and approximately 0.3 s before the start of an unusual emission.

Thermal analyses of the W7-X plasma vessel for operation phase 2

The W7-X plasma vessel (PV) is part of the cryostat wall and forms a vacuum and thermal barrier between the hot vessel interior and the superconducting coils. The vessel is thermally loaded from the inside and is actively cooled by numerous cooling water pipes in order to control its temperature.However, due to the complex geometry and large number of different types of ports, the cooling pipes are distributed irregularly with varying distances in between them. So the temperature distribution of the PV becomes quite inhomogeneous even with homogeneous load density, and impermissible temperature hot spots >130 °C may arise. Particularly for the upcoming long pulse operation phase 2 (OP2), it is necessary to know the locally allowed thermal loads on the PV. Therefore, temperature distribution maps for different heat loads from 1 kW/m2 to 12 kW/m2 were calculated in order to determine the corresponding hot spots.The available CAD geometry was not convenient for this extensive and complex analysis; therefore, a simplified shell-beam model was employed to get an approximate temperature distribution, and a correction formula was derived to calculate more accurate values at the positions of interest.In some areas with large distances between cooling pipes, additional copper stripes are welded onto the PV to improve the heat transfer. For these regions local models were built to calculate the temperature distributions, and, in the course of this analysis, to judge the effect of the copper stripes.The calculation results indicate positions to be additionally protected and will be used to determine safe operation limits.

Specific features of eddy current analysis with ANSYS® for fast plasma current decay event in W7-X

During the second operation phase (OP1.2) of the most advanced stellarator Wendelstein 7-X (W7-X), some events of fast plasma current decay with a time constant of about 1 ms were observed (for both toroidal and diamagnetic plasma currents). The events were triggered by Electron Cyclotron Current Drive (ECCD) and were much faster than the assumptions considered during the design phase of W7-X components: 100 kA and 2.7 MA plasma current discharge with the time constants of 140 and 50 ms for toroidal and diamagnetic currents respectively. During the observed 1 ms discharge of maximum expected plasma currents, significant eddy currents are expected to be induced in some components with resulting electro-magnetic (EM) forces. In order to avoid unnecessary changes on as-built components, evaluation of the eddy currents / EM forces with high accuracy is required, if the conservative simplified estimation shows criticality. As a result, a sophisticated EM model with plasma currents, superconducting (sc) coils, plasma vessel (PV) and multiple ports is developed in ANSYS® using the new element type SOLID236 / 237 (edge-based formulation), which is much more accurate than the legacy elements SOLID97 (vector potential formulation). The specific features of using SOLID236 elements for eddy current analysis are studied and presented in this paper. This includes the application of boundary conditions on sector EM model, modelling of excitation currents, field and eddy current accuracy, skin effect, shielding effect, voltage in conducting components, etc. Most of the issues are studied in comparison with SOLID97 elements. Finally, the pros and cons of SOLID236 and SOLID97 elements are summarized, and the experiences during the W7-X EM analyses are presented.

Alfvénic fluctuations measured by in-vessel Mirnov coils at the Wendelstein 7-X stellarator

The magnetic diagnostic system at the optimized stellarator experiment Wendelstein 7-X (W7-X) includes a total number of 125 Mirnov coils, which are mounted at the inner plasma vessel wall. A wide range of expected signal amplitudes, frequencies and mode numbers is covered to account for the complex three-dimensional geometry of W7-X. The performed amplitude and phase calibration of individual coils including all relevant data acquisition system components allows a detailed spectral and mode analysis. The damped multiple signal classification method and stochastic system identification techniques are shortly described and applied. Two observations are emphasized within this work. Firstly, independent of magnetic configuration and heating scenario (electron cyclotron resonance heating and/or neutral beam injection), broadband fluctuations around 180 kHz have been measured. The nature of these fluctuations is possibly associated with ellipticity-induced Alfvén eigenmodes in the outer regions of the plasma. The latter was inferred by studying corresponding theoretically predicted Alfvén continua calculated with the 3D-MHD continuum code CONTI. Secondly, fast collapses of plasma current and energy, which occurred during recent operational phases at W7-X, show a clear magnetic signature. Short time scale Alfvénic bursts are revealed, which are induced during these sawtooth-like collapses.

A Compact Two-Frequency Notch Filter for Millimeter Wave Plasma Diagnostics

Sensitive millimeter wave diagnostics in magnetic confinement plasma fusion experiments need protection from gyrotron stray radiation in the plasma vessel. Modern electron cyclotron resonance heating (ECRH) systems take advantage of multi-frequency gyrotrons. This means that the frequency band of some millimeter wave diagnostics contains more than one narrow-band gyrotron-frequency line, which needs to be effectively suppressed. A compact standard waveguide notch filter based on coupled waveguide resonators with rectangular cross-section is presented which can provide very high suppression of several gyrotron frequencies and has low insertion loss of the passband.

Soft x-ray tomography measurements in the Wendelstein 7-X stellarator

The soft x-ray tomography diagnostic in the stellarator Wendelstein 7-X consists of twenty pinhole cameras, up–down symmetrically arranged in a poloidal, triangular cross-section of the plasma vessel. The x-ray emissivity is measured with 16 bit amplitude resolution at 2 MHz sampling rate along 360 lines-of-sight by silicon photodiode arrays. In the recent operation campaign data acquisition (DAQ) has been working reliable for the conducted plasma pulse lengths <1 min, however the DAQ system are ready for the foreseen 30 min plasma pulse lengths of upcoming campaigns. The bandwidth of the preamplifiers is ≈200 kHz and the sensitive energy range is approximately 1–12 keV. The measurements indicate the up–down symmetric emissivity distribution in the triangular poloidal cross-section. First tomographic reconstructions of different magnetic field configurations are consistent with the theoretically calculated flux surface topology.

Inspection of W 7-X plasma-facing components after the operation phase OP1.2b: observations and first assessments

Due to its twisted magnetic field configurations, the orientation of the plasma-facing components (PFCs) of the Wendelstein 7-X stellarator is changed from bean shape to triangular shape along the plasma axis within a module. The PFCs consist of graphite (test divertor units, baffles, head shield tiles, toroidal divertor closures) and stainless steel components (wall panels, pumping gap panels, poloidal closures). In order to document the status of these components as well as to get the first footprints of the plasma-wall interactions (PWI) that occurred during the plasma campaign, a detailed inspection of the whole plasma vessel after the plasma campaign is of vital importance. Such an inspection was carried out after the operation phase OP1.2b completed in October 2018. A number of observations were made, such as, molten stainless steel drops, melting of a long term probe, deposition on steel panels, arc traces, loosely bound particles and deposition stripes on the plasma vessel hidden behind the graphite tiles. In order to understand the possible causes, their implication and steps to avoid some of these problems for the next phase of machine operation, an assessment was made. The melting events should be avoided in the future, the others are due to usual PWI mechanisms and will be monitored after each campaign.