Linear Plasma Device Research Articles

Optics of a multiple color gas puff imaging diagnostic on the HL-2A tokamak

The optics of a new Multiple Color Gas Puff Imaging (MC-GPI) diagnostic on the HL-2A tokamak is presented. MC-GPI utilizes the helium I (He I) line ratio technique to infer plasma density ne and electron temperature Te in edge plasma simultaneously. A compact four-channel beam splitter and dichroic mirrors enable the selection of the intensities of He I lines at specific wavelengths, i.e., λ=587.6 nm, λ=667.8 nm, λ=706.5 nm and λ=728.1 nm. The transmittance of these four specific He I lines after passing through the optical fiber bundle and the beam splitter is 34%, 33%, 33% and 30%, respectively. The temporal resolution of MC-GPI is 100μs and the spatial resolution is 2.5 mm × 2.5 mm. This diagnostic has been experimentally tested on an optical table as well as on a linear plasma device to verify its capability for ne and Te measurements. A newly designed light splitter with double-layer layout is under developing to absorb the stray light, aiming at improve the performance of the diagnostic.

Experimental studies on the propagation of whistler-mode waves in a magnetized plasma structure with a non-uniform density

Propagation of whistler-mode waves in a magnetized plasma structure is investigated in the Keda linear magnetized plasma device. The magnetized plasma structure has its density peak in the center, and the background magnetic field is homogeneous along the axial direction. A whistler-mode wave with a frequency of 0.3 times of electron cyclotron frequency (f ce) is launched into the plasma structure. The wave normal angle (WNA) is about 25°, and the wavefront exhibits a wedge structure. During propagation of the whistler wave, both the propagating angle and WNA slowly approach zero, and then the wave is converged toward the center of the structure. Therefore, the wave tends to be trapped in the plasma structure. The results present observational evidence of the propagation of a whistler-mode wave trapped in the enhanced-density structure in a laboratory plasma. This trapping effect is consistent with satellite observations in the inner magnetosphere.

Global sensitivity analysis of a coupled multiphysics model to predict surface evolution in fusion plasma–surface interactions

We construct a global sensitivity analysis framework for a coupled multiphysics model used to predict the changes in material properties and surface morphology of helium plasma-facing components in future fusion reactors. The model combines the particle dynamics simulator F-TRIDYN, that predicts the helium implantation profile, with the cluster dynamics simulator Xolotl, that predicts the growth and evolution of subsurface helium gas bubbles. In order to keep the sensitivity analysis tractable, we first construct a sparse, high-dimensional polynomial chaos expansion surrogate model for each output quantity of interest, which allows the efficient extraction of sensitivity information. The sensitivity analysis is performed for two problem settings: one for ITER-like conditions, and one that resembles conditions inside the PISCES-A linear plasma device. We present a systematic comparison of important parameters, for both F-TRIDYN and Xolotl in isolation as well as for the coupled model, and discuss the physical interpretation of these results.

Machine learning-aided line intensity ratio technique applied to deuterium plasmas

It has been demonstrated that the electron density, ne, and temperature, Te, are successfully evaluated from He I line intensity ratios coupled with machine learning (ML). In this paper, the ML-aided line intensity ratio technique is applied to deuterium (D) plasmas with 0.031 < ne (1018 m−3) < 0.67 and 2.3 < Te (eV) < 5.1 in the PISCES-A linear plasma device. Two line intensity ratios, Dα/Dγ and Dα/Dβ, are used to develop a predictive model for ne and Te separately. Reasonable agreement of both ne and Te with those from single Langmuir probe measurements is obtained at ne > 0.1 × 1018 m−3. Addition of the D2/Dα intensity ratio, where the D2 band emission intensity is integrated in a wavelength range of λ ∼ 557.4–643.0 nm, is found to improve the prediction of, in particular, ne, and Te. It is also confirmed that the technique works for D plasmas with 0.067 < ne (1018 m−3) < 6.1 and 0.8 < Te (eV) < 15 in another linear plasma device, PISCES-RF. The two training datasets from PISCES-A and PISCES-RF are combined, and unified predictive models for ne and Te give reasonable agreement with probe measurements in both devices.

Deuterium retention in reduced activation ferritic/martensitic steel EUROFER97 exposed to low-energy deuterium plasma

EUROFER97 steel samples were exposed to deuterium plasma in the linear plasma device PISCES-A with the ion energies of 90 and 140 eV at exposure temperatures between 360 and 773 K to deuterium ion fluences between 3.8 × 1023 and 6.3 × 1025 m−2. RBS analysis of these plasma-exposed samples showed that the uppermost surface layers were enriched with tungsten and tantalum. Deuterium depth profiles in the plasma-exposed samples were examined using the energy-scanning NRA technique allowing measurements of the deuterium depth profiles at depths of up to about 9 μm. With an increase in the exposure temperature from 360 to 773 K, the deuterium retention in the 1 µm thick sub-surface layer decreased almost monotonically from about 1 × 1019 D/m2 to 5 × 1017 D/m2. The deuterium concentration at a depth of 8 μm demonstrated a different temperature dependence - with an increase in the exposure temperature from 360 to 500 K, the deuterium concentration significantly decreased from about 4 × 10−3 to (4 ± 3) × 10−5 at.%. With a further increase in the temperature up to 773 K, the deuterium concentration at a depth of 8 μm became equal to or less than 5 × 10−5 at.%.

Radially inward particle transport driven by low-frequency instability in cylindrical magnetized plasma

Net radially inward turbulent particle flux was observed in a linear plasma device, the linear experimental advanced device (LEAD). The amplitude of the inward particle flux is comparable with outward particle flux. The inward particle flux peak value locates in the outer E r shear layer. Cross-phase evolution dominates the radial evolution of particle flux from outward to inward. In the frequency domain, a 1 kHz mode contributes most of the inward particle flux. Frequency-resolved energy transfer analysis infers that this mode obtains energy nonlinearly from very low frequency fluctuations below 0.5 kHz. Moreover, turbulence spreading could also transfer energy to the position where the inward flux peak locates from nearby positions, and thus acts as a nonlocal, nonlinear drive for the 1 kHz mode.

Uniform plasma generation with filament assisted DC discharge in a linear plasma device

Uniform and quiescent (δ n/n < 0.5%) laboratory plasma has been produced in a linear plasma device with a simple filament-assisted DC source without using any magnetic field for plasma confinement. A filament-assisted DC plasma source has been designed, fabricated in-house, and operated successfully to achieve the desired plasma parameters. A stainless steel(ss)-grid is placed in-front of the filament assembly and biased appropriately using a DC-regulated power supply to accelerate thermionically emitted electrons from the heated filaments along the length of the main chamber and facilitate the production of uniform plasma. Heating of the filaments was done by passing a current of ∼3.8 − 4.2 A through it using another DC-regulated power supply. 2-dimensional spatial profiles of plasma density (n e ), electron temperature (T e ) and plasma potential (V P ) obtained from the Langmuir Probe measurements by inserting 4 number of Langmuir Probes inside the plasma from 4 co-linear radial ports of the plasma chamber and scanning them radially with the help of indigenously built probe drive setups reveal spatially uniform plasma generation with n e in the range ∼(1 − 2) × 1015 m−3 and T e ∼ (2.5 − 3.5) eV. Variations of plasma parameters and its spatial uniformity with neutral pressure are also investigated. It is observed that the spatial uniformity of the plasma produced at neutral pressures in the range of ∼(3 − 6) × 10−4 mbar is very good with δ n/n < 0.5%.

Physical and technical basis of Materials Plasma Exposure eXperiment from modeling and Proto-MPEX results *

The Materials Plasma Exposure eXperiment (MPEX) is a steady-state linear plasma device that will address plasma-material interaction (PMI) science and enable testing of fusion reactor-relevant divertor plasma-facing materials. The MPEX source concept consists of a helicon plasma source to generate the plasma, electron cyclotron heating (ECH) for electron heating, and ion cyclotron heating (ICH) for ion heating. The MPEX source plasma is then transported axially to the PMI material target region to test material samples in fusion reactor-relevant divertor conditions. This paper will summarize the physical and technical basis of MPEX. The paper will first define the MPEX parameters and scenarios at the target relevant to PMI science for various fusion reactor-relevant divertor conditions and show plasma transport modeling results to set the MPEX source parameters. Recent experimental and modeling results from Proto-MPEX, a short-pulse experiment to develop the plasma production, heating, and transport physics for MPEX, will be shown. From these results, it will be shown that MPEX can reach its desired scenarios. The MPEX physical and technical basis will also determine important functional requirements for magnetic field, radiofrequency (RF) power, RF frequency, and neutral pressure in the helicon, ECH, ICH, and PMI regions that are required to achieve the desired MPEX scenarios. The necessity for key in-vessel components such as skimmers, limiters, and microwave absorbers will also be highlighted.

Study the erosion of Eurofer-97 steel with the linear plasma device GyM

This work reports on the investigation of Eurofer-97 erosion behaviour when exposed to the deuterium plasma of the linear device GyM. The erosion dependence of Eurofer-97 on the deuterium ion fluence, Φ≤2.3×1025 m−2, and temperature of the samples, T = 600 K and 990 K, was addressed. A bias voltage of −200 V was applied to GyM sample holder during the experiments. Samples were deeply characterised by: profilometry, scanning electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, Rutherford backscattering spectroscopy and particle-induced X-ray emission.The behaviour of Eurofer-97 erosion rate with the ion fluence strictly depends upon temperature. At 600 K, it was ∼0.14 nm/s after 4.7 × 1024 m−2, then decreased, reaching a steady state value of ∼0.01 nm/s from 8.0 × 1024 m−2. At 990 K instead, the erosion rate was roughly constant around 0.019 nm/s for Φ≤1.24×1025 m−2. The value at 2.35 × 1025 m−2 was slightly lower. The erosion rate at 990 K was greater than that at 600 K for every ion fluence.Microscopy and surface analysis techniques showed that Eurofer-97 erosion rate dependence on Φ at 600 K was primarily determined by the preferential sputtering of iron and other mid-Z elements of the alloy, leading to a surface rich in W and Ta difficult to be sputtered. The erosion behaviour at 990 K was dominated by the morphology dynamics, instead. The different properties of the morphology developed at the two temperatures can explain the higher erosion rate at 990 K for all the ion fluences.

Impact of vibrationally and electronically excited H2 on the molecular assisted recombination rate in detached plasma regimes

The vibrational redistribution of H2 molecules via the B1Σu+ and C1Πu states are included in the determination of molecular assisted recombination (MAR) rates and are found to have negligible impact in detached plasma regimes. In high recycling and detached divertor plasma conditions, MAR occurs through the higher levels (v≥4) of vibrationally excited H2 which are primarily populated through direct electron impact excitation or the electronic excitation of H2 molecules and the subsequent radiative decay to another vibrational state. We use the collisional-radiative model CRUMPET to evaluate MAR rate coefficients with electronic transitions included, which are applied to detached plasma parameters found in the linear plasma device Magnum-PSI.

Reducing tin droplet ejection from capillary porous structures under hydrogen plasma exposure in Magnum-PSI

Liquid metal based divertors could be a more robust alternative to a solid tungsten design for DEMO. The liquid is confined in a sponge-like tungsten layer, called a capillary porous structure (CPS). It has been found previously that under certain conditions, many tin droplets eject from a CPS when it is brought into contact with a hydrogen plasma. These would present a contamination issue for the plasma core. Stability analysis suggests that droplet ejection can be suppressed by reduction of the pore size. To test this, stainless-steel CPS targets with pore size ranging from 0.5–100µm filled with tin were exposed to identical loading conditions. This was done in the linear plasma device Magnum-PSI, capable of reaching divertor relevant plasma conditions. Furthermore, the influence of the CPS manufacturing techniques is considered by comparing the performance of a 3D printed, a mesh felts and a sintered CPS, all made from tungsten. Each target was surrounded by four witness plates, which were analysed post-mortem for Sn content by Rutherford backscattering. During plasma exposure, tin droplets were observed using a fast visible camera and plasma light emission via survey optical emission spectroscopy. The results imply that Sn erosion can be reduced by a factor of 50 when reducing the pore size. Moreover, it highlights the importance of avoiding overfilling of CPS targets with Sn.

Bubble Formation in ITER-Grade Tungsten after Exposure to Stationary D/He Plasma and ELM-like Thermal Shocks

Plasma-facing materials (PFMs) in the ITER divertor will be exposed to severe conditions, including exposure to transient heat loads from edge-localized modes (ELMs) and to plasma particles and neutrons. Tungsten is the material chosen as PFM for the ITER divertor. In previous tests, bubble formation in ITER-grade tungsten was detected when exposed to fusion relevant conditions. For this study, ITER-grade tungsten was exposed to simultaneous ELM-like transient heat loads and D/He (6%) plasma in the linear plasma device PSI-2. Bubble formation was then investigated via SEM micrographs and FIB cuts. It was found that for exposure to 100.000 laser pulses of 0.6 GWm−2 absorbed power density (Pabs), only small bubbles in the nanometer range were formed close to the surface. After increasing Pabs to 0.8 and 1.0 GWm−2, the size of the bubbles went up to about 1 µm in size and were deeper below the surface. Increasing the plasma fluence had an even larger effect, more than doubling bubble density and increasing bubble size to up to 2 µm in diameter. When using deuterium-only plasma, the samples showed no bubble formation and reduced cracking, showing such bubble formation is caused by exposure to helium plasma.

Hydrogen isotope analysis in W-tiles using fs-LIBS

Laser-Induced Breakdown Spectroscopy (LIBS) is a promising technology for in-situ analysis of Plasma-Facing Components in magnetic confinement fusion facilities. It is of major interest to monitor the hydrogen isotope retention i.e. tritium and deuterium over many operation hours to guarantee safety and availability of the future reactor. In our studies we use ultraviolet femtosecond laser pulses to analyze tungsten (W) tiles that were exposed to a deuterium plasma in the linear plasma device PSI-2, which mimics conditions at the first wall. A high-resolution spectrometer is used to detect the Balmer-alpha transition of the surface from implanted hydrogen isotopes (H and D). We use Calibration Free CF-LIBS to quantify the amount of deuterium stored in W. This proof-of-principle study shows the applicability of femtosecond lasers for the detection of low deuterium concentration as present in first wall material of prevailing fusion experiments.

Rapid growth of nanostructure on tungsten thin film by exposure to helium plasma

A fiberform nanostructure was synthesized by exposing high-density helium plasma to a 100 nm thick tungsten thin film in the linear plasma device NAGDIS-II. After helium plasma exposure, the cross-section of samples was observed by a scanning electron microscope, transmission electron microscope, and focused ion beam scanning electron microscope. It is shown that the thickness of the nanostructured layer increases significantly for only a short irradiation time. The optical absorptivity remains high, even though it is exposed to helium plasma for a short time. The usage of the thin film can shorten the processing time for nanostructure growth, which will be beneficial for commercial production.

Linear plasma device GyM for plasma-material interaction studies

GyM is a linear plasma device operating at Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Milan, with the original aim of studying basic plasma physics, such as turbulent processes. Since 2014, GyM experimental program has been mainly focused on the issue of plasma-material interaction (PMI) for magnetic confinement nuclear fusion applications. GyM consists of a stainless steel vacuum chamber (radius and length of 0.125 m and 2.11 m), a pumping system, a gas injection system, 10 magnetic field coils and two magnetron sources at 2.45 GHz, capable of delivering a total microwave power up to 4.5 kW. Highly reproducible steady-state plasmas of different gas species, at a maximum working pressure of ∼10−1 Pa, can be obtained by electron cyclotron resonance heating in the resonance layer at 87.5 mT. Plasmas of GyM have electron and ion temperature ≤15 eV and ∼0.1 eV, respectively. The electron density is in the range of 1015–1017 m−3 and the ion flux is ≤5 × 1020 ions⋅m−2s−1. Main plasma diagnostics of GyM comprise Langmuir probes, an optical emission spectrometer, a mass spectrometer and a fast camera system equipped with an image intensifier unit. For the purpose of investigating the topic of PMI, GyM is provided with two sample exposure systems. Both are biasable at a negative bias voltage down to −400 V, to tune the energy of the impinging ions. One of them is also equipped with a heating lamp and can reach and sustain a temperature of 990 K for several hours, thus allowing to study the role of sample temperature during the plasma-material interaction. This contribution presents the layout of GyM, the diagnostics, the sample exposure systems and the typical plasma parameters. A brief overview of the main PMI activities carried out so far and a description of future machine upgrades are also given.

Simulation of ion cyclotron resonance heating by using particle-in-cell method in MPS-LD linear plasma device

The auxiliary heating of electrons and ions in linear plasma devices (LPDs) is necessary to achieve the boundary plasma relevant environment of tokamaks, to investigate the boundary physics and plasma—material interactions. In this work, the simulation of ion cyclotron resonance heating (ICRH) in the LPD multiple plasma simulation linear device (MPS-LD) is carried out by using a 3D particle-in-cell method, and the wave—ion interaction mechanism based on a ‘beach-heating’ technique in the ion heating region is investigated. A left-handed, circularly polarized wave along the magnetic field lines is used to represent the electromagnetic wave in the model, after the analysis of the cold plasma dispersion relation. The mechanism of ion heating by collisionless damping absorption is demonstrated and explained by using the plasma current as the plasma response. The dependencies of the heating efficiency on the plasma density, magnetic field strength and magnetic field configuration are studied. The correlation between plasma density and magnetic field strength, which satisfies the heating efficiency, is found and it is in perfect agreement with the theoretical derivation. Finally, by using the designed parameters of MPS-LD provided by SOLPS-ITER, the prediction of ICRH is performed. The simulation result shows that the ion temperature can be heated higher than 40 eV and it satisfies the requirement for scrape-off layer/divertor simulation experimentally in MPS-LD.

Global SOLPS-ITER and ERO2.0 coupling in a linear device for the study of plasma–wall interaction in helium plasma

Plasma–wall interaction (PWI) is a great challenge in the development of a nuclear fusion power plant. To investigate phenomena like erosion of plasma-facing components, impurity transport and redeposition, one needs reliable numerical tools for the description of both the plasma and the material evolution. The development of such tools is essential to guide the design and interpretation of experiments in present and future fusion devices. This contribution presents the first global simulation of PWI processes in a linear plasma device mimicking the boundary plasma conditions in toroidal ones, including both the description of plasma and impurity transport and of plasma-facing material evolution. This integrated description is obtained by coupling two of the state-of-the-art numerical codes employed to model the plasma boundary and the PWI, namely SOLPS-ITER and ERO2.0. Investigation of helium plasma is also of primary importance due to the role helium will have during ITER pre-fusion power operation, when it is planned to be used as one of the main plasma species, as well as fusion ash in full power operation. The plasma background is simulated by SOLPS-ITER and the set of atomic reactions for helium plasmas is updated, including charge-exchange and radiative heat losses. ERO2.0 is used to assess the surface erosion in the GyM vessel, using different wall materials (e.g. carbon, iron or tungsten) and applying different biasing voltage. Eroded particles are followed within the plasma to assess their redeposition location. The ionization probability of the different materials in the GyM plasma is inferred through the energy distribution of impacting particles and its effects on migration are investigated.

MPEX High Heat Flux Plasma Dump Design, Manufacture, and Articles Test

The Material Plasma Exposure eXperiment (MPEX) device is a steady-state linear plasma device currently in the final design phase at the Oak Ridge National Laboratory. This device will reach ion fluences up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{31}\,\,\text{m}^{-2}$ </tex-math></inline-formula> and will be used to expose neutron-irradiated materials to divertor-relevant plasmas and to study the effects of plasma-material interactions. These studies will elucidate the complex effects of plasmas with divertor candidate materials capable of withstanding high heat flux and high fluences for next-generation fusion devices. Bidirectional plasma will be generated using a high-power (200 kW) helicon source. Plasma will be confined by superconducting magnets. The last plasma-facing component on the upstream side of the MPEX device is the dump, which has been designed to intercept plasma and energetic particles. The dump will have a total heat load of 9.2 kW. A copper alloy (Glidcop AL-15) was selected for use in the water-cooled flange design because of its high thermal conductivity, its retaining strength at elevated temperatures, and its ability to be used in the high-temperature braze joints used in this application. Titanium-zirconium-molybdenum (TZM) tiles are brazed to the Glidcop AL-15 flange using high-temperature braze alloy. External water-cooling channels are used on the dump flange to prevent water leakage inside the vacuum space. This article discusses the details of the high heat flux dump design, including the computational fluid dynamics (CFD) and structural analyses performed to validate the design to meet the operational requirements of the MPEX device.

Material Plasma Exposure eXperiment (MPEX) High Heat Flux Bellows for Component Alignment in High Microwave Environment

The Material Plasma Exposure eXperiment (MPEX) device is a linear plasma device developed to perform plasma material interaction experiments under the conditions prototypic of a fusion reactor divertor. MPEX has multiple systems that must be precisely aligned to the plasma axis, including an electron cyclotron heating system that emits up to 400 kW of microwave power into the vacuum vessel. Five distinct systems require precise alignment on the MPEX device, thus requiring four bellows, all of which are adjacent to the plasma at a relatively high heat flux of approximately 47 kW/m2 and microwave power regions. The MPEX high heat flux bellows (HHFB) is designed to deliver 6 degrees of freedom positioning. The HHFB includes titanium–zirconium–molybdenum (TZM) inserts that are brazed into a Glidcop AL-15 body using a high-temperature braze alloy, thus blocking direct line of sight to an edge-welded bellows from the plasma and microwave screen to block microwaves from the bellows. A custom ConFlat knife edge is machined into the Glidcop AL-15, so the vacuum flanges do not need a braze or weld joint on the vacuum interface. Fingerstock or copper mesh is used to restrict microwave power from entering the interstitial space between the water-cooled Glidcop AL-15 body and edge-welded bellows. Glidcop AL-15 was selected as the material of choice for the water-cooled body because it can maintain mechanical integrity at elevated temperatures, and it also allows for a high-temperature braze. TZM was selected for its machinability and compatibility with vacuum and plasma requirements. A test article is also being considered for a similarly shaped component, the MPEX limiter, that will demonstrate the integrity of the braze joint under high thermal load. Results from this testing will be extrapolated to deduce the lifetime and integrity of the HHFB design.

Magnetized plasma pressure filaments: Analysis of chaotic and intermittent transport events driven by drift-Alfvén modes

The origin of intermittent fluctuations in an experiment involving several interacting electron plasma pressure filaments in close proximity, embedded in a large linear magnetized plasma device, is investigated. The probability density functions of the fluctuations on the inner and outer gradient of the filament bundle are non-Gaussian and the time series contain uncorrelated Lorentzian pulses that give the frequency power spectral densities an exponential shape. A cross-conditionally averaged spatial reconstruction of a temporal event reveals that the intermittent character is caused by radially and azimuthally propagating turbulent structures with transverse spatial scales on the order of the electron skin depth. These eruption events originate from interacting pressure gradient-driven drift-Alfvén instabilities on the outer gradient and edge of the filament bundle. The temporal Lorentzian shape of the intermittent structures and exponential spectra are suggestive of deterministic chaos in the underlying dynamics; this conclusion is supported by the complexity–entropy analysis (CH-plane) that shows the experimental time series are located in the chaotic regime.