Linear Plasma Device Research Articles

Utilization of D2 molecular band emission for electron density measurement

D2 molecular band emission observed at a wavelength range of λ∼557−643nm is utilized to measure electron density, ne, in D plasmas of the PISCES-A and PISCES-RF linear plasma devices. The D2 band is divided at λ=593nm to make an intensity ratio, D2L (∼557–593nm)/D2R (∼593–643nm), where D2L consists predominantly of the g3Σg+3dσ, h3Σg+3sσ, i3Πg3dπ, j3Δg3dδ→c3Πu2pπ transitions, while D2R mainly includes the d3Πu3pπ→a3Σg+2sσ Fulcher band emission. It is experimentally found that D2L/D2R depends strongly on ne with little Te dependence in ranges of ne∼(0.031−6.1)×1018 m−3 and Te∼2.3−13.9eV. This observed trend is consistent with collisional-radiative model calculations using Yacora on the Web.

Two-dimensional magnetic field diagnostics of plasma based on nano-thin-film probe

In this paper, we propose a method to determine the diagnostics of magnetic fields based on arrays of nano-thin-film probes that are manufactured by using magnetron sputtering-based fabrication and arranged in a two-dimensional (2D) topological design. Measurements of the magnetic field and its wave number based on the proposed method were validated in a linear cylindrical plasma device, the linear experimental advanced device. We manufactured nano-thin-film probes, each with a thickness of 500 nm, a diameter of 9 mm, and a shape similar to the Greek letter “omega” by depositing silver nanoparticles generated through magnetron sputtering to improve the accuracy of the fabrication as well as their time response. The 2D topological arrangement of the array of probes enabled the determination of the diagnostics of the magnetic field and its wave number at a high spatial resolution. Measurements of the amplitude of the magnetic field and its wavenumber obtained using the proposed method were in good agreement with the results of theoretical simulations in COMSOL, which verifies its high reliability and accuracy in obtaining low-pressure plasma diagnostics. In future work, we plan to apply our diagnostic method based on the array of thin-film probes to scenarios that require a high spatial resolution.

Research on the characteristics of the Helium plasma beam in HIT-PSI

As a high heat flux linear plasma device designed for studying divertor materials in future fusion reactors, HIT-PSI(Plasma Surface Interaction device at Harbin Institute of Technology) has been successfully constructed and has maintained stable operation since its completion. The characteristics of He plasma beams in HIT-PSI are investigated by emission spectroscopy and an infrared camera, with preliminary irradiation experiments conducted by bombarding tungsten with the beam. For relatively conservative discharge parameters, HIT-PSI achieved a steady-state heat flux capacity of ∼40 MW/m2 using infrared measurements, with the full width at half maximum (FWHM) of the heat flux beam reaching 4 mm. These characteristics make HIT-PSI an advanced platform for testing divertor materials and plasma-facing components, providing essential experimental supports for research and development of high-performance divertor materials.

Deuterium plasma induced preferential erosion in ultra-high temperature ceramics TiB2 and ZrB2*

Abstract Steady-state deuterium plasma exposures were performed on ultra-high temperature ceramics titanium diboride (TiB2) and zirconium diboride (ZrB2) using the PISCES-RF linear plasma device (LPD) as early screening for first wall, plasma-facing applications. Deuterium plasma exposures were performed using 40 eV ion energies at 240, 525, and 800 °C sample temperatures and 90 eV ion energies at 240 °C sample temperatures to analyze TiB2 and ZrB2 surface morphology and chemistry evolution behavior. Post-plasma exposure chemistry characterization of the near surface ( < 50 nm) region of the samples all show transition metal enrichment, indicating boron preferential erosion. Transition metal to boron fractions vary with plasma exposure temperature under the 40 eV ion energy; metal enrichment is maximized at 800 °C and then minimized at 525 °C. SEM micrographs of all plasma exposed sample surfaces show no significant or noticeable plasma induced damage from cracking or blistering.

First spectroscopic studies in the plasma-beam installation

This paper presents the results of spectroscopic measurements of plasma in a linear simulator of a plasma-beam installation (PBI) for conducting materials science research. For the first time, non-contact optical diagnostic methods were employed to measure the plasma parameters in the PBI. These measurements were allowed for the clarification of the PBI plasma parameters and the identification of transition zones in different operational modes. Analysis of the plasma emission spectrum enabled the identification of impurity spectral lines and the estimation of key plasma parameters under varying experimental conditions. The spectra were recorded using two optical spectrometers covering a wavelength range of 200–800 nm. The experimental conditions varied in terms of electron beam accelerating voltage (1–5 kV), working gas pressure (1–6 mTorr), and target bias voltage (from −500 to −100 V). The Boltzmann plot method and Stark broadening of the Balmer lines were used to estimate electron temperature and density. Based on the obtained spectroscopic data, the electron density and the electron temperature were determined. The methods described in this study are applicable to linear plasma devices.

Assessments of the key plasma parameters with different scenarios on HIT-PSI using EMC3-EIRENE

Linear plasma devices offer an effective way to conduct plasma-wall interaction studies and contribute to a basic understanding of edge plasma physics. A new platform at Harbin Institute of Technology for Plasma Surface Interaction experiments (HIT-PSI) is a newly-built linear device at the stage of commissioning that is capable of simulating high heat power deposition on divertor targets similar to tokamak conditions. Therefore, numerical simulations to evaluate the plasma characteristics are essential for designing and guiding the experimental conditions in HIT-PSI. In this work, the three-dimensional edge transport code EMC3-EIRENE has been used to investigate the plasma parameter distributions in HIT-PSI with the puffing and pumping systems involved. The effects of the heating power and target position on the distribution of electron density, electron temperature, and particle and heat fluxes have been investigated by EMC3-EIRENE. Particularly, the reduction in the electron density with the puffing fluxes has also been studied by analyzing individual atomic and molecular processes. Finally, the influence of varying pumping speeds on plasma parameters has been investigated in detail by adjusting the recycling coefficients at the two pumping ports.

Impurity transport in PISCES-RF*

Abstract Linear plasma devices (LPD) utilizing a helicon plasma source, a high density light ion source, can generate impurities due to progressive erosion of the radio frequency (RF) transmission window caused by rectified sheath voltage. These source-born impurities can entrain and be transported by the plasma toward a target, affecting plasma-material interaction studies. Earlier work on material testing in Prototype-Materials Plasma Exposure eXperiment at ORNL revealed significant source impurity deposition on downstream targets. However, using a similar RF source, no target impurity deposition is observed in Plasma Interaction Surface Component Experimental Station (PISCES)-RF despite evidence of RF window erosion in the source region, thereby motivating the present work. Experimentally, using various magnetic field configurations upstream of the PISCES-RF plasma source and seeding titanium (Ti) impurities at various axial locations, impurity transport and deposition along the machine axis were investigated. It was found that Ti deposition was localized to the side of the plasma source where the Ti impurity was seeded. In contrast, aluminum (Al) deposition, originating from the sputtering of the helicon window, occurred predominantly upstream of the plasma source, suggesting an asymmetry in the axial transport of eroded RF window material. These observations suggest a stagnation of the parallel plasma flow immediately downstream of the plasma source, with impurity ions remaining unmagnetized near the source upstream. Al deposition in magnetic field-free regions in PISCES-RF indicates that sputtered Al impurities likely remained neutral due to their large ionization mean-free path under PISCES-RF conditions. Plasma modeling and simulation supported this, indicating that Al-neutrals transport toward the helicon source upstream for low electron density cases. It was found that the Larmor radius of the Al ions was greater than the plasma radius towards the source upstream and remained weakly magnetized in PISCES-RF, meaning that plasma source-born impurities are not efficiently entrained in the plasma flow. These findings provide critical insights into impurity transport in helicon plasma-based LPDs.

Design of a 3D-printed liquid lithium divertor target plate and its interaction with high-density plasma

A liquid Li divertor is a promising alternative for future fusion devices. In this work a new divertor model is proposed, which is processed by 3D-printing technology to accurately control the size of the internal capillary structure. At a steady-state heat load of 10 MW m−2, the thermal stress of the tungsten target is within the bearing range of tungsten by finite-element simulation. In order to evaluate the wicking ability of the capillary structure, the wicking process at 600 °C was simulated by FLUENT. The result was identical to that of the corresponding experiments. Within 1 s, liquid lithium was wicked to the target surface by the capillary structure of the target and quickly spread on the target surface. During the wicking process, the average wicking mass rate of lithium should reach 0.062 g s−1, which could even supplement the evaporation requirement of liquid lithium under an environment > 950 °C. Irradiation experiments under different plasma discharge currents were carried out in a linear plasma device (SCU-PSI), and the evolution of the vapor cloud during plasma irradiation was analyzed. It was found that the target temperature tends to plateau despite the gradually increased input current, indicating that the vapor shielding effect is gradually enhanced. The irradiation experiment also confirmed that the 3D-printed tungsten structure has better heat consumption performance than a tungsten mesh structure or multichannel structure. These results reveal the application potential and feasibility of a 3D-printed porous capillary structure in plasma-facing components and provide a reference for further liquid−solid combined target designs.

Hydrogen isotope effects on recombination dominant plasmas in NAGDIS-II

The detachment processes of the hydrogen (H) and deuterium (D) plasmas are comparatively investigated in the linear plasma device NAGDIS-II. The laser Thomson scattering measurements demonstrate that the recombination rate of the H plasma is greater than that of the D plasma as the neutral pressure increases in the molecular activated recombination (MAR) dominant detachment phase. As the recombination process by MAR is strongly dependent on the vibrational and rotationally excited states of the molecule, the rovibrational quantum state populations of the H and D molecules are measured using the Fulcher-α band spectroscopy. The results indicate that the vibrational temperature in the electronic ground state is considerably higher than the rotational temperature during detachment. The reaction rate coefficients for MARs due to charge exchange chains (CX-MAR) and dissociative attachment chains (DA-MAR) are calculated by the collision-radiation model under the measured temperature conditions. It can be observed that the CX-MAR is larger than the DA-MAR for both H and D, and that the CX-MAR of H is larger than the CX-MAR of D at electron temperatures T e above 1 eV. In consideration of the experimentally observed vibrational and rotational excitation temperatures, the reaction rate coefficients of CX-MAR and DA-MAR are increasing in the low T e region. These calculations are in accordance with the experimental results, which indicate that recombination processes due to MAR are more predominant in the H plasma compared to the D plasma. Furthermore, a transition from MAR to electron–ion recombination processes is observed in the D plasma at T e below 0.5 eV.

Development of a fixed Langmuir probe system for newly installed tungsten monoblock lower divertors in KSTAR

A fixed Langmuir probe system for newly installed tungsten monoblock lower divertors in KSTAR is developed. The probe structure is designed to effectively cooled via divertor coolant using inclined contact surfaces between the probe components. The probe body is tapered, narrowing up towards the divertor surface to enhance thermal contact conductance through mechanical forces exerted by the probe-divertor assembly. The probe structure is designed for easy replacement of a probe tip in the event of its damage and to accommodate different shapes of the tip. Currently, all the installed probe tips have 16° of inclination, i.e., one-sided rooftop shape, with respect to the surrounding divertor surface protruding 1.0 mm from the divertor surface. An individual probe system occupies two mono-blocks along the direction of the coolant channel. A single divertor cassette can accommodate 52 probes having a nominal poloidal spatial resolution of 12 mm. In the newly installed KSTAR divertor system, a total of 104 probes are installed at two toroidal locations. Thermal analysis on the probe structure indicates that the one-sided rooftop-shaped probe tips can withstand a localized heat flux of 10 MW/m2 for 2 s and 5 MW/m2 for steady-state operation. Experiments with a probe-divertor mockup are conducted in a linear plasma device under a magnetic field (|B|<360 G) and steady-state plasmas of a density of the order of 1011 cm−3. Possible analysis models for the I–V characteristics are discussed with practical applications to the measured data from KSTAR.

First results of multi‐fluid modeling of detached hydrogen plasmas in a linear plasma device using fluid code LINDA‐NU

AbstractIn order to simulate hydrogen (H) plasma in the linear plasma device NAGDIS‐II, we have modified the fluid code LINDA‐NU to allow the simultaneous calculation of multiple ion species consisting of hydrogen atomic ions () and molecular ions (). In this simulation, H and neutrals are assumed to be uniformly distributed in space in order to obtain initial qualitative results. The fraction of ions increases as the molecular density increases, and the recombination process between and electrons is observed to reduce the particle flux to the target plate. With an increase in H density, the electron density increases due to the decrease in ion flow velocity due to the change exchange process, and the electron temperature decreases to less than 1 eV, leading to the detached plasma formation attributed to the electron‐ion recombination process.

Deuterium retention in co-deposition with lithium in Magnum-PSI: experimental analysis and comparison with SOLPS-ITER

The vapor-box, a liquid metal design for the divertor, utilizes lithium recirculation through evaporation and condensation. Safety concerns arise from Li-D/T formation and co-deposition on vapor-box walls and the first wall, affecting tritium retention. Additively manufactured tungsten capillary porous structure (CPS) samples with Li were exposed to high heat flux D plasmas in the linear plasma device Magnum-PSI, to study D retention in Li–D co-deposition dependence on substrate temperature (200 ∘ C–428 ∘ C) and distance from the plasma beam center (25–85 mm). The D:Li ratio was determined via in-situ ion beam diagnostics with simultaneously analyzed Nuclear Reaction Analysis and Elastic Backscattering Spectroscopy spectra to maximize the precision. Experimental results approach close to the theoretical maximum at 40:60 D:Li ratio and deposited film thickness ranging from 0.02 to 3.2 µm. Witness plate temperatures above 400 ∘ C yielded Li films under 150 nm in thickness with lower D:Li ratios (5:95 D:Li ratio). At this temperature LiD decomposition pressure is comparable with vessel pressure during plasma. SOLPS-ITER simulations narrowed CPS surface temperature to 650 ∘ C–700 ∘ C, indicating Li+ plasma dominance near the target surface. Redeposition ratio of lithium on the CPS surface was determined to be around 80 % , matching quartz crystal microbalance results. However, SOLPS-ITER simulations lacked accuracy in recreating observed Li and D deposition layers on WPs, improvements are needed to model plasmas with significant Li quantities. Extension of SOLPS-ITER simulations to include LiD molecules and enhance heat flux accuracy is crucial for better alignment with experimental data.

Characterization of cascaded arc He plasma in a compact linear plasma device using voltammetry and optical emission spectroscopy

Cascaded arc plasma has been widely applied in linear plasma devices (LPDs) to produce high flux plasma for the study of plasma-material interaction. In this work, cascaded arc He plasma produced in an LPD with a compact arrangement is investigated by voltammetry and optical emission spectroscopy (OES). The results show that the cathode potential increases with the discharge current while it firstly decreases and then increases as increasing the gas flow rate. A local reverse electric field is observed at low gas flow rates between two cascaded plates (i.e. floating electrodes) near the cathode. The OES’ results reveal that as the gas flow rate increases, the intensity of He I lines increases and the electron excitation temperature (T exc ) decreases. As increasing the discharge current, the intensity of He lines exhibits various trends at different gas flow rates, showing a monotonic decline at 1.94 slm and a first increase followed by a reduction at 3.52 slm. The T exc increases with the discharge current. These findings could preliminarily shed light on the properties of cascaded arc of He plasma in the compact LPD and aid in the optimization of the device to generate the high-flux divertor-relevant plasma.

Study of Nanostructured Layer Growth on Plasma-Facing Materials Irradiated with Helium Plasma

Tungsten and stainless steel samples were irradiated with stationary helium plasma in the plasma linear multicusp plasma device. The surface of the material is modified under the influence of helium plasma with the formation of nanostructures and microstructures on the surface. The fluence of helium ions equal to 8 × 1027 ions/m2 was achieved on the tungsten sample. Depending on the helium ion fluence, fuzzlike layers, loops, and bubbles of 20- to 500-nm scale were formed on the tungsten surface. The fuzz layer thickness depends on the duration of plasma irradiation in a wide range of fluence. Saturation of the growth of the thickness of the tungsten fuzz layer was observed at a fluence of more than 8 × 1026 ions/m2. The growth of microstructures and nanostructures on the surface of stainless steel irradiated with helium plasma was observed. The growth of nanostructured layers is explained by a theoretical model considering the dynamics of adatoms under the influence of plasma.

Four-dimensional conditional averaging tomography of rotating plasma ejection from cylindrical detached plasma

Detached plasma formation is a way to reduce the heat load on the wall in magnetic fusion devices. This study proposes a novel analysis technique consisting of the conditional averaging, sliding window, and tomography to reveal the spatiotemporal behavior of the rotating radial ejection event of detached plasma, which further contributes to local heat load reduction. The used equipment is a high-speed camera and an electrostatic probe located at the periphery of the linear plasma device NAGDIS-II. By applying this method, four-dimensional (4D) behavior of the emission structure along time (1D) and space perpendicular and parallel to the magnetic field (3D) was clarified; a rotating distorted structure appears as a precursor, which is then scraped and transported radially and axially. The proposed method is widely applicable to short-term rigid-body rotating structures, especially in linear plasmas.

Observation of tantalum deposition and growth on TiB2 and ZrB2 from PISCES-RF deuterium and helium plasma exposures

Deuterium and helium plasma exposures on bulk TiB2 and ZrB2 samples were performed using the PISCES-RF linear plasma device. 40 and 90 eV deuterium ion plasma exposures were performed at 240 and 800 °C sample temperatures, and 80 eV helium ion plasma exposures were performed at 800 °C sample temperatures. Following plasma exposures, it was discovered that two plasma conditions (90 eV deuterium and 80 eV helium at 800 °C) resulted in thick (>200 nm) tantalum-rich (>10 at%) surface features on the targets, presumably from tantalum sourced from a tantalum adapter mask or cap used as part of the target holder. This work aims to characterize these tantalum-rich features and examine the mechanisms of impurity deposition.Plasma-induced surface morphology of the tantalum-rich surface layers depends on plasma properties and target temperature and chemistry. Greater titanium sputtering compared to zirconium resulted in more distinct surface features in the TiB2 samples compared to the ZrB2 samples via increased, prompt deposition onto tantalum surface impurities. There is still uncertainty as to why thick tantalum deposition only occurred under some plasma exposure conditions but not others; it is likely due to tantalum sputtering by a combination of boron molecules from the targets and carbon-impurities in the tantalum mask or targets. Impurity driven surface features are a well-documented phenomena in samples exposed to plasma from linear plasma device facilities—this work confirms the occurrence of this and emphasizes the need for chemistry characterization of isolated post-mortem surface features in plasma-exposed samples.

High-parameters experimental linear plasma device for fusion wall conditioning studies

In future, HELPD would provide valuable platform for investigating the issue in the W impurity control, towards the wall conditioning requirements in tokamak.The tungsten (W) impurity caused by sputtering of the first wall is considered as an issue influencing the plasma safe operation of EAST, ITER and other future fusion devices. As a supplement to the experiments on the tokamak, the linear plasma device (LPD) serves as an effective divertor simulator for the W impurity control research. This work presents the design and construction of High-parameters Experimental Linear Plasma Device (HELPD) in detail, including its layout and operation parameters. Besides the function of plasma exposure, the target sample could be exchanged in a separate material pretreatment vessel for wall conditioning, such as lithium coating and glow discharge cleaning (GDC) without breaking vacuum conditions. And the preliminary results show that the device could produce a steady state plasma of 1∼10 eV, and its flux could reach 1022 m−2s−1 with 10 kW helicon wave. In the future, HELPD would provide valuable platform for investigating the issue in the W impurity control，towards the wall conditioning requirements in tokamak.

Observation of the low electron density and electron temperature in an unmagnetized cascaded arc helium plasma by laser Thomson scattering approach

In this study, the electron density (ne ) and temperature (Te ) in an unmagnetized cascaded arc helium (He) plasma are precisely determined using cutting-edge laser Thomson scattering. In our experimental scope, ne is only 1018 m−3 and Te is less than 0.2 eV, both of which are substantially lower than in linear plasma devices (LPDs). The comparison of ne and Te values in He plasma with those in cascaded arc Ar plasma reveals that these two parameters are likewise significantly lower in He plasma than they are in Ar plasma on average. In comparison to Ar gas, the degree of ionization of He is low due to its high ionization potential, and diffusive loss dominates due to its light weight, both of which result in a lower ne . Meanwhile, these two characteristics render the three-body recombination interaction between electrons and He+ ions in He plasma insignificant, thus the electrons cannot be heated effectively, explaining why Te is lower. This study will provide foundational data and build the groundwork for a thorough knowledge of cascaded arc He plasma in LPDs and plasma windows.

D retention in e-beam powder-bed fused (3-D printed) tungsten exposed to high-flux deuterium plasma in Pisces-RF

Tungsten targets produced by the additive manufacturing (AM) method of electron-beam powder-bed fusion, or 3-D metal printing, are exposed to high flux D plasma in the Pisces-RF linear plasma device with the plasma-exposed surface normal to the AM build direction. D retention was measured by thermal desorption mass spectrometry following exposure to D plasma with an associated ∼50eV D+ ion flux. D+ fluence, and operational temperature, in the ranges 5×1024–5×1026 m−2 and 400–1000 K, are explored. D retention values for the AM W are compared to identically plasma exposed ’conventional’ sintered W and it is found that total D retention is similar. However, the D thermal release is notably different. Desorption from the AM W shows reduced D retention in traps typical of sintered W, and moderately increased trapping in defect types of higher trap release energy. The dependence of D retention on fluence is also different for the AM W, revealing an uptake slower than expected from Fickian diffusion, while that for sintered W is consistent and in agreement with previous poly-crystalline W results from Pisces-B. Hydrogen transport modeling of the fluence dependence suggests that interconnected pathways for D release back to the surface during plasma-exposure can account for the slower D uptake in the AM W.

Optical System Design for Laser-Induced Fluorescence Study of Helium Atoms on the Linear Plasma Device

According to the characteristic spectral lines of helium atoms, an optical system for the laser-induced fluorescence study of helium atoms has been designed. The design includes a helium spectral scheme and a laser injection and fluorescence collection system. The diode laser generates a 667.8-nm laser, and the laser is injected into the linear plasma device through an optical fiber. The fluorescence collection system detects 501.6-nm fluorescence signals. Experiments were carried out on the linear plasma device during helium discharge, simulating the helium ash environment at the boundary of the fusion reactor. The fluorescence collection was realized, and the fluorescence signals showed an increasing trend with laser power. Atomic density calibration and study will be performed based on the collected signals in the next step.