Linear Divertor Plasma Simulator Research Articles

Evaluation of axial decay length of plasma pressure in detached plasma

Abstract This study proposes a method for evaluating the axial decay length of the plasma pressure in detached helium plasmas with a fixed-point laser Thomson scattering and spectroscopic measurements in the linear divertor plasma simulator NAGDIS-II. By controlling the neutral gas pressure, the highly volume-recombining region called the “recombination front” was moved along the magnetic field. The differential terms for the electron pressure and the recombination-front position with respect to the neutral pressure were obtained. By using these two terms, the decay length of the plasma pressure was qualitatively calculated. In addition, from the movable Langmuir probe measurement, difference between the locally estimated decay length around the recombination front and the decay lengths over a wide axial range was discussed. The proposed method can be applied even in the huge heat flux condition where direct-contact measurements such as the Langmuir probe are not available.

Investigation of recombination front region in detached plasmas in a linear divertor plasma simulator

Abstract We have investigated detailed plasma structure around the recombination front region (RFR), where volume electron-ion recombination (EIR) strongly occurs in detached plasmas, by using two-dimensional (2D) movable probe system as well as laser Thomson scattering diagnostics. The 2D distribution of electron density, electron temperature, and plasma potential around the RFR, has been revealed for the first time. The spatial distribution of the particle loss rate due to EIR estimated from electron temperature and electron density profiles indicated that the three-dimensional structure of the RFR adopts a cone structure. Furthermore, in the peripheral region of the RFR, the radial profile of the electron density was widened locally with increasing cross-field transport caused by plasma fluctuation. From the heat transport analysis of electrons, it was found that the thermal relaxation process between electrons and ions is important for the cooling of electrons, and that the convection of electrons regulates electron heat transport near the RFR.

Development of a LIBS system for in situ surface measurements during plasma exposure in PISCES-A.

A laser-induced breakdown spectroscopy (LIBS) system has been developed with a Q-switched Nd:YAG laser (wavelength = 1064 nm and pulse width ∼5 ns) to conduct in situ surface measurements during plasma exposure in the PISCES-A linear divertor plasma simulator. The LIBS signal enhancement is obtained with both the magnetic field normal to the surface of a target and steady-state background plasma. Migration of sputtered Ta impurities onto the neighboring Cr surface is identified during He plasma exposure, only when cone structures are formed on the Cr surface. D retention in W during D plasma exposure is observed to decrease with increasing the sample temperature. The temporal evolution of D outgassing from W is measured in a time range of ∼10-420 s right after D plasma exposure. A power law fit, t -α , yields α ∼ 0.34 ± 0.09, which is nearly consistent with calculated and measured values.

Microstructure and deuterium retention of beryllium co-deposition layer formed under high density plasma exposure

A systematic study of the temperature effect on the microstructure and the deuterium retention property in beryllium co-deposition layers has been carried out using W samples exposed to D+Be and D+Be+He mixture plasmas in the linear divertor plasma simulator PISCES-B. A deposition layer consisting of small grains of ∼10nm with original hexagonal close-packed structure of beryllium was formed on a sample exposed to D+Be mixture plasmas at low temperature of 373K. He seeding to the mixture plasmas was found to cause amorphization of the layer. In contrast, columnar structure consisting of Be2C due to crystal growth appeared at high temperature exposure cases of > 773K both with and without He seeding. The formation of these deposition layers brought about a significant D retention. Assuming all retained D atoms uniformly distribute within the deposition layers, the D/Be ratios are estimated to be about 0.05 for the low temperature exposure case of 573K, and to be about 0.01 even for the high temperature at 773K. In addition, post-irradiation of 3keV-D2+ for Be deposited sample demonstrated that the hydrogen isotope retention behavior can be modified drastically, once the Be deposition layer is formed.

Statistical Analysis of Particle Flux Flowing into the End‐Target in between Attached and Detached States in the Linear Divertor Plasma Simulator NAGDIS‐II

AbstractWe have investigated the particle flux flowing into the axisymmetric end‐target in the transient state from attached to detached divertor conditions in the linear plasma device NAGDIS‐II. In the transient state, a dramatic decrease of the mean particle flux and a large‐amplitude fluctuation with negative and positive spikes were observed. We have analyzed the fluctuation with a newly suggested analysis technique: pre‐multiplied cubic spectrum with the wavelet transform. Analysis result indicates that these spikes consist of a few kilohertz components. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling of Linear Divertor Plasma Simulator Experiments with Three‐dimensional Target Structure by Using EMC3‐EIRENE Code

AbstractWe have adapted the EMC3‐EIRENE code for modeling of a linear divertor plasma simulator in order to demonstrate plasma‐wall interactions with three‐dimensional (3D) effects. 3D distributions of hydrogen plasma and neutrals can be successfully calculated for four different types of target plates: a V‐shaped target, inclined targets with open and closed structures, and a planer target. Hydrogen atoms and molecules are accumulated more effectively in the V‐shaped target plate, leading to a higher electron density with lower electron temperature than the planar target plate. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Systematic investigation of the formation behavior of helium bubbles in tungsten

The formation of He bubbles in tungsten under exposure to a He plasma was systematically investigated using low energy (∼50eV) He+ ions with a wide fluence range (∼1023 to 1026m−2) in the linear divertor plasma simulator PISCES-A at several temperatures (523–973K). TEM observations after thinning exposed W samples with FIB revealed that the layer thickness (>30nm) of He bubbles largely exceeds the ion implantation range of a few nm. The size of He bubbles was found to increase with an increase in the sample temperature: it was around 10nm at 973K, while only small He bubbles of 1–2nm were observed at <773K. In addition, to obtain information on the initial formation behavior, in-situ TEM observations during He ion irradiation were also performed.

Study of Optimum Conditions for the V-Shaped Target of a Gas Divertor

Experimental simulation of a V-shaped target for a gas divertor via detached plasma formation is presented using a linear divertor plasma simulator, TPD-Sheet IV. Three geometries of the opposite plate on the V-shaped target were investigated with variation of the contact gas flow rate. The ionization and recombination events are discussed for each target configuration using the collisional-radiative (CR) model. The results expect that a gas divertor with a V-shaped target effectively enhances plasma recycling and detachment.

Ion Acceleration by Ion-Cyclotron Resonance in Non-Uniform Magnetic Field Using the TPD-Sheet IV Linear Divertor Simulator

Ion acceleration of high density sheet plasma (ca.1018 m-3) in a non-uniform magnetic field by ion-cyclotron resonance (ICR) is investigated in a linear divertor plasma simulator, TPD-Sheet IV. The radio frequency (RF) electrodes consist of two parallel plates. The ion energy along the axis of the magnetic field or in the perpendicular direction was measured using a Faraday cup. The experiment was conducted using helium gas and a discharge current of 50 A. The ion energy in the direction perpendicular to the magnetic field line increases by with ion-cyclotron resonance. Ions are also accelerated along the axis of the magnetic field line due to the magnetic field gradient along the axis.

Be I and Be II spectroscopy in divertor plasma relevant conditions

Abstract Intensity ratios of various Be I and Be II lines measured in Be-seeded D and He plasmas in the PISCES-B linear divertor plasma simulator are compared with the corresponding ratios of the photon emissivity coefficient, PEC, calculated by ADAS. Agreement of measured intensity ratios with calculated PEC ratios is satisfactory within a factor of ∼2 for both Be I and Be II. It is proposed that a Be I line ratio of 234.8 nm/265.0 nm and a Be II line ratio of 467.3 nm/313.1 nm can be used to estimate the electron temperature, while a 265.0 nm/332.1 nm Be I line ratio is sensitive to the electron density. Further, S/XB values of a Be I line at 457.3 nm were experimentally determined from a ratio of the sputtered Be flux to the emission intensity. Measured values are systematically lower than calculated ADAS values, which may be explained by the increased sputtering yield of redeposited Be atoms.

核融合ダイバータの閉構造化に関する模擬実験

In this study, we present the experimental simulation of the deivertor target geometry via detached plasma formation of hydrogen plasma in a linear divertor plasma simulator TPD-SheetIV. Three types of target geometry (V-shaped, oblique, and vertical targets) have been investigated. The eletron temperature (Te) and density (ne) of the plasma are measured with Langmuir probe. It is also intended to show that the observed hydrogen Lyman spectra could be explained by recombination processes. The V-shaped target enhances the recycling and reduction in heat load.

Effects of Steady-State Plasma Exposure on Tungsten Surface Cracking due to Elm-Like Pulsed Plasma Bombardment

Sequential exposures of W surfaces to steady-state and pulsed (∼0.5 ms) plasmas have been performed in a linear divertor plasma simulator and a magnetized coaxial plasma gun to investigate effects of D blisters, nano-sized He bubbles, and He-induced W fuzz on surface cracking by pulsed plasma loads. Surface cracks appeared on samples containing D blisters or He bubbles following 10 shots at ∼0.5 MJ/m2 per shot, while a mirror-polished sample with no pre-plasma exposure did not exhibit cracks after similar transient exposures. Note that the cracking is limited to the edge region for a sample with D blisters. This means that the energy density threshold for surface cracking is lowered by the existence of D blisters and, especially, He bubbles. On the other hand, it is found that fuzzy surfaces possess a good resistance to surface cracking, although arcing is prone to occur.

Beryllium erosion induced by transient heat loads and subsequent reactions in a deuterium plasma

Pulsed laser irradiation (∼0.1–4 MJ/m 2 at 1–10 ms) to a beryllium (Be) target has been performed under steady-state deuterium (D) plasma exposure in the linear divertor plasma simulator PISCES-B to investigate the response of Be to transient heat loads such as edge localized modes. Emission intensities from Be atoms and beryllium deuteride (BeD) molecules are observed in front of the Be target by using two fast framing cameras simultaneously; those exhibit similar time evolution to each other, but spatial profiles are different. While the Be I light emission peaks just in front of the target, the BeD emission peaks away from the target. This indicates that Be atoms are directly ejected from the surface, and BeD molecules are volumetrically formed in the plasma. It is also found that the time evolution of light emission can be qualitatively well reproduced by the evaporation flux of Be atoms, estimated from the calculated surface temperature. The Be surface is thought to be eroded as atoms due mainly to evaporation during laser irradiation.

Sputtering properties of tungsten ‘fuzzy’ surfaces

Sputtering yields of He-induced W ‘fuzzy’ surfaces bombarded by Ar have been measured in the linear divertor plasma simulator PISCES-B. It is found that the sputtering yield of a fuzzy surface, Yfuzzy, decreases with increasing fuzzy layer thickness, L, and saturates at ∼10% of that of a smooth surface, Ysmooth, at L>1μm. The reduction in the sputtering yield is suspected to be due mainly to the porous structure of fuzz, since the ratio, Yfuzzy/Ysmooth follows (1−pfuzz), where pfuzz is the fuzz porosity. Further, Yfuzzy/Ysmooth is observed to increase with incident ion energy, Ei. This may be explained by an energy dependent change in the angular distribution of sputtered W atoms, since at lower Ei, the angular distribution is observed to become more butterfly-shaped. That is, a larger fraction of sputtered W atoms can line-of-sight deposit/stick onto neighboring fuzz nanostructures for lower Ei butterfly distributions, resulting in lower ratio of Yfuzzy/Ysmooth.

Measurements of Mo I S/XB values

The number of ionization events per photon, i.e. S/XB value, of Mo I lines has been determined by taking a ratio of the sputtered Mo flux, ΓSptMo, to the line-of-sight integrated Mo I emission intensity in weakly ionized (ionization degree of ∼1–10%) He plasmas in the linear divertor plasma simulator PISCES-B under the following plasma parameters: electron density ne ∼ (0.6–1.3) × 1019 m−3, electron temperature Te ∼ 10–30 eV, ion flux Γi ∼ (0.9–1.2) × 1023 m−2 s−1 and incident ion energy to the Mo target Ei ∼ 60 eV. For these conditions, most of the sputtered Mo atoms are ionized in the plasma column. While measured S/XB values for z 5P° → a 5S (550.6, 553.3 and 557.0 nm) are systematically higher than theoretically calculated values provided in the Atomic Data and Analysis Structure database by a factor of ∼2–5, agreement for the other transitions, y 7P° → a 7S (313.3, 317.0 and 319.4 nm) and z 7P° → a 7S (379.8, 386.4 and 390.3 nm), is more satisfactory (within a factor of 2).

Experimental determination of S/XB values of W I visible lines

The dependence of the number of ionization events per photon, or so called S/XB value, on the electron temperature, Te, for W I lines at λ=400.8, 429.4, 498.2, and 505.3 nm has been experimentally determined by measuring the line emission of W atoms sputtered by Ar plasma bombardment in the PISCES-B linear divertor plasma simulator [R. P. Doerner et al., Phys. Scr. T 111, 75 (2004)]. The measured S/XB values at Te>10 eV are found to agree with theoretical calculations at an effective temperature TW=0.1–0.3 eV, which is treated as a free parameter and affects the population of ground states of W atoms. However, compared with previously reported experimental values of S/XB for the 400.8 nm line, our S/XB values are ∼5–10 times higher. At Te<10 eV, it is seen that the S/XB values increase with decreasing electron density, ne, even with correction of the geometrical loss flux. This indicates that there is a ne dependence of S/XB, which is also confirmed by comparison of W I line intensity ratios with calculat...

Observations of suppressed retention and blistering for tungsten exposed to deuterium–helium mixture plasmas

Blister formation and D retention in W have been investigated for low energy (∼55 ± 15 eV), high flux (∼1022 m−2 s−1), high fluence (⩽4.5 × 1026 m−2) ion bombardment at moderate temperature (∼573 K) in mixed species D+He plasmas in the linear divertor plasma simulator PISCES-A. The amount of D retained in W is found to decrease significantly when compared with that in W exposed to pure D plasmas, as measured with high resolution thermal desorption spectroscopy. Scanning electron microscopy observations reveal the suppression of the blisters, a surface feature known to drive up retention, in the D + He mixture plasma exposed W samples. Reduced D retention is accompanied by the formation of nano-sized high density He bubbles in the near surface, observed with a transmission electron microscope (TEM). It is believed that the nano-bubbles act as a diffusion barrier to implanted D atoms and consequently reduce the amount of uptake in the W material. This newly observed effect implies that current predictions of D retention in W, in actual fusion devices, may be overestimated, since there will be He ash in fusion plasma. Toughness enhanced, fine-grained (grain size of ∼1 µm) W–TiC samples, exposed to pure D plasma conditions, also show little or no evidence of blistering. The measured D retention in the W–TiC samples was approximately 1 × 1019 D m−2 corresponding to about 2 × 10−7 of the implanted D fluence, and is very low compared with the retention in pure stress relieved W, which exhibited surface blisters and had a D retention of about 1 × 1021 D m−2.

In situ reflectivity of tungsten mirrors under helium plasma exposure

The formation of micron-sized helium bubbles/holes on tungsten surface due to helium ion irradiation has been investigated in the linear divertor plasma simulator NAGDIS-II [1]. In order to investigate the surface variation during the helium plasma exposure, the optical reflectivity is measured by using a He–Ne laser and a photodiode. When the incident helium ion energy is 15 eV, a great amount of bubbles and holes were formed by the exposure with the ion fluence of 1.8 × 10 27 m −2. The degradation of the reflectivity is enhanced when increasing the incident ion energy. When the incident ion energy is 30–50 eV, the surface is covered with fiberform nanostructured tungsten. The reflectivity recovery effect by a ruby laser with a long pulse duration of ∼0.6 ms is demonstrated. The reflectivity recovers to the initial value when the laser pulse energy exceeds ∼1.2 MJ m −2.

Erosion yields of deposited beryllium layers

Erosion yields of various Be surfaces exposed to deuterium plasma, including polycrystalline Be (PC-Be), magnetron and thermionic vacuum arc deposited surfaces, and in situ plasma-deposited Be layers on Be (DP-Be/Be) and on graphite (DP-Be/C) targets, are measured in the linear divertor plasma simulator PISCES-B. It is observed that the enhanced erosion (∼3 times higher than PC-Be) of DP-Be/Be occurs at sample temperature T s ∼ 310 K, while DP-Be/Be possesses the same yield as PC-Be at a higher T s ∼ 570 K. The erosion yield of DP-Be/C is found to be slightly decreased at T s ∼ 850 K. This is consistent with the more effective formation of Be 2C at higher T s, which has a higher surface binding energy than Be. It has been identified from spectroscopic measurements that Be is also eroded as BeD.

Spectroscopic determination of the singly ionized helium density in low electron temperature plasmas mixed with helium in a linear divertor plasma simulator

The spectroscopic method is developed to obtain the He+ ion density nHe+ in low electron temperature, Te=5–20eV, plasmas mixed with He. Plasmas were produced in the PISCES-B linear divertor plasma simulator [R. P. Doerner et al., Phys. Scr. T111, 75 (2004)] where the electron densities are ne=(1−15)×1018m−3 and the ionization degree is ∼1–10%. In the method, the He I line intensity IHeI at λ=447.1nm is used, instead of the He II line intensity in the conventional method. The radial confinement time of He+ ions is requisite, and is measured to be at a level of the Bohm confinement time. The He+ ion concentration, nHe+∕ne, is found to be proportional to IHeI, and to weakly depend on ne and Te. Because of the higher ionization energy of He than other species (D2, Ne, and Ar), the measured nHe+∕ne becomes systematically lower than the He gas pressure fraction, and agrees with data from an omegatron mass spectrometer. The omegatron measurement and estimates of the He+ ion loss rates indicate that the influences of vibrationally excited deuterium molecules on the particle balance of He+ ions are small at Te⩾10eV.