Yoshi Hirooka

Laboratory experiments on cluster/aerosol formation by colliding ablation plumes

First-of-a-kind experiments on cluster/aerosol formation by colliding ablation plumes have been conducted, radiating Al, Cu and C with 3ω-YAG laser at power densities between 2~30 J/cm2/pulse. Visible spectroscopy indicates that the excitation light intensities of Cu and Al plumes are not necessarily be doubled in collision, but can rather be weakened due to atomic and molecular reactions. For colliding C plumes, Swan band radiation has been observed, indicative of C2 and/or C2+ formation, and ion mass spectrometry has identified Cn+-clusters, including C+, C2+, C3+, C4+ and C5+. From ICCD camera observations, C plumes generated at power densities above ~15 J/cm2/pulse tend to split into two components with respective velocities, only the slow component of which appears to be interactive to form clusters. Nano structures like CNT have been identified in deposits from colliding C plumes.

Interpenetration and stagnation in colliding laser plasmas

We have investigated plasma stagnation and interaction effects in colliding laser-produced plasmas. For generating colliding plasmas, two split laser beams were line-focused onto a hemi-circular target and the seed plasmas so produced were allowed to expand in mutually orthogonal directions. This experimental setup forced the expanding seed plasmas to come to a focus at the center of the chamber. The interpenetration and stagnation of plasmas of candidate fusion wall materials, viz., carbon and tungsten, and other materials, viz., aluminum, and molybdenum were investigated in this study. Fast-gated imaging, Faraday cup ion analysis, and optical emission spectroscopy were used for diagnosing seed and colliding plasma plumes. Our results show that high-Z target (W, Mo) plasma ions interpenetrate each other, while low-Z (C, Al) plasmas stagnate at the collision plane. For carbon seed plasmas, an intense stagnation was observed resulting in longer plasma lifetime; in addition, the stagnation layer was found to be rich with C2 dimers.

Hydrogen Plasma-Driven Permeation Through a Reduced Activation Ferritic Steel Alloy F82H

Hydrogen plasma-driven permeation (PDP) experiments have been conducted, using a steady state linear plasma device for the membranes made of reduced activation ferritic steel (F82H) and stainless steel (SUS304). The steady state PDP ratios have been measured to be of the orders of 10-3 and 10-4 at ~220 °C for 1 mm thick F82H and SUS304 membranes, respectively. For F82H, the steady state PDP flux ratio has been found to be inversely proportional to membrane thickness at ~220 °C, indicating that permeation is diffusion-limited. From the temperature dependent PDP data for F82H an activation energy has been evaluated to be ~0.5 eV.

Plasma- and Gas-Driven Hydrogen Isotope Permeation Through the First Wall of a Magnetic Fusion Power Reactor

Abstract The first wall of a magnetic fusion power reactor is defined essentially as the plasma-facing walls of blankets. For the high temperature operation of self-cooled breeder blankets, the first wall is often designed to be less than 1cm thick to reduce mechanical stresses and as a result will be subjected to bi-directional hydrogen permeation by two distinctive mechanisms; in one direction by edge plasma-driven and in the other direction by bred tritium gas-driven permeation. Using a laboratory-scale plasma device and a one-dimensional diffusion model, plasma-driven and gas-driven hydrogen permeation behavior has been investigated under some of the conditions relevant to FLiBe-employed blankets. For a 5mm F82H membrane, the plasma-driven permeation flux at ~500 °C and the gas-driven hydrogen permeation flux at ~350 °C have been measured to be of the orders of 1013 H-atoms/cm2/s and 1014 H-atoms/cm2/s, respectively. From these data one predicts that gas-driven permeation could dominate the hydrogen isotope transport through the first wall.

Proof-of-principle experiments on the concept of moving-surface plasma-facing components—hydrogen recycling over a titanium-gettered rotating drum

In the magnetic fusion community much attention has recently been directed to particle control in the existing confinement experiments, as it is recognized that the core plasma performance can directly be affected by the edge conditions. However, it is still true that there is no clear strategy to cope with the needs for steady-state reactor operation. That is that, due to the saturation nature, wall conditioning such as boronization is not applicable for fuel and impurity particles control in steady-state devices, requiring a start from enabling concepts development. As a possible solution, the concept of moving-surface plasma-facing components (MS-PFCs) was proposed in our previous work and in the present work proof-of-principle experiments have been conducted. In this concept, the plasma-facing surface is mechanically circulated out-of-pile for regeneration of particle capturing capabilities by coating getter films on it. A prototypical MS-PFC test unit has been constructed for these proof-of-principle experiments, employing a rotating copper drum as the moving surface and titanium as the getter material. It has been indicated that, from Hα light intensity data taken in front of the rotating target, relative to no getter cases, approximately 6% reduced hydrogen recycling has been achieved at steady state. In support of these data, a first-order particle balance model predicts a 7% reduction in hydrogen recycling.

Carbon Plume Stagnation: Platform for Vapor Shield Study

Abstract Laser ablation scheme can cover pretty wide range of intensity regime as a heat source at its laser focus spot from 103 W/cm2 to 1014 W/cm2. These intensities cover the ones expected at the divertor (MFE) and the first walls (IFE) in a reactor. For example expected values are of 10 to 100 MW/m2 at MFE divertor and 109 W/cm2 or higher at IFE first walls. The ablation may include plasma, gas, liquid, or solid: all possible phases mixed at an extreme condition where temperature may exceed 1 eV with corresponding densities. The areas of these mixed phases at extreme conditions (MPEC) have not been systematically studied. The inside of the solid wall becomes so called “Warm Dense Matter” where the details of the states should still be clarified. In our experimental setting up, the ablated plumes can be aligned orthogonally and can cross each other. The collision processes include Coulomb, elastic, molecular, and cluster collisions at the cross point. The characteristics of this experimental platform are introduced and attractive application is indicated.

Aerosol Formation and Hydrogen Co-Deposition by Colliding Ablation Plasma Plumes of Carbon

Abstract Along with pellet implosions, the interior of an inertial fusion reactor will be exposed to intense and short pulse power fluxes, leading to materials ablation. Ablated materials will either collide with each other in the axis-of-symmetry region or be re-deposited elsewhere in the target chamber. The present work is intended to investigate the behavior of colliding ablation plasma plumes and that of materials re-deposition in hydrogenic atmosphere. Laser-ablation plasma plumes of carbon are set to collide with each other in a laboratory-scale experimental setup. Results indicate that carbon cluster ions are formed, including C2+ C3+ C4+ C5+ and C6+, some of which grow into aerosol in the form of micro/nano carbon structure. Also, it has been found that ablated carbon and hydrogen can form co-deposited layers with the H/C ratio, reaching the order of 0.1.

Lithium-Gettered Moving Surface Plasma-Facing Components for Particle Control in Steady State Magnetic Fusion Devices

Abstract In our previous work, the first proof-of-principle experiments were successfully conducted on the particle control capability based on the concept of moving-surface plasma-facing component (MS-PFC). Over a continuously titanium-gettered rotating drum, hydrogen recycling was found to be reduced down to levels around 94% even at steady state. These experiments on the MS-PFC concept have now been extended to the second stage where lithium is employed as the getter material, while using the same rotating drum. These experiments are intended to pilot the potential use of lithium as a flowing liquid facing the edge plasmas in steady state reactors beyond ITER. Reported in this paper are rather dramatic findings that hydrogen recycling is reduced down to levels around 76% and 86% at steady state over the rotating drum at the lithium deposition rates of 9.5 Å/s and 7.3 Å/s, respectively. These steady state recycling data have been nicely reproduced by a simple zero-dimensional particle balance model.

Modeling of materials mixing behavior among beryllium, carbon and tungsten as plasma-facing materials under deuterium and tritium plasma bombardment

Abstract If in-vessel components in a magnetic fusion device employ two or more different plasma-facing materials, materials mixing can occur due to the impurity transport along with erosion and redeposition. Materials mixing alters surface characteristics, which in turn affects plasma interactions behavior with in-vessel components. Currently, the use of beryllium, carbon and tungsten as plasma-facing materials is under consideration for in-vessel components for the International Thermonuclear Experimental Reactor (ITER). However, there is no standard model applicable to analyze the materials mixing behavior. In this study a zero-dimensional materials balance model has been applied to analyze the behavior of deposition and removal of plasma impurities over in-vessel components under deuterium and tritium (DT) particles bombardment for the purpose of evaluating the possibility of materials mixing via impurity deposition. It is found that materials mixing between beryllium and carbon is likely whereas tungsten tends to remain without beryllium and carbon impurity deposition. In contrast, the model predicts that beryllium and carbon plasma-facing components will be highly susceptible to the deposition of tungsten impurities.

Moving-Surface Plasma-Facing Components for Particle Control in Steady State Magnetic Fusion Devices

Abstract This paper will report on the proof-of-principle (POP) experiments conducted to demonstrate reduced wall recycling, using a laboratory-scale test unit, constructed based on the concept of moving-surface plasma-facing component (MS-PFC). In this concept, the moving-surface exposed to edge plasmas in steady state magnetic fusion devices is continuously deposited ex-situ with a getter material, so that particle trapping capabilities can be regenerated prior to the subsequent exposure. In our previous paper, the construction details of the MS-PFC test unit and the first results in the case of titanium gettering was reported, but in the present paper preliminary results in the case of lithium gettering will be presented for comparison. Results indicate that the Hα light intensity used as the measure of hydrogen recycling is reduced by ~6% due to titanium gettering and by ~12% due to lithium gettering, both at steady state.