Preparation for a neutronics experiment using a discharge fusion device and an imaging plate neutron detector

Tritium breeding ratio (TBR) is one of the most important parameters determining the tritium self-sufficiency of a deuterium-tritium fusion reactor. A neutronics experiment is planned to measure the spectral neutron fluences two-dimensionally (2D) using a discharge-type compact fusion neutron source and a neutron imaging plate (NIP), respectively. We report a calibration method for the NIP and optimization of the discharge condition to enhance the neutron production rate. A linear relationship between the neutron fluence in the NIP and photostimulated luminescence (PSL) per area was obtained in the range of the neutron fluence from 10³ to 10⁷ n/cm². A neutron production rate higher than 10⁷ n/s was successfully achieved by the optimized discharge condition. It is shown that a quantitative 2D measurement by the NIP is feasible using the linear relationship and a correlation coefficient on the energy spectrum

Numerical Modeling of the Coagulation and Porosity Evolution of Dust Aggregates

Porosity evolution of dust aggregates is crucial in understanding dust evolution in protoplanetary disks. In this study, we present useful tools to study the coagulation and porosity evolution of dust aggregates. First, we present a new numerical method for simulating dust coagulation and porosity evolution as an extension of the conventional Smoluchowski equation. This method follows the evolution of the mean porosity for each aggregate mass simultaneously with the evolution of the mass distribution function. This method reproduces the results of previous Monte Carlo simulations with much less computational expense. Second, we propose a new collision model for porous dust aggregates on the basis of our N-body experiments on aggregate collisions. We first obtain empirical data on porosity changes between the classical limits of ballistic cluster-cluster and particle-cluster aggregation. Using the data, we construct a recipe for the porosity change due to general hit-and-stick collisions as well as formulae for the aerodynamical and collisional cross sections. Simple coagulation simulations using the extended Smoluchowski method show that our collision model explains the fractal dimensions of porous aggregates observed in a full N-body simulation and a laboratory experiment. Besides, we discover that aggregates at the high-mass end of the distribution can have a considerably small aerodynamical cross section per unit mass compared with aggregates of lower masses. We point out an important implication of this discovery for dust growth in protoplanetary disks.Comment: 17 pages, 15 figures; v2: version to appear in ApJ (typos corrected

Resting CD4+ T Cells with CD38+CD62L+ Produce Interleukin-4 Which Contributes to Enhanced Replication of T-Tropic Human Immunodeficiency Virus Type 1

AbstractA significant increase in the CD38+ population among T lymphocytes has been observed in human immunodeficiency virus type 1 (HIV-1)-infected carriers. We previously reported a higher replication rate of T-tropic HIV-1 in the CD4+CD38+CD62L+ than CD38− subset under conditions of mitogen stimulation after infection. Here, we revealed a similarly high susceptibility in the CD38+ subset on culture with conditioned medium containing Th2 cytokine, interleukin (IL)-4 that was produced endogenously from this subset on stimulation with mitogen or anti-CD3 antibody for 3 days. The contribution of IL-4 to the upregulated production of virus in the CD38+ subset was confirmed by culture of this subset with recombinant human IL-4. In contrast, the rate of replication in the CD38− subset was not augmented in the conditioned medium from either subset or with IL-4. However, there were no differences in the surface expression of IL-4 receptor or HIV-1 receptors CD4 and CXCR4 between the two subsets. Thus, the CD4+CD38+CD62L+ subset comprises a specific cell population secreting endogenous Th2 cytokine that contributes to the efficient production of T-tropic HIV-1 through upregulation at a certain stage of the viral life cycle, probably after the adsorption step

Evaluation of tritium production rate in a blanket mock-up using a compact fusion neutron source

We report a neutronics study of a blanket mock-up using a discharge-type compact fusion neutron source. Deuterium–deuterium fusion neutrons were irradiated to the mock-ups composed of tritium breeder and neutron reflector/moderator. The tritium production rate (TPR) per source neutron was measured by a single-crystal diamond detector with a 6Li-enriched lithium fluoride film convertor after the calibration process. Despite the low neutron yield, energetic alpha and triton particles via 6Li(n, t)α neutron capture as well as 12C via elastic scattering were successfully detected by the SDD with high signal to noise ratios. The TPRs were experimentally evaluated with errors of 8.4%–8.5% at the 1σ level at the positions with high thermal neutron fluxes where the errors were dominantly introduced by uncertainties in the monitoring of the neutron production rate. The calculated to experimental (C/E) values of TPR were evaluated to be 0.91–1.27 (FENDL-2.1) and 0.94–1.28 (FENDL-3.1). As the neutron source can generate 14 MeV neutrons using a mixed gas of deuterium and tritium, this approach provides more opportunities for blanket neutronics experiments

Electrostatic Barrier against Dust Growth in Protoplanetary Disks. I. Classifying the Evolution of Size Distribution

Collisional growth of submicron-sized dust grains into macroscopic aggregates is the first step of planet formation in protoplanetary disks. These grains are expected to carry nonzero negative charges in the weakly ionized disks, but its effect on their collisional growth has not been fully understood so far. In this paper, we investigate how the charging affects the evolution of the dust size distribution properly taking into account the charging mechanism in a weakly ionized gas as well as porosity evolution through low-energy collisions. To clarify the role of the size distribution, we divide our analysis into two steps. First, we analyze the collisional growth of charged aggregates assuming a monodisperse (i.e., narrow) size distribution. We show that the monodisperse growth stalls due to the electrostatic repulsion when a certain condition is met, as is already expected in the previous work. Second, we numerically simulate dust coagulation using Smoluchowski's method to see how the outcome changes when the size distribution is allowed to freely evolve. We find that, under certain conditions, the dust undergoes bimodal growth where only a limited number of aggregates continue to grow carrying the major part of the dust mass in the system. This occurs because remaining small aggregates efficiently sweep up free electrons to prevent the larger aggregates from being strongly charged. We obtain a set of simple criteria that allows us to predict how the size distribution evolves for a given condition. In Paper II (arXiv:1009.3101), we apply these criteria to dust growth in protoplanetary disks.Comment: 20 pages, 22 figures, accepted for publication in Ap

Electrostatic Barrier against Dust Growth in Protoplanetary Disks. II. Measuring the Size of the "Frozen" Zone

Coagulation of submicron-sized dust grains into porous aggregates is the initial step of dust evolution in protoplanetary disks. Recently, it has been pointed out that negative charging of dust in the weakly ionized disks could significantly slow down the coagulation process. In this paper, we apply the growth criteria obtained in Paper I to finding out a location ("frozen" zone) where the charging stalls dust growth at the fractal growth stage. For low-turbulence disks, we find that the frozen zone can cover the major part of the disks at a few to 100 AU from the central star. The maximum mass of the aggregates is approximately 10^{-7} g at 1 AU and as small as a few monomer masses at 100 AU. Strong turbulence can significantly reduce the size of the frozen zone, but such turbulence will cause the fragmentation of macroscopic aggregates at later stages. We examine a possibility that complete freezeout of dust evolution in low-turbulence disks could be prevented by global transport of dust in the disks. Our simple estimation shows that global dust transport can lead to the supply of macroscopic aggregates and the removal of frozen aggregates on a timescale of 10^6 yr. This overturns the usual understanding that tiny dust particles get depleted on much shorter timescales unless collisional fragmentation is effective. The frozen zone together with global dust transport might explain "slow" (\sim 10^6 yr) dust evolution suggested by infrared observation of T Tauri stars and by radioactive dating of chondrites.Comment: 14 pages, 13 figures, accepted for publication in Ap

Geometrical Cross Sections of Dust Aggregates and a Compression Model for Aggregate Collisions

Geometrical cross sections of dust aggregates determine their coupling with disk gas, which governs their motions in protoplanetary disks. Collisional outcomes also depend on geometrical cross sections of initial aggregates. In the previous paper, we performed three-dimensional N-body simulations of sequential collisions of aggregates composed of a number of sub-micron-sized icy particles and examined radii of gyration (and bulk densities) of the obtained aggregates. We showed that collisional compression of aggregates is not efficient and that aggregates remain fluffy. In the present study, we examine geometrical cross sections of the aggregates. Their cross sections decreases due to the compression as well as their gyration radii. It is found that a relation between the cross section and the gyration radius proposed by Okuzumi et al. is valid for the compressed aggregates. We also refine the compression model proposed in our previous paper. The refined model enables us to calculate the evolution of both gyration radii and cross sections of growing aggregates and reproduces well our numerical results of sequential aggregate collisions. The refined model can describe non-equal-mass collisions as well as equal-mass case. Although we do not take into account oblique collisions in the present study, oblique collisions would further hinder compression of aggregates

Evaluation of 3D printed buckyball-shaped cathodes of titanium and stainless-steel for IEC fusion system

An inertial electrostatic confinement (IEC) fusion device accelerates ions, such as deuterium (D) or tritium (T), to produce nuclear fusion and generate neutrons. The IEC's straightforward configuration consists of a concentric spherical transparent cathode at a negative bias surrounded by a grounded spherical anode. The effects of cathode properties on the neutron production rate (NPR) remain, to date, inadequately studied. This study aims to determine the impact of the cathode material on the NPR by investigating fusion reactions on the cathode surface. Two buckyball-shaped cathodes made of stainless steel (SS) and titanium (Ti), both of 5 cm diameter, fabricated by selective laser melting and 3D printing, are used for this investigation. A SS spherical chamber of 25 cm inner diameter is used as an anode in this experiment. A performance evaluation of surface fusion reaction in the IEC using SS and Ti grids is conducted by examining the NPR as a function of the applied voltage and grid currents at different gas pressures. So far, IEC with Ti and SS cathodes achieves NPRs of 2.32 and 1.41 × 10⁷n/s, respectively, at 5.6 kW (70 kV, 80 mA). The normalized NPRs (NPR/I-cathode) from IEC using SS and Ti cathodes are compared. The results demonstrate that fusion reaction occurs on the cathode surface, and fusion increases with the applied voltage. The measured NPR/I-cathode using the Ti cathode is higher than that of the SS cathode by factors of 1.36–1.64 across the 20–70 kV range. Moreover, fusion on the Ti cathode surface enhances the total NPR significantly compared to the SS cathode under the same conditions. The Ti's considerable ability to accumulate D ions and molecules compared with that of SS explains the difference of measured NPR results