Deuterium Gas Pressure Research Articles

Novel fast-neutron activation counter for high repetition rate measurements

A fast-neutron beryllium activation counter has been constructed for neutron measurements on a high repetition rate deuterium plasma focus. Beryllium activation is especially suitable for measurements of DD neutron yields. The cross section for the relevant reaction, Be9(n,α)He6, results in a maximum sensitivity at the characteristic energy of the DD neutrons (∼2.5MeV) and practically no sensitivity to neutrons with energies <1MeV. The short half-life (0.8s) for the decay of He6 enabled the shot-to-shot neutron yield from the plasma focus to be measured for repetition rates from 0.2to3Hz (and for a range of deuterium gas pressures). With careful analysis, the shot-to-shot yield can be measured up to a maximum repetition rate of 3Hz, beyond which the pileup of counts from the previous shots reduces the accuracy of the measurements to an unacceptable level. This new beryllium activation counter has been cross-checked against an indium activation counter to obtain absolute neutron yields. At a charging voltage of 12.5kV (bank energy of 2.2kJ), the average neutron yield was found to be (7.9±0.7)×107 per shot (standard deviation of 4×107). It was found that activation of the plasma focus construction materials (especially aluminum) must be taken into account.

Generation of Deuteron Beam from the Plasma Focus

The generation of deuteron beam from a 3 kJ Mather type plasma focus is studied. Two simple ion collectors made of copper plates are employed and are placed at the axis of the electrodes to detect the ion beam. The deuteron beam intensity at various deuterium gas pressures is determined together with ion beam energy using the time of flight method. For a series of discharges of the present plasma focus system operated at 15 kV discharge voltage, ion beams of energies ranging from 50 to 200 keV have been measured. The suitable deuterium filling pressure for ion beam production for electrodes lengths of 16 cm, 22 cm and 27 cm are 1 mbar, 0.7 mbar and 0.5 mbar respectively.

Hydrogen retention of carbon dust prepared by arc discharge and electron beam irradiation

Abstract Tritium retention of carbon dust co-deposited with fuel hydrogen is so large that it is required to evaluate the tritium inventory for estimating potential hazards of next step fusion reactors such as International Thermonuclear Experimental Reactor (ITER). In the present work, the authors prepared carbon dust samples by two methods. One was deuterium (D2) arc discharge with carbon electrodes to simulate co-deposition of carbon atoms with hydrogen atoms. The other was electron beam irradiation to carbon fiber composite (CFC) target in hydrogen gas to simulate disruptions. The dependence of retained hydrogen amount on hydrogen gas pressure and substrate temperature was investigated. The surface morphology and the crystal structure were also examined. In the case of D2 arc discharge, the concentration of retained deuterium was increased with the deuterium gas pressure, but did not depend on substrate temperature from 573 to 723 K. Under the anticipated ITER divertor condition, namely deuterium gas pressure and substrate temperature were less than 1 Pa and 573 K, the deuterium concentration was 0.2 in the atomic ratio, D/C. In the case of electron beam irradiation, the amount of retained hydrogen was the same as the background level.

Study of the ZrCr 0.7Ni 1.3 multiphased system under hydrogen pressure by in-situ neutron diffraction

Both X-ray and neutron diffraction Rietveld analyses show that the as-cast ZrCr 0.7Ni 1.3 alloy crystallises with the C15-type Laves phase type structure, with the presence of >30 wt.% Zr 7Ni 10 and a few weight percent chromium. After annealing at 1373 K, we observed the disappearance of chromium metal and a reduction by more than half of Zr 7Ni 10. P– C– T measurements performed on the as-cast material show an absorption capacity of about 0.9 H/AB 2 at 323 K and under P=10 bar. In-situ neutron diffraction experiments performed at 323 K under deuterium gas pressure reveal that, after full hydrogenation of the sample, the Zr 7Ni 10 phase transforms into an amorphous phase, but recrystallises under vacuum when the temperature is raised above 373 K.

カーボンダストの重水素保持特性

In a nuclear fusion device, plasma facing materials (PFMs) such as graphite are bombarded by extremely high heat and particle fluxes during an abnormal discharge called as disruption. As a result of the disruption, the PFMs are eroded and deposited on the wall. In the case of graphite plasma facing material, carbon dusts are produced. Flaky carbon dust was produced from isotropic graphite by electron beam evaporation. The absorption properties of deuterium in carbon dust were investigated by changing an absorption temperature (773-1173 K), time (10 min-24 hr) and deuterium gas pressure (1-100 Pa). The amount of adsorbed deuterium was evaluated by using thermal desorption spectroscopy. Deuterium was desorbed from the carbon dust at the temperature higher than 873 K in form of HD and D2. At the absorption temperature of 973 K and the pressure of 100 Pa, the amount of deuterium absorbed in the dust saturated within 5 hr. This saturation level was one or two orders of magnitude larger than that of isotropic graphite. The heat of solution for deuterium was 0.27 eV which was almost the same as that of isotropic graphite.

Direct-Drive Implosion Experiments on the SG-II Laser Facility

SG-II, a 8-beam Nd:glass laser with an output energy capability of 6kJ at 1.053μm, was built and direct-drive implosions were successfully performed early in 2000. Both exploding pusher and ablative targets were imploded using glass capsules with diameters of 200 and 500μm, and a wall thickness of about 1μm. The deuterium and tritium (DT) gas pressure filled in these capsules were 2.0 and 0.5MPa, respectively. Sophisticated diagnostics were deployed to measure laser absorption, hot electron temperature and fraction, thermal electron temperature, neutron yields, ion temperature, temporally resolved x-ray images, fuel areal density, alpha particle image, and so on. Significant results, such as neutron yields up to 4 × 109 for exploding pushers with 100-ps laser pulse irradiation and 6 × 108 for ablative targets with 1-ns pulses and clear x-ray images to see the compression process, were obtained. Numerical simulations were conducted to optimize target and laser parameter design and to duplicate the results afterwards with the specific shot parameters used in the experiment.

Development of high pressure deuterium gas targets for the generation of intense mono-energetic fast neutron beams

Two different technical solutions to the problem of generation of mono-energetic fast neutron beams on the gaseous targets are presented here. A simple and cost-effective design of a cooled windowed gas target system is described in the first part of this paper. It utilises a thin metallic foil window and circulating deuterium gas cooled down to 100 K. The ultimate beam handling capability of such target is determined by the properties of the window. Reliable performance of this gas target system was achieved at 1 bar of deuterium gas, when exposed to a 45 μA beam of 5 MeV deuterons, for periods in excess of 6 h. Cooling of the target gas resulted in increased fast neutron output and improved neutron to gamma-ray ratio. The second part of this paper discusses the design of a high pressure, windowless gas target for use with pulsed, low duty cycle accelerators. A rotating seal concept was applied to reduce the gas load in a differentially pumped system. This allows operation at 1.23 bar of deuterium gas pressure in the gas cell region. Such a gas target system is free from the limitations of the windowed target but special attention has to be paid to the heat dissipation capability of the beam dump, due to the use of a thin target. The rotating seal concept is particularly suitable for use with accelerators such as radio-frequency quadrupole (RFQ) linacs that operate with a very high peak current at low duty cycle. The performance of both target systems was comprehensively characterized using the time-of-flight (TOF) technique. This demonstrated that very good quality mono-energetic fast neutron beams were produced with the slow neutron and gamma-ray component below 10% of the total target output.

Enhancement of critical temperature up to 100 K of YBCO ozone annealed pellets by deuterium absorption

In this paper we report preliminary results about transition temperatures of YBa 2Cu 3O 7 pellets loaded with high pressure deuterium gas. After the gas loading non-superconducting phases were not detected by X-ray diffraction. The complex magnetic ac susceptibility has been measured as a function of the temperature. One D-YBCO pellet exhibited a magnetic critical temperature ( T c) as high as 100 K and the absence of a significant weakening of the superconducting grain coupling. Another sample, with a T c of 95.8 K in free air, showed a T c onset as high as 102 K under 35 bar of deuterium gas pressure.

Alloying Effects on Hydrogen Gettering Properties for Zr-Al System

This paper discusses alloying effects on absorption and desorption kinetics of deuterium for Zr-Al alloys, studied with mass analyzed thermal desorption spectroscopy using a conventional high vacuum system. It is found that the absorption rate of deuterium was proportional to the 1/2 power of deuterium gas pressure. On the other hand, the desorption process obeyed the second order kinetics with respect to the amount of absorbed deuterium. The temperature dependence of the rate constants revealed that the activation energies for both the absorption and desorption processes were lowered by the increase in the Al content in the alloys. Through potential diagrams for the absorption and desorption of deuterium, it was also found that the heat of deuterium (hydrogen) solution decreased with increasing Al composition. In addition, the x-ray diffraction spectroscopy showed the formation of a Zr{sub 4}Al{sub 3} phase in the Zr{sub 3}Al{sub 2} sample owing to repeated absorption and desorption cycles. The results suggest that the electronic factors, for example, work function, electron density, d-band character and so on, play an important role for the alloying effects rather than crystallographic structures.

In search of nuclear fusion in electrolytic cells and in metal/gas systems

It has been reported recently in the literature that unexpected thermal and nuclear effects (production of excess heat, neutrons, γ-rays, and tritium) can occur during the electrolysis of heavy water at palladium or titanium electrodes, or during temperature and pressure cycling of the titanium/deuterium gas system. We have attempted to reproduce some of these experiments. A variety of electrochemical cells having palladium cathodes in the form of wires, tubes, sheets, and rods have been used to electrolyze heavy water containing 0.1 mol.dm−3 LiOH, 0.1 mol.dn−3 LiOD or 0.5 mol.dm−3 D3PO4. Current densities of up to 200 mA.cm−2 were applied. The mass of the palladium cathodes covered the range from 1–40 grams and the surface area varied from 8–140 cm2. Neutron detection systems with low constant backgrounds were used to search for neutron emission during electrolysis. These included3He- and10BF3-based detectors. After running some of the cells for more than 30 days, no neutron emission above background could be detected. This puts upper limits of 0.5 s−1 and 2×10−23 fus. D-D.s−1 on the neutron emission and the fusion rate, respectively. A sensitive and accurate heat-flow calorimeter was built and used to monitor the energy balance of some of the cells during electrolysis. No unexpected heat effects were observed. This puts an upper limit of 0.13 W.cm−3 on the specific excess power. No enrichment of the electrolyte in tritium was evident after electrolysis. Experiments were also performed with the titanium/ deuterium gas system. These consisted of exposing titanium metal to a deuterium gas pressure of 40 atmospheres, lowering the temperature to −196°C, releasing the pressure and gradually warming the titanium to room temperature. No neutron emission above background was observed during these experiments, which puts upper limits of 0.5 s−1 and 4×10−25 fus.D-D.s−1 on the neutron emission and fusion rate, respectively.

Rotational Instability in the Theta Pinch

An experimental study of the rotation and associated instability was made in a theta pinch with a peak field of 85 kG reached in 2.3 μsec. The starting deuterium gas pressure was in the range 50–150 μ Hg, and for operation on the second half-cycle of the sinusoidal driving field the magnetic flux trapped within the plasma varied between ±100 kMx. In most conditions the circular cross section of the plasma became perturbed along its length by the growth of two flutes which rotated at an angular velocity of up to 6 × 107 rad/sec. It is deduced from measurements on the rotation that the plasma acquires its angular momentum at the start of the half-cycle, and the results are interpreted on the basis that the angular momentum is initially in a diamagnetic (azimuthal) ion current. A model is described for establishing this current as a consequence of contact between the plasma and the walls. Results on the time of onset of the instability, and on the growth rate of the flutes, point to the existence of some damping or stabilizing process; it is found that field diffusion, or a rapid axial contraction of the plasma (due to closed reversed field loops), can lead to instability, and when these are reduced the stable time is increased. The possible influence of finite Larmor radius stabilization is discussed. The stability condition, originally derived for low β but recently shown to have more general validity, is in qualitative agreement with the results.