Neutron emission during a nanosecond discharge in deuterium in a nonuniform electric field

Neutron emission during a nanosecond deuterium discharge formed at the nonuniform electric field is investigated. A stable neutron yield is observed when the cathode is made of metallic plates covered by a layer of deuterated zirconium and the anode is made of stainless steel in the form of a tube. It is shown that, when the deuterium pressure equals several Torr, neutrons are emitted from both deuterated and deuterium-free cathodes. The influence of the anode design on the neutron yield is studied.     

Neutron production with mixture of deuterium and krypton in Sahand Filippov type plasma focus facility

This Letter reports the order of magnitude enhancement in neutron yield from Sahand plasma focus device with krypton seeded deuterium operation. The highest average neutron yield of 2.2×¹⁰⁹ neutrons per shot was achieved at 1.00 Torr deuterium with 3% krypton which is higher than the best average neutron yield of 3.18×¹⁰⁸ neutrons per shot for pure deuterium operation. Estimation of average neutron energy showed that the maximum and minimum average energies are 2.98±0.6 MeV at 16 kV in 0.25 Torr deuterium with 3% Kr and 2.07±0.2 MeV at 18 kV operation in 0.5 Torr deuterium with 3% Kr, respectively. The anisotropy of neutron emission from Sahand DPF showed that the neutrons are produced mainly by beam-target mechanisms.     

Search for neutron production during heavy water electrolysis on palladium electrodes

The results are presented from a number of experiments in search for neutrons from the heavy water electrolysis with use of different palladium cathodes. The upper limit for the yield of neutrons from a cell with a 2.34 cm³ palladium cathode, which operated continuously for seven days, is 0.2 neutron/s (0.01 neutron/s per gram of palladium). This limit is 10⁵ times lower than the yield claimed recently by Fleischamann and Pons.     

Subthreshold sputtering at high temperatures

The sputtering of tungsten from a target at a temperature of 1470 K during irradiation by 5-eV deuterium ions in a steady-state dense plasma is discovered. The literature values of the threshold for the sputtering of tungsten by deuterium ions are 160–200 eV. The tungsten sputtering coefficient measured by the loss of weight is found to be 1.5×10^?4 atom/ion at a deuterium ion energy of 5 eV. Previously, such a sputtering coefficient was usually observed at energies of 250 eV. The sputtering is accompanied by a change in the target surface relief, i.e., by the etching of the grain boundaries and the formation of a wavy structure on the tungsten surface. The subthreshold sputtering at a high temperature is explained by the possible sputtering of adsorbed tungsten atoms that are released from the traps around the interstitial atoms and come to the target surface from the space between the grains. The wavy structure on the surface results from the merging of adsorbed atoms into ordered clusters.     

Study of the impact of steady-state plasma on radiation-damaged tungsten

Experimental studies of tungsten (as a candidate plasma-facing material for a fusion reactor) whose properties will degrade as a result of its contact with near-wall plasma and irradiation with neutrons are performed. The effect of a high level of radiation damages (1–100 displacements per atom) on deuterium accumulation and erosion caused by tungsten irradiation with deuterium plasma was studied. Radiation damages are obtained as a result of the irradiation of tungsten samples with high-energy ions in an accelerator (He⁺², C⁺³, 4–10 MeV). Then the samples are exposed to steady-state deuterium plasma at the LENTA facility (National Research Centre Kurchatov Institute). The effects of the erosion of tungsten and accumulation of hydrogen isotopes in it are studied. Modification of the surface microstructure and radiation swelling is observed. The helium and deuterium concentrations were measured using the methods of nuclear elastic backscattering and elastic recoil detection analysis. An increased accumulation of deuterium in the damaged layer to a depth of about 5 μm is revealed.     

Effect of the configuration of the discharge chamber of the plasma focus on the neutron yield

Preliminary probe experiments on the Filippov-type plasma focus with the energy E = 70 kJ and a current of about 1 MA show significant stray currents flowing near the insulator. To suppress them and optimize the discharge circuit, the main discharge chamber elements, i.e., the insulator, anode and cathode liner, were changed. As a result, a 30-fold increase in the setup neutron yield to Y _n = 5· 10¹⁰ neutrons per pulse was detected.     

Accumulation of deuterium in radiation-damaged tungsten

Experimental studies have been performed to determine the effect of high-level radiation damage on the accumulation of deuterium and erosion of tungsten samples exposed to deuterium plasma. Tungsten samples were exposed first to fast helium ions having an energy of 3–4 MeV (providing from one to ten displacements per atom) and then to deuterium plasma up to a dose of 10²⁵ ion/m². The effects of deformation and modification of the surface microstructure have been observed. The concentrations of helium and deuterium have been measured by the methods of elastic nuclear proton backscattering and nuclear recoil detection of helium ions. A high concentration of deuterium in the damaged layer of a tungsten sample has been measured, and helium has been detected in a layer ～5 μm thick. The proposed method shows promise for determining the lifetime of materials used in fusion reactors and measuring the concentration of tritium accumulated in these materials.     

Analysis of a tungsten surface irradiated by fast ions and deuterium plasma

The effects occurring on the surface of tungsten under irradiation with fast ions with an energy in the megaelectrolvolt range and with high fluxes of hydrogen (deuterium) plasma are considered. These effects are radiation damage of the surface layer of the material, its erosion and deuterium retention in it. Irradiation with helium ⁴He²⁺ (3.2–4.0 MeV) and carbon ¹²C³⁺ (10 MeV) ions is performed using a cyclotron at the National Research Center Kurchatov Institute. The thickness of the damaged layer is 3.5–6 μm. The irradiated samples are exposed to steady-state deuterium plasma using a LENTA linear plasma facility to reach a plasma ion fluence of 10²¹–10²² cm^?2. Tungsten erosion and modification of the structure of the damaged layer are analyzed at a plasma-ion energy of 250 eV. Deuterium retention in the damaged layer is studied by elastic recoil detection analysis. The deuterium concentration and its penetration depth into the material are measured. The data obtained for different kinds of fast ions used in the work are compared.     

Observation of tungsten particles in vacuum arcs by laser inducedfluorescence

We have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode in high current period by using a laser induced fluorescence method in tungsten vacuum arcs of 60 Hz sinusoidal current with the peak value of 3.3, 6.7, and 9.8 kA. In the case of 6.7 kA, the arc mode was the anode spot mode. Because of the generation of the anode spot, the tungsten ion density near the anode was higher than near the cathode and the density near the anode was about ten times as high as the case of 3.3 kA which was the diffuse mode. In the case of 9.8 kA, which was the intense arc mode, the density near the anode was not significantly different from the case of 6.7 kA. The density near the cathode was higher than near the anode and tungsten ions were observed till about 30 μs after current zero while they disappeared at current zero in the other cases     

Nuclear fusion induced by a super-intense ultrashort laser pulse in a deuterated glass aerogel

A SiO₂ aerogel with absorbed deuterium is proposed as a target for the fusion reaction d + d → He³ + n induced by a superintense ultrashort laser pulse. The multiple inner ionization of oxygen and silicon atoms in the aerogel skeleton occurs in the superintense laser field. All the formed free electrons are heated and removed from the aerogel skeleton by the laser field at the front edge of the laser pulse. The subsequent Coulomb explosion of the deuterated charged aerogel skeleton propels the deuterium ions up to kinetic energies of ten keV and higher. The neutron yield is estimated at up to 10⁵ neutrons per laser pulse for ～200–500 ps if the peak intensity is 10¹⁸ W/cm² and the pulse duration is 35 fs.     

Solid state effects during deuterium implantation into copper and titanium

Results of neutron counting experiments during deuterium implantation into titanium and copper are reported. Models for neutron yield have been developed by taking into account different solid state effects like energy degradation of incident ions, energy dependent d-d fusion cross section and diffusion of implanted deuterium possibly influenced by surface desorption and formation of metal deuterides. The asymptotic time dependence of the neutron yield during implantation has been compared with the experimental results. Using these results, solid state processes that might occur during deuterium implantation into these metals are inferred.     

Effect of cathode structure on neutron yield performance of a miniature plasma focus device

In this Letter we report the effect of two different cathode structures - tubular and squirrel cage, on neutron output from a miniature plasma focus device. The squirrel cage cathode is typical of most DPF sources, with an outer, tubular envelope that serves as a vacuum housing, but does not carry current. The tubular cathode carries the return current and also serves as the vacuum envelope, thereby minimizing the size of the DPF head. The maximum average neutron yield of (1.82±0.52)×¹⁰⁵ n/shot for the tubular cathode at 4 mbar was enhanced to (1.15±0.2)×¹⁰⁶ n/shot with squirrel cage cathode at 6 mbar operation. These results are explained on the basis of a current sheath loading/mass choking effect. The penalty for using a non-transparent cathode negates the advantage of the smaller size of the DPF head.     

Deuterium-ion implantation into composite structures with tungsten coatings

The results of studying the accumulation and thermal desorption of ion-implanted deuterium from tungsten films deposited on various composite substrates are presented. The influence of the tungsten-film thickness and irradiation dose with D⁺ ions of intermediate energy on the amount of trapped deuterium and the shape of the spectra of the thermal desorption of deuterium in vacuum are studied. Possible mechanisms of these processes are discussed.     

X-ray observations of tungsten wire array Z-pinch implosions on QiangGuang-1 facility

Z-pinch experiments with two arrays consisting, respectively, of 32 4-μm- and 6-μm-diameter tungsten wires have been carried out on QiangGuang-1 facility with a current rising up to 1.5MA in 80ns. At early time of implosion, x-ray framing images show that the initial emission comes from the central part of arrays, and double clear emission rings, drifting to the anode and the cathode at 5×10⁶cm/s and 2.4×10⁷cm/s respectively, are often produced near the electrodes. Later, in a 4-μm-diameter tungsten wire array, filamentation caused by ohmic heating is prominent, and more than ten filaments have been observed. A radial inward shift of arrays starts at about 30\,ns earlier than the occurrence of the x-ray peak power for both kinds of arrays, and the shrinkage rate of emission region is as high as 1.7×10⁷cm/s in a 4-μm-diameter tungsten wire array, which is two times higher than that in a 6-μm one. Emission from precursor plasmas is observed in implosion of 6-μm-diameter tungsten wire arrays, but not in implosion of a 4-μm-diameter tungsten wire array. Whereas, in a 4-μm-diameter tungsten wire array, the soft x-ray emission shows the growth of m=1 instability in the plasma column, which is caused by current. The reasons for the discrepancy between implosions of 4-μm- and 6-μm-diameter tungsten wire arrays are explained.     

Neutron spectrometer based on a proton telescope with electronic collimation of recoil protons

A prototype of a neutron spectrometer based on a gas proportional counter with recoil-proton registration is created at the Frank Laboratory of Neutron Physics at the Joint Institute for Nuclear Research (FLNP JINR) in Dubna. The spectrometer is developed to measure the kinetic energy of protons scattered elastically at small angles that are produced by (n, p) reaction in an environment containing hydrogen. The elaborated prototype consists of two cylindrical proportional counters used as cathodes. They are placed in a gas environment with a common centrally situated anode wire. Studies on the characteristics of the neutron spectrometer were conducted using ²⁵²Cf and ²³⁹Pu-Be radioisotope neutron sources. Measurements were made with monoenergetic neutrons produced by the ⁷Li(p, n)⁷Be reaction when a thin lithium target was bombarded with a proton beam from an EG-5 electrostatic accelerator, as well as with neutrons from the reaction D(d, n) ³He with a gas deuterium target.     

An investigation of the ion beam of a plasma focus using a metalobstacle and deuterated target

The results of an experimental study of the ion beam properties of a 3.3-kJ, 15-kV plasma focus are reported. The plasma focus is operated at 3 torr deuterium, and neutron yield measurements are made with a copper obstacle or deuterated target placed at various axial positions from the end of the anode. It is found that in the normal operation of the focus without an obstacle or target, less than 15% of the neutrons are produced within the pinch column, and that more than 85% of the neutrons arise from the deuterium-ion-beam bombardment of the deuterium gas in the region 20-60 mm from the end of the anode. The neutron production is highest between 30-40 mm from the anode. The results are valid only for a small focus     

The structure of He and the neutron electric form factor from polarization observables

As there are no free neutron targets one has to resort to the nuclear targets deuterium or helium. In order to reduce nuclear effects blurring the information on nuclear form factors the technique of double polarization experiments had been developed. Recent experiments at MAMI measuring the electric form factor of the neutron are discussed. In addition, other single and double nucleon knockout experiments are described which support the buildup of a consistent interpretation of nuclear structure in ³He.     

DD reaction enhancement and X-ray generation in a high-current pulsed glow discharge in deuterium with titanium cathode at 0.8–2.45 kV

The DD reaction yield (3-MeV protons) and the soft X-ray emission from a titanium (Ti) cathode surface in a periodic pulsed glow discharge in deuterium were studied at a discharge voltage of 0.8–2.45 kV and a discharge current density of 300–600 mA/cm². The electron screening potential U_e = 610 ± 150 eV was estimated in the range of deuteron energies 0.8 keV < E_d < 2.45 keV from an analysis of the DD reaction yield as a function of the accelerating voltage. The obtained data show evidence for a significant enhancement of the DD reaction yield in Ti in comparison to both theoretical estimates (based on the extrapolation of the known DD reaction cross section for E_d ≥ 5 keV to low deuteron energies in the Bosch-Halle approximation) and the results of experiments using accelerators at the deuteron energies E_lab ≥ 2.5 keV and current densities 50–500 μ A/cm². Intense emission of soft X-ray quanta (10¹³–10¹⁴ s^?1 cm^?2) was observed at an average energy of 1.2–1.5 keV. The X-ray emission intensity and the DD reaction yield enhancement strongly depend on the rate of deuterium diffusion in a thin subsurface layer of Ti cathode.     

Experimental investigation of the anode region of a free-burning atmospheric-pressure inert-gas arc I. General characteristics of the discharge. Low-current regime

This paper reports the experimental investigation of the anode region of a free-burning inert-gas arc at atmospheric pressure in the current range from a few amperes to hundreds of amperes. The tungsten thermionic-emission cathode and the large-diameter water-cooled copper anode that were used permitted the anode arc root to assume its natural form. The general characteristics of the discharge are given and results are presented from investigations of the anode region at low currents, where the anode arc root is single and constricted, but erosion-free. Measurements of the plasma parameters as well as the current density in the arc root are reported, and a comparison is made between the values obtained and those characteristic of the region of the cathode arc root. Zh. Tekh. Fiz. 67, 35–40 (January 1997)     

Investigation of the aging of gas-discharge detectors of particles with the use of nuclear reaction analysis

This paper reports on the results of an investigation into the aging of drift tubes with anodes fabricated from gold-plated tungsten wires under continuous irradiation with a ⁹⁰Sr beta source. The accumulated charge is as high as 9 C cm^?1. The process of aging is studied in a gaseous mixture containing Ar, CO₂, and CF₄. It is revealed that the aging brings about a severe damage of the gold coating on a tungsten wire and an increase in the diameter of the wire in the irradiated zone from 35 to 42 μm. The observed swelling of the anode wire under forces applied from the inside is apparently a new mechanism of aging of anode wires. The swelling effect is assumed to be caused by the oxidation of the tungsten wire. It is demonstrated using the nuclear reaction analysis that the deposits containing tungsten, oxygen, carbon, and fluorine are distributed along the anode wire in the direction of the gas flow. The oxygen and fluorine distributions over the depth and along the length of the anode wire are determined.