Time resolved metal impurity fluxes in the dite tokamak

Measurements are presented of the time resolved flux of metallic impurities to a carbon probe placed at the limiter radius of the DITE tokamak. Average fluxes of titanium and molybdenum of the order 4 × 10¹² mm^?2 s^?1 are measured. It is concluded that peaks in the metal flux are correlated with arcing and with disruptions initiated by neutral beam injection. The decrease in flux by almost one order of magnitude compared with earlier studies may be related to a recent redesign of the molybdenum limiters which reduces the minor radius of the plasma. It is observed that the deposit density on the probe is not a linear function of exposure to the plasma and also that insertion of the probe causes a change to the local plasma environment.     

Determination of plasma parameters in rf glow-discharge plasma

The Langmuir probe measurements were carried out in a planar rf discharge of N₂ and Ar gases. The dc characteristic I-U curves are calculated from the measured rf characteristic I-U curves for frequencies 10-60 MHz. The measured parameters such as electron temperature and electron density are compared with the simulated results. At gas pressures 10-40 Pa, the sheath thickness at the powered electrode was proportional to f^−0.5 for simulation and f^−2/3 by using the electrical parameters of the probe measurements and to p^−0.5 for both cases.     

Online time-differential perturbed angular correlation study with an O beam - Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe ¹⁹F, the β⁻ decay product of ¹⁹O (t_1/2 = 26.9 s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣V_zz∣ = 2.91(17) × 10²² V m⁻², suggests that the incident ¹⁹O atoms are stabilized at an interlayer position with point group C_3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive ¹⁹O beam.     

Monte-Carlo study of induced radioactivity in probe for low-energy proton beam

Induced radionuclides generated from the probe which is bombarded by proton beam will turn the detector into a typical external irradiation radiation source. Thus, it is beneficial for developing radiation protection to calculate the types and the activities of radionuclides. Here we applied both a theoretical analysis and a Monte-Carlo method to compute the induced radioactivity in a copper probe irradiated by proton beam. Various kinds of radionuclides saturation activity obtained by these two different methods were compared. The comparisons of the results cast by the two methods show the similar saturation activities for ⁶³Zn and ⁶⁵Zn. However, the Monte-Carlo method conducted by the software FLUKA is able to provide a more complete consideration on nuclear reaction, and to calculate both the direct and indirect radioactivity under different irradiation time. Furthermore, by employing the FLUKA Monte-Carlo program, the induced radioactivity of three types of probe materials (Cu, Ta and W) under low-energy (below 20 MeV) proton beam irradiated were also separately simulated and tantalum is considered as the best material for low-energy proton interceptive diagnostics probe due to the higher energy threshold of nuclear reaction and the lower radioactivity.     

Atom probe tomography characterization of radiation-sensitive KS-01 weld

The microstructure of a radiation-sensitive KS-01 test weld has been characterized by atom probe tomography. The levels of copper, manganese, nickel and chromium in this weld were amongst the highest of all the steels used in Western reactor pressure vessels. After neutron irradiation to a fluence of 0.8 × 10²³ n m⁻² (E>1 MeV) at a temperature of 288 °C, this weld exhibited a large Charpy T_41J shift of 169 K, a large shift of the fracture toughness transition temperature of 160 K, a decrease in upper shelf energy from 118 to ∼78 J, and an increase in the yield strength from 600 to 826 MPa. However, the mechanical properties data conformed to the master curve. Atom probe tomography revealed a high number density (∼3 × 10²⁴ m⁻³) of Cu-, Mn-, Ni-, Si- and P-enriched precipitates and a lower number density (∼1  × 10²³ m⁻³) of P clusters.     

Transient field measurements with 80Se ions in glassy metals

Measurements of Transient Magnetic Fields (TF) were performed on ⁸⁰Se(2⁺) ions in amorphous Fe₈₀B₂₀ alloys employing ⁴⁸Ti beams. The motivation was to study the influence of the amorphous structure of the host on the strength of TF, when the 2s-state of probe ions is responsible for the field. The results are compared with those of crystalline Fe, where large attenuations have been observed. The significantly smaller attenuation in the present conditions demonstrates the superiority of metallic glasses for g-factor measurements.     

Atomic scale characterization of ion-induced amorphization of GaAs and InAs using perturbed angular correlation spectroscopy

The perturbed angular correlation technique has been utilized to understand the production and nature of the implantation induced crystalline to amorphous transformation in GaAs and InAs. This technique, which is based upon the nuclear hyperfine interaction of the electric-quadrupole moment of the probe nucleus with the electric field gradient from extra nuclear charges, requires introduction of radioactive probe nuclei in host material. The radioactive probes ¹¹¹In/¹¹¹Cd were produced with the 14UD heavy-ion accelerator via nuclear reaction that recoil implants the ¹¹¹In nuclei deep into single crystals of GaAs (1 0 0) and InAs (1 0 0). After removal of radiation damage, caused by recoil implantation, single crystals of GaAs (1 0 0) and InAs (1 0 0) were implanted with stable ⁷⁴Ge ions (MeV) over a wide dose range at liquid nitrogen temperature. The irradiated samples were investigated with respect to the damage production. The crystalline, disordered and amorphous probe environments were identified from the measurement. The evolution of damage is described within the framework of different amorphization models. In GaAs, amorphization is obtained by direct-impact amorphization and by the growth of amorphous zones due to defect-stimulation at crystalline/amorphous interface. In InAs, the amorphization is first initiated by accumulation of simple point defects and then direct-impact/defect-stimulated mechanism contributes to further stimulate the transformation.     

A Powerful Remote Source of O Atoms for the Removal of Hydrogenated Carbon Deposits

We describe a system to deliver a large flux of O atoms for the removal of hydrogenated carbon films from surfaces in remote areas of tokamaks with carbon divertors. The oxygen plasma is generated via electrode-less radiofrequency discharge in a discharge chamber connected to a remote chamber by a 2 m long complex-shaped glass tube 4 cm in diameter. The density of O atoms in the remote chamber was measured with a nickel catalytic probe and its variation with discharge power obtained. The density was close to the detection limit of the probe (around 1 × 10¹⁹ m^?3) as long as the vacuum system was pumped with a rotary pump at a nominal pumping speed of 80 m³ h^?1. The density increased well over 10²⁰ m^?3 when a Roots pump was added. The effective pumping speed at the current setup was up to 200 m³ h^?1. At such conditions, the maximal O-atom density at 2 m from the source was up to 3 × 10²⁰ m^?3. The density depended on the pressure as well as the discharge power. The behavior of O-atom density far away from the source was explained by gas phase and surface phenomena. The effective pumping speed was found to be of crucial importance. The setup was used for removal of model hydrogenated carbon films. Experiments were performed at sample temperatures up to 600 K and etching rates up to 50 nm/s were obtained. We found that the experimental setup is suitable for removal of hydrogenated atoms on a large scale.     

Magnetic Probe Measurements in INTI Plasma Focus to Determine Dependence of Axial Speed with Pressure in Neon

Current sheath dynamics generated in INTI plasma focus device operated with neon gas has been studied. A 3-turn Rogowski coil design has been used to measure derivative current. A new magnetic probe was designed and used to study of current sheath arrival time, current profile and velocity variation in the axial phase at different experimental conditions. The current sheath’s average velocity was found to vary with pressure^?0.51 with a R² value of 0.9 which agrees well with the theoretically expected variation of pressure^?0.5.     

Transport of caesium through a nuclear graphite. part I. Microgravimetric studies and electron probe microanalysis

Adsorption of caesium (≈40 mg/g maximum) on a gilsonite graphite at 473 K and subsequent desorption in the range 973–1373 K have been followed in a vacuum microbalance and caesium concentration profiles in the graphites have been measured by electron probe microanalysis at stages in the adsorption-desorption programme. An analysis is developed which enables apparent diffusion coefficients, D, to be determined from desorption rates. Electron probe microanalysis provides evidence for two-phase or multi-phase diffusion of caesium in the graphites during adsorption and desorption and there is evidence that diffusion of caesium is sensitive to the thermal history of graphite. The variation of all reported values of D (m²/s) with temperature is log $\text{D = ?6.34 ?}\text{5450}\text{T}$. The limitations of the method of analysis due to nonuniform caesium concentrations, finite surface exit resistance and multi-phase diffusion are also considered.     

Depth-dependent magnetic characterization of Fe films on NiO(0 0 1)

We used nuclear hyperfine spectroscopies and a ⁵⁷Fe probe layer approach to study the depth-dependent magnetic properties of ultrathin Fe films on NiO(0 0 1), a system exhibiting exchange bias. Conversion electron Mössbauer spectroscopy and nuclear resonance scattering of synchrotron radiation were employed. The samples were two Fe films with a thickness (8–10 ML) slightly above the critical thickness for the onset of ferromagnetism at room temperature, in which a 2 ML-thick probe layer, enriched in the ⁵⁷Fe Mössbauer isotope, was embedded at different depths from the Fe/NiO interface. Both techniques indicate that inside the film Fe has a metallic character, while at the interface with NiO different Fe phases are present. The main conclusion is that already a few monolayers from the interface with NiO the magnetic properties of Fe are bulk-like.     

Characterization of plasma jet ejected from a parallel-plate rail gun for simulating edge localized mode

A small plasma gun with parallel-plate configuration is fabricated to generate a bunch of plasma which is similar to ELM (edge localized mode) plasma, by taking advantages of its simplicity and cost-effectiveness. Prior to explore how to control the ELM-like plasma so as to relieve heat load on the divertor target, characteristics of a plasma jet ejected from the plasma gun are investigated using a quadruple Langmuir probe which is appropriate for measuring rapidly varying plasma parameters such as electron density, temperature, and ion velocity at the same time. The plasma density and ion velocity measured at 112 mm away from the exit are 3 × 10¹⁹ m^?3 and 11 km/s, respectively, which seem to be suitable for investigating next step research on the control of ELM-like plasma using various methods such as electromagnetic waves and high-voltage pulses. Also, the quadruple Langmuir probe is proven to be adequate for use in such experiments.     

Local measurement of interfacial area, interfacial velocity and liquid turbulence in two-phase flow

Double sensor probe and hotfilm anemometry methods were developed for measuring local flow characteristics in bubbly flow. The formulation for the interfacial area concentration measurement was obtained by improving the formulation derived by Kataoka and Ishii. The assumptions used in the derivation of the equation were verified experimentally. The interfacial area concentration measured by the double sensor probe agreed well with one by the photographic method. The filter to validate the hotfilm anemometry for measuring the liquid velocity and turbulent intensity in bubbly flow was developed based on removing the signal due to the passing bubbles. The local void fraction, interfacial area concentration, interfacial velocity, Sauter mean diameter, liquid velocity, and turbulent intensity of vertical upward air–water flow in a round tube with an inner diameter of 50.8 mm were measured by using these methods. A total of 54 data sets were acquired consisting of three superficial gas flow rates, 0.015–0.076 m s⁻¹, and three superficial liquid flow rates, 0.600, 1.00, and 1.30 m s⁻¹. The measurements were performed at the three locations: L/D=2, 32, and 62. This data is expected to be used for the development of reliable constitutive relations which reflect the true transfer mechanisms in two-phase flow.     

Transient magnetic fields and electron polarizations of H-like oxygen ions in crystalline iron and amorphous iron-boron compounds

Transient magnetic fields (TF) have been measured for oxygen ions with the ¹⁶O(3⁻) state as probe traversing thin layers of crystalline Fe and amorphous Fe₈₀B₂₀ compounds. The TF associated with polarized 1s electrons have strengths of B_TF(Fe) = 412 (38) T in Fe and B_TF(FeB) = 368 (62) T in FeB implying degrees of polarization of p_1s(Fe) = 0.13 (1) and p_1s(FeB) = 0.14 (3), respectively. The strength parameter a = 12.1 (2.3) T deduced for FeB agrees very well with other data rendering it highly suitable for TF measurements.     

Investigation of oxide particles in unirradiated ODS Eurofer by tomographic atom probe

Oxide dispersion strengthened steels possess better high-temperature creep and radiation resistance than conventionally produced ferritic/martensitic steels. This behaviour is mainly caused by the presence of highly dispersed and extremely stable oxide particles with diameters of a few nanometers. In this work the nanostructure of ODS Eurofer steel was investigated by means of tomographic atom probe and correlations with recent TEM and SANS studies were derived. The present investigation revealed nanoscaled clusters of typically 2 nm diameter containing not only yttrium and oxygen but also vanadium and nitrogen. Moreover, concentration of vanadium in particles was found to be higher than that of yttrium, which indicates the importance of these elements in cluster formation. The estimated average cluster number density is about 2 × 10²⁴ m⁻³. These enriched zones might be evidently attributed to precursors of the larger precipitates observed by TEM. This conclusion is also supported by the similarities of the chemical composition inside enriched zones seen in both atomic probe and TEM data.     

A Pulsed Neutron Source for Thermal Reactor Physics

A compact 200 kV Cockcraft type pulsed neutron source was designed and constructed as a versatile tool for various pulsed neutron experiments in thermal reactor physics. Special care was taken to obtain the maximum peak intensity of pulsed neutrons against the average background neutrons induced from accelerator operation. To this end, two particular design features have been adopted. One is pulsing of the R-F ion source probe voltage combined with pulsing of the deflector voltage of the post accelerated D⁺ ion beam. The other feature is the use of a beam analyzing magnet to the accelerated beam. With these devices, a peak to background ratio of 10⁵ was obtained.

Pulsed neutrons of 14 MeV can be generated at pulse widths from 0.13 μs 500 ms, with a neutron yield of 3.7×10⁴–2×10⁹ n/pulse. The repetition rate of the pulses can be changed independently of the pulse width. An arrangement for long focusing beam adjustment provides for selection of the target position in a wide range.     

Separation of Nd metal by using disproportionation reaction of Nd(II) in molten chlorides

In order to separate neodymium (Nd) from lanthanides in chloride melts, the electrochemical characteristics Nd ions in molten LiCl-CaCl₂ eutectic were studied. The formal redox potentials of the Nd³⁺|Nd²⁺ and Nd²⁺|Nd couples in molten LiCl-CaCl₂ eutectic at 823 K were determined to be −2.745 ± 0.005 and −3.081 ± 0.005 V vs. Cl₂|Cl⁻. Under the controlled potential electrolysis by applying negative potential to form Nd²⁺, Nd²⁺ was disproportionated to Nd³⁺ and metallic Nd fog according to the reaction; 3Nd²⁺ ? 2Nd³⁺ + Nd. When a quartz glass was immersed in the melt during the electrolysis, Nd was coated on the quartz surface. The chemical composition of the recovered Nd was analyzed to be Nd metal and Nd₂O₃ by scanning electron microscopy, X-ray diffractometry, and electron probe microanalysis. The same electrolytic method was carried out under the coexistence of Nd³⁺ and lanthanum ion (La³⁺). Nd³⁺ was separated from La³⁺ and recovered to be Nd₂O₃.     

Optimized H extraction in an argon-magnesium seeded magnetized sheet plasma

The enhancement and optimization of H⁻ extraction through argon and magnesium seeding of hydrogen discharges in a magnetized sheet plasma source are reported. The paper first presents the modification of the production chamber into a hexapole multicusp configuration resulting in decreased power requirements, improved plasma confinement and longer filament lifetime. By this, a wider choice of discharge currents for sustained quiescent plasmas is made possible. Second, the method of adding argon to the hydrogen plasma similar to the scheme in Abate and Ramos [Y. Abate, H. Ramos, Rev. Sci. Instr. 71 (10) (2000) 3689] was performed to find the optimum conditions for H⁻ formation and extraction. Using an E × B probe, H⁻ yields were investigated at varied argon-hydrogen admixtures, different discharge currents and spatial points relative to the core plasma. The optimum H⁻ current density extracted at 3.0 cm from the plasma core using 3.0 A plasma current with 10% argon seeding increased by a factor of 2.42 (0.63 A/m²) compared to the measurement of Abate and Ramos [Y. Abate, H. Ramos, Rev. Sci. Instr. 71 (10) (2000) 3689]. Third, the argon-hydrogen plasma at the extraction chamber is seeded with magnesium. Mg disk with an effective area of 22 cm² is placed at the extraction region’s anode biased 175 V with respect to the cathode. With Mg seeding, the optimum H⁻ current density at the same site and discharge conditions increased by 4.9 times (3.09 A/m²). The enhancement effects were analyzed vis-à-vis information gathered from the usual Langmuir probe (electron temperature and density), electron energy distribution function (EEDF) and the ensuing dissociative attachment (DA) reaction rates at different spatial points for various plasma discharges and gas ratios. Investigations on the changes in the effective electron temperature and electron density indicate that the enhancement is due to increased density of low-energy electrons in the volume, conducive for DA reactions. With Mg, the density of electrons with electron temperature of about 3 eV increased 3 orders of magnitude from 2.76 × 10¹² m⁻³ to 2.90 × 10¹⁵ m⁻³.     

Portable spectroscopic fast neutron probe and 3He detector dead-time measurements

This paper presents dead-time calculations for the Portable Spectroscopic Fast Neutron Probe (N-Probe) using a combination of the attenuation law, MCNP (Monte Carlo N-particle Code) simulations and the assumption of ideal paralyzing and non-paralyzing dead-time models. The N-Probe contains an NE-213 liquid scintillator detector and a spherical ³He detector. For the fast neutron probe, non-paralyzing dead-time values were higher than paralyzing dead-time values, as expected. Paralyzing dead-time was calculated to be 37.6 μs and non-paralyzing dead-time was calculated to be 43.7 μs for the N-Probe liquid scintillator detector. Dead-time value for Canberra ³He neutron detector (0.5NH1/1K) was also estimated using a combination of subcritical assembly measurements and MCNP simulations. The paralyzing dead-time was estimated to be 14.5 μs, and the non-paralyzing dead-time was estimated to be 16.4 μs for ³He gas filled detector. These results are consistent with the dead-time values reported for helium detectors.     

Tomographic atom probe characterization of the microstructure of a cold worked 316 austenitic stainless steel after neutron irradiation

For the first time, chemical analyses using Atom Probe Tomography were performed on a bolt made of cold worked 316 austenitic stainless steel, extracted from the internal structures of a pressurized water reactor after 17 years of reactor service. The irradiation temperature of these samples was 633 K and the irradiation dose was estimated to 12 dpa (7.81 × 10²⁵ neutrons.m⁻², E > 1 MeV). The samples were analysed with a laser assisted tomographic atom probe. These analyses have shown that neutron irradiation has a strong effect on the intragranular distribution of solute atoms. A high number density (6 × 10²³ m⁻³) of Ni-Si enriched and Cr-Fe depleted clusters was detected after irradiation. Mo and P segregations at the interfaces of these clusters were also observed. Finally, Si enriched atmospheres were seen.