Energetic Tritons Research Articles

A comprehensive evaluation of the thermal neutron detection efficiency by a single crystal CVD diamond detector with a LiF thermal neutron converter

Thermal neutron detection efficiency of the single crystal CVD diamond detector (SDD) equipping 1.9 μm-thick 6LiF thermal neutron converter, which induces the energetic ions through the 6Li(n,α)3H reaction, was evaluated comprehensively using a charge-integral based pulse shape discrimination (PSD) method and radiation transport calculation by PHITS. The PSD method applied in this work successfully separated the pulses induced by energetic tritons, alpha particles and other types of radiation. The detection efficiency of tritons was consistent with the radiation transport calculation, although that of alpha particles was lower than the estimation. The overall thermal neutron detection efficiency was obtained by summing detection efficiencies of tritons and alpha particles. Approximately 29% of 6Li(n,α)3H reaction occurred in an effective 1.9 μm-thick 6LiF thermal neutron converter could be detectable as the pulses of tritons (∼ 73%) and alpha particles (23%).

Investigation on tritium retention and surface properties on the first wall in the large helical Device

• Examined tritium (T) retention in material probes on the first wall after the LHD 2017 campaign. • Studied the relationships between surface microstructures and T retention in material probes. • Much larger amounts of T remained in probes located at deposition-dominant area. • Among probes located at erosion-dominant area, remaining T in the probe on which energetic tritons impinged was much larger than others. In the Large Helical Device (LHD), the first deuterium plasma experiment was conducted in 2017. To investigate tritium migration in the LHD vacuum vessel, long-term material probes were installed on the first wall before the deuterium plasma experiment. After the experiment, the microstructure and amount of tritium remaining in each probe were analyzed. The results showed that a relatively large amount of tritium remained in the probes on the first wall, forming a thick deposition layer, rather than in the probes located in the erosion-dominant area. In the deposition layers on the probes, the dominant element is carbon, which can be generated on the divertor tiles made of graphite. The result of orbit calculation of the energetic tritons in the case of the standard magnetic configuration in the LHD showed that approximately 40% of the tritons generated by deuterium–deuterium fusion reactions were promptly lost mainly to the divertor. Thermalized tritons also flew to the divertor along with the background plasma. The divertor tiles, on which the tritons impinged, were eroded by the divertor plasma, and carbon atoms and tritiated hydrocarbon molecules were generated and deposited on the first wall. This can be the dominant mechanism of tritium retention in the first wall. Among the material probes located in the erosion-dominant area, the amount of tritium remaining in the probe on which the energetic tritons impinged was relatively large. The results of the tritium balance analysis show that the first wall is not the dominant reservoir of tritium in the LHD.

Investigation of the distribution of remaining tritium in divertor in LHD

Abstract To reveal the triton transport and the tritium migration in a deuterium plasma experiment in the Large Helical Device (LHD), the distribution of the remaining tritium in divertor tiles made of graphite after the first deuterium plasma experimental campaign in 2017 was investigated. In this study, tritium contents in divertor tiles have been measured by using a full-combustion method. The asymmetric tritium retention in divertor tiles located at symmetric positions, which was found in the previous study by the surface tritium measurement using an imaging plate technique, has been confirmed by the results of the full-combustion method. The asymmetry is considered to be attributed to the asymmetric distribution of lost-points of energetic tritons in divertor. A depth profile of remaining tritium in a divertor tile estimated by using a combination of the imaging plate technique and a sputtering treatment shows that the peak of the profile locates at several micro-meters from the surface. This result suggests that the majority of the remaining tritium impinged upon the divertor tile as energetic tritons. In this study, a distribution of lost-points of energetic tritons has been calculated by using a Lorentz orbit following code (LORBIT) with taking into account divertor components. The obtained distribution has been compared with measured tritium distributions on divertor tiles. The measured and calculated distributions are similar to each other, but they are not the same. The difference between them can be attributed to plasma exposures during and after the deuterium plasma campaign.

Investigation of remaining tritium in the LHD vacuum vessel after the first deuterium experimental campaign

Remaining tritium in the vacuum vessel after the first deuterium plasma experimental campaign conducted over four months was investigated in the large helical device (LHD) for the first time in stellarator/heliotron devices by using the tritium imaging plate technique. In-vessel components such as divertor tiles and first wall panels, and long-term material probes retrieved from the vacuum vessel were analyzed. The in-vessel component in which tritium remained most densely is the baffle part of divertor tiles made of graphite retrieved from the inboard-side divertor. Asymmetric tritium retention is observed on divertor tiles located at magnetically symmetric positions, and can be attributed to the toroidal field direction dependence of the asymmetric loss of energetic tritons generated by deuterium–deuterium nuclear fusion reactions. On the first wall, tritium remained in a deposited layer, which mainly consists of carbon.

Use of the σ-Rays from the 6Li(t,p1)8Li* Reaction for Studying Knock-On Tritons in the Core of Fusion Plasmas

We discuss use of σ-rays for diagnosing energetic particles in burning plasma. Our attention is focused on 0.981-MeV σ-rays emitted in the 6Li(t,p1)8Li* reaction governed in the plasma by energetic tritons. These quanta are shown to have an important application for diagnosing knock-on tritons and α-particles confined in burning DT plasmas.

Use of the [formula omitted] nuclear reaction for diagnostics of energetic particles in burning plasmas

Application of reaction-produced γ -rays to diagnostics of energetic particles in burning plasmas is analyzed. Particularly, we focus on 0.981 MeV γ -rays emitted in the Li 6 ( t , p 1 γ ) Li 8 nuclear reaction solely governed in the plasmas by energetic tritons. It is shown that these γ quanta can serve as a promising tool to diagnose α knock-on tritons and α -particles confined in burning DT plasmas. Key parameters of the α knock-on triton population and the α -particle confinement property can be obtained by comparing the experimental γ -ray yield and spectrum with theoretical slowing-down calculations. Even if the γ -ray spectral shape cannot be acquired, one can monitor densities of these tritons and α -particles at energies of 0.6–1.8 MeV and 2.0–3.5 MeV, respectively, in a nearly steady-state plasma. The 0.981-MeV photons also would help to display time evolution of the α -particle population in experiments of deuterium plasmas with pulsed tritium beam shots.

Modeling of neutron emission spectroscopy in JET discharges with fast tritons from (T)D ion cyclotron heating

The measurement of fast ion populations is one of the diagnostic capabilities provided by neutron emission spectroscopy (NES). NES measurements were carried out during JET trace tritium campaign with the magnetic proton recoil neutron spectrometer. A favorable plasma scenario is (T)D where the resulting 14 MeV neutron yield is dominated by suprathermal emission from energetic tritons accelerated by radio frequency at their fundamental cyclotron frequency. Information on the triton distribution function has been derived from NES data with a simple model based on two components referred to as bulk (B) and high energy (HE). The HE component is based on strongly anisotropic tritium distribution that can be used for routine best-fit analysis to provide tail temperature values (THE). This article addresses to what extent the THE values are model dependent by comparing the model above with a two-temperature (bi-) Maxwellian model featuring parallel and perpendicular temperatures. The bi-Maxwellian model is strongly anisotropic and frequently used for radio frequency theory.

Tritium distribution on plasma-facing tiles from ASDEX Upgrade

Tritium surface distribution on the plasma-facing tiles used in the divertor region and the central column of ASDEX Upgrade was measured by Tritium Imaging Plate Technique. The tritium intensity is high at the dome baffle underneath the X point, the outer divertor region and the mid-plane or lower side of the central column. In contrast, little tritium was retained in the carbon deposition found in the inner strike point module tiles. Such profile is very similar to that observed in JT-60U, indicating that long-term tritium retention in ASDEX Upgrade was dominated by the implantation of energetic tritons escaping from the main plasma without losing their initial energy (1MeV) rather than the codeposition of eroded carbon and hydrogen isotopes. However, such tritium profile was modified by carbon deposition and thermal load, particularly on the divertor target tiles and the central column.

Measurement of deuterium and tritium retentions on the surface of JT-60 divertor tiles by means of nuclear reaction analysis

It is important to measure retentions and depth profiles of deuterium and tritium on plasma facing materials to understand plasma-wall interactions. The nuclear reaction analysis (NRA) has been established using a deuteron beam with an adjustable energy. By means of the NRA, depth profiles of deuterium and tritium on the surface of JT-60 divertor graphite tiles irradiated with DD burning were obtained. The NRA method showed that the average concentrations of the deuterium retained in the surface of two different were 1.3 × 10 16 and 0.3 × 10 16 cm −2. α-particles from the T(d, α)n reaction were measured from a dome unit tile of JT-60U W-shaped divertor and the measured activity of tritium was about 18 kBq/cm 2. The peak energy of the α spectrum obtained with the NRA was about 1.2 MeV. This result implies that energetic tritons were implanted in the divertor graphite tile.

The effect of coatings on retention and permeation in SS 316L APT tritium production tubes

The accelerator production of tritium (APT) design calls for thousands of thin-walled tubes to be filled with 3He gas. The reaction of the spallation neutrons with this gas will result in the bombardment of the interior of these tubes with energetic tritons and protons. For APT to be able to meet its tritium production goals, it is necessary that the holdup of the tritium in the tube walls be minimized. To examine the tritium retention characteristics of stainless steel, one of the tube reference materials, accelerator implantation experiments were performed. In these experiments, deuterium was used in place of tritium to eliminate the problem of tritium contamination. Deuterons with energies up to 200 keV and protons with energies up to 600 keV were implanted into stainless steel (SS 316L) samples to fluences as high as 5 × 10 22 D/m 2 and 5 × 10 22 p/m 2. Thermal desorption spectroscopy showed that approximately 3% of the deuterium was retained within the sample. Approximately 0.5% of the deuterium permeated through to the back surface of the sample where a zirconium getter trapped the deuterium. The deuterium trapped in the zirconium layer was measured by nuclear reaction analysis. Eight-micron thick copper and nickel coatings were applied to the implantation side of the stainless steel substrate in an attempt to reduce the retention and permeation of the deuterium. The copper-coated stainless steel was not successful in reducing the retention and permeation, however the nickel coated stainless steel reduced both the retention and permeation substantially.

Radiogenic helium isotope fractionation: the role of tritium as 3He precursor in geochemical applications

Reduced 4He/ 3He ratios, e.g., down to ≈1/100 times those expected from radiogenic production, were observed in sedimentary rocks. Formation and history of these rocks eliminate a contribution of mantle 3He-bearing fluid. To explain the difference between the observed and the calculated production 4He/ 3He ratios Loosli et al. (1995) and Tolstikhin et al. (1996) suggested a different behaviour of helium and tritium in damage tracks produced by emission of these nuclides. Generally, the tracks cross grain boundaries or some imperfections within a rock or mineral allowing a fast loss of noble 4He and 3He atoms. However, radiogenic 3He has the precursor 3H, generated in the exothermic 6Li(n t, α) 3H + 4.5 MeV reaction. The energetic tritons produce damage tracks comparable with those from α-decay of U and Th series. If 3H is chemically bound within a track, and the track is able to recover via some diagenetic process before the 3H decay, then 3H and daughter 3He atoms are trapped within the recovered track. This mechanism would explain the shorter residence time of 4He in the rocks/minerals than of 3He; therefore, 4He/ 3He ratios could decrease through time. To check this mechanism 4He, 3H, and 3He (from 3H-decay) were produced by the above reaction in special targets, consisting of layered composites of thin sections of quartz, sample, Li-bearing cover, sample, and quartz. The samples were the same rocks in which reduced 4He/ 3He ratios have been previously observed. Each target was placed in a quartz ampoule, which was then pumped out, sealed off, and then exposed to the flux of thermal neutrons in a reactor. After irradiation and cooling down (total duration 145 days), the nuclides produced during ( 3H, 3He, 4He) and after ( 3He) irradiation were measured in the gas phase above the targets and compared with their total quantities expected from the Li abundance and the integrated neutron flux. The ratios obtained were 3H(gas)/ 3H(total) < 0.05 and 3He(gas)/ 3He(total) varying from 0.2 to 0.9. The average residence times τ of 3H and 3He, respectively, were estimated to be ≈16 and ≈0.25 yr for this first time interval, which included the irradiation of the targets. After these first measurements, the targets were kept in a vacuum system under room temperature for 210 days and the amounts of 3H and 3He, which accumulated above the targets during this second time interval under fully controlled conditions, were also measured. Much slower rates of gas loss from the same targets with average residence times of τ( 3H) ≈ 600 yr and τ(He) ≈ 1.6 yr resulted for this second time interval. Probably these longer residence times are closer to those in the relevant natural environments, the 3H residence time being much longer than the 3H half-life. In all cases the inequality τ( 3He) ≪ τ( 3H) is valid. This confirms the proposed scenario envisaging longer retention of 3H than He in damage tracks. Within the frame of this scenario the life-time of 3H gives a time constraint on diagenetic processes; at least one to several newly formed atomic layers should appear during ∼10 yr to recover the tracks.

Bulk ion heating with ICRH in JET DT plasmas

Reactor relevant ICRH scenarios have been assessed during DT experiments on the JETtokamak using H mode divertor discharges with ITER-like shapes and safety factors. Deuteriumminority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, anICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermaldeuterons and thermal tritons. The Q value of the steady state discharge reached 0.22 for thelength of the RF flat-top (2.7 s), corresponding to three plasma energy replacement times. TheDoppler broadened neutron spectrum showed a deuteron energy of 125 keV, which was optimumfor fusion and close to the critical energy. Thus, strong bulk ion heating was obtained at the sametime as high fusion efficiency. Deuterium fractions around 20% produced the strongest ionheating together with a strong reduction of the suprathermal deuteron tail. The ELMs had low amplitude and high frequency and each ELM transported less plasmaenergy content than the 1% required by ITER. The energy confinement time, on the ITERH97-Pscale, was 0.90, which is sufficient for ignition in ITER. 3He minority heating, in approximately50:50 D:T plasmas with up to 10% 3He, also demonstrated strong bulk ion heating. Central iontemperatures up to 13 keV were achieved, together with central electron temperatures up to 12 keV. The normalized H mode confinement time was 0.95. Second harmonic tritium heatingproduced energetic tritons above the critical energy. This scheme heats the electrons in JET,unlike in ITER where the lower power density will allow mainly ion heating. The invertedscenario of tritium minority ICRH in a deuterium plasma was demonstrated as a successfulheating method producing both suprathermal neutrons and bulk ion heating. Theoreticalcalculations of the DT reactivity mostly give excellent agreement with the measured reactionrates.

Study of radiation damage inLi2O by means of electron-spin resonance

ESR spectra with a hyperfine structure consisting of more than 20 peaks were observed in Li/sub 2/O single crystals, irradiated by energetic tritons and helium ions formed by 6 x 10/sup 22/ to 6 x 10/sup 25/ /sup 6/Li(n,..cap alpha..)-/sup 3/H reactions per m/sup 3/ introduced with the order of 10/sup 20/ to 10/sup 23/ thermal neutrons per m/sup 2/. The g value of the spectra was 2.002 +- 0.001, and the hyperfine structure depended on the orientation of the specimens relative to the applied magnetic field. From the angular dependence the spectra were attributed to F/sup +/ centers (an oxygen vacancy trapping an electron). In Li/sub 2/O single crystals irradiated to the order of 10/sup 23/ thermal neutrons per m/sup 2/, colloidal Li metal centers were not observed, in direct contrast with the situations in sintered Li/sub 2/O pellets. Isochronal and isothermal annealing experiments were done and the recovery behavior of the F/sup +/ centers was observed. From the isothermal annealing experiments, the activation energy for recovery of the F/sup +/ centers was determined to be about 135 kJ/mol (1.4 eV).

A new method for the determination of tritium-diffusion coefficients

A new method for the determination of volume diffusion coefficients of tritium in metallic foil was developed. This method is based on the production of a rectangular concentration profile of the radionuclide in the sample. This was accomplished by irradiating the sample with energetic tritons formed in a Cu-target bombarded with, for instance, 104 MeV α-particles. The main advantage of the method described is that the influence of the oxide surface layer on the hydrogen diffusion is avoided.

Confinement of Fusion-Produced Tritium in the Princeton Large Torus

Tritium is produced in deuterium discharges in the Princeton Large Torus by the reaction D($d$,$p$)T. These tritons undergo reactions D($t$,$n$)$^{4}\mathrm{He}$ creating 14-MeV neutrons which have been detected by two independent techniques at a level of 1% of the 2.5-MeV neutrons from the reaction D($d$,$n$)$^{3}\mathrm{He}$. The magnitude of the 14-MeV neutron emission is consistent with finite banana width, neoclassical predictions for the confinement of the energetic tritons.