Tracer Impurities Research Articles

Commissioning of the tracer-encapsulated solid pellet (TESPEL) injection system for Wendelstein 7-X and first results for OP1.2b

Abstract A new tracer-encapsulated solid pellet (TESPEL) injection system was successfully commissioned for the stellarator fusion experiment Wendelstein 7-X (W7-X) during its OP1.2b operational campaign. TESPELs are polystyrene encapsulated solid pellets loaded with tracer impurities that have been employed in other stellarator devices for impurity transport studies. During the OP1.2b campaign approximately 140 pellet injections were performed with a successful delivery rate of 89%, thus this system has proven to be very reliable. Here, the experimental set-up and methodology are described first. In addition, it is outlined how, through the analysis of TESPEL time-of-flight signals and of the temporal evolution of line emissions originating from shell and tracer species as well as comparisons with ablation models, the radial localization of the deposited tracer is determined. This contribution also provides a general overview of the TESPEL injector performance during OP1.2b, discusses the global effects of TESPEL injections on W7-X plasmas and reports on first results in terms of a summary of TESPEL injections, plasma response to TESPELs, the post-deposition evolution of tracer spectral emission lines and soft x-ray emissions.

Dynamics of enhanced neoclassical particle transport of tracer impurity ions in ion temperature gradient driven turbulence

Tracer impurity transport in ion temperature gradient-driven (ITG) turbulence is investigated using a global full-f gyrokinetic simulation including kinetic electrons, bulk ions, and low to medium Z tracer impurities, where Z is the charge number. It is found that in addition to turbulent particle transport, enhanced neoclassical particle transport due to a new synergy effect between turbulent and neoclassical transports makes a significant contribution to tracer impurity transport. Bursty excitation of the ITG mode generates non-ambipolar turbulent particle fluxes of electrons and bulk ions, leading to a fast growth of the radial electric field following the ambipolar condition. The divergence of E × B flows compresses up-down asymmetric density perturbations, which are subject to transport induced by the magnetic drift. The enhanced neoclassical particle transport depends on the ion mass because the magnitude of up-down asymmetric density perturbation is determined by a competition between the E × B compression effect and the return current given by the parallel streaming motion. This mechanism does not work for the temperature and, thus, selectively enhances only particle transport.

Observation of the TESPEL-injected impurities behaviour by the PHA system at Wendelstein 7-X

The Pulse Height Analysis (PHA) system at Wendelstein 7-X (W7-X) is a broad energy range spectrometer dedicated to the observation of impurity behaviour during the plasma discharges. The system measures spectra in the range between 0.5 and 20 keV, from integrated signals along three lines of sight that pass close to the plasma center. During the recent operational phase of W7-X, the TESPEL (Tracer-Encapsulated Solid PELlet) injector has been commissioned. By this injector, polystyrene pellets with a single tracer impurity as well as multiple tracer impurities have been injected into the W7-X plasmas. The multiple tracer impurities injection allows studying the behavior of various impurities in the same plasma condition. The time resolution of the PHA system in the order of 60–100 ms, is smaller than typical confinement times and sufficient to observe differences in temporal behaviour of injected impurities. The collected PHA spectra consist of Kα lines of vanadium, manganese and nickel in case of the triple tracers, and of iron or copper in case of the single tracer. The temporal evolution of these Kα lines has been obtained in various plasma conditions, which will be presented and discussed in this paper.

Development of a tracer-containing compact-toroid injection system.

The accumulation and behavior of impurities is one of the most important subjects in the development of magnetically confined fusion reactors because impurities can potentially cause cooling and worsen the confinement of the hot core plasma. Tracer-encapsulated solid pellets (TESPELs) have demonstrated some results for impurity injection for fusion-reactor plasma studies [N. Tamura et al., J. Phys. Conf. Ser. 823, 012003 (2017)]. However, the TESPEL technique has several shortcomings, for example, the penetration depth and the amounts of tracer impurities. In the present study, we have developed a tracer-containing, compact-toroid (TCCT) injection system that utilizes a magnetized coaxial plasma gun (MCPG). The discharge current through the MCPG sputters and ionizes the electrode material, and the Lorenz self-force accelerates it as a plasmoid. The MCPG easily accelerates a magnetized plasmoid to speeds greater than the ion thermal velocity of several tens of kilometers per second. The accelerated and ejected plasmoid that contains the tracer ions is itself a warm, ionized plasma. Therefore, a TCCT can potentially be injected into the core region of a target plasma with less adverse effect.

Dissociation of methane and nitrogen molecules and global transport of tracer impurities in an ASDEX Upgrade L-mode plasma

We model the dissociation of injected methane (13CH4) and nitrogen (15N2) molecules and the subsequent transport of tracer ions in ASDEX Upgrade (AUG) low confinement (L-mode) plasma conditions resembling a tracer injection experiment conducted in 2011. Based on simulations with the ERO code, the dissociation is predicted to occur relatively close to the injection port in the far-scrape-off layer (far-SOL) plasma with the dissociation location moving closer to the injection location with increasing plasma density and heating power. Simulations of global transport of the tracer ions resulting from the dissociation using the ASCOT code predict that the decreasing penetration depth of the molecules (dissociation in the far-SOL) increases the ratio between main chamber and divertor deposition.

Transport characteristics of tracer and intrinsic impurities depending on the density of LHD plasmas

By injecting a tracer-encapsulated solid pellet (TESPEL) with triple tracers, V (Z = 23), Mn (Z = 25) and Co (Z = 27), into a plasma in the Large Helical Device (LHD), it was found in the high-density case (typically ne = (5–7) × 1019 m−3) that the Kα emissions from the intrinsic impurities were strongly suppressed, while those from all the three tracers were retained for a long time. In the medium-density case (typically ne = (3–4) × 1019 m−3), Kα emissions from the intrinsic impurities were observed clearly, while those of the tracer impurities were observed to decay much faster than the high-density case. When the intrinsic impurities penetrate into the plasma core through the plasma periphery in the plasma build-up phase under relatively low-density conditions, then such impurities are found to be kept for a long time in the later phase under high-density conditions. By implementing supersonic Ar gas puffing in addition to the TESPEL injection, Ar Kα emission was clearly observed together with Kα emissions from the tracers in the medium-density case. In contrast to this, Ar Kα emission was completely suppressed in the high-density case. This result shows that the suppression of the intrinsic impurity coming from outside the plasma is indeed working in the high-density case, while the impurities deposited inside the plasma are kept for a long time.

Multiple-tracer TESPEL injection for studying impurity behaviour in a magnetically confined plasma

A new diagnostic method with tracer-encapsulated solid pellet (TESPEL) injection with multiple tracers is developed to study impurity behaviour in a magnetically confined plasma. If a pellet contains multiple tracers, it becomes possible to compare the behaviour of different impurities simultaneously under the same plasma conditions. We injected a TESPEL into the Large Helical Device mainly with triple tracers: vanadium (V), manganese (Mn) and cobalt (Co). The Li-like lines in the vacuum ultraviolet range and the Kα lines in the soft x-ray range from these tracers are simultaneously observed with a time resolution of 50 ms. As the charges of the nuclei of intrinsic impurities, chromium (Cr) and iron (Fe), are in between those of the tracers, the behaviour of Cr and Fe can be studied quantitatively by knowing the number of tracer particles and also by comparing the emission intensity change due to the electron temperature change. It is observed that the tracer impurities remain in the plasma core region when the plasma density is higher than 5 × 1019 m−3. It is also observed that the intrinsic impurities cannot enter the core region when the plasma density is higher than the same level, although the two phenomena appear to be independent.

Dissipative homogeneous Maxwell mixtures: ordering transition in the tracer limit

The homogeneous Boltzmann equation for inelastic Maxwell mixtures is considered to study the dynamics of tracer particles or impurities (solvent) immersed in a uniform granular gas (solute). The analysis is based on exact results derived for a granular binary mixture in the homogeneous cooling state (HCS) that apply for arbitrary values of the parameters of the mixture (particle masses m i , mole fractions c i , and coefficients of restitution α ij ). In the tracer limit (c 1 → 0), it is shown that the HCS supports two distinct phases that are evidenced by the corresponding value of E 1/E, the relative contribution of the tracer species to the total energy. Defining the mass ratio $${\mu\equiv m_1/m_2}$$ , there indeed exist two critical values $${\mu_{\rm HCS}^{(-)}}$$ and $${\mu_{\rm HCS}^{(+)}}$$ (which depend on the coefficients of restitution), such that E 1/E = 0 for $${\mu_{\rm HCS}^{(-)}<\mu<\mu_{\rm HCS}^{(+)}}$$ (disordered or normal phase), while $${E_1/E\neq 0}$$ for $${\mu<\mu_{\rm HCS}^{(-)}}$$ and/or $${\mu>\mu_{\rm HCS}^{(+)}}$$ (ordered phase).

Derivation of local impurity transport quantities from soft-x radiation evolution during tracer injection at W7-AS

A frequently used method for determination of impurity transport coefficients is the analysis of the radial and temporal time evolution of the impurity density of injected tracer impurities. In cases when the impurity density cannot be measured directly, e.g., by active charge exchange recombination spectroscopy, the impurity density has to be reconstructed from impurity radiation as measured, e.g., by the soft-x camera. Due to the energy integrated information, assumption about the spatial ionization state distribution, e.g., coronal equilibrium, is necessary. This might not be valid in cases of large transport. Consequently, the transport quantities, derived under the preliminary assumption of radial coronal distribution, are used again in an iterative way to improve the density reconstruction in order to approach more realistic transport quantities. The procedure was applied to plasma discharges in the stellarator Wendelstein 7-AS.

Determination of Non-Protein-Bound Plasma 1,25-Dihydroxyvitamin D by Symmetric (Rate) Dialysis

Most of the total circulating 1,25-dihydroxyvitamin D (1,25(OH)2D) is bound to plasma proteins, mainly vitamin D-binding protein (DBP) and albumin. Only a small fraction in plasma exists in the free form. It is widely assumed that the non-protein-bound free hormone reflects the biologically active fraction. We describe a dialysis method for the determination of plasma free 1,25(OH)2D which is relatively easy to perform. In this symmetric or “rate” dialysis method, identical samples are placed at both sides of a membrane. At one side, tritiated 1,25(OH)2D is added and the rate of transfer of this tritiated 1,25(OH)2D through a dialysis membrane is directly related to the free fraction of plasma 1,25(OH)2D.This method is much less susceptible toward tracer impurities than indirect equilibrium dialysis and centrifugal ultrafiltration. Moreover, it requires much less tracer. The intraassay coefficient of variation for the determination of the free fraction is 1.0%; the interassay variation is 7.7%. Comparison of the free fraction of 23 samples assessed with both centrifugal ultrafiltration and symmetric dialysis showed much higher values using the former method. No significant correlation between the two methods was found. The free fraction of 1,25(OH)2D in normal subjects as assessed with symmetric dialysis ranges from 0.049 to 0.103%.

Impurity Diffusion in SiGe Alloys: Strain and Composition Effects

ABSTRACTThe advent of epitaxial-growth techniques dedicated to the growth of epitaxial layers of group-IV semiconductors and their alloys has opened up new possibilities for novel types of diffusion experiments and thus for critical tests of existing diffusion theories. It is now possible to grow test structures for diffusion consisting of well-defined narrow distributions of tracer impurities in epitaxial layers of biaxial-strained or relaxed alloys of different compositions. This enables studies of alloy effects and/or strain effects on diffusion with varying strain energy for a well-defined type of strain (tensile or compressive). A review is presented on recently published results on impurity diffusion in strained and relaxed, epitaxial SiGe alloy layers. As almost all these studies have focused on boron and antimony diffusion, these two impurities will have the leading roles in this review.

Impurity Diffusion in SiGe Alloys: Strain and Composition Effects

ABSTRACTThe advent of epitaxial-growth techniques dedicated to the growth of epitaxial layers of group-IV semiconductors and their alloys has opened up new possibilities for novel types of diffusion experiments and thus for critical tests of existing diffusion theories. It is now possible to grow test structures for diffusion consisting of well-defined narrow distributions of tracer impurities in epitaxial layers of biaxial-strained or relaxed alloys of different compositions. This enables studies of alloy effects and/or strain effects on diffusion with varying strain energy for a well-defined type of strain (tensile or compressive). A review is presented on recently published results on impurity diffusion in strained and relaxed, epitaxial SiGe alloy layers. As almost all these studies have focused on boron and antimony diffusion, these two impurities will have the leading roles in this review.

Radiation Enhanced Tracer Diffusion in Intermetallic Alloys

ABSTRACTRadiation enhanced diffusion in the intermetallic alloy Cu3Au was measured directly for the first time. Ni and Pd tracer impurities were used to probe the diffusion of atoms occupying Cu and Au sublattice site's, respectively. Measurements were performed temperatures both above and below the ordering temperature, of TC =390°C The ability to grow high quality Cu3Au films by MBE with buried marker layers in the form of either (Cu2Ni)Au or Cu3(AuPd), made these measurements possible. Under irradiation with 0.75MeV Ar+ ions, the diffusion of Ni is approximately equal to that of Pd above Tc, but smaller below Tc. In the absence of irradiation, the thermal diffusion coefficients reveal a similar trend. Activation energies for RED for the two species are similar both above and below Tc whereas, in absence of irradiation, the activation enthalpies below Tc are different. These results are interpreted on the basis of lattice order.

Measurement of the unbound fraction of long chain nonesterified fatty acids (NEFA) in serum: Methodological considerations

Long-chain nonesterified fatty acids (NEFA) are extensively bound to albumin; knowledge of their unbound concentrations is important in evaluating the numerous biologic effects attributed to these compounds. We measured the unbound fraction of five long-chain NEFA in serum using the equilibrium partition of 14C-NEFA between heptane and aqueous phases. Commercial 14C-NEFA preparations gave non-linear estimates of unbound fraction with serum dilution, consistent with the presence of polar tracer impurities, but 14C-NEFA purified by alkaline ethanol extraction gave an approximately linear relationship between unbound fraction and serum dilution over a 4096-fold range of dilution, provided that pH of the aqueous phase remained stable. Mean unbound percentages were: myristic acid 0.0066, linolenic acid 0.0019, arachidonic acid 0.0017, oleic acid 0.00078 and palmitic acid 0.00061. These data suggest that some previous studies appear to have overestimated the free fraction of long-chain NEFA at physiological albumin concentrations by at least one order of magnitude.

Ion beam mixing and radiation-enhanced diffusion in metallic glasses

Measurements of ion beam mixing and radiation-enhanced diffusion in metallic glasses were made in the temperature range 10–525 K. It was observed that the mixing of tracer impurities at low temperatures was greater in the metallic glasses than in the constituent elements for NiZr and CuEr systems, and that it was about the same for the NiTi system. It was also observed in the NiZr glass that the rate of ion mixing is somewhat larger for copper than for gold tracer impurities but does not depend on the exact composition of the alloy. At higher temperatures, the rate of ion mixing was found to be dependent on temperature and ion flux in amorphous NiZr (a-NiZr), indicating that radiation-enhanced diffusion occurs in metallic glasses. The apparent activation enthalpy for diffusion in a-NiZr during ion irradiation is about 0.5 eV, and depends on the square root of the flux.

Low-temperature ion-beam mixing in metals.

A systematic study of ion-beam mixing of tracer impurities in thin metal films at low temperatures has been conducted. We have investigated the dependence of ion mixing on two matrix properties: atomic mass and cohesive energy. We have also studied the dependence of ion mixing on tracer impurity properties: its heat of mixing with the matrix and its thermal diffusivity in the matrix. The matrices investigated were thin films of C, Al, Ti, Fe, Ni, Cu, Mo, Ru, Ag, Hf, Ta, W, Pt, and Au. The tracer impurities, Al, Ti, Cr, Mn, Fe, Ni, Cu, Y, Nb, Mo, Ru, Ag, In, Sb, Hf, Ta, W, Pt, Au, and Bi, were deposited as \ensuremath{\le}15 A\r{} layers near the midplanes of the specimens. All the tracer and matrix elements, except C, were deposited sequentially, without breaking vacuum. The samples were irradiated with 300--1000-keV Kr ions to doses ${10}^{15}$--${10}^{16}$ ions/${\mathrm{cm}}^{2}$ at temperatures of 6 and/or 77 K. Most samples were analyzed at the irradiation temperature by He backscattering. A strong correlation between ion mixing and the matrix properties, atomic mass, and cohesive energy, was observed. A correlation between ion mixing and tracer impurity diffusion was also observed but not between ion mixing and the heat of mixing or relative mass of the impurity with the matrix. The results are interpreted within the framework of a thermal spike model of cascade diffusion.

Ion Beam Mixing In Binary Amorphous Metallic Alloys

ABSTRACTIon beam mixing (IM) was measured in homogeneous amorphous metallic alloys of Cu-Er and Ni-Ti as a function of temperature using tracer impurities, i.e., the so called “marker geometry”. In Cu-Er, a strong temperature dependence in IM was observed between 80 K and 373 K, indicating that radiation-enhanced diffusion mechanisms are operative in this metallic glass. Phase separation of the Cu-Er alloy was also observed under irradiation as Er segregated to the vacuum and SiO2 interfaces of the specimen. At low-temperatures, the amount of mixing in amorphous Ni-Ti is similar to that in pure Ni or Ti, but it is much greater in Cu-Er than in either Cu or Er.