Tracer-encapsulated Solid Pellet 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.

Overview of the TJ-II stellarator research programme towards model validation in fusion plasmas

TJ-II stellarator results on modelling and validation of plasma flow asymmetries due to on-surface potential variations, plasma fuelling physics, Alfvén eigenmodes (AEs) control and stability, the interplay between turbulence and neoclassical (NC) mechanisms and liquid metals are reported. Regarding the validation of the neoclassically predicted potential asymmetries, its impact on the radial electric field along the flux surface has been successfully validated against Doppler reflectometry measurements. Research on the physics and modelling of plasma core fuelling with pellets and tracer encapsulated solid pellet injection has shown that, although post-injection particle radial redistributions can be understood qualitatively from NC mechanisms, turbulence and fluctuations are strongly affected during the ablation process. Advanced analysis tools based on transfer entropy have shown that radial electric fields do not only affect the radial turbulence correlation length but are also capable of reducing the propagation of turbulence from the edge into the scrape-off layer. Direct experimental observation of long range correlated structures show that zonal flow structures are ubiquitous in the whole plasma cross-section in the TJ-II stellarator. Alfvénic activity control strategies using ECRH and ECCD as well as the relation between zonal structures and AEs are reported. Finally, the behaviour of liquid metals exposed to hot and cold plasmas in a capillary porous system container was investigated.

Development of a double-barreled Tracer Encapsulated Solid Pellet (TESPEL) injection system for LHD

For performing a study of impurity transport efficiently by using a Tracer-Encapsulated Solid Pellet (TESPEL) in Large Helical Device (LHD), a new double-barreled TESPEL injection system was recently developed and installed on the LHD. The TESPEL in each injection line can be injected with different timing and acceleration pressure which can be set individually. To prevent the high-pressured gas for the TESPEL acceleration from getting into the vacuum vessel of the LHD, a 3-stage differential pumping scheme was applied. Since the available space is very limited, the 3-stage differential pumping system is shared with each TESPEL injection line. And for the same reason, one of the guide tubes in one injection line of the new TESPEL injection system was purposely bent (bending radius of about 40 m). The guide tubes in the other injection line of the new TESPEL injection system are straight. We performed a laboratory test to check the TESPEL transferring capability with the guide tubes in the new injection system. The points of TESPEL impact on the target foil are found to be within the circles defined by a possible pellet divergence full angle, 2 degrees. Therefore, we confirmed that the TESPEL can pass through both the slightly bent and the straight injection lines of the new injection system without any problem. The blank shot test shows that the increment of the total pressure even by using both the injection lines simultaneously is much below the base pressure of the LHD vacuum vessel during the discharge. This result successfully demonstrated the capability of the applied 3-stage differential pumping scheme in the new injection system.

A new multi-tracer pellet injection for a simultaneous study of low- and mid/high-Z impurities in high-temperature plasmas

A new multi-tracer technique in the Tracer-Encapsulated Solid Pellet (TESPEL) method has been developed in order to acquire simultaneously the information about the behaviors of various impurities, i.e., to study concurrently the behaviors of low- and mid/high-Z impurities in magnetically confined high-temperature plasmas. In this new technique, an inorganic compound (for example, lithium titanate, Li2TiO3) is proposed to be used as a tracer embedded in the core of the TESPEL, instead of pure elements. The results of the proof-of-principle experiment clearly demonstrate the applicability of the new multi-tracer technique in the TESPEL method for the simultaneous study of behaviors of low- and mid/high-Z impurities in high-temperature plasmas.

Investigation of TESPEL cloud dynamics in Wendelstein 7-X stellarator

Tracer-encapsulated solid pellet (TESPEL) was injected from the outboard midplane into Wendelstein 7-X (W7-X) plasmas. Viewing from behind the flight path, the distribution of the light emissions from the TESPEL cloud particles (H, C and C2+) was recorded using an ultrafast visible camera with a temporal resolution up to 2 µs, which allows resolving both the pellet cloud evolution and the detachment of the drifting cloud. It was observed that both the B┴ and the B║ aligned dimensions of the pellet cloud fluctuate quasi-periodically, this being associated with a vertical cloud movement and an eruption/ejection of part of the cloud. It is found that, first, the pellet cloud expands parallel to the magnetic field lines for about 10 µs. Then, when the cloud reaches a certain B║ size the ionized part of the cloud (plasmoid) moves vertically (typically upwards) and detaches itself from the pellet within 10 µs. One possible explanation for the upward movement of the pellet cloud is the presence of a vertical component of grad(B) pointing downward resulting in an upward cloud drift.

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.

Integration of the TESPEL injection system at W7-X

Since impurities determine the plasma performance in magnetically-confined fusion plasmas, analyzing their behavior and controlling their confinement is significantly important for a stable and steady-state operation of fusion devices. A comparison of the impurity transport in the core region of large stellarators, such as Wendelstein 7-X and Large Helical Device (LHD) is quite useful to gain a better understanding of the impurity transport in reactor-relevant stellarator plasmas. Therefore, a Tracer-Encapsulated Solid Pellet (TESPEL) injection system has been newly installed for the operational phase OP1.2b of W7-X. The injector was designed and manufactured by NIFS to achieve a similar TESPEL deposition location as in LHD. Technical guidelines and functional specifications, addressing the specific conditions at W7-X had been implemented in close collaboration between NIFS and IPP. This resulted in a compact 3-stage differential pumping system considering spatial restrictions due to existing and future diagnostics, material requirements and limitations due to the magnetic stray field in the vicinity of the turbo pumps. This contribution reports on the design and the integration of the new TESPEL injection system to W7-X and first achievements during the commissioning of the system.

Tracer-Encapsulated Solid Pellet (TESPEL) injection system for Wendelstein 7-X.

Impurity confinement in fusion plasmas is mainly determined by transport mechanisms in the core region. For the Wendelstein 7-X stellarator, its island divertor is expected to screen effectively external impurity sources in the scrape-off layer at higher densities. However, the unique feature of Tracer-Encapsulated Solid Pellet (TESPEL) injection, releasing impurities at a well-localized radial position directly in the core plasma, enables investigating such transport mechanisms. This paper reports on the detailed design of a completely new TESPEL injection system, which has been designed by the National Institute for Fusion Science, Toki, Japan, and is currently being installed at Max-Planck-Institut für Plasmaphysik, Greifswald, Germany, for the Wendelstein 7-X. This injector consists of a storage and injection unit, attached to a system of guiding tubes which run through 3 successive differential pumping stages. A light-gate system and an optical observation system are used to determine the location of the deposited tracers. Laboratory tests carried out by shooting TESPELs onto a sample foil showed good performance after careful realignment of the guiding tubes.

Comparison of cryogenic (hydrogen) and TESPEL (polystyrene) pellet particle deposition in a magnetically confined plasma

A cryogenic pellet injector (PI) and tracer encapsulated solid pellet (TESPEL) injector system has been operated in combination on the stellarator TJ-II. This unique arrangement has been created by piggy-backing a TESPEL injector onto the backend of a pipe-gun–type PI. The combined injector provides a powerful new tool for comparing ablation and penetration of polystyrene TESPEL pellets and solid hydrogen pellets, as well as for contrasting subsequent pellet particle deposition and plasma perturbation under analogous plasma conditions. For instance, a significantly larger increase in plasma line-averaged electron density, and electron content, is observed after a TESPEL pellet injection compared with an equivalent cryogenic pellet injection. Moreover, for these injections from the low-magnetic-field side of the plasma cross-section, TESPEL pellets deposit electrons deeper into the plasma core than cryogenic pellets. Finally, the physics behind these observations and possible implications for pellet injection studies are discussed.

Observation of the ECH effect on the impurity accumulation in the LHD

Recent experiments in the Large Helical Device (LHD) clearly show the ability of electron cyclotron heating (ECH) to control accumulation of impurities, as demonstrated by external injection of a tracer-encapsulated solid pellet (TESPEL). The impurity introduced locally into the core region of the LHD plasma by the TESPEL has strongly accumulated in a high-density LHD plasma, as well as an intrinsic impurity, such as a carbon. When a high-power (1.5 MW) ECH is applied just after the TESPEL injection for such a plasma, the accumulation of the impurity injected by the TESPEL was almost completely suppressed. This result indicates that applying ECH enhances the outward radial impurity flux. Although a neoclassical ambipolar radial electric field in stellarators has a stronger impact on the transport, particularly on the impurity transport, than in tokamaks, the sign of the radial electric field measured and calculated is directed to a negative even in the 1.5 MW ECH case, resulting in the inward impurity flux. By revisiting the neoclassical formulation, an increment of ion temperature and its gradient could be one of the candidates for the explanation, since the ion temperature is also increased globally by ECH.

Improvements in a Tracer-Encapsulated Solid Pellet and Its Injector for More Advanced Plasma Diagnostics

A Tracer-Encapsulated Solid Pellet (TESPEL) has been developed for promoting a precise study of the impurity transport in a magnetically-confined high-temperature plasma. This paper gives a brief report of the recent improvements in the TESPEL and its injector for more advanced plasma diagnostics. The TESPEL can be considered as a double-layered impurity pellet. This structure enables us to produce a both poloidally and toroidally localized “tracer” impurity source in the plasma, and to specify the total amount of the tracer impurity deposited in the plasma precisely. Recent experiments on the Large Helical Device by using the TESPEL suggest that the importance of the impurity source location in the impurity transport study. Thus we have developed new-type TESPELs, which are greatly improved in regard to the above-mentioned unique features. In addition, we also developed a new TESPEL injector, which enables us to inject the TESPEL obliquely into the plasma. This injector can also contribute to a further shallower penetration of the TESPEL into the plasma.

Mitigation of the tracer impurity accumulation by EC heating in the LHD

The mitigation of a tracer impurity accumulation in the core region of high-temperature helical plasma was clearly observed by applying electron cyclotron heating (ECH) in the large helical device (LHD). In the LHD, the accumulation of impurities toward the centre of the plasma has been observed in a high-density regime. In this study, for observing clearly the behaviour of impurity ions in the plasma core, the extrinsic ‘tracer’ impurity was injected into that region by means of a tracer-encapsulated solid pellet (TESPEL). The high-density LHD plasma without ECH definitely shows the strong impurity accumulation, and then it causes the reduction in electron and ion temperatures in the core region. When ECH was applied just after the TESPEL injection, the accumulation of the tracer impurity ions was mitigated. Even after ECH was switched-off, the intensities of the line emissions from the highly-ionized tracer impurity were increased very slightly. The micro-turbulence measurement with a 2-dimensional phase contrast imaging diagnostic during ECH does not support the view that the change in the micro-turbulence would enhance the outward flow (an increase in a diffusive flux, a decrease in an inward convective flux and/or a change the direction of the convective flux from inward to outward) of the impurity ions. Moreover, at this moment, there is no conclusive data regarding a radial electric field measured with a charge exchange spectroscopy diagnostic to support the view that the change in the radial electric field would be attributed to the increment in the outward flow of the impurity ions from the core region of the LHD plasma.

Tracer-Encapsulated Solid Pellet (TESPEL) injection system for the TJ-II stellarator.

A tracer-encapsulated solid pellet (TESPEL) injection system for the TJ-II stellarator was recently developed. In order to reduce the time and cost for the development, we combined a TESPEL injector provided by National Institute for Fusion Science with an existing TJ-II cryogenic pellet injection system. Consequently, the TESPEL injection into the TJ-II plasma was successfully achieved, which was confirmed by several pellet diagnostics including a normal-incidence spectrometer for monitoring a tracer impurity behavior.

Development of a new tracer-encapsulated solid pellet injection system for more precise control of tracer-impurity-deposit location in LHD.

A new tracer-encapsulated solid pellet (TESPEL) injection system has been developed additionally for the LHD heliotron. This system has three-dimensionally bended guide tubes, which allows us to inject the TESPEL obliquely on a poloidal cross-section of the LHD plasma. Consequently, this system enables us to control a tracer-impurity-deposited location more precisely. Moreover, this system can make it possible for the tracer impurity to be deposited even only inside the ergodic region (outside a last closed flux surface of the LHD plasma). A stereoscopic fast camera system has been also developed for capturing a three-dimensional TESPEL trajectory.

Creation of Impurity Source inside Plasmas with Various Types of Tracer-Encapsulated Solid Pellet

A Tracer-Encapsulated Solid Pellet (TESPEL) was developed for promoting an impurity transport study in a magnetically-confined plasma. One of the advantages of the TESPEL is that it can make a three-dimensionally localized impurity source in the plasma. This enables us to inject the tracer impurity inside or in the vicinity of the region of interest. Recently, a new-type TESPEL with a thinner outer shell has been developed in order to achieve a shallower deposition of the tracer impurity. With the TESPEL having the thinner shell, we have achieved about 4 cm shallower deposition of the tracer impurity, compared with the case of the conventional thick-shell type TESPEL with the same outer diameter of about 700 µm. Moreover, for the achievement of the further shallower deposition of the tracer impurity, we also developed the TESPEL with a tracer-impurity-doped thin shell. After the injection of the TESPEL with the tracer-impurity-doped thin shell, the line emissions from the highly-ionized doped impurity are clearly observed with a vacuum ultraviolet spectrometer, which clearly demonstrates its ability to carry the impurity as a new tool.

Measurements of Radiation Power from High Z Impurities with TESPEL and Comparison with Theoretical Calculations

Utilizing the advantage of the Tracer-encapsulated Solid Pellet (TESPEL) injection method, the radiation power emitted from heavy atoms such as tungsten is measured knowing the total amount of the heavy atoms. The experimentally obtained radiation power is compared with the theoretical calculations. The TESPEL method is found to be useful for evaluating the absolute radiation power from the heavy atoms, although the present status is preliminary and further data should be accumulated. c � 2014 The Japan Society of Plasma Science and Nuclear Fusion Research

Observation of visible forbidden lines from highly charged tungsten ions at the large helical device

Visible line emission from highly charged tungsten ions has been observed at the large helical device (LHD) using a tracer encapsulated solid pellet. One of the measured lines is assigned to a magnetic-dipole (M1) line of the ground-term fine-structure transition of W26+. The other line is unidentified but probably due to a highly charged tungsten ion. Photon emission was observed at 40 lines of sight divided along the vertical direction of a horizontally elongated poloidal cross section of the LHD plasma. The line-integrated intensity of the M1 line along each line of sight indicates a peaked profile at the plasma center where the electron temperatures are high enough so that tungsten ions are highly ionized.

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.

Impurity Transport Study with TESPEL Injection and Simulation

Impurity behaviors in LHD are studied by a Tracer-encapsulated Solid Pellet (TESPEL) injection. By containing multiple tracers in a TESPEL, the different tracer species have been compared simultaneously under the same plasma condition. The density disturbance on the bulk plasma by TESPEL is typically less than 10 %. The amount of the tracer particles deposited locally inside a plasma is about a few 1017 particles which is smaller than that of the bulk plasma by a factor of three orders of magnitude. Triple tracers, V, Mn and Co are used, because the charges of nuclei of intrinsic impurities, Cr and Fe are in between those of the tracers. The impurity confinement behavior depends substantially on the electron density. In case of the density higher than ne = 5×1019 m−3, the tracer impurity in the plasma core was kept for a long time, while it decays in order of 500 ms in the medium density case. Such temporal behavior is compared with a STRAHL simulation code assuming diffusion coefficient and convection. The general behavior fits well with the emissivity value integrated along the sight line.

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.