Development of TESPEL Configurations for Plasma Diagnostics

Abstract

A Tracer-encapsulated Solid Pellet (TESPEL) injection method has been shown to be useful for many fields of plasma diagnostics. In order to provide more flexibility for such plasma diagnostics, we are developing various types of TESPELs. Here we report a new type TESPEL with a thin shell made of poly-dichlorostyrene useful for the impurity transport study. The new multilayer TESPEL configuration is also discussed. This type of TESPEL is not only useful for impurity transport study but also for the neutron observation due to the transient increase of the deuterium in the local position in the plasma for the upcoming LHD D-D experiment. A Tracer-encapsulated Solid Pellet (TESPEL) injection method has contributed so far to the various fields of plasma diagnostics as reported [1]. However, we noticed that more detailed control of the tracer deposition location is necessary in particular for promoting the impurity transport study. Thus, we are developing new TESPEL configurations for aiming at more flexible tracer deposition with a multilayer concept. From the technical viewpoint, regarding the production of the conventional TESPEL, we need to first drill a polystyrene ball, then insert relevant tracers, and, finally, cover the hole with a tiny polystyrene ball as a lid. Making TESPELs thus requires much time. Moreover, the tracer material is often in an irregular form, so it is difficult to estimate the precise amount of the tracer, although it is possible to estimate the amount roughly from the visually observed volume. In order to solve these problems, a new type TESPEL with mixing tracers as a compound or as a colloidal dispersion in the base polystyrene becomes a candidate.Recently we reported two types of TESPEL configurations [1], namely, (i) thick-shell-type and (ii) thin-shelltype [2] for the accurate impurity transport diagnostics. In particular, it was shown that the impurity transport feature depends on the source location of the impurity and also on the collisionality (between impurities and bulk ions).Regarding (i), the polystyrene ball (polymer in the form of (C 8 H 8 ) n ) with a diameter of up to 0.9 mm is drilled with a typical diameter of 0.2 mm, and the tracers are put inside the polystyrene ball, and then the hole is covered with a tiny polystyrene ball (a typical diameter of 0.2 mm) as a lid. In case (ii), the polystyrene shell with a typical shell thickness of ∼75 µm is produced for the shallower

A review of impurity transport characteristics in the LHD

“…In this case, with type (iii), the thin shell is made of Poly-2,6dichlorostyrene: (C 8 H 6 Cl 2 ) n . In fact, quite different impurity transport features were observed in cases (ii) and (iii) (see figure 1 of[59]). Namely, the Li-like emission of the tracer V deposited inside (ρ ~ 0.85) decays much more slowly than that of the tracer Cl deposited in the outer region (ρ = 0.85-1).…”

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

“…In the research of magnetically confined high temperature plasmas aiming for establishing a fusion reactor, impurity transport is one of the critical issues to be clarified. In order to promote a definitive study of impurity transport in the magnetically confined plasma, a Tracer-Encapsulated Solid Pellet (TESPEL) 1 has been developed at National Institute for Fusion Science in Japan. Simply stated, the TESPEL is a double-layered impurity pellet.…”

A spherical shell pellet injection system for repetitive laser engagement

“…The shell target contains a CD with a diameter of 488 ± 3.8 µm and a thickness of 6.9 ± 0.3 µm. These shells were fabricated through a density-matched emulsion method [31][32][33], which allows a shell diameter and thickness range from 100 to 1500 µm, and 3 to 15 µm, respectively with a sphericity of 99.8%, thickness uniformity of 99.3%, and surface smoothness of below 0.1 µm [31]. The shell surface roughness was not directly measured in our experiment but optical microscopic images (see figure 1 left bottom) indicate that they seem to be below the resolution threshold: 10 µm.…”