LNF:Estudios del confinamiento mejorado y de impurezas en los Stellarators TJ-II y W7-X mediante inyección de pastillas criogénicas (Pellets) y encapsulados de impurezas (TESPEL): Difference between revisions

The goal of this project, which falls within the realm of magnetic confinement nuclear fusion, is to continue research initiated in projects ENE2013-48679-R and FIS2017-89326-R on fuelling and impurity control in plasmas created in the stellarators TJ-II (Ciemat, Madrid) and W7-X (Greifswald, Germany). Further research to resolve these issues is critical to demonstrate steady-state operation of helical-type fusion reactors, in particular to identify operational scenarios that ensure adequate plasma fuelling and avoidance of impurity accumulation. This project will contribute to the development and scientific exploitation of stellarators, a priority highlighted in the document "Fusion Electricity: a roadmap to the realization of fusion energy" (EFDA 2012).

1. The first aim is continue our research on plasma fuelling using cryogenic pellets. Many aspects related to the pellet ablation and to the effects of fuel pellets on plasma magnetic activity, plasma turbulence and plasma performance are still not fully understood. For this work, the medium-sized heliac TJ-II will be used. It is equipped with a cryogenic pellet injector (PI) for producing solid hydrogen pellets that can be injected at high velocity into the plasma from its outer plasma edge. In previous projects, it was found that a pellet injection enhances plasma performance significantly in the TJ-II (increased core density and ion temperature, higher stored energy, longer particle confinement, higher plasma beta). More recently, it was possible to reach record values for plasma performance using a train of such pellets. Understanding the physics behind such observations will be a key part of this project.

2. The second aim is to continue to support impurity transport and accumulation studies in TJ-II and W7-X. Under the umbrella of a trilateral collaboration (2020-2029) with the National Institute for Fusion Science (Japan) and IPP-Max-Planck (Greifswald, Germany), Tracer-Encapsulated Solid Pellet (TESPEL) injections systems are now operated on both TJ-II and W7-X. TESPELs are polystyrene spheres (diameter <1 mm) loaded with impurity tracers (atomic elements other than fuel). This allows delivering a precise quantify of tracer to a preselected location in the plasma core, after which its transport and confinement can be studied. An important aspect of the collaboration has been the establishment of a laboratory to fabricate TESPELs at Ciemat for both devices (project FIS2017- 89326-R). Key parts of this current project are to continue TESPEL fabrication for TJ-II and W7-X at this laboratory, thereby allowing Ciemat to maintain this fruitful collaboration, and to upgrade a vacuum ultraviolet spectrometer on TJ-II to provide important spectral line data for impurity identification in W7-X.

The aim of this project, which falls within the realm of magnetic confinement fusion, is to continue and broaden the research initiated in projects ENE2013-48679-R, FIS2017-89326-R and PID2020-116599RB-I00 on fuelling and impurity control in plasmas created in the stellarators TJ-II (Ciemat, Madrid), W7-X (Greifswald, Germany) and LHD (Toki, Japan). This research aims to investigate issues related to these two issue which are critical for achieving steady-state operation of helical-type fusion reactors. In particular, it is necessary to identify operational scenarios that ensure adequate plasma fuelling and short impurity confinement times, in particular, for heavy ions. It is intended that this work will support the European stellarator programme and contribute to the development and scientific exploitation of stellarators, a priority highlighted in the document "Fusion Electricity: a roadmap to the realization of fusion energy" (EFDA 2012).

1. The first aim is to investigate aspects of plasma fuelling that are still not fully understood and the effects of fuel pellets on plasma magnetic activity, plasma turbulence and plasma performance. For this, the medium-sized heliac TJ-II will be used. It is equipped with a cryogenic pellet injector (PI) for producing solid hydrogen pellets that can be injected at high velocity into the plasma. It is intended to investigate pellet fuelling as a means to enhance plasma confinement (higher stored energy, longer particle confinement) and to identify and explore new pellet phenomena. While TJ-II is equipped with a large number of modern diagnostics, it is proposed to develop a new system to measure pellet cloud density and temperature to extend knowledge of pellet physics.

2. The second aim is to continue to support impurity transport studies in TJ-II, W7-X and LHD. Under the umbrella of a trilateral collaboration (2020-2029) with the National Institute for Fusion Science (Japan) and IPP-Max-Planck (Greifswald, Germany), the Tracer-Encapsulated Solid Pellet (TESPEL) method is now employed on TJ-II, W7-X and LHD. TESPELs are polystyrene spheres (diameter <1 mm) loaded with small quantities of selected impurities (atomic elements other than fuel). Thus, TESPEL allows delivering a precise quantify of tracer to a preselected location in the plasma core, after which its transport can be studied. An important aspect of previous projects was the establishment of a laboratory to fabricate TESPELs at Ciemat. Thus, key parts of this current project are to continue TESPEL fabrication at this laboratory, thereby allowing Ciemat to maintain its fruitful collaborations with W7-X and LHD. Finally, during recent experiments with TESPEL at LHD, project members observed that mid-/high- Z atomic ions can be flushed out of the high-density plasmas if Li-granules are dropped continuously into the plasma edge. Compared to reference plasmas, confinement times of high-Z ions were reduced by a factor 4 or more. Further work will be done here to explore this new operational scenario for stellarators.  

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