LNF:Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL)
LNF - Nationally funded project
Title: Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL)e
Reference: PID2020-116599RB-I00
Funding Umbrella: Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023
Funding Programme: Proyecto de investigación subvencionado por el Ministerio de Ciencia e Innovación
Subprogramme: Proyectos de I+D+i Retos Investigación
Programme and date: Proyectos I+D+i 2020
Programme type/ Modalidad:
Area/subarea: Physical Sciences / Physics and its applications
Principal Investigator(s): Kieran Joseph McCarthy María Isabel García Cortés
Project type: Investigación Orientada Tipo B
Start-end dates: 01/09/2021 - 31/08/2025
Financing granted (direct costs): 130000 €
Description of the project
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).
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.
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 hare 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 PI and TESPEL systems on TJ-II share a common injection guide lines. This unique set-up allows direct comparative studies of ablation, deposition and plasma response to be made thereby facilitating the understanding of common physics. Given that fuelling and impurity control are critical issues for stellarator steady-state operation, the project will allow us to continue to contribute to, and participate in, research programmes on W7-X, the stellarator of reference. Finally, team members have significant experience in the formation of young researchers at Master and PhD levels and in disseminating research to second level students and to the general public. A PhD student will undertake research in these areas during the project.
Main Results
1. A pellet-induced enhanced confinement regime (or PiEC) has been identified in Neutral Beam Injection (NBI) heated discharges made in TJ-II after the injection of a single cryogenic fuel pellet into its plasma core [1]. In addition to the expected increase in core electron density, the plasma diamagnetic energy content, as determined using a diamagnetic loop, is seen to rise by up to 40%, with respect to reference discharges without pellet injection. Furthermore, the energy confinement time is significantly enhanced when compared to predictions obtained using the 2004 International Stellarator Scaling law, ISS04. Indeed, the operational regimes of other stellarator devices, such as LHD and W7-X, can be similarly extended to performance well beyond those obtainable with gas puffing alone.
2. New studies, performed with multiple pellet injections, have extended the TJ-II operational regime well beyond limits previously achieved in this device using NBI heating and gas puff [2]. In order to achieve best results, it has been determined that the plasma target electron density should to be in the range 1x10^19 m^-3 to 2.5x10^19 m^-3 and time separations between pellets should be close to energy confinement times, around 10 ms. It is also found, using a Charge Exchange Recombination Spectroscopy diagnostic, that while the plasma electron temperature is almost unaffected by such pellet injections, the majority ion temperature irises significantly due to reduced ion radial heat fluxes during the PiEC phase. It is also found that enhanced performance is independent of whether co- or counter-NBI heating beam is employed. Finally, record stored diamagnetic energy content and plasma beta values are achieved when the largest available pellets are employed. The results indicate that pellet injections extend the operational regime well beyond limits previously achieved in TJ-II without pellets.
3. As noted above, improvement confinement associated with the injection of pellets has been observed in TJ-II during NBI phase of its plasmas. Using a simple model, the modification of turbulent transport by a pellet injection and how this modification affects particle confinement time has been studied [3]. The results indicate a relationship between improved confinement and the evolution of shear flows due to turbulence, especially near low order rational surfaces. Furthermore, experiments show that an additional pellet, or pellets, may enhance the confinement improvement produced by the first. This effect is reproduced in the model when the second density pellet is launched soon after the first one. For this to occur, the second pellet must be injected in the transient period, before the plasma returns to the steady state.
4. Pellet injection experiments are performed for several magnetic configurations of the TJ-II stellarator in order to increase our understanding of the role played by rational surfaces in plasmoid drift and deposition profiles in stellarators [4]. The analysis of plasmoid drifts during experiments is supported by simulations made with the code HPI2. Such plasmoid drifting is found to be significantly reduced, as is observed in tokamaks, in the vicinity of rational surfaces (rational surfaces have magnetic field lines that are periodic; i.e., the magnetic field lines close back on themselves). This is attributed to the fact that plasmoid external charge reconnection lengths become shorter close to rational surfaces, resulting in more effective damping of plasmoid drift. Although in stellarators, the effect of plasmoid external currents on drift is expected to be negligible, compared with plasmoid internal currents, this latter effect is clearly measurable in TJ-II. In addition, code simulations reveal that enhanced drift reductions near rational surfaces lead to significantly different deposition profiles for standard magnetic configurations in TJ-II. This implies that it should be possible to identify magnetic configurations that will result in more efficient pellet fuelling.
References
[1] Enhanced confinement induced by pellet injection in the stellarator TJ-II I. García-Cortes, K. J. McCarthy, T. Estrada, V. Tribaldos, B. van Milligen, E. Ascasíbar, R. Carrasco, A. A. Chmyga, R. García, J. Hernández-Sánchez, C. Hidalgo, S. Kozachek, F. Medina, D. Medina-Roque, M. A. Ochando, J. L. de Pablos, N. Panadero, I. Pastor and TJ-II Team, Phys. Plasmas 30 (2023) 072506. https://doi.org/10.1063/5.0151395
[2] Multi-pellet injection into the NBI-heated phase of TJ-II plasmas, K. J. McCarthy, I. García-Cortés, A. Alonso, A. Arias-Camisón, E. Ascasíbar, A. Baciero, A. Cappa, R. Carrasco, O. O. Chmyga, T. Estrada, R. García, J. Hernández-Sánchez, F. J. Herranz, O. S. Kozachok, B. López Miranda, F. Medina, D. Medina-Roque, B. van Milligen, M. Navarro, M. A. Ochando, J. L. de Pablos, N. Panadero, I. Pastor, J. de la Riva, M. C. Rodríguez, D. Tafalla, V. Tribaldos and TJ-II Team, Nucl. Fusion 64 (2024) 066019, https://doi.org/10.1088/1741-4326/ad4047.