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Title: Modification of the Alfvén wave spectrum by pellet injection

Abstract

Alfvén eigenmodes driven by energetic particles are routinely observed in tokamak plasmas. These modes consist of poloidal harmonics of shear Alfvén waves coupled by inhomogeneity in the magnetic field. In this work, further coupling is introduced by 3D inhomogeneities in the ion density during the assimilation of injected pellets. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum. We find that the frequencies of Alfvén eigenmodes drop dramatically when a pellet is injected in JET. From these observations, information about the changes in the ion density caused by a pellet can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, the 3D MHD codes Stellgap and AE3D were generalised to incorporate 3D density profiles. Additionally, a model for the expansion of the ionised pellet plasmoid along a magnetic field line was derived from the fluid equations. Thereby, the time evolution of the Alfvén eigenfrequency is reproduced. By comparing the numerical frequency drop of a toroidal Alfvén eigenmode (TAE) to experimental observations, the initial ion density of a cigar-shaped ablation region of length 4 cm is estimated to be n*=6.8×1022m-3 at the TAE location (r/a≈0.75). The frequency sweeping of an Alfvén eigenmode ends when themore » ion density homogenises poloidally. Modelling suggests that the time for poloidal homogenisation of the ion density at the TAE position is τh=18±4 ms for inboard pellet injection, and τh=26±2 ms for outboard pellet injection. By reproducing the frequency evolution of the elliptical Alfvén eigenmode (EAE), the initial ion density at the EAE location (r/a≈0.9) can be estimated to be n*=4.8×1022 m-3. Poloidal homogenisation of the ion density takes 2.7 times longer at the EAE location than at the TAE location for both inboard and outboard pellet injection.« less

Authors:
ORCiD logo [1]; ORCiD logo [2];  [3];  [3]; ORCiD logo [4]; ORCiD logo [5]
  1. Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies; Culham Science Centre, Abingdon (United Kingdom). Culham Centre for Fusion Energy (CCFE), EURATOM/UKAEA Fusion Association
  2. Culham Science Centre, Abingdon (United Kingdom). Culham Centre for Fusion Energy (CCFE), EURATOM/UKAEA Fusion Association
  3. Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies
  4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  5. Consorzio RFX, Padova (Italy)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES); EUROfusion Consortium; RCUK Energy Programme
OSTI Identifier:
1648967
Grant/Contract Number:  
AC05-00OR22725; FG02-04ER54742; EP/P012450/1; 633053
Resource Type:
Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 59; Journal Issue: 10; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; MHD spectroscopy; Alfvén eigenmodes; pellet injection

Citation Formats

Oliver, H. J.C., Sharapov, S. E., Breizman, B. N., Fontanilla, A. K., Spong, D. A., and Terranova, D.. Modification of the Alfvén wave spectrum by pellet injection. United States: N. p., 2019. Web. https://doi.org/10.1088/1741-4326/ab382b.
Oliver, H. J.C., Sharapov, S. E., Breizman, B. N., Fontanilla, A. K., Spong, D. A., & Terranova, D.. Modification of the Alfvén wave spectrum by pellet injection. United States. https://doi.org/10.1088/1741-4326/ab382b
Oliver, H. J.C., Sharapov, S. E., Breizman, B. N., Fontanilla, A. K., Spong, D. A., and Terranova, D.. Wed . "Modification of the Alfvén wave spectrum by pellet injection". United States. https://doi.org/10.1088/1741-4326/ab382b. https://www.osti.gov/servlets/purl/1648967.
@article{osti_1648967,
title = {Modification of the Alfvén wave spectrum by pellet injection},
author = {Oliver, H. J.C. and Sharapov, S. E. and Breizman, B. N. and Fontanilla, A. K. and Spong, D. A. and Terranova, D.},
abstractNote = {Alfvén eigenmodes driven by energetic particles are routinely observed in tokamak plasmas. These modes consist of poloidal harmonics of shear Alfvén waves coupled by inhomogeneity in the magnetic field. In this work, further coupling is introduced by 3D inhomogeneities in the ion density during the assimilation of injected pellets. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum. We find that the frequencies of Alfvén eigenmodes drop dramatically when a pellet is injected in JET. From these observations, information about the changes in the ion density caused by a pellet can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, the 3D MHD codes Stellgap and AE3D were generalised to incorporate 3D density profiles. Additionally, a model for the expansion of the ionised pellet plasmoid along a magnetic field line was derived from the fluid equations. Thereby, the time evolution of the Alfvén eigenfrequency is reproduced. By comparing the numerical frequency drop of a toroidal Alfvén eigenmode (TAE) to experimental observations, the initial ion density of a cigar-shaped ablation region of length 4 cm is estimated to be n*=6.8×1022m-3 at the TAE location (r/a≈0.75). The frequency sweeping of an Alfvén eigenmode ends when the ion density homogenises poloidally. Modelling suggests that the time for poloidal homogenisation of the ion density at the TAE position is τh=18±4 ms for inboard pellet injection, and τh=26±2 ms for outboard pellet injection. By reproducing the frequency evolution of the elliptical Alfvén eigenmode (EAE), the initial ion density at the EAE location (r/a≈0.9) can be estimated to be n*=4.8×1022 m-3. Poloidal homogenisation of the ion density takes 2.7 times longer at the EAE location than at the TAE location for both inboard and outboard pellet injection.},
doi = {10.1088/1741-4326/ab382b},
journal = {Nuclear Fusion},
number = 10,
volume = 59,
place = {United States},
year = {2019},
month = {9}
}

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