Modification of the Alfvén wave spectrum by pellet injection

Oliver, H. J.C.; Sharapov, S. E.; Breizman, B. N.; Fontanilla, A. K.; Spong, D. A.; Terranova, D.

doi:10.1088/1741-4326/ab382b

Modification of the Alfvén wave spectrum by pellet injection

Journal Article · Wed Sep 04 00:00:00 EDT 2019 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/ab382b· OSTI ID:1648967

^[1]; ^[2]; Breizman, B. N. ^[3]; Fontanilla, A. K. ^[3]; ^[4]; ^[5]

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
Culham Science Centre, Abingdon (United Kingdom). Culham Centre for Fusion Energy (CCFE), EURATOM/UKAEA Fusion Association
Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Consorzio RFX, Padova (Italy)

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 τ_h=18±4 ms for inboard pellet injection, and τ_h=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.

View Accepted Manuscript (DOE)

Research Organization:: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES); EUROfusion Consortium; RCUK Energy Programme

Grant/Contract Number:: AC05-00OR22725; FG02-04ER54742

OSTI ID:: 1648967

Alternate ID(s):: OSTI ID: 22929947

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 10 Vol. 59; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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