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Title: Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

Abstract

Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after secondary deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard x-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5–6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal beam instability is accessed by ramping up the current. The instability leads to a sawtooth-like relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposedmore » as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at ≈ 100 GHz, suggesting resonant absorption of the ECE at low frequencies.« less

Authors:
 [1];  [2];  [2];  [3];  [4];  [5];  [2];  [6];  [5]; ORCiD logo [7];  [8];  [9];  [6];  [8]; ORCiD logo [7];  [9]
  1. Oak Ridge Associated Univ. (ORAU), Oak Ridge, TN (United States)
  2. General Atomics, San Diego, CA (United States)
  3. Max-Planck-Inst. für Plasmaphysik, Greifswald (Germany)
  4. Univ. of Texas, Austin, TX (United States)
  5. Univ. of Milano-Bicocca (Italy)
  6. Univ. of California, San Diego, CA (United States)
  7. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  8. Consiglio Nazionale delle Ricerche (CNR), Milan (Italy)
  9. Univ. of California, Irvine, CA (United States)
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States); Univ. of California, Irvine, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
OSTI Identifier:
1615237
Alternate Identifier(s):
OSTI ID: 1648942; OSTI ID: 1782097
Grant/Contract Number:  
FC02-04ER54698; SC0020337; AC05-00OR22725
Resource Type:
Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 60; Journal Issue: 056008; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; tokamak disruptions; runaway electrons; energy distribution; current profile; MHD instabilities; electron cyclotron emission

Citation Formats

Lvovskiy, Andreas, Paz-Soldan, Carlos, Eidietod, N. W., Aleynikov, P., Austin, M. E., Dal Molin, A., Liu, Y.Q., Moyer, R. A., Nocente, M., Shiraki, Daisuke, Giacomelli, L., Heidbrink, W. W., Hollmann, E. M., Rigamonti, D., Spong, Donald A., and Tardocchi, M. Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability. United States: N. p., 2020. Web. doi:10.1088/1741-4326/ab78c7.
Lvovskiy, Andreas, Paz-Soldan, Carlos, Eidietod, N. W., Aleynikov, P., Austin, M. E., Dal Molin, A., Liu, Y.Q., Moyer, R. A., Nocente, M., Shiraki, Daisuke, Giacomelli, L., Heidbrink, W. W., Hollmann, E. M., Rigamonti, D., Spong, Donald A., & Tardocchi, M. Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability. United States. https://doi.org/10.1088/1741-4326/ab78c7
Lvovskiy, Andreas, Paz-Soldan, Carlos, Eidietod, N. W., Aleynikov, P., Austin, M. E., Dal Molin, A., Liu, Y.Q., Moyer, R. A., Nocente, M., Shiraki, Daisuke, Giacomelli, L., Heidbrink, W. W., Hollmann, E. M., Rigamonti, D., Spong, Donald A., and Tardocchi, M. Mon . "Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability". United States. https://doi.org/10.1088/1741-4326/ab78c7. https://www.osti.gov/servlets/purl/1615237.
@article{osti_1615237,
title = {Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability},
author = {Lvovskiy, Andreas and Paz-Soldan, Carlos and Eidietod, N. W. and Aleynikov, P. and Austin, M. E. and Dal Molin, A. and Liu, Y.Q. and Moyer, R. A. and Nocente, M. and Shiraki, Daisuke and Giacomelli, L. and Heidbrink, W. W. and Hollmann, E. M. and Rigamonti, D. and Spong, Donald A. and Tardocchi, M.},
abstractNote = {Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after secondary deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard x-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5–6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal beam instability is accessed by ramping up the current. The instability leads to a sawtooth-like relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposed as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at ≈ 100 GHz, suggesting resonant absorption of the ECE at low frequencies.},
doi = {10.1088/1741-4326/ab78c7},
journal = {Nuclear Fusion},
number = 056008,
volume = 60,
place = {United States},
year = {2020},
month = {4}
}

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