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

Journal Article · Mon Apr 06 00:00:00 EDT 2020 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/ab78c7· OSTI ID:1615237

Lvovskiy, Andreas ^[1]; Paz-Soldan, Carlos ^[2]; Eidietod, N. W. ^[2]; Aleynikov, P. ^[3]; Austin, M. E. ^[4]; Dal Molin, A. ^[5]; Liu, Y.Q. ^[2]; Moyer, R. A. ^[6]; Nocente, M. ^[5]; Shiraki, Daisuke  ^[7]; Giacomelli, L. ^[8]; Heidbrink, W. W. ^[9]; Hollmann, E. M. ^[6]; Rigamonti, D. ^[8]; Spong, Donald A.  ^[7]; Tardocchi, M. ^[9]

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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)

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.

• Research Organization: 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 Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: FC02-04ER54698; SC0020337; AC05-00OR22725
• OSTI ID: 1615237
• Alternate ID(s): OSTI ID: 1648942; OSTI ID: 1782097
• Journal Information: Nuclear Fusion, Vol. 60, Issue 056008; ISSN 0029-5515
• Publisher: IOP Science
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
tokamak disruptions
runaway electrons
energy distribution
current profile
MHD instabilities
electron cyclotron emission