The non-thermal origin of the tokamak low-density stability limit

Journal Article · Wed Apr 13 00:00:00 EDT 2016 · Nuclear Fusion

DOI:https://doi.org/10.1088/0029-5515/56/5/056010· OSTI ID:1329374

Paz-Soldan, C. ^[1]; La Haye, R. J. ^[1]; Shiraki, D. ^[2]; Buttery, R. J. ^[1]; Eidietis, N. W. ^[1]; Hollmann, E. M. ^[3]; Moyer, R. A. ^[3]; Boom, J. E. ^[4]; Chapman, I. T. ^[5]

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1. General Atomics, San Diego, CA (United States)
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
3. Univ. of California-San Diego, La Jolla, CA (United States)
4. Max-Planck-Institut fur Plasmaphysik, Garching (Germany)
5. JET, Culham Science Centre, Abingdon (United Kingdom)

DIII-D plasmas at very low density exhibit onset of n=1 error eld (EF) penetration (the `low-density locked mode') not at a critical density or EF, but instead at a critical level of runaway electron (RE) intensity. Raising the density during a discharge does not avoid EF penetration, so long as RE growth proceeds to the critical level. Penetration is preceded by non-thermalization of the electron cyclotron emission, anisotropization of the total pressure, synchrotron emission shape changes, as well as decreases in the loop voltage and bulk thermal electron temperature. The same phenomena occur despite various types of optimal EF correction, and in some cases modes are born rotating. Similar phenomena are also found at the low-density limit in JET. These results stand in contrast to the conventional interpretation of the low-density stability limit as being due to residual EFs and demonstrate a new pathway to EF penetration instability due to REs. Existing scaling laws for penetration project to increasing EF sensitivity as bulk temperatures decrease, though other possible mechanisms include classical tearing instability, thermo-resistive instability, and pressure-anisotropy driven instability. Regardless of rst-principles mechanism, known scaling laws for Ohmic energy con nement combined with theoretical RE production rates allow rough extrapolation of the RE criticality condition, and thus the low-density limit, to other tokamaks. The extrapolated low-density limit by this pathway decreases with increasing machine size and is considerably below expected operating conditions for ITER. While likely unimportant for ITER, this e ect can explain the low-density limit of existing tokamaks operating with small residual EFs.

• Research Organization: Univ. of California-San Diego, La Jolla, CA (United States); General Atomics, San Diego, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES); European Commission (EC)
• Contributing Organization: JET Contributors; Max-Planck-Institut fur Plasmaphysik, Garching (Germany); Culham Science Centre, Abingdon (United Kingdom)
• Grant/Contract Number: FC02-04ER54698; AC05-00OR22725; FG02-07ER54917; 633053; EP/I501045
• OSTI ID: 1329374
• Alternate ID(s): OSTI ID: 1247027; OSTI ID: 1372314
• Journal Information: Nuclear Fusion, Vol. 56, Issue 5; ISSN 0029-5515
• Publisher: IOP Science
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
runaway electron
error fields
error field penetration
density limit
tearing mode