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Title: Evolution of E × B shear and coherent fluctuations prior to H-L transitions in DIII-D and control strategies for H-L transitions

While operating a magnetic fusion device in H-mode has many advantages, care must be taken to understand and control the release of energy during the H-L back transition, as the extra energy stored within the H-mode transport barrier will have the potential to cause damage to material components of a large future tokamak such as ITER. Examining a scenario where the H-L back transition sequence begins before the E × B shearing layer decays on its own, we identify a long-lived precursor mode that is tied to the events of the H-L sequence and we develop a robust control strategy for ensuring gradual release of energy during the transition sequence. Back transitions in this scenario commonly begin with a rapid relaxation of the pedestal, which was previously shown to be inconsistent with ideal peeling-ballooning instability as the trigger, despite being otherwise similar to a large type-I Edge Localized Mode (ELM). Here, this so-called transient occurs when the E × B shearing rate ωE×B is significantly larger than the turbulence decorrelation rate ωT, indicating that this is not the result of runaway turbulence recovery. The transient is always synchronous with amplitude and propagation velocity modulations of the precursor mode, which hasmore » been dubbed the Modulating Pedestal Mode (MPM).The MPM is a coherent density fluctuation, which, in our scenario at least, reliably appears in the steep gradient region with f ≈ 70 kHz, kθ ≈ 0.3 cm–1, and it exists for ≳100 ms before the onset of back transitions. The transient may be reliably eliminated by reducing toroidal rotation in the co-current direction by the application of torque from counter-injecting neutral beams. The transient in these “soft” H-L transitions is then replaced by a small type-III ELM, which is also always synchronous with the MPM, and MPM shows the same behavior in both hard and soft cases.« less
Authors:
 [1] ;  [2] ;  [3] ;  [2] ;  [4] ;  [5] ;  [6] ;  [4] ;  [6] ;  [2] ;  [2] ;  [7] ;  [8] ;  [2] ;  [2] ;  [2]
  1. Princeton Univ., Princeton, NJ (United States); Univ. of California San Diego, La Jolla, CA (United States)
  2. General Atomics, San Diego, CA (United States)
  3. Oak Ridge Associated Univ., Oak Ridge, TN (United States)
  4. Univ. of Wisconsin, Madison, WI (United States)
  5. Univ. of California, Los Angeles, CA (United States)
  6. Univ. of California San Diego, La Jolla, CA (United States)
  7. Princeton Univ., Princeton, NJ (United States)
  8. Univ. of California, Davis, CA (United States)
Publication Date:
Grant/Contract Number:
FC02-04ER54698; AC02-09CH11466; AC05-06OR23100; FG02-07ER54917; FG02-89ER53296
Type:
Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 22; Journal Issue: 11; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Research Org:
General Atomics, San Diego, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; H-L transition; H-mode; tokamak; heat load; DIII-D
OSTI Identifier:
1350090
Alternate Identifier(s):
OSTI ID: 1226662