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

Journal Article · 19 November 2015 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.4935919 · OSTI ID:1350090

Eldon, David  ^[1]; Boivin, Rejean L. ^[2]; Chrystal, Colin  ^[3]; Groebner, Richard J. ^[2]; McKee, George R. ^[4]; Schmitz, Lothar ^[5]; Tynan, G. R. ^[6]; Yan, Zheng ^[4]; Boedo, Jose A. ^[6]; Burrell, Keith H.  ^[2]; King, J. D. ^[2]; Kolemen, E. ^[7]; Luhmann, Jr., N. C. ^[8]; Muscatello, C. M. ^[2]; Osborne, T. H. ^[2]; Snyder, P. B. ^[2]

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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)

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 ω_E×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 has 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, k_θ ≈ 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.

• Research Organization: General Atomics, San Diego, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: FC02-04ER54698; AC02-09CH11466; AC05-06OR23100; FG02-07ER54917; FG02-89ER53296
• OSTI ID: 1350090
• Alternate ID(s): OSTI ID: 1226662
• Journal Information: Physics of Plasmas, Vol. 22, Issue 11; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English