Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations
- General Atomics, San Diego, CA (United States)
- Univ. of California, Los Angeles, CA (United States)
- Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
- Univ. of Wisconsin, Madison, WI (United States)
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
New experimental studies and modelling of the coherent Edge Harmonic Oscillation (EHO), which regulates the conventional Quiescent H-mode (QH-mode) edge, validate the proposed hypothesis of edge rotational shear in destabilizing the low-n kink peeling mode as the additional drive mechanism for the EHO. The observed minimum edge ExB shear required for the EHO decreases linearly with pedestal collisionality v$$*\atop{e}$$, which is favorable for operating QH-mode in machines with low collisionality and low rotation such as ITER. In addition, the QH-mode regime in DIII-D has recently been found to bifurcate into a new ‘wide-pedestal’ state at low torque in double-null shaped plasmas, characterized by increased pedestal height, width and thermal energy confinement [Burrell POP 23 (2016) 056103, Chen NF 57 (2017) 022007]. This potentially provides an alternate path for achieving high performance ELM-stable operation at low torque, in addition to the low-torque QH-mode sustained with applied 3D fields. Multi-branch low-k and intermediate-k turbulences are observed in the ‘wide-pedestal’. New experiments support the hypothesis that the decreased edge ExB shear enables destabilization of broadband turbulence, which relaxes edge pressure gradients, improves peeling-ballooning stability and allows a wider and thus higher pedestal. The ability to accurately predict the critical ExB shear for EHO and maintain high performance QH-mode at low torque is an essential requirement for projecting QH-mode operation to ITER and future machines.
- Research Organization:
- Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE
- Contributing Organization:
- The DIII-D Team
- Grant/Contract Number:
- FG02-08ER54999; AC02-09CH11466; FG02-94ER54235; FG02-08ER54984; FC02-04ER54698
- OSTI ID:
- 1373372
- Alternate ID(s):
- OSTI ID: 1374546
- Journal Information:
- Nuclear Fusion, Vol. 57, Issue 8; ISSN 0029-5515
- Publisher:
- IOP ScienceCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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