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Title: Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations

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 E×B shear required for the EHO decreases linearly with pedestal collisionality $$\nu _{\text{e}}^{\ast}$$ , 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 2016 Phys. Plasmas 23 056103, Chen 2017 Nucl. Fusion 57 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 E×B shear enables destabilization of broadband turbulence, which relaxes edge pressure gradients, improves peeling-ballooning stability and allows a wider and thus higher pedestal. In conclusion, the ability to accurately predict the critical E×B 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.
Authors:
 [1] ;  [1] ;  [1] ;  [2] ;  [3] ;  [1] ;  [1] ;  [4] ;  [1] ;  [5] ;  [2] ;  [5] ;  [1] ;  [1] ;  [4]
  1. General Atomics, San Diego, CA (United States)
  2. Univ. of California, Los Angeles, CA (United States)
  3. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  4. Univ. of Wisconsin, Madison, WI (United States)
  5. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Publication Date:
Grant/Contract Number:
FG02-08ER54999; AC02-09CH11466; FG02-94ER54235; FG02-08ER54984; FC02-04ER54698
Type:
Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 57; Journal Issue: 8; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Research Org:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); General Atomics, San Diego, CA (United States)
Sponsoring Org:
USDOE
Contributing Orgs:
The DIII-D Team
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; QH-mode; EHO; low torque; ELM free; E×B shear; pedestal; edge broadband MHD
OSTI Identifier:
1373372
Alternate Identifier(s):
OSTI ID: 1374546

Chen, Xi, Burrell, K. H., Osborne, T. H., Barada, K., Ferraro, N. M., Garofalo, A. M., Groebner, R. J., McKee, G. R., Petty, C. C., Porkolab, M., Rhodes, T. L., Rost, J. C., Snyder, P. B., Solomon, W. M., and Yan, Z.. Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations. United States: N. p., Web. doi:10.1088/1741-4326/aa7531.
Chen, Xi, Burrell, K. H., Osborne, T. H., Barada, K., Ferraro, N. M., Garofalo, A. M., Groebner, R. J., McKee, G. R., Petty, C. C., Porkolab, M., Rhodes, T. L., Rost, J. C., Snyder, P. B., Solomon, W. M., & Yan, Z.. Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations. United States. doi:10.1088/1741-4326/aa7531.
Chen, Xi, Burrell, K. H., Osborne, T. H., Barada, K., Ferraro, N. M., Garofalo, A. M., Groebner, R. J., McKee, G. R., Petty, C. C., Porkolab, M., Rhodes, T. L., Rost, J. C., Snyder, P. B., Solomon, W. M., and Yan, Z.. 2017. "Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations". United States. doi:10.1088/1741-4326/aa7531. https://www.osti.gov/servlets/purl/1373372.
@article{osti_1373372,
title = {Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations},
author = {Chen, Xi and Burrell, K. H. and Osborne, T. H. and Barada, K. and Ferraro, N. M. and Garofalo, A. M. and Groebner, R. J. and McKee, G. R. and Petty, C. C. and Porkolab, M. and Rhodes, T. L. and Rost, J. C. and Snyder, P. B. and Solomon, W. M. and Yan, Z.},
abstractNote = {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 E×B shear required for the EHO decreases linearly with pedestal collisionality $\nu _{\text{e}}^{\ast}$ , 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 2016 Phys. Plasmas 23 056103, Chen 2017 Nucl. Fusion 57 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 E×B shear enables destabilization of broadband turbulence, which relaxes edge pressure gradients, improves peeling-ballooning stability and allows a wider and thus higher pedestal. In conclusion, the ability to accurately predict the critical E×B 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.},
doi = {10.1088/1741-4326/aa7531},
journal = {Nuclear Fusion},
number = 8,
volume = 57,
place = {United States},
year = {2017},
month = {6}
}