Evolution of ELMs, pedestal profiles and fluctuations in the inter-ELM period in NBI and ECH dominated discharges in DIII-D

Banerjee, Santanu; Mordijck, Saskia; Barada, Kshitish Kumar; Zeng, L.; Groebner, Richard J.; Osborne, Tom H.; Rhodes, Terry L.; Snyder, Philip B.; Grierson, Brian A.; Diallo, Ahmed

doi:10.1088/1741-4326/abe8b1

Evolution of ELMs, pedestal profiles and fluctuations in the inter-ELM period in NBI and ECH dominated discharges in DIII-D

Journal Article · Mon Feb 22 00:00:00 EST 2021 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/abe8b1· OSTI ID:1766952

^[1]; Mordijck, Saskia ^[2]; Barada, Kshitish Kumar ^[3]; Zeng, L. ^[3]; Groebner, Richard J. ^[4]; Osborne, Tom H. ^[4]; Rhodes, Terry L. ^[4]; ^[4]; ^[5]; ^[5]

College of William and Mary, Williamsburg, VA (United States); General Atomics
College of William and Mary, Williamsburg, VA (United States)
Univ. of California, Los Angeles, CA (United States)
General Atomics, San Diego, CA (United States)
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

In DIII-D, it has been observed that ELM frequency decreases by 40% and ELM spacing becomes more regular in time when heating is changed from pure neutral beam injection (NBI) to predominantly electron cyclotron heating (ECH) in ITER Similar Shape plasmas. In comparison with pure the NBI discharges, pedestal fluctuations in magnetics and density increase in the ECH dominated discharges. Recovery of the pedestal profiles like electron density (ne), temperature (Te) and pressure (pe) shows marked differences for these two heating schemes. Average profiles in the last 30% of the ELM cycle shows higher Te, lower ne, and similar pe at the pedestal top for the ECH discharge when compared to the NBI discharge. Here, the gradient of Te (∇Te) is also steeper at the pedestal in the ECH discharge. Magnetic fluctuations show 3 distinct modes in 13~116 kHz in the ECH discharges only. ne fluctuations show two modes evolving in the inter-ELM period of the ECH discharge, a low frequency (400 kHz) quasi-coherent mode (LFQC) and high frequency (~2 MHz) broadband (HFB) fluctuations. Evolution of these modes has marked correspondence with the inter-ELM ∇Te recovery. A sharp decrease in the Dα baseline is observed whenever the LFQC weakens and the HFB grows, prior to each large ELM. Transport coefficients obtained from TRANSP shows that MTM and/or TEM are plausible candidates for the observed fluctuations. Linear gyrofluid simulation (TGLF) corroborates this characterization. TGLF shows that the linear growth rate of the most dominant mode peaks at ion-scale (kθρs ~ 0.4) at the pedestal steep gradient and the frequency is in the electron diamagnetic direction. It is proposed that increased fluctuations in the ECH dominated case, due to increased ∇Te, caused an increase in fluctuation-driven transport in the pedestal and slowed the pedestal recovery between ELMs, leading to a reduction in the ELM frequency.

View Accepted Manuscript (DOE)

Research Organization:: College of William and Mary, Williamsburg, VA (United States); General Atomics, San Diego, CA (United States); Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States); Univ. of California, Los Angeles, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: AC02-09CH11466; FC02-04ER54698; FG02-08ER54984; SC0019302

OSTI ID:: 1766952

Alternate ID(s):: OSTI ID: 1820176
OSTI ID: 1774491
OSTI ID: 23129731

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 5 Vol. 61; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ELMs
pedestal
plasma
tokamaks
transport
turbulence

Evolution of ELMs, pedestal profiles and fluctuations in the inter-ELM period in NBI and ECH dominated discharges in DIII-D

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