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

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

Journal Article · Tue Apr 13 00:00:00 EDT 2021 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/abe8b1· OSTI ID:1774491

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

^[3];

^[4];

^[4]

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 (n_e), temperature (T_e) and pressure (p_e) shows marked differences for these two heating schemes. Average profiles in the last 30% of the ELM cycle shows higher T_e, lower n_e, and similar p_e at the pedestal top for the ECH discharge when compared to the NBI discharge. Here, the gradient of T_e (∇T_e) 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. n_e 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 ∇T_e recovery. A sharp decrease in the D_α 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 (k_θρs ~ 0.4) at the pedestal steep gradient and the frequency is in the electron diamagnetic direction. Here, it is proposed that increased fluctuations in the ECH dominated case, due to increased ∇T_e, 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.

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Research Organization:: 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)

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

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

OSTI ID:: 1774491

Alternate ID(s):: OSTI ID: 1766952; OSTI ID: 1820176

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

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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