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

Abstract

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 andmore » 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.« less

Authors:

Banerjee, Santanu  ^[1];

• Search DOE PAGES for author "Banerjee, Santanu"
• Search DOE PAGES for ORCID "0000-0003-0859-8855"

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

• Search DOE PAGES for author "Snyder, Philip B."
• Search DOE PAGES for ORCID "0000-0002-0613-4232"

Grierson, Brian A.  ^[4];

• Search DOE PAGES for author "Grierson, Brian A."
• Search DOE PAGES for ORCID "0000-0001-5918-6506"

Diallo, Ahmed  ^[4]

• Search DOE PAGES for author "Diallo, Ahmed"
• Search DOE PAGES for ORCID "0000-0002-0706-060X"

---

+ Show Author Affiliations

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

Publication Date:

Tue Apr 13 00:00:00 EDT 2021

Research Org.:

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 Org.:

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

OSTI Identifier:

1774491

Alternate Identifier(s):

OSTI ID: 1766952; OSTI ID: 1820176

Grant/Contract Number:  

SC0019302; FG02-08ER54984; AC02-09CH11466; FC02-04ER54698

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 61; Journal Issue: 5; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

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