Evolution of ELMs, pedestal profiles and fluctuations in the inter-ELM period in NBI- and ECH-dominated discharges in DIII-D
- 22 February 2021
- journal article
- research article
- Published by IOP Publishing in Nuclear Fusion
- Vol. 61 (5), 056008
- https://doi.org/10.1088/1741-4326/abe8b1
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 (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. 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.Keywords
Funding Information
- U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences (DE-AC02-09CH11466 DE-FC02-04ER54698 DE-FG02-08ER54984 DE-SC0019302)
This publication has 58 references indexed in Scilit:
- Gyrokinetic analysis and simulation of pedestals to identify the culprits for energy losses using ‘fingerprints’Nuclear Fusion, 2019
- High fusion performance in Super H-mode experiments on Alcator C-Mod and DIII-DNuclear Fusion, 2019
- Access to pedestal pressure relevant to burning plasmas on the high magnetic field tokamak Alcator C-ModNuclear Fusion, 2018
- Pedestal structure and stability in H-mode and I-mode: a comparative study on Alcator C-ModNuclear Fusion, 2013
- Dynamical evolution of pedestal parameters in ELMy H-mode in the National Spherical Torus ExperimentNuclear Fusion, 2011
- A first-principles predictive model of the pedestal height and width: development, testing and ITER optimization with the EPED modelNuclear Fusion, 2011
- Inter-ELM behaviour of the electron density and temperature pedestal in ASDEX UpgradePlasma Physics and Controlled Fusion, 2010
- Pedestal stability comparison and ITER pedestal predictionNuclear Fusion, 2009
- A comparison of H-mode pedestal characteristics in MAST as a function of magnetic configuration and ELM typePlasma Physics and Controlled Fusion, 2009
- Temporal evolution of H-mode pedestal in DIII-DNuclear Fusion, 2009