Title: Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

Abstract

Experiments in the DIII-D tokamak show that many overlapping small-amplitude Alfv en eigenmodes (AEs) cause fast-ion transport to sharply increase above a critical threshold, leading to fast-ion density profile resilience and reduced fusion performance. The threshold is above the AE linear stability limit and varies between diagnostics that are sensitive to different parts of fast-ion phase-space. A comparison with theoretical analysis using the nova and orbit codes shows that, for the neutral particle diagnostic, the threshold corresponds to the onset of stochastic particle orbits due to wave-particle resonances with AEs in the measured region of phase space. We manipulated the bulk fast-ion distribution and instability behavior through variations in beam deposition geometry, and no significant differences in the onset threshold outside of measurement uncertainties were found, in agreement with the theoretical stochastic threshold analysis. Simulations using the `kick model' produce beam ion density gradients consistent with the empirically measured radial critical gradient and highlight the importance of including the energy and pitch dependence of the fast-ion distribution function in critical gradient models. The addition of electron cyclotron heating changes the types of AEs present in the experiment, comparatively increasing the measured fast-ion density and radial gradient. Our studies provide themore » basis for understanding how to avoid AE transport that can undesirably redistribute current and cause fast-ion losses, and the measurements are being used to validate AE-induced transport models that use the critical gradient paradigm, giving greater confidence when applied to ITER.« less

Authors:

Collins, C. S. ^[1]; Heidbrink, W. W. ^[2]; Podestà, M. ^[3]; White, R. B. ^[3]; Kramer, G. J. ^[3]; Pace, D. C. ^[1]; Petty, C. C. ^[1]; Stagner, L. ^[2]; Van Zeeland, M. A. ^[1]; Zhu, Y. B. ^[2]

---

+ Show Author Affiliations

1. General Atomics, San Diego, CA (United States)
2. Univ. of California, Irvine, CA (United States)
3. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Publication Date:

Fri Jun 09 00:00:00 EDT 2017

Research Org.:

General Atomics, San Diego, CA (United States)

Sponsoring Org.:

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

Contributing Org.:

The DIII-D Team

OSTI Identifier:

1374553

Grant/Contract Number:  

FC02-04ER54698; FG03-94ER54271; AC02- 09CH11466

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 57; Journal Issue: 8; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; energetic particles; fast-ion transport; Alfven eigenmodes; critical gradient