Giovanni Rossi (UCLA), Tom Neiser (UCLA), and Ziyan "Zoe" Zhu (UCLA)

Date: 
Friday, October 21, 2016 - 2:00pm to 3:00pm
Series: 
Plasma Seminars

Plasma Seminar

BaPSF, Rehab Building Room 15-70 
Friday, October 21, 2016
2-3PM

"Electromagnetic turbulence and transport in increased β LAPD Plasmas"
by Giovanni Rossi (UCLA)

The new LaB6 plasma source in LAPD has enabled the production of magnetized, increased β plasmas (up to ~15\%). We report on the modifications of pressure-gradient-driven turbulence and transport with increased plasma β. Density fluctuations decrease with increasing β while magnetic fluctuations increase. B⊥ fluctuations saturate while parallel (compressional) magnetic fluctuations increase continuously with β. At the highest β values δB||/δB⊥ ~ 2 and δB/B ~ 1\%. The measurements are consistent with the excitation of the Gradient-driven Drift Coupling (GDC). This instability prefers k|| = 0 and grows in finite β plasmas due to density and temperature gradients through the production of parallel magnetic field fluctuations and resulting ∇B|| drifts. Comparisons between experimental measurements and theoretical predictions for the GDC will be shown. Direct measurements of electrostatic particle flux have been performed and show a strong reduction with increasing β. No evidence is found (e.g. density profile shape) of enhanced confinement, suggesting that other transport mechanisms are active. Preliminary measurements indicate that electromagnetic transport due to parallel magnetic field fluctuations at first increases with β but is subsequently suppressed at higher β values.

"Multi-scale Simulations of DIII-D near-edge L-mode plasmas"
by Tom Neiser (UCLA)

In order to self-consistently describe the L-H transition we have to be able to quantitatively characterize near-edge L-mode plasmas (ρ=0.8). Instructed by a linear analysis, we perform nonlinear gyrokinetic simulations of a DIII-D L-mode discharge. Comparison between single-scale and multi-scale simulations reveals that stability of ion temperature gradient (ITG) turbulence affects cross-scale coupling. When ion transport is stabilized by zonal flows, electron temperature gradient (ETG) streamer amplitude is reduced but persists at sub-ion-scales, causing radial electron heat transport to dominate. When ITG modes are unstable, we find that ion heat transport dominates, in agreement with experimental data. Moreover, nonlinear de-stabilization of ion transport occurs at higher critical gradients for multi-scale than for single-scale simulations, showing an enhanced Dimits shift. All simulations are performed with the GENE code (genecode.org).

"Evidence for Chaotic Edge Turbulence in the Alcator C-Mod Tokamak"
by Ziyan "Zoe" Zhu (UCLA)

Turbulence greatly reduces the confinement time of magnetic-confined plasmas; understanding the nature of this turbulence and the associated transport is therefore of great importance. This research seeks to establish whether turbulent fluctuations in Alcator C-Mod are chaotic or stochastic. This has an important impact on transport caused by turbulence in C-Mod: stochastic fluctuations sample all of phase space and can lead to diffusive transport, whereas chaotic fluctuations live in a restricted phase space (e.g. on attractors) and a diffusive description may not be valid. By analyzing the time series from an O-Mode reflectometer, turbulent edge density fluctuations in Ohmic plasmas and L-mode plasmas in the Alcator C-Mod tokamak are shown to be chaotic. Supporting evidence for chaos in the edge region includes: the observation of an exponential power spectra (which is associated with Lorentzian-shaped pulses in the time series) and the location of the signal in the Complexity-Entropy plane (C-H plane) and its corresponding Brandt-Pompe (BP) probability distribution. These analysis techniques will be briefly introduced along with a discussion of the analysis results. Different diagnostic techniques, such as Gas Puff Imaging (GPI), could be used to confirm the results.

Event Attachments: 
Location: 
BaPSF, Rehab Building Room 15-70