The goal of this research is to investigate experimentally the detailed dynamics of transitions between states of unstable drift fluctuations (including broadband turbulence), and fluctuation-suppressed states in a controlled laboratory environment. These experiments will be complemented by direct comparisons with a nonlinear fluid turbulence code, adapted specifically to model this experiment. The proposed experiments will take place in the dual-source HELCAT (HELicon-CAThode) device at the University of New Mexico (UNM). HELCAT is a flexible device that provides unique capabilities important to these experiments. The two plasma sources- operated independently or simultaneously - can generate plasmas with edge fluctuations ranging from single drift modes to broadband drift turbulence. Additionally, plasma biasing to affect Er × Bz flow profiles has been demonstrated, and collisionality, important in drift wave damping, has been varied by more than an order of magnitude. The numerical code developed by the co-PI at the University of Alaska Fairbanks, a 2-D fluid slab model of electrostatic fluctuations with detailed diagnostics to understand the dynamics of the interaction between the fluctuating modes, will be modified to allow the drive to mimic the experimental drive. The code has an external flow that can be turned on to look at the impact of flow on the fluctuating modes.
The research facility is well suited for training students for careers in fusion energy or studying turbulent transport in plasmas, a process important in space and astrophysical plasma as well as magnetically confined laboratory plasmas. The research could have a significant impact on efforts to create predictive models for turbulent transport in plasmas. This research is relevant to basic plasma physics, complex dynamics, space plasmas and magnetically confined plasmas.
This proposal was submitted to the NSF-DoE Partnership in Plasma Science and Engineering joint solicitation 08-589. This award is being funded by the Division of Physics of the Mathematical and Physical Sciences Directorate.
Turbulence and turbulence-driven transport of particles, heat, and momentum in the presence of spatially-varying (sheared) flows are ubiquitous in magnetized plasmas, including laboratory, space, fusion, and high density plasmas. In all of these types of plasmas, turbulence and turbulent transport can affect the overall dynamics and confinement properties of the plasma system. While sheared flows are generally associated with the creation (drive) of turbulence in fluids, sheared flows in plasma can both drive and suppress turbulent fluctuations. The goal of this research has been to elucidate the dynamics of the interaction between weakly turbulent fluctuations and sheared plasma flows in a controlled laboratory environment. Experiments were conducted in the HelCat (Helicon-Cathode) plasma device at the University of New Mexico, shown in Figure 1. HelCat is a 4 meter long, 50 cm diameter basic plasma science device that has both a radio frequency (RF) Helicon source and a thermionic Cathode source. It is suitable for a number of fundamental plasma physics studies. Unstable fluctuations, including turbulence, exist naturally at the edge of HelCat plasmas, driven by the plasma pressure gradient. In these experiments, plasma flows were controlled by biasing (applying voltage) to electrodes in contact with the plasma so as to produce an azimuthal Eradial x Bz force, causing the plasma rotation profile to be modified. Since the rotation profile is spatially-varying, a change in the shear of the flow results. Two types of electrodes utilized in these studies are shown in Figure 2. In addition to experiments, numerical modeling was performed in order to help understand the nature of the turbulent fluctuations (i.e. the type of underlying plasma instabilities that lead to turbulence), and the interactions between the turbulent fluctuations and plasma sheared flows. Two types of numerical models were used: 1) a one-dimensional linear stability solver (LSS code) that numerically solves eigenmode equations given inputs of plasma density, temperature, and potential to find which types of instabilities may be present, and 2) a three-dimensional "global" code that solves the full Braginskii equations in the electrostatic limit (GBS code) which solves for the complete plasma state, including profiles, fluctuations, and flows [B.N. Rogers and Paolo Ricci (2010). Phys Rev Lett 104, 225002]. The GBS code is a large scale model that typically takes weeks to run on a large parallel supercomputer. Comparisons between experimental data and the LSS code indicate that the fundamental type of turbulent fluctuations present in HelCat helicon plasmas is resistive drift waves (RDW’s), but that Interchange Mode (IM) and Kelvin-Helmholtz (KH) instabilities may be present when electrode biasing is applied. These results provide a detailed understanding of the basic "linear" physics at work. Figure 3 shows an example output from the GBS code When electrode biasing is applied, clear effects on plasma fluctuations, and the associated turbulent transport of particles, are observed. An example is shown in Figure 4, where density fluctuations over the whole duration of the plasma are shown, together with a trace of applied bias voltage on end ring electrodes. It can be seen that during the first, lower voltage, electrode pulse, fluctuations are reduced in amplitude, while during the second, higher voltage, pulse they are completely suppressed. During the times when bias pulses are applied, it is observed that there is an associated azimuthal flow profile (shear) change at the plasma edge. During biasing, more than one type of instability can be present, which can lead to increased dynamical complexity, including chaotic fluctuations. An example is shown in Figure 5, where the frequency spectrum of density fluctuations before and during a bias pulse is shown. Nonlinear time series analysis, including computation of the correlation dimension of the fluctuation signal, indicates that the fluctuations in the biased case shown are chaotic (correlation dimension, D > 2). When high levels of bias (voltage) are applied to the electrodes, a large scale plasma instability, where the plasma density and potential modulate by 50% - 80%, is observed. An example of this behavior is shown in Figure 6. This behavior has been identified as the Potential Relaxation Instability (PRI) [Sato, N. et al. (1976) Phys. Fluids, 19]. In the presence of the PRI, the density and potential undergo periodic collapses and build ups, consistent with a moving double layer. This behavior is also consistent with a bistable plasma potential profile that is predicted by Loizu, et al. [J. Loizu, P. Ricci, and C. Theiler (2011). Phys Rev E 83, 016406] to occur in the presence of strong electron sheaths, and may indicate the presence of a limit cycle. While the PRI is not related to the types of plasma turbulence that was the subject of this work, these results do help to establish the limits to flow and fluctuation control that can be achieved by electrode biasing.
