Electrical and Computer Engineering ETDs

Publication Date

6-9-2016

Abstract

Experimental studies investigating turbulence, and sheared flow were carried out using the linear Helicon-Cathode (HelCat) device. Without any outside influence, such as biasing or momentum input, fluctuations were observed. The dynamics of these fluctuations was found to vary with magnetic field, varying in both frequency and mode number. At low-field, the fluctuations were observed to be coherent, while at high fields, they were found to have turbulent characteristics. The first goal of this work was to identify the drive of the fluctuations. This was completed by taking detailed radial measurements at two axial positions at five magnetic fields: 19.2 mT, 44 mT, 61.6 mT, 79.2 mT, and 129.5 mT. Experimental results were compared to linear theories, and analysis was completed to identify the mode at each magnetic field. In addition, the results were verified using Linear Stability Solver code (LSS), which utilized the electrostatic Braginskii equations. Both results indicated that the mode was a hybrid mode between the Kelvin-Helmholtz instability and the drift-wave instability. The generation of this hybrid mode is believed to be caused by changes in the axial parameters, particularly the density and potential gradients. In order to affect these fluctuations, and explore the effect of sheared flow, a gridded electrode was placed 0.7 m downstream from the source. This grid was nearly $70\\%$ transparent, and had a diameter of 17 cm, which covered the majority of the plasma radius, which is estimated to be 20-30 cm in diameter. The electrode was biased with respect to the chamber wall, thereby generating a radial electric field. This in turn is modifies the azimuthal flow, which is believed to be E$\ imes$B dominated. Grid bias was found to have various effects depending on the bias and magnetic field strength. It was found that the application of negative grid bias ($V_g=-40$ V) once coherent fluctuations were driven into a broad-band turbulent mode. The electric field in was observed to become strongly negative (inward pointing) with the negative bias. At lower magnetic fields, this resulted in a steeper electric field gradient, which corresponds to a steeper shear flow. However, at high magnetic fields, the shear was not as steep, indicating a change in the drive-physics. With an increase towards positive bias, the fluctuation magnitude of density and potential was exhibited to decrease. However, it was found that the density-potential cross-phase, and fluctuating radial flux, increased initially. This initial increase changed as the turbulent fluctuations were observed to transition to a coherent mode, where it was observed that the flux more closely followed changes in the density and potential fluctuation magnitude. With a positive bias, the fluctuation levels could generally be reduced, and at certain radii, suppressed. Again, changes were observed to occur in the radial electric field, with the field now exhibiting both positive (outward pointing) and negative (inward pointing) components. At low magnetic fields, the shear decreased at inner radii, but increased at the edge, which was consistent with the drive regions of the Kelvin-Helmholtz mode whose drive was stronger at inner radii, and the drift-mode whose drive dominated at the edge. However, with higher magnetic field, positive bias lead to stronger shear overall. In the higher magnetic field case, B=129.5 mT, a positive bias up to $V_g=+40$ V was applied, and appeared to have minimal effect. However, flux data and fast imaging revealed that the positive bias lead to a shift in the turbulence. With negative bias, the flux was strongly outward, but the positive bias lead to a fully inward transport of flux. This was captured and verified with fast imaging. The exact nature and effect of this inward flux is still not well understood. The last part of this work focused on a purely non-linear instability known as the potential relaxation instability, or PRI. This instability was first generated by accident, and was found to exist only at low magnetic fields. It occurred when a high positive bias was applied; this resulted in the rise of large scale fluctuations, with density fluctuations magnitudes $\ ilde{n}/n>50\\%$. Plasma from unbiased boundaries was found to travel axially towards the biased electrode, and to be accelerated to supersonic values $v\\approx 2-3c_s$, where $c_s$ is the nominal ion sound speed.

Keywords

helcat, plasma, turbulence, biasing, helicon

Sponsors

National Science Foundation

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Advisor

Gilmore, Mark

First Committee Member (Chair)

Fisher, Dustin

Second Committee Member

Ricci, Paolo

Third Committee Member

Schamiloglu, Edl

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