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Experimental, Numerical and Analytical Studies of the MHD-Driven Plasma Jet, Instabilities and Waves


Zhai, Xiang (2015) Experimental, Numerical and Analytical Studies of the MHD-Driven Plasma Jet, Instabilities and Waves. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z90G3H3Q.


This thesis describes a series of experimental, numerical, and analytical studies involving the Caltech magnetohydrodynamically (MHD)-driven plasma jet experiment. The plasma jet is created via a capacitor discharge that powers a magnetized coaxial planar electrodes system. The jet is collimated and accelerated by the MHD forces.

We present three-dimensional ideal MHD finite-volume simulations of the plasma jet experiment using an astrophysical magnetic tower as the baseline model. A compact magnetic energy/helicity injection is exploited in the simulation analogous to both the experiment and to astrophysical situations. Detailed analysis provides a comprehensive description of the interplay of magnetic force, pressure, and flow effects. We delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms.

When the experimental jet is sufficiently long, it undergoes a global kink instability and then a secondary local Rayleigh-Taylor instability caused by lateral acceleration of the kink instability. We present an MHD theory of the Rayleigh-Taylor instability on the cylindrical surface of a plasma flux rope in the presence of a lateral external gravity. The Rayleigh-Taylor instability is found to couple to the classic current-driven instability, resulting in a new type of hybrid instability. The coupled instability, produced by combination of helical magnetic field, curvature of the cylindrical geometry, and lateral gravity, is fundamentally different from the classic magnetic Rayleigh-Taylor instability occurring at a two-dimensional planar interface.

In the experiment, this instability cascade from macro-scale to micro-scale eventually leads to the failure of MHD. When the Rayleigh-Taylor instability becomes nonlinear, it compresses and pinches the plasma jet to a scale smaller than the ion skin depth and triggers a fast magnetic reconnection. We built a specially designed high-speed 3D magnetic probe and successfully detected the high frequency magnetic fluctuations of broadband whistler waves associated with the fast reconnection. The magnetic fluctuations exhibit power-law spectra. The magnetic components of single-frequency whistler waves are found to be circularly polarized regardless of the angle between the wave propagation direction and the background magnetic field.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Plasma, MHD, jet, Astrophysical jet, Rayleigh-Taylor instability, Kink instability, Whistler wave, magnetic probe, plasma diagnostics, High Voltage probe
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Minor Option:Applied And Computational Mathematics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Bellan, Paul Murray
Thesis Committee:
  • Bellan, Paul Murray (chair)
  • Phinney, E. Sterl
  • Ott, Christian D.
  • Tsurutani, Bruce
  • Vahala, Kerry J.
Defense Date:9 June 2015
Non-Caltech Author Email:zxzhaixiang (AT)
Funding AgencyGrant Number
U.S. Department of EnergyDE-FG02-04ER54755
U.S. Department of EnergyDE-SC0010471
National Science Foundation1059519
Air Force Office of Scientific ResearchFA9550-11-1-0184
Record Number:CaltechTHESIS:06092015-162916583
Persistent URL:
Related URLs:
URLURL TypeDescription MHD simulation of the Caltech plasma jet experiment: first results MHD simulation of the Caltech plasma jet experiment: first results earth-isolated optically coupled wideband high voltage probe powered by ambient light
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9014
Deposited By: Xiang Zhai
Deposited On:20 Jul 2015 20:51
Last Modified:04 Oct 2019 00:09

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