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Battery-Powered RF Pre-Ionization System for the Caltech Magnetohydrodynamically-Driven Jet Experiment: RF Discharge Properties and MHD-Driven Jet Dynamics


Chaplin, Vernon Hampden (2015) Battery-Powered RF Pre-Ionization System for the Caltech Magnetohydrodynamically-Driven Jet Experiment: RF Discharge Properties and MHD-Driven Jet Dynamics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9T43R08.


This thesis describes investigations of two classes of laboratory plasmas with rather different properties: partially ionized low pressure radiofrequency (RF) discharges, and fully ionized high density magnetohydrodynamically (MHD)-driven jets. An RF pre-ionization system was developed to enable neutral gas breakdown at lower pressures and create hotter, faster jets in the Caltech MHD-Driven Jet Experiment. The RF plasma source used a custom pulsed 3 kW 13.56 MHz RF power amplifier that was powered by AA batteries, allowing it to safely float at 4-6 kV with the cathode of the jet experiment. The argon RF discharge equilibrium and transport properties were analyzed, and novel jet dynamics were observed.

Although the RF plasma source was conceived as a wave-heated helicon source, scaling measurements and numerical modeling showed that inductive coupling was the dominant energy input mechanism. A one-dimensional time-dependent fluid model was developed to quantitatively explain the expansion of the pre-ionized plasma into the jet experiment chamber. The plasma transitioned from an ionizing phase with depressed neutral emission to a recombining phase with enhanced emission during the course of the experiment, causing fast camera images to be a poor indicator of the density distribution. Under certain conditions, the total visible and infrared brightness and the downstream ion density both increased after the RF power was turned off. The time-dependent emission patterns were used for an indirect measurement of the neutral gas pressure.

The low-mass jets formed with the aid of the pre-ionization system were extremely narrow and collimated near the electrodes, with peak density exceeding that of jets created without pre-ionization. The initial neutral gas distribution prior to plasma breakdown was found to be critical in determining the ultimate jet structure. The visible radius of the dense central jet column was several times narrower than the axial current channel radius, suggesting that the outer portion of the jet must have been force free, with the current parallel to the magnetic field. The studies of non-equilibrium flows and plasma self-organization being carried out at Caltech are relevant to astrophysical jets and fusion energy research.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:plasma, laboratory astrophysics, astrophysical jet, magnetohydrodynamics, MHD, pre-ionization, RF amplifier, electrically floating, global discharge model, two-fluid model, 1D model, argon, helicon, inductively coupled plasma, spectroscopy, Langmuir probe, high speed imaging, afterglow, transport, self-organization, partially ionized plasma, gas breakdown
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Bellan, Paul Murray
Thesis Committee:
  • Bellan, Paul Murray (chair)
  • Giapis, Konstantinos P.
  • Hillenbrand, Lynne A.
  • Shepherd, Joseph E.
Defense Date:29 April 2015
Non-Caltech Author Email:vernon.chaplin (AT)
Record Number:CaltechTHESIS:05012015-172120954
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8844
Deposited By: Vernon Chaplin
Deposited On:19 May 2015 16:19
Last Modified:30 May 2023 22:25

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