Power Absorption Mechanism in a Non-Uniform Helicon Plasma
- Paper number
IAC-06-C4.P.4.02
- Author
Mr. Charles Lee, The University of Texas at Austin, United States
- Coauthor
Mr. Dan Berisford, The University of Texas at Austin, United States
- Coauthor
Dr. Roger Bengtson, University of Texas at Austin, United States
- Year
2006
- Abstract
This study is based on the on-going collaboration between the University of Texas (UT) at Austin and Ad Astra Rocket company to support the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) project . The VASIMR engine (originally developed at NASA’s Advanced Space Propulsion Laboratory) uses an RF-driven helicon to create plasma for a space propulsion system. The VASIMR rocket is currently under development for commercial applications. Past research has demonstrated that helicon plasmas can have high degrees of ionization. They have been used as plasma sources for everything from semiconductor applications to rocket propulsion. Although helicon sources are widely used because of the possibility of high ionization efficiency, the actual power absorption mechanism is not well understood. The classic dispersion relation for a uniform plasma dictates that a long spatial length is needed to deposit the wave energy; however, experiments have shown much shorter observed decay lengths. Because of this apparent contradiction, this experiment will look for mechanisms to explain the observed behavior. A team at UT has suggested one such mechanism. They suggest a leaky, cavity-like structure is created when a density gradient is present (non-uniform plasma assumption). This gradient provides the confining mechanism for the helicon wave which reflects most of the incoming surface wave. The dispersion relation varies from the classical Whistler wave due to the non-uniform plasma density assumption. The cavity set up by the density gradient effectively “quantizes” the wavenumber, thereby allowing only certain resonant frequencies. The specific allowable modes in this cavity are uniquely determined by the field structure. My experiment will measure detailed electron density profiles and magnetic amplitude and phase measurements to determine the electromagnetic wave structure. I will carry out the experiment on the UT helicon machine consisting of argon gas pumped into a vacuum vessel at a pressure of one milliTorr. The power is deposited by a 1KW generator at an operating frequency of 13.56MHz. I will present the design of Langmuir and magnetic B-dot probes used to collect the density and field data in both axial and radial directions. The density data will be used as input to FEMLab computer models of wave propagation. The output is a 2-D map of the wave structure which will be compared to the data collected by the magnetic probe.
- Abstract document
- Manuscript document
IAC-06-C4.P.4.02.pdf (🔒 authorized access only).
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