Direct comparison of whistler mode radiation between an electric dipole and magnetic dipole antenna in a laboratory plasma
ORAL · Invited
Abstract
High-energy electrons from the solar wind or human activity can become trapped in the Van Allen belts or form artificial radiation belts, posing a hazard to spacecraft. Whistler waves can precipitate these electrons into the atmosphere, making wave-based remediation a promising strategy. Recent missions such as the Van Allen Probes and Demonstration and Science Experiment (DSX) have advanced our understanding of electron loss and wave injection in space, but no clear consensus exists on the most effective whistler wave source for spacecraft-based remediation.
Results of a laboratory plasma experiment comparing the efficiency of exciting whistler waves by electric and magnetic dipole antennas are presented here. To quantify coupling efficiency, we compute the ratio of the measured wave power propagating along the plasma column to the input power at the antenna. For the first time, the complex impedance on a magnetic dipole has been directly measured by measuring the voltage and current directly on the loop itself. A significant decrease in the real part of the impedance is observed as plasma density decreases, consistent with reduced coupling as the plasma can no longer support whistler waves and the antenna is unable to excite free-space waves at the driving frequency. This trend also provides experimental validation of our impedance diagnostic, confirming sensitivity to plasma-loaded impedances. To support space-based deployment, we investigate how the antenna size relative to the parallel wavelength (R/λ∥) affects coupling efficiency for whistler waves, enabling direct experimental tests of previously unvalidated theoretical predictions for the magnetic dipole (Kondrat’ev et al., 1992). For the electric dipole, we similarly examine how coupling efficiency scales with antenna length, motivated by DSX observations in which the measured radiation resistance’s dependence on length deviated from theoretical expectations (Song et al., 2023).
Results of a laboratory plasma experiment comparing the efficiency of exciting whistler waves by electric and magnetic dipole antennas are presented here. To quantify coupling efficiency, we compute the ratio of the measured wave power propagating along the plasma column to the input power at the antenna. For the first time, the complex impedance on a magnetic dipole has been directly measured by measuring the voltage and current directly on the loop itself. A significant decrease in the real part of the impedance is observed as plasma density decreases, consistent with reduced coupling as the plasma can no longer support whistler waves and the antenna is unable to excite free-space waves at the driving frequency. This trend also provides experimental validation of our impedance diagnostic, confirming sensitivity to plasma-loaded impedances. To support space-based deployment, we investigate how the antenna size relative to the parallel wavelength (R/λ∥) affects coupling efficiency for whistler waves, enabling direct experimental tests of previously unvalidated theoretical predictions for the magnetic dipole (Kondrat’ev et al., 1992). For the electric dipole, we similarly examine how coupling efficiency scales with antenna length, motivated by DSX observations in which the measured radiation resistance’s dependence on length deviated from theoretical expectations (Song et al., 2023).
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Presenters
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Jesus A Perez
University of California, Los Angeles
Authors
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Jesus A Perez
University of California, Los Angeles
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Patrick Pribyl
University of California, Los Angeles, University of California , Los Angeles
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Quinn R Marksteiner
Los Alamos National Laboratory
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Walter N Gekelman
University of California, Los Angeles