Currents And Radiation Induced By Moving Tethers And Magnetic Antennas

A relative motion between a current-carrying conductor and a magnetized plasma is common in space plasmas or drifting laboratory plasmas. Even for de currents flowing in insulated conductors, the motion results in induced time-varying plasma currents. Such "eddy" currents can excite electromagnetic eigenmodes resulting in radiation of waves (e.g., whistlers, Alfven waves), well known from the study of the contactors of electrodynamic tethers.
¹
In addition to this Cherenkov-type emission, an imposed ac current in an antenna excites waves at the driving frequency. The resultant radiation patterns and efficiencies have received little attention in experiments. In the present work,
²
magnetic probe measurements are performed in a large laboratory plasma (1 m ⊥ B
₀
, 2.5 m ∥ B
₀
, n
_c
⩽ 10
¹²
cm
^-3
, B
₀
⩽ 20 G) containing fixed conductors (straight wires, loop antennas) through which pulsed currents are passed. The Cherenkov-like radiation patterns are obtained by the superposition of waves from appropriately delayed pulses at displaced exciter positions (Huygen's principle of wave construction). In our experiment, short current pulses (Ms; 1 μs) excite low-frequency whistlers whose superposition yields a whistler "wedge" for a straight conductor (e.g., a tether) and more structured radiation patterns for circular conductors ( e.g., circular antennas). Radiated vs. near-zone stored energies are estimated. The results are relevant to VLF antennas in space.