Investigating physics of nanosecond-pulsed argon plasma discharges for a VLF plasma antenna

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Liu, Connie Y.
Walker, Mitchell L. R.
Cohen, Morris B.
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Very low frequency (VLF) waves (3-30 kHz) are useful in communication and navigation, can deeply penetrate the ground and ocean surface, and help with satellite protection by removing energetic charged particles in the Van Allen radiation belts that damage satellite electronics. However, current VLF antenna arrays take 1000s of acres because they are efficiency-limited – the signal propagates down the antenna and reflects faster than the signal period, which interferes with and cancels the outgoing signal. A top-hat loaded antenna is a solution that radiates more efficiently but is constrained to a small bandwidth. Replacing the metal conductor in a conventional antenna with a series of individually-controlled plasma cells in a plasma antenna could overcome both efficiency and bandwidth limitations. Modulating the plasma conductivity in each segment would turn a portion of the antenna on or off and suppress reflected waves in the time-domain by removing the necessary electrically conducting pathway. The two main research goals were to further understand the physics of pulsed plasmas by investigating ionization and recombination of pulsed plasmas on the nanosecond timescale and how operating conditions affect the time-resolved conductivity of a pulsed plasma. A single plasma cell was investigated by generating nanosecond-pulsed, argon plasma at various pulse frequencies, widths, and pressures. Argon emission lines were analyzed with an ICCD-spectrometer assembly gating at 4 ns, and relative intensities of strong argon neutral and ion lines were used in line-ratio calculations. These experimentally-determined ratios were compared to theoretical ratios generated from PrismSPECT, a collisional-radiative spectral analysis software, to obtain time-resolved electron temperature (~ 1 eV), electron density (1014 − 1015cm−3), and plasma frequency (~200 GHz). Those results were used to discover trends and extract sets of plasma parameters for the rapid ionization and recombination needed for a successful VLF plasma antenna design. Further investigations into the physics of nanosecond-pulsed plasmas could include analysis of wavelength transitions and processes as well as the effects of electrode geometry on plasma properties. Additional future work needed for a VLF plasma antenna demonstration would entail developing the signal propagation technology needed for transmission through the plasma antenna cell.
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