INVESTIGATION OF PLASMAS SUSTAINED BY HIGH REPETITION RATE SHORT PULSES WITH APPLICATIONS TO LOW NOISE PLASMA ANTENNAS

In the past two decades, great interest in weakly ionized plasmas sustained by high voltage nanosecond pulsed plasmas at high repetition rates has emerged. For such plasmas, the electron number density does not significantly decay between pulses, unlike the electron temperature. Such conditions are favorable to reconfigurable plasma antennas where the low electron temperature may enable the reduction of the Johnson–Nyquist thermal noise if an antenna is operated in the plasma afterglow. Moreover, it may be possible to sustain such conditions with RF pulses. Doing so could enable a plasma antenna that transmits the driving frequency when the pulse is applied and receives other frequencies with low thermal noise between pulses.

To study nanosecond pulsed plasmas, experiments were performed in a parallel-plate electrode configuration in argon and nitrogen gas at a pressure of several Torr and repetition frequencies of 30-75 kHz. To measure the time-resolved electron number density in the afterglow of each pulse, a custom 58.1 GHz homodyne microwave interferometer was constructed. The voltage and current measurements were made using a back current shunt (BCS). Initial analysis of the measured electron density in both plasmas indicated that the electron thermalization was much faster than the electron decay. In the nitrogen plasma, dissociative recombination with cluster ions was the dominant electron loss mechanism. However, the dissociative recombination rates of the electrons in the argon plasma suggested the presence of molecular impurities, such as water vapor. Therefore, to better understand the recombination mechanisms in argon plasma with trace amounts (0.1% or less by volume) of water vapor under the experimental conditions, a 0-D kinetic model was developed and fit to the experimental data. The influence of trace amounts of water on the electron temperature and density decay was studied by solving electron energy and continuity equations. It was found that in pure argon, Ar⁺ ions dominate while the electrons are very slow to thermalize and recombine. Including trace amounts of water impurities drastically reduces the time for electrons to thermalize and increases their rate of recombination.

In addition to large quasi-steady electron number densities and low electron temperature in the plasma afterglow, plasmas sustained by nanosecond pulses use a lower power budget than those sustained by RF or DC supplies. The efficiency of the power budget can be characterized by measuring the ionization cost per electron, defined as the ratio of the energy deposited in a pulse to the total number of electrons created. This was experimentally determined in air and argon plasmas at 2-10 Torr sustained by 1-7 kV nanosecond pulses at repetition frequencies of 0.1-30 kHz. The number of electrons were determined from the measured electron density through microwave interferometry and assuming a plasma volume equivalent to the volume between electrodes. The energy deposited was calculated from voltage and current measurements using both a BCS as well as high frequency resistive voltage divider and fast current transformer (FCT). It was found that the ionization cost in all conditions was within a factor of three of Stoletov’s point (the theoretical minimum ionization cost) and two orders of magnitude less than RF plasma.

Having shown that it is possible to generate high electron density, low electron temperature plasmas with nanosecond pulses, it was necessary to now create a plasma antenna prototype. Initially, commercial fluorescent light bulbs were used and ignited using surface wave excitation at various RF frequencies and powers. The S₁₁ of the antenna response was measured by a VNA through a novel coupling circuit, while the deposited power was measured using a bi-directional coupler. Next, a custom plasma antenna was created in which the pressure and gas composition could be varied. In addition to the S₁₁ and deposited power, the antenna gain, and the electron number density were also measured for a pure argon plasma antenna at pressures of 0.3-1 Torr. Varying the applied power shifts the antenna resonance frequency while increasing the excitation frequency caused an increase in measured electron density for the same deposited power. Initial tests using direct electrode excitation of a twin-tube integrated compact fluorescent light bulb with nanosecond pulses have successfully been achieved. Future efforts include designing the proper circuitry to time-gate out the large pulse voltage to facilitate safe antenna measurements in the plasma afterglow.