<p> 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.</p>
<p>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<sup>+</sup> 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.
</p>
<p>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.</p><p>
</p><p>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 S<sub>11</sub>
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 S<sub>11</sub> 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.<br></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/9958139 |
| Date | 17 October 2019 |
| Creators | Vladlen Alexandrovich Podolsky (7478276) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/INVESTIGATION_OF_PLASMAS_SUSTAINED_BY_HIGH_REPETITION_RATE_SHORT_PULSES_WITH_APPLICATIONS_TO_LOW_NOISE_PLASMA_ANTENNAS/9958139 |
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