<p>Nonlinear transmission lines (NLTLs) provide a solid state
alternative to conventional vacuum based high power microwave (HPM) sources.
The three most common NLTL implementations are the lumped element, split ring
resonator (SRR), and the nonlinear bulk material based NLTLs. The nonlinear
bulk material implementation provides the highest power output of the three
configurations, though they are limited to pulse voltages less than 50 kV;
higher voltages are possible when an additional insulator is used, typically SF<sub>6</sub>
or dielectric oil, between the nonlinear material and the outer conductor. The
additional insulator poses a risk of leaking if structural integrity of the
outer conductor is compromised. The desire to provide a fieldable NLTL based
HPM system makes the possibility of a leak problematic. The work reported here develops
a composite based NLTL system that can withstand voltages higher than 50 kV and
not pose a risk of catastrophic failure due to a leak while also decreasing the
size and weight of the device and increasing the output power.</p>
<p>Composites with barium strontium
titanate (BST) or nickel zinc ferrite (NZF) spherical inclusions mixed in a
silicone matrix were manufactured at volume fractions ranging from 5% to 25%.
The dielectric and magnetic parameters were measured from 1-4 GHz using a
coaxial airline. The relative permittivity increased from 2.74±0.01 for the polydimethylsiloxane
(PDMS) host material to 7.45±0.33 after combining PDMS with a 25% volume
fraction of BST inclusions. The relative permittivity of BST and NZF composites
was relatively constant across all measured frequencies. The relative
permeability of the composites increased from 1.001±0.001 for PDMS to 1.43±0.04
for a 25% NZF composite at 1 GHz. The relative permeability of the 25% NZF
composite decreased from 1.43±0.05 at 1 GHz to 1.17±0.01 at 4 GHz. The NZF
samples also exhibited low dielectric and magnetic loss tangents from
0.005±0.01 to 0.091±0.015 and 0.037±0.001 to 0.20±0.038, respectively, for all
volume fractions, although the dielectric loss tangent did increase with volume
fraction. For BST composites, all volume fraction changes of at least 5%
yielded statistically significant changes in permittivity; no changes in BST
volume fraction yielded statistically significant changes in permeability. For
NZF composites, the change in permittivity was statistically significant when
the volume fraction varied by more than 5% and the change in permeability was
statistically significant for variations in volume fraction greater than 10%.
The DC electrical breakdown strength of NZF composites decreased exponentially
with increasing volume fraction of NZF, while BST composites exhibited no
statistically significant variation with volume fraction. </p>
<p>For composites containing both BST
and NZF, increasing the volume fraction of either inclusion increased the
permittivity with a stronger dependence on BST volume fraction. Increasing NZF
volume fraction increased the magnetic permeability, while changing BST volume
fraction had no effect on the composite permeability. The DC dielectric
breakdown voltage decreased exponentially with increased NZF volume fraction.
Adding as little as 5% BST to an NZF composite more than doubled the breakdown
threshold compared to a composite containing NZF alone. For example, adding 10%
BST to a 15% NZF composite increased the breakdown strength by over 800%. The
combination of tunability of permittivity and permeability by managing BST and
NZF volume fractions with the increased dielectric breakdown strength by
introducing BST make this a promising approach for designing high power
nonlinear transmission lines with input pulses of hundreds of kilovolts.</p>
<p>Coaxial nonlinear transmission
lines are produced using composites with NZF inclusions and BST inclusions and
driven by a Blumlein pulse generator with a 10 ns pulse duration and 1.5 ns
risetime. Applying a 30 kV pulse using the Blumlein pulse generator resulted in
frequencies ranging from 1.1 to 1.3 GHz with an output power over 20 kW from
the nonlinear transmission line. The output frequencies increased with
increasing volume fraction of BST, but the high power oscillations
characteristic of an NLTL did not occur. Simulations using LT Spice demonstrated
that an NLTL driven with a Blumlein modulator did not induce high power
oscillations while driving the same NLTL with a pulse forming network did. </p>
<p>Finally, a composite-based NLTL
could be driven directly by a high voltage power supply without a power
modulator to produce oscillations both during and after the formed pulse upon
reaching a critical threshold. The output frequency of the NLTLs is 1 GHz after
the pulse and ranged from 950 MHz to 2.2 GHz during the pulse. These results
demonstrate that the NLTL may be used as both a pulse forming line and high
power microwave source, providing a novel way to reduce device size and weight,
while the use of composites could provide additional flexibility in pulse
output tuning. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/14485128 |
| Date | 06 May 2021 |
| Creators | Andrew J Fairbanks (10701090) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Novel_Composites_for_Nonlinear_Transmission_Line_Applications/14485128 |
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