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JITTER AND RECOVERY RATE OF A TRIGGERED SPARK
GAP WITH HIGH PRESSURE GAS MIXTURES
Technical Issues:
Payoff:
High rep-rate low loss switch for pulsed ringdown applications.
End Goals:
Allow accurate switching for a pulsed ring
down phased array antenna that has both
good recovery rate and low jitter
Approach:
Construct a system that will allow high
frequency, high voltage switching to
monitor the recovery rate and jitter of
-Use hermetically sealed high pressure spark gap
design
-Introduce a simple effective gas mixing subsystem
-Fast diagnostics and data acquisition techniques
-Modular design for both simple system integration
and minimal corona and breakdown possibilities
-System integrity at high voltages and high
pressures
Accomplishments:
-
different gases and gas mixtures from
atmospheric to high pressures (1000 psi)
-
Construct a parallel test system for material
lifetime and geometry evaluation
-
Completed project design and construction
Integration and improvement of project
subsystems
Basic diagnostics setup and initial testing
Triggered repetitive operation (100Hz, 65 kV,
400 psi nitrogen)
Performed initial lifetime testing
James Dickens, [email protected], 806-742-1254
PROJECT DESIGN IMAGES
Charge Line
1” Lexan
Cover
Exhaust
RG 220 (10m)
50 Ohm, 100
ns pulse, ~1 nF
Pressure
monitor
Gas mix
output
Diagnostics
>400V, 1.5 A power
supply
>10V trigger
Trigger
SOS pulser
100 kV, 10 ns rise-time
1kHz in burst mode
Vacuum
HV
Charger
>50kV, 25mA
Switch Design
Gas input
Copper
tungsten
electrode
Gas flow
Set screw RG220
fitting
Kel-F
lining
G-10
housing
Spark Gap
G-10
Housing
Al Baffle
CuW
Electrodes
Al Connecting
Pieces
KEL-F
Liner
Polished CuW Electrodes
Eroded CuW Electrodes
•
•
•
Electrode wear after ~104
shots
Example of minimal erosion
Ablation measurements
indicate negligible material
loss
PROJECT IMAGES
BNC 565
Pulse/Delay
Generator
XHR 600 1.7 DC
Power Supply
50 Ω Load
Feed-through for seal
and corona reduction
HV
125 KΩ Charging
Resistor
Charge
Line
Project wave forms
Rep-rated Self Break
(30 kV, 30psi Nitrogen)
Signal from
Capacitive Vprobe
Integral of
Capacitive Vprobe signal
Externally triggered
35 kV, 10Hz operation
BNC trigger to capture
10th pulse
Triggered 35kV, 10Hz pulses
Lifetime Test Setup
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•
•
•
Main and peaking gaps pressurized to ~500psig
Charging voltage = 90kVDC
Trigger pulse is created by peaking gap self-break
Voltage probes on the load side of peaking and main
gap record pulse
FY07-FY08 SCHEDULE
Improve system connections for enhanced power transfer and
corona reduction
Test with higher voltage and pressure to improve rise-time and
jitter
Compare rise-time and jitter of different gasses
Introduce gas mixtures and record effects on jitter and rise-time
Pulsed Ring-down
Multi-Element Antenna
Payoff:
Far field energy deposition for neutralization
of Improvised Explosive Devices (IEDs) at
long range distances.
End Goals: Be able to accurately model and
simulate various multi-element antenna
structures and the effects upon the
performance of a pulsed ring-down phased
array.
Approach:
Construct an accurate model of a single
element pulsed ring-down antenna using the
Comsol Multi-physics software package
allowing exotic antenna structures to be
evaluated before they are physically
constructed.
Technical Issues:
• Initial condition integration into model.
•
Accurately accounting for material
properties and effects.
•
Proper modeling of a closing switch and
the effects of jitter.
Accomplishments:
• Achieved accurate results of multiple
antenna structures in a 2-D and 3-D
regime using transient analysis.
• Constructed a two element array to
demonstrate beam steer and the effect of
high switch jitter.
• Achieved numerical results for energy
density and magnitude at various far
field points.
Monoconical Antenna 2-D
Electric Field 2-D
Dual Dipole Array 3-D
Electric Field 3-D
2-D and 3-D Modeling
Beam Steering
Far Field Results
PRDS array
Example: radiated electric field for four dipole sources
(spaced ½ wavelength apart), with no switch jitter
Simulated single source
radiated electric field
waveform:
Peak electric field vs. direction, measured
relative to that received from a single source:
90
4
120
1
60
3
wfma 0
150
2
30
1
1
0
100
200
0
a
1
2
3
180
0
PRDS array
Example: radiated electric field for four dipole sources
(spaced ½ wavelength apart), with uniformly distributed
switch jitter from 0 to ½ period (1 single shot)
Simulated single source
radiated electric field
waveform:
Peak electric field vs. direction, measured
relative to that received from a single source:
90
4
120
1
60
3
wfma 0
150
2
30
1
1
0
100
a
200
0
1
2
3
180
0
PRDS array –
Monte Carlo simulation
• Difficult to solve analytically for output variable statistical
distributions given switch jitter distributions
• Use Monte Carlo method: simulate many firings of an array to
build up output statistics
• Inputs: array parameters, simulated or experimentally
measured switch jitter distributions
• Status: basic simulation is functional
PRDS array –
advanced concept
•
•
•
•
Sources mounted on multiple
vehicles
Firing controlled using GPS timing,
coordinated to place “hot spot” on
desired location
High rep-rate sources could be
controlled to rapidly scan an area
Modeling to include GPS timing and
position errors in addition to
individual switch jitter
FY07-FY08 SCHEDULE
• Complete the Comsol model that accounts for
material properties, initial charging conditions, and
closing switch characteristics.
• Compare model to experimental results and adjust
accordingly to match.
• Design and model various antenna structures along
with the performance results when in an array.
• Examine the affect of jitter on a compact array (2 ft5ft antenna distance) and a large mobile array (2 m –
15 m antenna distance)
Ultra-Fast Gas Switching
Technical Issues:
• Scaling laws and physics of ultra-fast
switching are unknown
Payoff: Scaling laws and design criteria for ultrafast switching.
End Goals: Improve transmission line switching
for antenna coupling.
Approach:
• Empirical analysis of fast switching gas
• Pulses: <150 ps rise, <300 ps FWHM
• V(t), I(t) with 50 ps sampling rate
• X-ray analysis through fast PMT
• Streak-camera luminosity analysis
• FEM analysis of geometric gap transition
• Distributed Monte-Carlo electron motion /
amplification simulations
Accomplishments:
• Empirical results
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–
–
–
•
Gap currents determined through lumped
parameter modeling
Formative delay times quantified
Runaway electron analysis
Ultra-fast luminosity imaging
Monte-Carlo Analysis
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–
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Determination of electron multiplication rates
Direct calculation of space charge formation
Results support empirical analysis
PROJECT IMAGES
1) Experimental Setup
2) Essential Experimental Results
Formative delay
times as a function of
pressure for different
voltage amplitudes
from 40-150 kV.
• Background gases are Argon and Dry
Air with pressures from high vacuum to
atmosphere.
• Rexolite lens between coaxial to
biconical geometric transition limits
wave distortion.
FEM simulation of open gap for line
characterization (time not to scale).
Streak-Camera
results show
breakdown structure
as a function of time.
The images show a
region of high
ionization near the
cathode. The slope
in the luminosity
shows the transit
time for the gap.
PROJECT IMAGES
3) Monte Carlo Simulation
• Simulations run on 32
node Beowulf cluster.
• Capable of > 5 Gflop/s
• Efficient internode
communication using
the standard message
passing interface (MPI)
• Simulation based off null-collision method
for determining collision type.
4) Simulation Results
•
Electron amplification rates for varying
pressures and field amplitudes can be
combined with models to predict delay times.
•
Space charges in the vicinity of the cathode
lead to local fields on the order of the applied
field.
•
Ionization mapping shows a high ionization
region near the cathode similar to the
empirical results. Past this region electrons
tend to accelerate to runaway velocities
limiting further ionization.
Cathode
Anode