Motivation

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• Motivation
• Compact Pulsed Power, miniaturization of
  components
• Better understanding of breakdown in liquid
  dielectrics
• Clarify breakdown phenomenology for nanosecond
  time scale
• Cryogenic operation (Increased power density for
  semiconductors,
• faster closing time for switches)
• Liquid nitrogen as fast switching medium
• Fast rise-time lt2ns
• High hold-off at small gaps
• Non-pressurized switches.

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• Accomplishments
• Electrical Diagnostics in LN2
• Current measurements 10-4 103 Ampere
• 300ps resolution
• Optical Diagnostics
• Multi-port fiber optic diagnostics with 800ps
  PMTs
• 2-D imaging with 2.5 ns exposure
• Streak camera imaging with 5ps resolution
• Designed and built 200kV high speed pulse chamber
  for
• non-cryo liquids

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• Experimental Setup
• DUAL POLARITY CHARGED TRANSMISSION LINE SETUP
• Facilitates fast electrical diagnostics
• Only half the breakdown voltage applied to
  feedthroughs
• DIAGNOSTICS USED
• Current (transmission line current sensor,
  0.1V/A, lt300 ps risetime, threshold 1 mA)
• Voltage divider (11000, 300 ps risetime)
• Luminosity (PMT, 0.8 ns risetime)
• IN PROGRESS
• High speed photography

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- charging
charging
Supercooled LN2, no thermal bubbles
Basic setup with dual polarity charging
(Vgap,max200 kV)
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LN2
Gap
Dielectric
Outer Conductor
Inner Conductor
Steel Container
Basic setup feedthrough and gap
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VOLTAGE
T1
TIME
CURRENT
T
Expected current and voltage waves in
double-polarity charges transmission line
setup T1 single line transit time, T transit
time sensor-gap
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MEASURED CURRENT, VOLTAGE high pass, dc not
measured
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Prebreakdown current and luminosity spikes
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Orig. pulse
Reflected at charging side
Reflected at gap capacitance
Current waveform of pre-discharge pulses,
schematically
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Examples for pre-discharge current spikes (upper
trace) and luminosity (lower trace) Current
amplitude 0.5 A, duration 1 ns, corresponds to
3x109 electrons, estimated photons per pulse
1010
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Current (upper trace) and voltage (lower trace)
at breakdown
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CONCLUSIONS Pre-breakdown pulses start several
microseconds before breakdown, occur
stochastically Duration on the order of one
nanosecond, amplitude 0.5 A ? 3x109
electrons Luminosity pulses with duration several
nanoseconds, 1010 photons emitted Main breakdown
shows current risetimes I/(dI/dt) on the order of
1 ns
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• CONCLUSIONS, contd
• Nanosecond current pulses associated with light
  emission are expected to be due to field emitted
  electrons, bubble formation, and further charge
  amplification in the gas phase
• Pre-breakdown processes are repeated until one
  site reaches critical amplification which leads
  to breakdown
• The current amplification parameter I/(dI/dt) is
  on the order of several 10-10 s, which implies
  current risetimes on the order of one nanosecond
  (at an average field of 200 kV/cm and a
  macroscopic field enhancement of 10)

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Work in progress High speed photography (spatial
resolution 10??m, exposure time 3 ns) for pre-
and main breakdown Optical emission spectroscopy
with temporal and spatial resolution Current
measurement with higher resolution (min. current
lt 10-7 A _at_ 2?s, 10-4 A _at_ 1 ns)
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Preliminary results High speed photography of
pre-discharges
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Reference picture showing electrode
contour (Cathode left, gap width 1 mm)
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Negative Side Current
Positive Side Current
Intensity during main-breakdowngt104 intensity
during prepulse dynamic range of camera
insufficient
Center for Pulsed Power and Power Electronics (P
3
E)
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To PMTs
Optical fibers
Cyl. lenses
Time resolved zone-imaging of gap
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CURRENT, 100A/V
VOLTAGE, 4kV/V
Current and voltage signal during mainbreakdown
(voltage from capacitive divider, i.e. 0
corresponds to 24 kV, -6 corresponds to 0)
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CURRENT
LUMINOSITY
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CURRENT
LUMINOSITY
TIME (?s)
(?s)
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• Future Plans
• Examine oil and fluorinated liquids near boiling
  point (i.e. pull vacuum and heat)
• Attempt 10-7 current measurement with low noise
  system
• Develop physical model for early phase liquid
  breakdown
• Examine acoustic shock signatures of pre-pulse
  phenomena
• Utilize our new high speed digital oscilloscope
  (6 GHz analog bandwidth with 50ps between samples
  single shot) to measure pre-pulse duration
• Literature states 10ns pre-pulse duration, but we
  think diagnostics is the limiting factor. We
  currently measure 1-2ns pre-pulse duration, but
  this is still diagnostic limited.

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Motivation for near boiling point measurements
J. Gerhold, Cryogenic Liquids A Prospective
Insulation Basis for Future Power Equipment,
IEEE Transactions on Dielectrics and Electrical
Insulation, Vol. 9, Number 1, Feb. 2002, pg 68-75.
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High Voltage Liquid Breakdown Test Chamber
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Test Chamber Design Simulations and Calculations