AE41A-0145
The observation of x-ray bursts
produced by 1.5 MV laboratory
sparks in air
Z. Saleh, J. R. Dwyer, and H. K. Rassoul
Department of Physics and Space Sciences, Florida Institute of Technology,
Melbourne, FL 32901; email: jdwyer@fit.edu
J. Jerauld and M. A. Uman
Department of Electrical and Computer Engineering, University of Florida,
Gainesville, FL 32611
J. A. Plumer
Lightning Technologies Inc., Pittsfield, MA 01201
Abstract
X-ray observations of 1.5 MV laboratory sparks were performed at
the high-voltage laboratory of Lightning Technologies Inc. (LTI) in
Pittsfield, MA, using the same three x-ray instruments that had been
used previously for measuring x-ray emission from both rockettriggered lightning and natural lightning at the University of
Florida/Florida Tech International Center for Lightning Research and
Testing (ICLRT). Observations were made during fourteen 1.5 to 2.0
m high-voltage discharges in air produced by a 1.5 MV Marx circuit.
It was found that all 14 discharges generated x-rays in the ~30 to 150
keV range. The x-rays, which arrived in discrete bursts, less than 0.5
microseconds in duration, occurred from both positive and negative
polarity rod-to-plane discharges as well as from small, 5 - 10 cm
series spark gaps within the Marx generator. The x-ray bursts usually
occurred when either the voltages across the gaps were the largest or
were in the process of collapsing and were remarkably similar to the
x-ray bursts previously observed from lightning. Our results imply
that runaway breakdown is occurring in relatively small high-voltage
sparks and that the physics of such sparks involves more than just a
conventional breakdown. This finding implies that the physics used
for decades to describe discharges in air may be inadequate, even for
relatively small sparks. It also opens up the possibility of using
laboratory sparks to study the poorly understood phenomenon of
runaway breakdown [Gurevich et al. 1992; Gurevich and Zybin
2001]. Because runaway breakdown has been shown to be associated
with lightning, thunderstorms electrification and terrestrial gammaray flashes (TGFs) seen from space, all of which are extremely
difficult to study, the laboratory study of x-rays from sparks should
greatly add to our understanding of these atmospheric phenomena
[Dwyer et al. 2005].
Figure 1. Schematic of instrument used to measure x-rays from
lightning and laboratory sparks [Dwyer et al. 2003, 2005].
Figure 2. Natural and triggered lightning make x-rays [Dwyer et al.
2003; 2004, 2005b; Dwyer 2004; Moore et al. 2001]
Figure 3. Positive rod-to-ground plane 1.5 MVspark. The spark gap
was 2 m through air.
Figure 4. The Left panel shows two x-ray pulses produced by a 1.5
MV positive rod-to-ground-plane spark with a 2 m gap. The Marx
circuit is fired at time t = 0. The black diamonds are the anode signal
from the 12.7 cm NaI/PMT detector, and the red curve is the detector
response function. For comparison, the right panel shows the
measurement of one 662 keV gamma-ray from a Cs-137 radioactive
source, placed temporarily on top of the box. The offset in the x-axis
in the right panel was adjusted so that the pulses occur at the same
positions in both panels. The good fit of the response function to the
anode data demonstrates that the signals are indeed produced by xrays.
Figure 5. X-ray waveforms
and the gap potential. A
change in pulse size of -0.25 V
corresponds to a deposited
energy of 662 keV in all
detectors. In the upper panel,
from top to bottom,
respectively, the signals are
from two control detectors, an
attenuated detector (0.32 cm
thick bronze cap), a detector
covered by a wire mesh, an
un-attenuated detector (no
bronze cap) and a larger 12.7
cm detector. The bottom
panel shows the gap voltage,
measured with a resistive
divider.
Figure 6. Negative rod-to-ground plane 1.5 MV spark. The spark
gap was 1.5 m through air.
Figure 7. X-ray waveform
from the 12.7 cm detector and
the gap potential for a
negative polarity spark. The
vertical dashed lines indicate
the times of the x-ray bursts.
The first x-ray burst occurred
at the time of the first sparks
among the series gaps within
the Marx generator. The next
x-ray burst occurred at the
time of peak potential
difference across the 1.5 m
gap, and the final x-ray burst
occurred during the latter
stages of the collapse of the
voltage across the 1.5 m gap.
Figure 8. The world’s
largest air insulated Van de
Graaff machine at the
Boston Museum of Science
does not make x-rays. No
x-rays were observed from
either positive or negative
sparks with air gaps up to
several meters and from a
range of about 1 m.
(Thanks to Earle Williams
for helping with these
measurements)
Summary
 X-ray bursts in the ~30 -150 keV range where measured from 1.5
MV sparks in air produced by a Marx generator.
 The x-ray bursts were produced for both positive and negative
polarity rod-to-ground plane sparks.
 The x-rays arrived in discrete bursts <0.5 microseconds in
duration when the voltage across the gap was the largest or in the
process of collapsing.
 X-rays were also observed from the small 5 - 10 cm spark gaps
within the Marx generator.
 These observations imply that runaway air breakdown is
occurring in relatively small laboratory sparks [Gurevich 1961].
 The x-rays are very similar to the x-rays previously observed from
natural and rocket-triggered lightning.
 No x-rays were observed from large sparks from the Van de
Graaff machine at the Boston Museum of Science.
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