DEVELOPMENT OF INTENSE LIGHT FLASHES AND THEIR
APPLICATION TO DISCHARGE INITIATION
by
MICHAEL PAUL ALLEY, B.S. IN E.P.
A THESIS
IN
ELECTRICAL ENGINEERING
Submitted to the Graduate Faculty
of Texas Tech University in
Partial Fulfillment of
the Requirements for
the Degree of
MASTER OF SCIENCE
IN
ELECTRICAL ENGINEERING
December, 1982
s
w-
I'/j^, <
ACKNOWLEDGEMENTS
I am deeply indebted to Professor Erich E. Kunhardt for his
patience and understanding in the preparation of this thesis. Also
I would like to thank Professor Lynn Hatfield and Professor Russell
Seacat who served on my committee and who offered me advice and
provided some of the instrumentation.
T. M. Delacruz machined many
of the components incorporated in the experiment, while Greg Vincent,
Michael White, and Jerry Seams provided technical assistance.
n
CONTENTS
ACKNOWLEDGEMENTS
ii
ABSTRACT
iv
LIST OF FIGURES
I.
II.
III.
IV.
v
INTRODUCTION
1
DEVELOPMENT OF FLASHER
3
Flasher Design
3
Flasher Studies
7
APPLICATION TO DISCHARGE SYSTEM
16
Theory
16
Experiment
24
CONCLUSIONS
31
REFERENCES
33
m
ABSTRACT
An ultraviolet light flasher has been built which consists of
a spark gap (V^ = 7 kV) mounted at the end of a coaxial transmission
line so that current pulses of five nanosecond width are produced.
Intensity studies of the flasher in the spectral range capable of
producing photoelectric emission from the electrode in a pulsed discharge experiment were performed and statistical plots showing the
variation in intensity at particular wavelengths are presented.
The
light from the flasher was focused on an electrode arrangement in a
pressure-vacuum chamber.
Current curves for single avalanches were
recorded and the statistical distribution of the number of electrons
produced by the flasher were found.
IV
LIST OF FIGURES
Figure
1. Ultraviolet Flasher Circuit
4
Figure
2. Transmission Line
5
Figure
3. Photograph of Flasher Pulse
6
Figure
4. Optical Lens Arrangement for U.V. Flasher
8
Figure
5. Transmission Characteristics for Quartz
9
Figure
6.
Flasher Intensity Distributions
for 200, 240, and 250 nm
10
Figure
7.
Intensity Distribution for One Flash of U.V. Source.. 11
Figure
Figure
8.
9.
Photoelectric Yield for Brass
Statistical Plot of the Projected
Photoel ectron Rel ease
14
Intensity Distribution of U.V. Source
with Graphite Electrodes
15
Figure 10.
Figure 11.
13
Electrical Circuit for Observing Electron
Avalanches Across a Gap
17
Figure 12.
Electron Current of an Avalanche
19
Figure 13.
Ion Current of an Avalanche
21
Figure 14.
Plot of U^(t)
23
Figure 15.
Discharge Chamber Electrode Arrangement
25
Figure 16.
Discharge I n i t i a t i o n Chamber
26
Figure 17.
AC-coupled Amplifier
28
Figure 18.
Photograph of U^(t)
29
Figure 19.
Statistical Distribution of nQ
30
CHAPTER I
INTRODUCTION
The time elapsed from the application of a high voltage to
a gas filled gap to the collapse of the voltage across the gap consists of two parts:
the statistical time lag, which is the time
for a free electron(s) to appear at a suitable position in the gas,
and the formative time lag, which is the time for the initial free
electron(s) to grow in number and initiate a discharge.
To measure the formative time in a pulsed breakdown experiment,
2
the statistical time lag must be eliminated. Free electrons arise
naturally in the atmosphere from cosmic rays and the penetration
of strong radiation from the sun. However, the rate of production
from these effects is small and reliance on them to produce free
electrons in pulsed breakdown experiments to measure formative
lags is not possible. Therefore, to insure the presence of free
electrons at the cathode when the voltage pulse is applied to a
3
gap, other methods of generating free electrons are used.
Ultra-
violet radiation from external sources has long been used as a
source of electrons in the study of gas breakdown.
In a pulsed
breakdown experiment, the radiation, which is focused on the
cathode, produces free electrons from the cathode surface by means
of photoelectric emission.
This radiation serves two purposes:
first, it eliminates the statistical time lag, and
secondly, it specifies the point on the cathode from which the
2
spark originates (that point being the focus of the radiation on the
cathode surface).
The specification of the spark's position in the
gap allows optical diagnostics of its development.
An ultraviolet light flasher has been developed for application
5
to pulsed breakdown study. The flasher consists of an inhomogeneous
electrode arrangement mounted at the end of a coaxial transmission
line. Light from this spark gap is utilized as the flasher.
The
flasher spark gap has a breakdown voltage of seven kilovolts and
produces a current pulse five nanoseconds wide.
Intensity studies
of the flasher in the spectral range capable of producing photoelectric emission in a pulsed discharge experiment were performed
and statistical plots showing the variation in intensity at particular
wavelengths are presented.
The light flashes from the flasher were focused on the cathode
r
of a Rogowski profile
electrode arrangement mounted on a pressure-
vacuum chamber. Current curves for single avalanches (initiated by
the flasher) similar to those found by Raether were recorded.
From
these curves, the statistics of the number of electrons produced by
the flasher were found. Two electrode materials, brass and graphite,
2
were used in the main chamber with the flasher producing 10 electrons
per flash from the brass and no measurable amount from the graphite.
CHAPTER I I
DEVELOPMENT OF FLASHER
Flasher Design
Figure 1 shows the main components for the c i r c u i t of the
flasher.
A high voltage power supply charges the main gap G, as well
as a capacitor C^ to 17.5 kV.
The main gap G, consists of two alumi-
num hemipheres separated by a few millimeters.
A trigger pin, which
is connected to a krytron pulse c i r c u i t , is inserted between the two
hemispheres and when triggered causes the gap to break down.
When
this happens, the capacitor is shorted to ground and discharges i t s
energy into a transmission l i n e (see Figure 2 ) , which has a characteristic impedance of 110 ohms.
The rate at which the transmission l i n e
is charged is determined by the time constant RoC-i.
At the end of
the transmission line there is an a i r gap (the flasher spark gap),
whose electrode separation d is adjusted so that i t breaks down where
the voltage appears across i t .
The flasher gap consists of two brass
electrodes, the cathode being conical and machined at a 45-degree angle
to a fine t i p and the anode being a hemisphere.
The cathode's shape
not only enhances the electric f i e l d but also induces the spark to
occur at the t i p .
In order to produce a f a s t - r i s i n g pulse in the flasher gap,
3 9 in
the flasher gap is overvolted. ' ' ^
Figure 3 depicts this fast rise-
time as well as the f i v e nanosecond pulse width which is controlled
by the length of the transmission l i n e .
was measured to be seven k i l o v o l t s
The
and
pulse
the
height
A
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Figure 3.
Photograph of Flasher Pulse
consistency of the pulse is exemplified by the picture.
The photo-
graph shows ten pulses superimposed upon one another.
Flasher Studies
An optical arrangement was developed (Figure 4) to make the
5
flasher suitable for incorporation into a discharge system. Quartz
lenses with the transmission characteristics of Figure 5 focus the
flash onto a focal plane 56 cm from the axis of the flasher spark gap,
the size of the focal spot being a rectangle of 2 mm x 1 mm. The
quartz effectively blocks a l l radiation whose wavelength is below
180 nm and thus provides a minimum wavelength that needs to be considered in any spectral studies.
A profile of the intensity spectrum of the flasher was performed
with a spectral radiometer (EG&G model 550-1) for the wavelengths
that could produce photoelectric emission from either a brass or
12
graphite surface. The screen of the radiometer was placed at the
focal point of the optical arrangement.
The radiometer gave an
integral reading of the energy produced from a single flash at a particular wavelength.
After hundreds of shots, intensity distributions
of particular wavelengths were determined.
these plots for 200, 240, and 250 nm.
Figure 6 gives three of
A total plot of the intensity
for the spectral region 180-300 nm is given in Figure 7 with s t a t i s tical mean bars at selected wavelengths.
Three hundred nm was chosen
as the highest wavelength because the ionization potentials for brass
12
and graphite f a l l just beneath t h i s .
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30
t
20
10
0
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16 )0
_.
1^30
2 00 22.0 2^1-0 2 60 28 0 3C)0
3:20
Wavelength in Nanometers
Figure 5. Transmission Characteristics for Quartz
'
10
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12
The intensity peak at 220 nm in Figure 7 corresponds to a second
order transition in the copper atoms of the brass electrodes. The
13
actual line occurs at 219.226 nm.
The intensity profile of the
flasher was roughly matched with the emission rate curve for brass
14
(Figure 8 ) .
The electron work function for brass was estimated at
4.24 eV (290 nm). Considering Figure 7 and Figure 8 and the size of
the focused flash, a statistical plot of the projected photoelectron
release was estimated (Figure 9) for a single flash on a brass surface
2
5 X 10 photoelectrons was the average.
There was no useful emission rate curve obtained for graphite
15
although the work function 4.35 eV (285 nm) is well-known.
The electrodes of the flasher gap were changed from brass to
graphite and a different intensity profile was obtained (Figure 10).
The peak observed between 240 and 250 nm corresponds to a firstorder transition in carbon, the actual line occuring at 247.86 nm.
However, it was found that the graphite cathode tip eroded quickly
and was unsuitable for the experiments to be performed.
Therefore,
the brass electrodes were used in all succeeding experiments. A
set procedure was developed to maintain the consistency of the
flasher spark gap. After ewery five hundred shots, the flasher
gap was subjected to a fan for five minutes and then allowed to
discharge for five minutes (once ewery three seconds).
The main
gap G, as well as the flasher gap was also cleaned and polished
every ten thousand shots.
13
-4
X 10
50 o3
:3
40
to
30
C7<
o
U
-p
o
20
(^
10 ..
Wavelength in Nanometers
Figure 8. Photoelectric Yield for Brass
14
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CHAPTER III
APPLICATION TO DISCHARGE SYSTEM
Theory
The number of photoelectrons produced at a cathode surface by an
intense light flash can be determined by applying a potential difference UQ between the cathode and an anode and measuring the drift
17 18
currents of the formed electrons and ions. ' Consider an experimental
arrangement as shown in Figure 11. A constant voltage source UQ is
connected across a gap (with capacity C) through a resistance R.
An
appropriate filter is placed between the voltage source and the gap.
It is assumed that the electrodes are parallel and plane and that
there exists a gas between them at a particular pressure p.
If the
cathode of this gap is illuminated by an intense light flash so that
n« photoelectrons are produced, then ng electrons will start an
avalanche.
For this current avalanche to be measured, UQ must be so
high that sufficient amplification of the avalanche occurs but low
enough that the probability of producing secondary avalanches by
photons and ions is small.
In this case, the current will only be
a function of the Townsend coefficients, alpha and beta, and not the
17 18
other secondary processes. '
If np electrons start simultaneously from the cathode of the
plane parallel gap (separated by a distance d) and drift in the field
E = UQ/d with a constant velocity v_, the current that flows during
the transit time T_ = d/v_ of the avalanche electrons will be:
16
17
^filter
Uo_-L%
^filter
Figure 11. Electrical Circuit for Observing Electron
Avalanches Across a Gap
18
enQ
enQV_
M t ) = -^
=-f-
0 ^ t < T_
where e is the charge of an electron.
The number of electrons increases with growing distance x from
the cathode because of ionizing collisions with the molecules of
gas between the electrodes.
The electrode component of the current
thus becomes a function of time t = x/v_.
en_(t)
I_(t) = —.j^
0 ^ t ^ T_
where n_(t) = nQ exp (a v_t), a being the first Townsend coefficient.
Therefore,
enQ
I_(t) = -j^
exp (a v_t)
0 ^ t $ T_
(1)
Ijt) = 0
T_ < t
The time constant of this electron component is:
T
e
= 1/a V
—
and the graph of the negative current carriers is shown in Figure 12.
The positive ions returning to the cathode form the ion component
They do not suffer ionizing collisions since the cross-section for
such a process is so small. The current component for the ions is:
enp,
I^.(t) = -j^ (exp (a v_t) - exp (a v^t)) 0 ^ t ^ T_
(I)
19
if
^
exp^cdX.
en-
0
Figure 12. Electron Current of an Avalanche
20
enQ
I+(t) = y ^ (exp (a d) - exp (a v^t)) T_ ^ t ^ T_ + T^
(3)
where v_j_ is the drift velocity of the positive ions and is assumed
to be much slower than that for the electrons. Typically, the drift
5
velocity for positive ions is on the order of 10 cm/sec while
electrons travel about one hundred times faster.
The curve for the
*f
)•
positive ion current is shown in Figure 13 and the time constant for
the positive ions is given by:
T+ = 1/aV^ .
(4)
The carrier current I^^(t) = I_(t) + I (t) is produced by the
motion of the charged particles and must be supplemented by the
displacement current CU (U - voltage across the gap) to give the
total current I in the external circuit.
dU^
I = I + C -r^
cr
dt
(5)
The voltage across the output resistance R is given by
dU^
U^(t) = RI = RI^^ + RC - 3 ^
and using UQ = U^(t) + U^(t), UQ being constant,
dU^
U,(t) = RI^^ - RC - ^
(6)
21
»•
.1
en.
expC-^) •
T +T
- +
Figure 13.
Ion Current of an Avalanche
22
where C = C + C ... Such capacities C ,. arise from the physical
structure of the experiment, particularly at the junction between
the anode and the amplifier.
An exact solution of this equation is given as:
t
U^(t) = 1/C (exp (-t/RC))(Jexp ( Y / R C ) I^^(Y)dY+ CU^(O)) (7)
If, however, RC > > T ^ = 1/av^, then the solution reduces to t
r
Using the equation for I
JQ
cr
, the electron and ion component of the
resistance voltage is given as:
enQ
U (t) = — ^ (exp (a v_t) - 1)
en^
U^(t) = - ^ (exp {od)}
0 ^ t $ T_
(8)
T ^t ^T^
This is assuming that exp (a v_j^t) can be neglected in the expression
for I (t). The curve of U (t) is shown in Figure 14. The amplitude
of the ion component reaches its maximum at t = T^.
enQ
U^(t) = - ^ ( e x p (ad))
(9)
and nQ becomes
U (T )C
r^ -'
"0 = exp (a d) e
(10)
I t is seen that U (T_) amounts to 'vl/ad of the total voltage
enJC exp(a d); this can be interpreted as i f all the electrons are
produced in the mean distance 1 / ^ before the anode.
The electrons
23
exp (ad) -
Figure 14.
Plot of U^(t)
24
participate therefore a t the total voltage drop eT)JC exp(cxd) only
the quotient ~(l/a) : d . The ionic component of U ( t ) proceeds almost
linearly with time, since the cloud of positive ions exp(ad) returns
with constant d r i f t velocity v^ to the cathode producing a constant
current and thus a l i n e a r voltage r i s e .
After the time T
the
capacity of the gap is recharged with the time constant RC.
Experiment
Chamber
The experimental arrangement used to measure nQ ( i . e . , the i n i t i a l
photoelectrons) was designed so that
Equation 10.
RC»T^
and nQ is obtained from
A 4x4" Corning pyrex cross housed an electrode arrangement
that was to be flashed by the u l t r a v i o l e t source.
The chamber electrode
arrangement (Figure 15) consisted of two Rogowshi-profiled electrodes
(either brass or graphite), the cathode mounted on a t i l t - t a b l e to insure
parallelism and the anode situated above on a threaded rod to allow for
gap separation adjustment.
The electrodes had a total diameter of 3.78"
with a f l a t plane diameter of 2.52".
Polished with fine emory cloth
beforehand, the electrodes were subjected to a glow discharge f o r t h i r t y
_5
minutes in the pyrex chamber which could be baked, evacuated to 10
torr, and then f i l l e d with Nitrogen gas (purity - 99,998%).
The capac-
i t y of the chamber gap was calculated theoretically for a one cm separation and found to be 6.44 pF.
This discharge i n i t i a t i o n chamber is shown in Figure 16.
A quartz
window was mounted on one of the side ports to allow u l t r a v i o l e t l i g h t
from the flasher to enter the chamber.
A high voltage power supply
(Spellman model 40PN60) with a load regulation of 0.01% was hooked
through an additional f i l t e r R C into the cathode.
s s
An AC
25
Threads for gap
separation adjustment
Rogowski
Profile
Tilt table
Ball bearing and
Spring mechanism
Figure 15. Discharge Chamber Electrode Arrangement
26
I
Quartz
c
^
Window
Zh
I
r~
Screen Room
Figure 16. Discharge Initiation Chamber
27
coupled amplifier (Figure 17) with a high input impedance (15 Meg)
had its input at the back of the anode. The amplifier, capable of
3
gains on the order of 10 with low noise sensitivity, fed a Tektronix
7104 oscilloscope.
The reason for the high input impedance was to
ensure that R C > T ^ .
A screen room surrounding the chamber provided
the ground plane for the experiment.
The flasher as well as every
component of the chamber had to be shielded against noise.
Results
The potential across the chamber gap was set between 95-98% of
19 20
V^, the self breakdown voltage, * and the cathode (brass) was flashed
by the ultraviolet source.
Photographs of U (t) were obtained (Figure
18) and match well with the theoretical curve (Figure 14). Note that
U (t) is inverted due to the amplifier.
Part A of the photograph
represents the linear ion component of U (t). The electron component
cannot be seen because the risetime of the amplifier was not fast
enough.
Part B of the photograph represents the recharging the chamber
gap with RC time constant.
Many photographs were taken at various pressures and a distribution of the number of photoelectrons produced in brass was found
(Figure 19). The distribution of photoelectrons results from a number
of random processes:
1) fluctuations in the U-V spectrum; 2) avalanche
size statistics;^^'^^'-^^ 3) fluctuations in the yield of photoelectrons;
etc. The distribution may be used to get an indication of the mean
number of photoelectrons generated, so that we can judge the regime
of operation, i.e., "single electron" (~1 to 100 photoelectrons)
or "multi-electron" (-10^ photoelectrons) initiation.
' '
28
Note that the number of photoelectrons obtained is of the same order
of magnitude as expected from theoretical considerations.
2
about 10 electrons were emitted from the brass.
Typically
The graphite electrodes were also flashed but no detectable
voltage was observed.
The photon energy from the flasher was
sufficient to have produced photoelectrons from the graphite (work
function 4.5 eV). This observation has yet to be explained.
29
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j / ^ 0 0 /
-ANV*CM
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30
[
B
Pressiire
27.9 torr
Voltage
2.2 kV
(98% V )
Amplifier gain 950
Gap Separation
1 cm
Figure 18. Photograph of U^(t)
31
o
o
•M
Xi
•r%.
4->
to
Q
ro
U
+->
to
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4->
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as
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—I
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sq.unoo
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VO
CHAPTER IV
CONCLUSIONS
An ultraviolet flasher with a consistent voltage pulse was developed for application to discharge initiation.
However, the consistency
of the voltage pulse was not reflected in the consistency of the ultraviolet light, as evidenced by the spectral radiometer reading and actual
avalanche results.
This is due to a combination of reasons - an impor-
tant one being the surface condition of the flasher electrodes.
The
intensity spectrum of light in the ultraviolet light range is very
dependent upon the electrode surface (see Figure 7 and 10) and any
alteration in the surface can alter significantly the output of the
light.
These alterations in the surface occurred in the gradual erosion
of the flasher electrode surface.
Also, despite efforts to maintain a
sharp tip on the flasher cathode, discharges occurred on either side of
the tip and this altered the amount of light which traveled through the
lens arrangement.
The use of brass electrodes in the discharge chamber produces an
average of 10 electrons per flash which was below the estimated value
of 5 X 10
photoelectrons projected from the radiometer data and
photoelectric curve.
However, it is satisfactorily close.
The distri-
butions were similarly shifted (compare Figure 9 and Figure 19). This
discrepancy can be accounted for by the surface condition of the chamber
cathode which was probably oxidized despite the glow discharge and
vacuum.
The inability of graphite electrodes to produce any recognizable
data in the discharge chamber can be accounted for by the low voltages
32
33
used across the chamber gap.
The reason for the lower voltages
(less than 95%) was due to the instability of V^ for the graphite
electrodes. The lower voltages meant a lower alpha which drops off
significantly at voltages below V
amplification in the avalanches.19
and a lower alpha meant insufficient
REFERENCES
1. Kunhardt, E. E., IEEE Trans, on Plasma Science PS-8, 130, 1980.
2.
Grey Morgan, C , "Fundamentals of Electric Discharges in Gases,"
Handbook of Vacuum Physics 2. Edited by A. H. Beck.
The Macmillan
Co., New York, 1965.
3. Teich, T. H. and Branston, D. W., IEEE Gas Discharge Conference,
London, p. 109, 1974.
4. Zworykin, V. K. and Ramberg, E. G., Photoelectricity and Its
Applications.
New York, John Wiley & Sons, 1949.
5. Levinson, S., "Investigations of Overvoltage Breakdown," M.S. thesis,
Dept. of Electrical Engineering, Texas Tech University, August 1981.
6. Cobine, J. D., Gaseous Conductors, Dover, New York 1958.
7. Raether, H., Electron Avalanches and Breakdown in Gases.
London,
Butterworth, Chapter 2, 1964.
8. Fischer, H., Journal Opt. Soc. of America 51, 543, 1961.
9. Godlove, T. F., Jourhalof Applied Physics 32, 8, 1961.
10. Andrew, S. I. and Vanyukov, M. P., Soviet Phys. Tech. Phys. 6^, 700,
1962.
11. The Ealing Corporation Optics Catalog, Newport Beach, California
92660, p. 464, 1962.
12.
Handbook of Chemistry and Physics, R. C. Weast and M. J. Astle, eds.,
61st Edition, p. E-83, CRC Press, Boca Raton, Florida, 1981.
13. M.I.T. Wavelength Tables, Handbook of Chemistry and Physics, R. C.
Weast and M. J. Astle, eds., 61st Edition, p. 415, CRC Press, Boca
Raton, Florida, 1981.
14.
DeVoe, C. F., Physics Review 5C^, 481, 1936.
34
35
15.
Dice, R. and Hershey, J., Esco Products Catalogue, Esco Labs,
1977.
16.
Reinman, A. L., Proceedings of the Physics Society 50, 496, 1938.
17.
Raether, H., Electron Avalanches and Breakdown in Gases.
London,
Butterworth, Chapter 2, 1964.
18.
Llewellyn-Jones, F. P., Ionization and Breakdown in Gases, London,
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