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7A D-R14l 989 A PROGRAMI TO IMPROVE SOLARBBLIND IJV (ULTRAVIOLET) FLASH i/i,LAMPS(U) SCI-TEC INC PRINCETON NJ W J MILLER OCT 83
I TR-83-8i6 N0804-80-C-0763UNLSIF IED F/G 13/1 NEEEEE|hEE|h|hE
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MICROCOPY RESOLUTION TEST CHARTINATIONAtL VIU t OF STYAOAROS - t943 A
777£ V/V/
I A PROGRAM TO IMPROVE
0 SOLAR BLIND UV FLASH LAMIPS
-CA-
*.%*705' 31 2
-A I/SciTec Report No. TR-83-016
!I
£ A PROGRAM TO IMPROVE
4 SOLAR BLIND UV FLASH LAMPS
Prepared for
OFFICE OF NAVAL RESEARCH
DEPARTMENT OF THE NAVY" 800 N. QUINCY ST.
ARLINGTON, VA 22217
~October 1983
ByIC• ,WILLIAM J. MILLER T~
SCITEC, INC.A; P.O. BOX CN 5203
: 1101 N. STATE ROAD, BLDG. NPRINCETON, NJ 08540
~(609) 921-3892
.(APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED)
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READ INSTRUCTIONSREPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM
1. REPORT NUMBER GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER
4. TITLE (aind Subtitle) S. TYPE OF REPORT & PERIOD COVERED ,
. A Program to Improve Solar Blind UV 1 Sep 80 - 31 May 82Flash Lamps 6. PERFORMING ORG. REPORT NUMBER
TR-83-0167. AUTHOR(q) 8. CONTRACT OR GRANT NUMBER(.)'
William J. Miller N00014-80-C-0763
9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK
AREA & WORK UNIT NUMBERS
SciTec, Inc.P.O. Box CN 5203Princeton, NJ 08540
II. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE
Office of Naval Research October 1983Department of the Navy 13. NUMBER OF PAGES
800 N. Quincy St., Arlington, VA 22217 5614. MONITORING AGENCY NAME 6 ADDRESS(If different frout Controlling Office) IS. SECURITY CLASS. (of this report)
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16. DISTRIBUTION STATEMENT (of thie Report)
Approved for public release; distribution unlimited.
17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, if different from Report)
Same
- IS. SUPPLEMENTARY NOTES
Co,
Dr. Van 0. Nicolai, Contract Monitor
19. KEY WORDS (Continue on reverse aide if necessary and Identify by block number)
Ultraviolet Lamps710 Light Sources
Xenon LampsSolar Blind UV
20. .48STRACT (Continue an reveree side If neceesary and identify by block number)
A study was conducted of UV light sources, particularly Xenon/H 2 flashlamps, in an effort to achieve higher efficiencies in the upper end of thesolar blind wavelength region for application to UV communication devices.Lamps containing a number of inert gases, with and without added Hr, were
subjected to detailed spectroscopic examination and their UV outputefficiencies compared to that of conventional commercially available lamps of
DD OM 1473 EDITION OF I NOV 65 IS OBSOLETE UNCLASSIFIED
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.- ,.
similar physical configuration. A highly versatile power supply capable ofdelivering very short pulses at variable frequency and power level wasdesigned and built for this purpose. It incorporates the capability to hold offcapacitor recharge for specifiable times between pulses in order to minimizepremature lamp firing which produces holdover and renders the source uselessfor FM or pulse position modulation operation. Modest increases in efficiencywere achieved by increasing Xe pressure and operating the lamps at thehighest power and voltage levels compatible with the structural strength ofthe lamp and the frequency range selected for operation. Xe/H 2 continue tobe judged the optimum fill gas in preference to He, Ne, or Ar mixtures. Themaximum attainable efficiency, 'defined as light output in the 260-280 nmrange divided by the total electrical power to the lamp electrodes, appears trbe about 0.4% achievable over a fairly broad range of electrical operatingparameters, H2 partial pressure and arc gap.
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TABLE OF CONTENTS
- Page
1. INTRODUCTION. .. ....... ....... .......... 1
2. BACKGROUND. .. .... ....... ....... ...... 3
2.1 UV Lamps. ... ....... ....... ........ 3
2.2 Pertinent Spectral Reference Data. .. ...... ...... 9
2.3 Atmoheric Transmission Considerations .. ...... ... 11
1 3. APPARATUS .. ...... ........ ....... ... 23
3.1 Experimental Flashlamps. .. ..... ....... ..... 23
3.2 GaslHandling System .. ...... ....... ...... 23
3.3 Electro Optics .. .... ....... ........... 26
3.4 Electronics. .. ..... ....... ....... ... 26
4. RESULTS. .. ....... ....... ....... ..... 38
4.1 Discharge Lamp Spectral Properties .. .. ...... .... 38
4.2 Radiometry. .. .... ....... ....... .... 42
4.3 Gas Composition Effects .. .. ...... .......... 49
5. CONCLUSIONS. .. .... ....... ....... ...... 54
~~F',
AcceS~~n or
LIST OF FIGURES
Figure Page
I Comparative Output Power for 3 UV Lamps for 10 nm
Band Passes ...... .... ....................... 4
P -2 Effect of Discharge Voltage on Xe Arc Lamp Spectrum . . 5
3 Comparative Light Output Efficiencies for Lamps ofFigure 1 ....... .... ......................... 7
4 Atmospheric Transmittance for 1 km Range 0.0 to" 100 ppb Ozone ....... .... ... .... ...... 20
5 Atmospheric Transmittance for 5 km Range 0.0 to4100 ppb Ozone ......... . . . . ... .... .... 21
6 Laboratory Flash Lamp ....... ............. ..... 24
7 Gas Handling Apparatus ..... ...... ........... .... 25
8 Transmission Characteristics of Solar Blind FiltersUsed ..... ... .......................... .... 27
9 Standard Lamp Output 500 mA, 30 cm Distance ...... ... 28
10 Monochrometer/PMT Wavelength Response Curve ... ...... 2911 Lamp Power Supply Diagram ....... ................ 3112 Clock Module Schematic ........ ................. 32
13 Recharge/Holdoff Timing Module Schematic ........... .... 33
14 Recharge Driver Module .... .................. .... 34
15 Lamp Main Power Module Schematic .............. .... 36
16 Lamp Trigger Power Module Schematic .............. .... 37
17 Spectral Output of Commercial Xe Flash Lamp .... ...... 39
18 Spectrum of a High Pressure Xe Lamp ..... ........... 40
19 Spectrum of an Ne-Filled Flash Lamp ............. .... 41
20 Spectrum of a He-Filled Flash Lamp ........ .... 43
21 Radiometer Band Pass Responses to ExperimentalXe Lamps ....... ........................ .... 45
22 UV Output as a Function of Voltage for VariousDischarge Frequencies ..... ... .................. 47
1i
9!.
LIST OF FIGURES (Continued)
Figure-Page
23 Effect of Frequency and Gap Size on Lamp
Op Efficiency ...... ......................... .... 48
24 Effect of Electrode Spacing on Maximum LampPower Input ....... ....................... .... 50
' : 25 Lamp Efficiencies as a Function of Power Input"-for Various Xe/H 2 Mixtures . . . . . . . . . . . . . . . . 51
'9 .9
4 .N
ii
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'-.
LIST OF TABLES
Table Page
1 Total Radiant Power and Lamp Efficiencies inSolar Blind for Various Xe/H 2 Lamps ....... ......... 10
2 Principal Emission Lines for Mercury in the WavelengthInterval 2500-3000A ...... ................... .... 12
S3 Principal Emission Lines for Neutral and Ionized Heliumin the Wavelength Interval 2500-3000A . ...... .... 12
4 Principal Emission Lines for Neutral and Ionized Neonin the Wavelength Interval 2600-3000A ............. .... 13
5 Principal Emission Lines for Neutral and Ionized Argonin the Wavelength Interval 2600-3000A ............. .... 15
6 Principal Emission Lines for Ionized Xenon in theWavelength Interval 2600-3000A ...... .............. 17
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IINTRODUCTION
The requirement for more intense and efficient ultraviolet light sources
has received considerable impetus in recent years for several reasons,
including the development of an ultraviolet (UV) communications system
(Ref. 1). State-of-the-art UV lamps presently available are used primarily
. , in spectroscopic applications, either for wavelength calibration, as sources
for absorption spectrophotometry or as solar simulators. Technically, these
light sources appear to have changed very little in decades. Contemporary
requirements for such lamps include the ability to perform in a pulsed mode
with controllable pulse duration, reproducible high intensity characteristics,
* long life and greater efficiency particularly in the solar blind region with as
little interference from visible IR and near UV radiation as possible. The
applications envisioned for these light sources often require a high degree
of portability thus prohibiting the use of large bulky power supplies such as
those typically used to operate high pressure are lamps and thereby creating
the need for high power efficiency. High pressure arc lamps, because of
their inherent high temperature characteristics, also produce large quantities
of visible, near UV and IR radiation which, in these applications, have to
be reduced or eliminated by selective filtering. The simultaneous
attainment of satisfactory rejection of high intensity sideband radiation and
the adequate transmission of UV puts severe practical demands on light
4 filters which can result in thermal stability problems and low output light
levels. Substantial improvements in the performance and efficiency of UV
flash lamps would therefore make it unnecessary to pursue other possibly
more difficult problems.
Low and intermediate (0.01 - 5 atm) pressure electrical discharge
lamps are the most efficient UV sources currently available and appear to
be the ones best suited to communications applications. Their performance
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*however is a limiting factor in the development of more effective UVcommunication systems. The objective of this program was the improvement
of the UV output efficiency and performance characteristics of these
i- flashlamps. The pursuit of this objective involved two different
approaches. The first consisted of a detailed quantitative investigation of- .. the performance and emission characteristics of existing state-of-the-art
lamps and extensive documentation of the effects of changes in their
* operating parameters. Secondly, an attempt was made to formulate novel
gas mixtures and electronic techniques which would provide more light in.- the spectral regions of prime interest without sacrificing overall lamp
," efficiency and without compromising other requisite lamp characteristics
" isuch as size, portability, pulse frequency and duration and lamp life. The
* first objective was met. Tentative conclusions based on preliminary
experiments utilizing isolated lamp samples were verified and lamp
performance characteristics were determined using a wide variety of gas
mixtures, fill pressures, arc gaps and excitation pulse configurations.
Modest increases in lamp efficiency were attained and data establishing p
probable limitations on the further extension of such improvement were
N .obtained. None of the more novel attempts to achieve higher efficiency
were successful.
.
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2. BACKGROUND
2.1 UV Lamps
The three discharge lamps most commonly employed as UV light sources
are filled with Hg, Xe or D 2 * The emission characteristics of these low
pressure lamps under their typical recommended operating conditions are
given in Figure 1. It can be seen that in the wavelength region between
200 - 300 nm, the Hg "pen lamp" is by far the strorip,- t emitter. This
emission, however, arises from an atomic resonance ransition and the
resultant narrow line comprises the only significant radi .,on throughout the
g entire solar blind portion of the UV. The width of thi '~is on the order
of 0.10 nm. Typical Xe lamp emission, under ordinary - sating conditions
.~ ~ with about 5-10 watts input power yields about 0.02 watts of output (Ref.
2) over the entire 200 - 300 nm bandpass compared to about 0.3 watts for
the low pressure Hg lamp which consumes slightly less power. The
D 2discharge lamp intensity is considerably lower than either of the lattertwo. This type of lamp has, however., become the calibration reference
standard (Ref. 3) for UV sources largely because of its remarkable
reproducibility and stability.
The spectral distributions of emissions from these lamps are not only
functions of the gas pressure within the lamp, but also functions of the1. -electrical characteristics of the discharge. Figure 2 (from Ref. 2) shows
the spectral output from a high pressure Xe arc lamp powered by 50
joules/second at two different voltages (Ref. 4). High voltages appear to
produce more UV radiation at the expense of emission in the visible and IR.
As a first approximation, the limitations on the amount of light that can be
obtained in the UV can be considered to be imposed by the physical
strength of the bulb envelope, and by the decay characteristics of the light
output pulse with time. For the same amount of input electrical power,
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o /l " Iji}2 2 1.'i '\-iII II
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"- "0III I I I I I I- .200 400 600 800 I000 1200
! 'i Wavelength, nm
..,. Figure 2. Effect of Discharge Voltage on Xe Arc Lan'p Spectrum-
'..
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U shorter pulses decrease bul life. Conversely at any fixed value of pulseduration, increased input power decreases bulb life. The approximate
-3 relationship between input power, I, and pulse duration, t, for constant. -.- bulb life and given bulb size is I = at . The root cause of these
limitations is heat. If cooling can be provided or if the quartz envelope
- can be moved away from the discharge, more power and/or longer pulses
,.. .,.p can be tolerated without adversely decreasing bulb life or output.
The efficiencies of the various UV light sources currently available varyat least as greatly as do their spectral energy distributions. Efficiency
figures for the sources of Figure 1 are given in Figure 3. These values
.r >were obtained by simply dividing the power output exhibited in Figure 1 by
the electrical power input to just the lamp (not the entire device) in each
case. The ex'reiae efficiency of the low pressure Hg lamp is immediately
, apparent. More than 90% of the radiation is in the 254 nm line and,
overall, about 10% of the input energy is converted to light at this
. wavelength. The seemingly attractive features of the Hg lamp in terms of
both preferential output and conversion efficiency are unfortunately offset
by other undesirable factors. First, the fact that the emitting species is
initially a liquid with a low nonoptimum vapor pressure means that a warm
up period is required, and that the intensity is somehow dependent on the
lamp temperature which changes continuously during the period of operation.
-.. , Secondly, the lifetime of the excited state (Ref. 5) and the absorption
, "cross sections for the Hg ground state produce a pulse of long duration
_ .- which limits the frequency of device operation. Despite the fact that the
Hg 254 nm line is due to a "forbidden" P1 S0 transition, it has a
radiative lifetime of about 3 nsec - a value much more characteristic of a
S-fully allowed transition than one involving a change of multiplicity. The
large emission and absorption coefficients corresponding to this short
- .. -lifetime, coupled with the fact that the lower level of the transition is the
- atomic ground state of the atom dictate that light will be absorbed and
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reemitted many times in even the low pressure Hg atmosphere of these
lamps before it reaches and passes through the walls of the quartz vessel.
"" " Measurements on Hg pen lamps fabricated from only 1 mm bore quartz
tubing indicate that a typical practically obtainable pulse width
(half-maximum) is about 52 microseconds (Ref. 6). Such a pulse width is
unacceptably long for FM devices. Finally, and most importantly, it will
be seen that 254 nm does not lie within the optimum spectral region with
" respect to atmospheric transmission.
-, The standard D2 lamp contains a very small discharge. Its lightemanates from an aperture which is typically only 1-2 mm in diameter.The glow itself within the quartz tube envelope is a few mm long and about
1 mm in diameter. Most of the continuum emission is due to the
a3 Zg b 3 transition (Ref. 7), the lower state of which is
repulsive. Higher current densities or gas pressures in H2 or D2 lamps
lead to greater degrees of molecular dissociation and to displacement of the
most intense emission to wavelengths both above and below the desired solar
blind region. Atomic hydrogen or D has no lines of any significance in the
K •bandpass of interest and its presence is therefore of no radiative
advantage, whatsoever, in a LIV lamp. In fact, its very high diffusion
, coefficient and reactivity are sources of potential problems with envelopei
integrity and electrode metallurgy. High power versions of D lamps are
therefore unlikely to serve the purposes of a UV lamp very well.
-. It is clear from the foregoing considerations why Xe lamps were chosen
for the UV communication system. In the case of the Xe discharge lamp
used in the transmitter described in Ref. 1, a small amount of H2 is added
1K:: .to the lamp gas fill not for its contribution to the emission but to increase
the rate of pulse decay, shorten the pulse width and prevent the discharge
.- *_ ,from holding over and becoming continuous with respect to time. The
H2 may produce this effect by enhancing the rate of removal of current-
conducting charged species created during the pulse. This may occur
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through an increase in the rate of recombination via the formation of
negative ions which have much larger neutralization cross sections than do". :~ electrons in collisions with positive atomic ions. Alternatively, the H2 may
act by quenching easily ionizable Xe metastable states via
electronic/vibrational energy transfer. In general, processes such as the
latter are too slow to be of significance but the multitude of energy levels
associated with many electron atoms and ions such as Xe, Xe+ and
Xe+ + greatly enhances the probability of occurrence of exact resonances at
which energy transfer is extremely efficient.
A recent study (Ref. 8) of means to improve UV communication system
Scomponents sponsored by NOSC revealed that some improvement in UV
output efficiency could be obtained by increasing the Xe pressure within the
- -.. .;lamp and that the lamp performance was only slightly sensitive to H2 partial
. ': pressure or electrode spacing. The study involved the use of 5 specially
designed lamps, two of which were nominally state-of-the-art conventional
U design, one with reduced H content, another with a 50% larger gap than
normal between the electrodes and the fifth with two times the normal fill
-. pressure of Xe. The results are presented in Table 1. It can be seen that
only the higher pressure lamp performed more efficiently than the two
conventional ones and that the improvement achieved is on the same order
as that of the differences between the nominally identical lamps 1 and 2.
The limited number of devices employed in this study made it impossible to
establish trends from this information, but at least one positive indication
Sin terms of improved performance was obtained.
2.2 Pertinent Spectral Reference Data
bSince line emission is an extremely important component of the
... J,.-, radiation from both Xe and Hg discharges, it seemed appropriate to give
,- - . consideration to the comparative abundances of the spectral lines of each of
'M the inert gas atoms and ions. The lamp fill gases of principal interest in
-9-
. Input Power, output UV, Efficiency)¢Watts Milliwatts %
Standard "Old" #1 25.6 51.3 0. 20
Standard "New" #1 25.9 69.9 0. 27
SStandard "New" #2 24.3 63.3 0. 26
Low H2 Design 25.6 64.0 0.25.'
Large Arc Gap 25.7 54.0 0.21
SHigh Xe Pressure 25.2 78.0 0.31 -
Table 1. Total Radiant Power and Lamp Efficiencies in!-Solar Blind for Various Xe/H 2 Lamps (Ref. 8)
* 4
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.' ...
i this study were the inert gases He, Ne, Ar, Xe and H2 and Hg. The
principal spectral lines for each of the elements involved and their
important ions are summarized in Tables 2 through 6. All of the inert gas
data was taken from Ref. 9; the Hg line intensities are from a Handbook
(Ref. 10).
Interestingly, neutral Xe has no strong lines in the spectral region of
'- '.' interest; all of the significant line radiation observed is attributable to
Xe+ and Xe++. On this basis, then, Ar, which also displays no atomic
- line emission but which does have a multitude of Ar+ ionic lines, could not
be eliminated as a candidate fill gas. Continuing this line of evaluation,
one would be led to conclude that He would be an unlikely emitter and that
Ne would hold great promise by virtue of the abundance of lines in the
spectra of both neutrals and ions. It will be seen in the description of
. .results to follow that none of these generalizations proved valid and thata priori rationalizations of this nature were, in the end, useless.
2.3 Atmospheric Transmission Considerations
., . As noted by Neer and Schlupf in Ref. 8, total solar blindness is not a
stringent requirement even in UV communication systems where background
- ,. radiation can be an important problem. Variations in received signal
intensity induced by changes in absorption due to atmospheric 03 is a far
- more significant consideration. The effects of absorption and molecular
scattering combined with the properties of solar blind filter materials led
these workers to the conclusion that optimum spectral region for these
purposes is 270 - 280 nm. The atmospheric transmission data presented in
Figures 4 and 5 illustrate the most important of these effects. At realistic
atmospheric 03 concentrations, the region below 280 nm can be seen to
, , become virtually opaque at distances greater than a few kilometers. As
- the practical ramifications of these absorption and scattering effects became
4 evident during the course of our experimental program, attention was quite
, -11-
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K. A I E Sv £. eV Transition
2967.28 100 4.67 8.85 6d 3 D_.6p 3po 0-1
2893.60 40 4.89 9.17 88 3S-6p3 po 1-12752.78 140 4.67 9.17 8S 3S- 63 po 1-02536.52 2000 0 4.89 6p 3 P*--6s 21s 1-0
Table 2. Principal Emission Lines for Mercuryin the Wavelength Interval 2500-3000A(Ref. 10)
I1. A I Elf,.eV Ell eV Transition
2945,106 100 19,82 24,03 2s SS-5p Po 1-2. 1 02829.076 40 19,82 24,20 2s 'S-6p -P0 1-2. 1 o
it2763,8N4 20 19.82 24,30 2s 3S-7p 3' 1-2. 1 (s2723.191 10 19.82 24.37 2s :S-8p 3P* 1-2. 1 .02696.119 7 19.82 24.42 2s S-9p 3p 1-2. 1, 02677,135 5 19.82 24,45 2s 'S-0p 'P 1-2. 1. 02663,271 4 19.82 24,47 2s S-ltp 3po 1-2, 1. 02652.848 3 19,82 24.49 2s 3S-12p 3P I-2. 1 . 0
% 2644,802 2 19,82 24,50 2s IS-13p P° I-2. 1
He I,
i 2733,32 100 48,37 52,90 Ud2D-61IF* etc.2511. 22 50 48.37 5.3,30 3d 2D-71 nF etc. 1= 'i:_Y1. I/
Table 3. Principal Emission Lines for Neutral andIonized Helium in the Wavelength Interval2500-3000A (Ref. 9)
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naturally focussed increasingly on the upper end of the solar blind region.
Much of our prior interest in the possibility of exploiting the extreme power
efficiencies displayed by low pressure Hg lamps, therefore was shown to
-.have been in vain. Subsequently, other candidate emitters were
investigated which it was hoped could be made to radiate in the moretransparent regions of the upper solar blind bandpass.
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3. APPARATUS.
3.1 Experimental Flashlamps
The desire to change gas mixtures and electrodes spacings often and
quickly made it extremely impractical to have a number of conventional
lamps fabricated for each parametric variation to be examined. A
somewhat cumbersome but highly versatile flashlamp design was therefore
conceived as indicated in Figure 6 from two commercially available Cajon
vacuum fittings and a 1.8 centimeter silica tube. Provision was made foremptying or refilling the lamp tube through a 0.6 centimeter diameter
" opening at one end of the apparatus and O-ring sealed feed throughs were
employed at each of the two ends to facilitate movement of tapered
, tungsten electrodes mounted on the ends of 1.6 mm diameter rods.
The trigger electrode comprising a 0.1 mm diameter wire rested against
the inside wall of the discharge tube and connected to the high voltageanode. The trigger electrode ground ring was painted on the outside of the
tube opposite the end of the trigger electrode. The entire configuration
was designed to allow both electrodes to be electrically isolated from all
other components of the system.
3.2 Gas Handling System
A schematic of the vacuum train and gas handling system is presented
in Figure 7. The entire apparatus is fabricated from Pyrex glass, stainless
steel and high vacuum fittings. It is capable of being evacuated to below
10 - Torr. The pumping was provided by a Varian model HS-2, 2 inch
diameter oil diffusion pump backed by an Alcatel model ZM 2004A 250 I/min
* mechanical pump. Pressure measurements were made using manometers, a; 'a
McLeod gauge which was interchangeable with the manometer, and a
Datametrics barocel passidence capacitance manometer manifolded to a
number of points throughout the system.
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The different sized storage reservoirs and the cold traps used to hold
condensed gases at liquid nitrogen temperatures provided means of preparing
mixtures of various gases in any desired proportions. The entire system
was accessible to a high pumping speed, low pressure manifold at a number
* of points along the train at the approximate locations indicated in Figure 7.
Seals between the glass gas handling system and the stainless steel vacuum
manifold were made with compression fittings equipped with high vacuum 0
rings.
3.3 Electro Optics
Spectrally resolved information on the various lamp emissions was
obtained using a model HR320 Instruments SA F/4.8 spectrograph with a
2400 g/mm grating blazed at 300 nm. The maximum resolution of thisinstrument is about 0.05 nm at 300 nm. A Hamamatsu R106 photomultiplier
tube served as the detector both in this instrument and in a radiometer
equipped with interchangeable filters. The bandpasses of the several filters
used in this study are given in Figure 8. These filter transmission
characteristics as well as the throughput efficiency of the spectrograph as a
function of wavelength were determined using a D2 discharge lamp as an
absolute calibration standard. The output of this lamp is given in Figure 9;
4 it was obtained from Optronic Laboratories, Inc. and is traceable to NBSstandards. The resultant throughput curve for the monochrometer/PMT
combination is presented in Figure 10. This data is normalized to the
maximum sensitivity value which occurs at about 325 nm.
3.4 Electronics
A power supply to drive the experimental test lamp was designed and
" built specifically for the purposes of this study. The requirements of the
program called for a high degree of flexibility in a number of electrical
parameters including frequency, voltage, capacitance and holdoff time.
a. -26-
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200 220 240 260 280 300Wavelength, nm
Figure 8. Transmission Characteristics ofSolar Blind Filters Used
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Figure 9. Standard Lamp Output'A -. 500 mA, 30 cm distance
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objectives in one piece of equipment. The device eventually built, which
resulted from a compromise between our wishes and our resources -- and
'V which consumed a much larger proportion of the time and money available
than originally anticipated -- is depicted schematically in Figure 11.
Timing for the discharge and capacitor recharging is controlled by a
clock module equipped with external thumbwheel dials for frequency
selection. This built-in clock module also provides an accurate frequency
reference for control of the holdoff interval between pulses. The clock
and holdoff timing modules, together with the recharge driver circuit which
they control, and a common power supply module are contained in a
. "control chassis" separated and isolated from the "head chassis" power
circuit enclosure which abuts the lamp itself. It proved impossible to
• .- ~ include the trigger and main power modules in the same enclosure as the
- control circuits. Noise from the power circuits apparently confused the
logic functions in the clock and/or timing modules and produced erratic
behavior.
The clock, holdoff timing and recharge driver module details are
, presented in Figures 12, 13, and 14, respectively. The discharge
frequency is controlled by the clock and is continuously variable from 0-30
kHz. A set of thumbwheel switches for frequency selection are provided on
the face of the control unit. Provision is also made for bypassing the
internal clock and triggering the device with an external signal. The
recharge holdoff unit provides a controllable time interval between the end
of one discharge pulse and the beginning of the capacitor recharge
operation for the next pulse. During this interval, no power is applied to
* the lamp driver circuits and the ionized gas between the electrodes remains
undisturbed and free to relax. This precludes premature breakdown which
might otherwise occur as the trigger and main capacitors are being
recharged. The duration of the holdoff interval is selectable via an
.le 3
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between pulses and, as a practical matter, capacitor recharge occupies
" "about a third of the interval; the holdoff maximum is therefore limited to
about two thirds of the interpulse time gap.
The trigger and main power module schematics are presented in Figures
-. 15 and 16. They both provide pulses to the same lamp module which
comprises a coupling transformer and diodes to prevent damage to the power
module circuit. Both power circuits are contained in the same chassis
enclosure and are connected to the control unit through separate cables.
Attempts to combine connecting wires within the same multiconductor
shielded cable caused problems similar to those encountered when the driver
-. circuits were positioned close to the logic elements, namely erratic behavior
and breakdown at high voltages. Recharge voltages up to 270V can be
accommodated by the configuration depicted and external switches allow
selection of main capacitor values of 0.1 and 0.03 pF and trigger
capacitances of 0.045 or 0.033 IiF.
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3. 4. RESULTS
4.1 Discharge Lamp Spectral Properties
* The first spectral studies were performed on the 4 state-of-the-art
discharge lamps built by USSI especially for the experimental program
described in Ref. 8. Scans were made from 200 - 300 nm at a discharge
frequency of 5 kHz and a total energy input of about 10 watts. The
discharge voltage was 150 V. The four resultant spectra, a sample of
which is given in Figure 17 are essentially indistinguishable from one
another. The spectra appear to comprise a large number of strong atomic
and ionic Xe lines superimposed on a wek', continuum or very closely spaced
, weaker lines. The most abundant and strongest lines can be seen to lie
between 225 and 260 nm. The region of greatest interest, i.e., that
between about 265 and 280 can be seen to be characterized by weaker lines
and a proportionately stronger continuum.
Close examination of the spectra of the two atm. lamp provided by
USSI reveals a slightly larger proportion of background continuum radiation
than the so called standard lamps. Subsequent experiments in which the Xe
"* pressure was raised to 4 atm. confirmed this effect and similar results were
obtained by increasing the discharge pulse frequency and thereby the power
input. A typical spectrum displaying either or both of these effects is
shown in Figure 18. It should be noted that in addition to what appears to
be a more intense continuum, a great many more atomic and ionic lines are
also present.
, LA 2Neon/H 2 filled lamps are much less intense than their Xe counterparts
throughout the 200-400 nm examined region and their spectra are largely
devoid of strong atomic or ionic lines. A typical spectrum is shown in
Figure 19. The emission features above 290 nm are almost all attributable
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to neutral Ne(I), those in the vicinity of 270 nm are due to Ne+(Il) and
those below about 260 nm are due to Ne++(III). The intensity scale in this" . figure is not directly comparable to those in Figures 11 and 12; the overall
intensity levels here are considerably lower. The pressure and frequency
parametric behavior noted in Xe/H 2 lamps were qualitatively reproducible in
these Ne discharges. The continuum radiation becomes stronger and the
wavelength distribution of higher intensities is shifted toward the red.
Helium/H 2 and Ar/H 2 discharges are also much less intense in the UV
than Xe/H 2 discharges. Atomic and ionic line strengths are weak comparedto the continuum radiation and the continua are similar in behavior to those
observed with Ne/H 2 mixtures. The He lamps at low power are noteworthy
in that the spectra are dominated by emission from contaminants. Figure
20 illustrates the nature of the emission from such a low power He
* ,.* discharge. Helium lines are readily identifiable at 295 and 319 nm and a
weak continuum underlies the entire spectral region, as in the case of the
other noble gas discharges. However the strong line at 248 and the group
of lines above 250 nm are attributable to C-atom emission and the emission
' .- features above 350 nm are strongly reminiscent of OH emission. Thepredominant contaminant in even quite pure He is normally natural gas,
mainly CH 4 which in an electrical discharge produces C-atom emission and,
in the presence of even very small quantities of 02 will produce the
observed radiation features.
4.2 Radiometry
After conducting a number of the spectroscopic studies described
above, it became apparent that the extensive parametric studies
contemplated to characterize the effects on the spectral nature of the
- emissions of varying gas composition, mixing rat: )s, electrical properties,
.- frequency, etc. would be much more expeditiously handled by observing
S'the output of radiometers equipped with the several bandpass filters
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* .described in Section 2 rather than by comparing detailed spectra. Observed
effects in the bandpass between 270 and 282 nm were deemed to be
particularly important in view of the practical atmospheric transmission
limitations which define the optimum bandpass for UV communication
- purposes.
A comparison of radiometer outputs at identical power input conditions
for the four standard lamps prepared by USSI are displayed in Figure 21.
All of this data has been corrected for filter transmission and
photomultiplier response and therefore comprise direct comparisons of UV
source intensities. Differences between the various bandpasses are much
more readily detected here than in the spectral data of Figures 17 and 18
where the influence of contributions of narrow lines to total intensities are
difficult to quantitatively evaluate by inspection. The normal pressure and
higher pressure lamps can be seen to display slightly higher intensities in
the bandpasses labeled A and D at opposite ends of the solar blind region
. than do the high H2 and narrow gap versions of these lamps. The
differences are small, however, and translate to but small increases in
efficiency as reported in Ref. 8. In view of this finding that parametric
variations in lamp operating characteristics appear to affect different
segments of the solar blind UV to the approximately same extent and due to
our priority of interest in the wavelength region 270 - 280 nm, all
subsequent radiometric experiments in which various means were sought to
r * increase lamp efficiency were conducted using a radiometer fitted with
filter C (see Figure 8).
Not all lamp parameters could be varied independently of one another.
S-"the limits for the variation of each depends in some way on the set of
prescribed values assigned to other parameters. Higher voltages, for
,.. .example, appear to produce a much desired increase in UV intensities at the
expense of intensities at higher wavelengths. However, the voltages
attainable are a function of the arc gap size and the discharge pulse
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4. 5
0220 240 260 280
Wavelength, nm
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•Figure 21. Radiometer Band Pass Responses toExperimental Xe Lamps
, -45-
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frequency. Some of the interdependencies of this complex of variables are
indicated in Figure 22 in which a straightline relationship is exhibited
between log I and log V2 . Unfortunately, though, there are limits to the
enhancement in performance to be attained by simply turning up the voltage
supplied to the lamp. As the voltage is increased, for example, in this
1.3 mm arc gap lamp configuration at a fixed discharge pulse frequency of,
say, 15 kHz "holdover", or a transition from stable pulsed operation to a
continuous arc occurs. In this 15 kHz case, acceptable performance is lost
at fairly low voltages. Conversely, higher voltages can be sustained
without holdover occurring only by employing lower pulse frequencies. In
this 1.3 mm arc gap case, the lamp output in stable pulsed form maximizes
at about 8 kHz. As anticipated, these performance characteristics are also
a sensitive function of the arc gap size. Each spacing displays a somewhat
different dependence on pulse frequency for optimum performance.
The nature of the interdependencies between discharge frequency, gap
size and voltage are partially illustrated by the data of Figure 23. This
data was taken using a lamp filled with the same gas mixture as that
employed in Figure 22 namely, 1 atm. Xe with 0.06 Torr H2 . The
I/V quotient (I = intensity) can be seen to vary nearly linearily with
discharge frequency indicating a nearly constant conversion efficiency ofelectrical to optical energy. A 0.5 mm gap width, however, can be seen
to be incapable of operating at frequencies greater than about 1500 Hz
without exhibiting sputtering, which creates inefficiency, and holdover,
which is unacceptable operation. Increasing the gap size, in general,
increases the frequency and power levels at which the lamp can be
operated. However, as larger gaps and higher frequencies are reached, the
conversion efficiency, i.e., the light intensity per unit power input, falls
off. In the frequency range 5-10 kilohertz, the most effective gap width
* . appears to be about 2 mm.
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so configured can be seen to operate in a stable manner over a narrower
band of frequencies with somewhat reduced light output. Lamps with large
gaps appear to operate intermittently with resultant lower output levels.
Figure 24 is a typical illustration of the dependence of performance on gap
size. The optimum configuration for this, a lamp filled with 1 atm Xe and
, 0.04 Torr H2 , can be seen to lie in the interval between 2-3 mm. At
smaller electrode separations, this lamp can accommodate only small
amounts of input power before holdover occurs and at larger separations, as
noted above, erratic behavior sets in, in which the lamp displays irregular
intermittent periods of holdover and sputtering.
4.3 Gas Composition Effects
In addition to the electrical and physical configuration parameters
examined, a number of experiments were conducted in which a variety of
gas mixtures were formulated and tested in efforts to achieve greater UV
efficiencies. As indicated by the results reported above, it was necessary
in those studies to vary the lamp configuration and electrical input
* iparameters, as well as the fill gas, since it was impossible to know, a
priori, which set of parameters would be most appropriate for any given gas
mixture. As noted in Section 3.1, the UV outputs from lamps in which Ne,
. Ar or He were substituted for Xe were considerably reduced regardless of
arc gap spacing or electrical input and in spite of apparently satisfactory' lamp stability. Many gas mixtures made up of these other noble gases,
with or without H2 , could, in fact, be made to function satisfactorily
'.*, from an operational point of view, but the lamps filled with Xe/H 2 mixtures
consistently displayed higher UV efficiencies.5'.
- Figure 25 summarizes the results of a very extensive test series in
which the partial pressures of both Xe and H were varied over the limits2
practically accessible in this apparatus. Not all of the results obtained are
-49-
- t"! 'l" j j , . ',i j . ,.' " ,- .- ,- ".. . , '., ,,. .,,. , ., - .'. '.,
*" .1*
10-
008' 0 0
.00
,6- 0 c 60 CI
0D
. , c,C 4
E
-0
.::2P- 0 Vmox - 150 V
20 Vmax 2 0 0 V
0 I 2 3 4
Electrode Separation, mm*i.Figure 24. Effect of Electrode Spacing on
Maximum Lamp Power Input
-50-
* ' q ' %.' V..'* "., - -. , ". .,'N , . ...* "* ,* .." ." * °. " - . . . ,W
r , ...- #.' .-. ', "-'-=",_'q ' ,
L7:- .>
'..2 AAe° a.0.2- O 0
0.1
0.08 0
• 0 .0 6 00
0.04-
". GAS MIXTURE
• TXe H 2
Y .0 0To rr To rr• G o.02- ,,0OR 0 1400 .040 0 1650 .04
.N , A 2170 .04
o •0 2530 .04
0.008 760 .06
. * 0 760 1.00
0.006 U 760 5.00
"amq 0
0.004 -
I I I I I i ! I
0 2 4 6 8 10Power Input, 0.5 CV 2 F, Watts
¢.4
Figure 25. Lamp Efficiencies as a Function of Power Inputfor Various Xe/H 2 Mixtures
-51-4',
_-<a.... '_-*,- . ?
I *4
p !presented here; those from unstable discharges or less than optimum
configurations are not given. The radiometer response was calibrated by
directly comparing the responses to the test lamp and to the standard
deuterium lamp viewed from the same distance using identical instrument
settings and bandpass filters. Radiant power was then computed byintegrating the output curve of Figure 9 over the appropriate bandpass
interval and multiplying the result by the ratio of the observed signal
,. strengths. Input power was taken, as indicated, as the sum of the main
and trigger circuit input wattages calculated from the capacitance and the" maximum recharge voltage. The lamp efficiency, c, is thus defined as the
- "ratio
R /R )2(t( W(X) dXf. Radiant Power Out K d 1 max
Electrical Power In 22 .. CV f
where t(X) and tmax are the filter transmissions at a given wavelength X ,
and at the filter transmission maximum, respectively, W(,) is the deuterium
lamp output power, in watts, at wavelength (Fig. 9), Rx and Rd are
the radiometer responses to the test lamp and the standard lamp,
respectively, C is the total capacitance in the input power circuit, V is its
voltage and f is the discharge frequency.
The UV output efficiency can be seen to rise rapidly with increasing
power, at least at low power input levels. Up to about 4W input, the
- data closely follow exponential behavior in accord with the simple
expression
6 2~
" = 5.4x10 exp (0.5 cv 2 f)
At higher power input levels, the gain in efficiency drops until at the
highest power levels attained, about 10W, it appears that a maximum has
nearly been reached or at about 0.25%.
-52-
*2.* ,. -
The optimum lamp contents, i.e., the mixtures which could
accommodate the highest levels of input power without the occurrence of
holdover or erratic performance comprised high pressures of Xe with someS,-'. small amount of H At high Xe pressures, performance was not especially
sensitive to the amount of H2 present; lamps with several other H partial2 2pressures ranging from 0.04 to about 0.3 Torr were tested at an Xe
"*- :. .pressure of 2530 Torr, for example, but the results were nearly identical to
those given for 0.04 Torr. Large H2 partial pressures do not appear to
enhance lamp performance; the H2 pressure in a lamp filled with 1 atm Xe
was varied from 0.04 to 5.0 Torr with almost no change in efficiency or
operating characteristics. As noted previously, high Xe pressures do enable"" i lamps to be operated at higher power and with somewhat improved UV
4 output. The degree of improvement realized diminishes, however, as the
. pressure and power input are increased. At the highest powers tested, the
.. changes produced by variations in pressure or power were small. On the
basis of this data, it would appear that the maximum efficiency attainable
is approximately 0.3%. This correlates regrettably well with the observa-
tions of Ref. 8 in which power levels nearly twice as high as those
employed here were found to produce efficiencies just slightly lower than
the highest of those shown in Figure 25.
,5%~
*1 -53
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° - . - - - - .- . 7 - .4 . -
S5. CONCLUSIONS
Although a great deal of information on the characteristics of pulse
discharges and UV lamp operating characteristics were obtained in this
program, the primary conclusions in terms of stated objectives must be
characterized as negative in nature. No novel gas mixtures or electronic
methodology was found which had any significant salutory effect on the
solar blind UV output efficiency of these lamps. Some modest improvements
were achieved in the operation of Xe filled flashlamps and some inherent
limitations in their performance characteristics were defined. Output
efficiency in the 260 - 280 nm range of prime interest was found to- increase with increasing input power and voltage. The degree of
/ improvement, however, diminishes as the power level or voltage is raised
and the efficiency appears to asymptotically approach a limiting value on
the order of about 0.4 percent. Conventional lamp designs need only small
changes to accommodate this sort of performance improvement.
-U Specifically, gap size must be adjusted slightly to accommodate desiredcombinations of frequency, voltage and power. The limits on input power
are the physical strength of the lamp itself which is mainly a function of
envelope thickness and the tendency of the lamp to holdover and become a
continuous arc. Thicker envelopes decrease light output by virtue of a
diminution in transmission. The tradeoff between transmission and
efficiency factors is a fairly straightforward engineering problem.
Holdover, on the other hand, can be minimized by electronic means but
represents an unavoidable limit for a lamp of any configuration.
The optimum fill gas for these lamps remains Xe with small amounts of
co-added H2 . Higher fill pressures produce somewhat higher efficiencies,
probably by simply increasing the density of emitters in the interelectrodearc channel. Practical considerations of lamp strength and electrode seal
-54-
..,
K q
directly analogous to that which limits input power. However, the gain in
- output efficiency attainable by increasing lamp fill pressure does not appear
to be as great as that derivable from raising the power input. Lamps
, :-. c o n t a in i n g t w o , t h r e e a n d f o u r a t m . o f f ill g a s d is p la y e d o n ly s m a ll*differences
in output efficiency.
6.
-55
q
'.,
o'.
i
"-55
6. REFERENCES
1. Neer, M.E. and Schlupf, J.M., "The Development and Testingh of an
. "Ultraviolet Voice Communication System", Final Report Contract No.
N66001-75-C-0180, A.R.A.P. Report No. 394, July 1979.
2. Compiled from Data Sheet No. F1005D-1, E.G.&G. Electro Optics
Division, Salem, MA, April 1973 and from Oriel Corporation, 1979
Catalog, Oriel Corporation, Stanford, CO, pp. D7 and D22.
3. Ott, W.R. and Wiese, W.L., Opt. Eng. 12 86 (1973).
h 4. Data Sheet F1002C-2, E.G.&G. Electro Optics Division, Salem, MA.
5. Corliss, C.H. and Bozman, W.R., NBS Monograph No. 53, 1962.
6. Mackey, Pollack, Applied Optics 2 542 (1963).
7. Herzberg, G., Spectra of Diatomic Molecules, D. Van Nostrand, NY,
41950, p. 530.
8. Neer, M.E. and Schlupf, J.M., "Ultraviolet Transceiver Component
Improvements", Final Report NOSC Contract N66001-80-C-0166, SciTec
Report No. TR-81-003, February 1981.
9. Striganov, A.R. and Sventitskii, N.S., Tables of Spectral Lines of
Neutral and Ionized Atoms, IFI Plenum Inc., New York, 1968.
10. Handbook of Chemistry and Physics, The Chemical Rubber Co., 38th
Edition, 1956-57.
-56-
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