Air spark fiducial for ultrahighspeed photographyH. M. Graham and G. A. Leavitt Citation: Review of Scientific Instruments 44, 1630 (1973); doi: 10.1063/1.1686016 View online: http://dx.doi.org/10.1063/1.1686016 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/44/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An UltraHighSpeed Detector for Synchrotron Radiation Research AIP Conf. Proc. 705, 945 (2004); 10.1063/1.1757952 Ultrahighspeed filming of atomizing jets Phys. Fluids 22, 1054 (1979); 10.1063/1.862711 Microtemporal Synchronization System for UltrahighSpeed Photography J. Acoust. Soc. Am. 37, 1216 (1965); 10.1121/1.1939621 UltraHighSpeed Photographs Refuting ``Cohesion in Plasma'' J. Appl. Phys. 31, 1225 (1960); 10.1063/1.1735808 UltraHighSpeed Flash Cinemicrograph Rev. Sci. Instrum. 28, 254 (1957); 10.1063/1.1715851

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Air spark fiducial for ultra-high-speed photography*

H. M. Graham and G. A. Leavitt

Lawrence Livermore Laboratory, University of California, Livermore, California 94550 (Received 24 May 1973; and in final form, 6 August 1973)

A triggered air gap has been developed for inserting timing marks on the fllm of streaking cameras used in u1tra-high-speed photography. The gap has a rise time ten times faster than existing xenon flashlamps. The peak output of 7 X 106 cd/cm2 (7Mcd/cm~ achieves the same fllm density as the lamps. This paper describes both the design used to achieve low inductance in the gap and the driving circuitry.

INTRODUCTION

For nearly ten years xenon flash lamps have been used to provide a set of timing marks for the streaking cameras used in hydrodynamic studies at the Lawrence Livermore Laboratory. These lamps were satisfactory at the lower streaking speeds, but their 100 nsec rise time was too slow to take advantage of faster camera speeds. Above 6 mm/~sec the timing marks were excessively blurred.

Figure 1 shows a camera arrangement and how the timing marks arrive at the film plane of the camera. Two flash sources are used to place "start" and "stop" marks along the lower edge of the film, one source being used for the "start" flash arid a second source for the "stop" flash. The time between flashes is determined by moni­toring the pulses of current through the sources. The current pulses, along with a set of timing pulses, are dis­played on a raster oscilloscope. The light is piped from the flash sources to the entrance slit of the camera by fiber optics. At the slit, half of the round fiber bundles is blocked so that a D-shaped spot is focused on the film. The straight edge of the D is the fiducial. If the rise time

Streaking camera

Film

::;:::===::::::::...-/=-Event

• Timing

Distance

Time

Start . flash

/~---74-~ t} f!iffiing device ~ I

Stop flash

FIG. 1. Camera arrangement showing how fiducial marks are put on film.

of the source is not sufficiently fast for the streaking speed, the fiducial is blurred on the picture. The calibra­tion of the film is determined by dividing the distance between the fiducials by the time between the current pulses.

DEVELOPMENT

We found we could not improve the rise time and retain a flash lamp as the source because the flash lamp has a finite resistance, too much inductance, and cannot be pre-ionized. Other commercial sources that were tested were either too dim or not fast enough.

March l suggested an air gap consisting of 1.27 cm diam brass balls between parallel aluminum plates. The balls were surrounded by four "doorknob" capacitors that also held the plates apart. This configuration has the low inductance needed for a fast rising pulse and can store the energy for high output brightness. The four 4000 pF capacitors we used stored 7 J at 30 k V. Allowing the gap to break down without triggering gave a 3-4 nsec rise time with an output brightness (as measured with an S-4 photosurface) equal to the xenon lamp. The discharge was observed through a space between two of the capacitors.

We modified the gap along the ideas of Lebessart2 and, with the same input energy, increased the output bright­ness to 5 or 6 times that of the ball gap. The modification consisted of adding a hollow cylinder of Teflon between

Light

Ught

FIG. 2. Optical fiducial as suggested by Lebessart.

1630 Rev. Sci. Instrum., Vol. 44, No. 11, November, 1973 Copyright © 1973 by the American Institute of Physics 1630

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H. M. Graham and G. A. Leavitt: Spark Fiducial

(a) n·Trigger c ! (Minor gap) LJ. Light output

(Moin gop)

(b) Retainer plate

FIG. 3. Optical fiducial. (a)-Schematic diagram; (b)-mechanical arrangement; and (c)-enlargement of the gap area.

the plates, in the center of which was a tungsten rod that was electrically connected to the high voltage plates (Fig. 2). The arc between the rod and the ground plate was confined by the Teflon and viewed through a hole in the plate. The increased light output was due in part to this view along the length of the discharge rather than across it.

This gap, which initially had a rise time of 1{)-20 nsec, aged to a rise time of 80 or 90 nsec (about the same as the xenon lamp) and proved difficult to trigger reliably. This aging was probably due to capacitor deterioration.

We redesigned the gap and incorporated the trigger electrode into it. This configuration is essentially two gaps (one of which is triggered) in series with the storage capacitor [see Fig. 3(a)]. When one gap is triggered, all the stored energy in the capacitor is dumped across the remaining gap, creating a fast rise time flash.

+4.2 kV'----e-----.:::.=...:.....,

IK Monitor~-e--f--.--C::::l--llf--'-r"""" (oscilloscope )

gop 5](2

FIG. 4. Schematic diagram of the charging and trigger circuits.

Rev. Sci. Instrum., Vol. 44, No. 11, November, 1973

1631

TABLE I. A comparison of several light sources.

Rise Pulsewidth Peak Peak time FWHM intensity luminance

Source Ref. Type (nsec) (nsec) (k cd) (Mcd/cm2)

FX-12-Q.25 xenon 100 1500 2500 7 LLL air 6 50 20 7 Fisher b air 2 9 20 13 Moden air 100 7000 50 Norton d air 125 250 1300 60

• Edgerton, Germeshhausen and Grier, Electronic Products Division. b H. Fischer, Proceedings of the Fifth International Congress on

High Speed Photography, edited by J. S. Courtney-Pratt, Society of Motion Picture and Television Engineers, 1962, p. 13.

c J. C. Moden, Ref. b, p. 17. d P. Nolan, Ref. b, p. 1l.

The mechanical arrangement is shown in Figs. 3 (b) and 3 (c). This final design consists of two parallel copper plates separated by a 0.0076-cm (3 mil) Mylar sheet. The resulting capacitance is about 2400 pF. A nylon bushing separates the tungsten trigger electrode from the positive plate. The copper plates and Mylar are clamped together in a canvas Bakelite housing that is 17.78 cm in diam and 3.2 cm thick. The plates of the capacitor are made from 1.27 cm copper, providing a structure that is sufficiently rigid to minimize the possibility of changing the capaci­tance or the gap dimensions during handling. A fiber optic support with two ports is shown in the figure. More ports could be added if it were desired to insert the same timing mark on the film of more than two cameras.

FIG. 5. A comparison of fiducial markers, all at X9 enlargement. (a)-Xenon lamp fiducial at 6 mm/J.lsec; (b)-air gap fiducial at 6 mrn/J.lsec; and (c)-air gap fiducial at 12 rnm/J.lsec.

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1632 H. M. Graham and G. A. Leavitt: Spark Fiducial

OPERATION

There are two gaps in the system. The main gap is between the central (or trigger) electrode and the ground plate. It is from this gap that most of the l!gh: is emitted. The hole was given a conical shape and IS slIver plated to increase the light reflected to the light pipes. The minor gap is between the trigger electrode and the positively charged plate. The main gap is about 0.15 cm, and the minor gap about 0.05 cm.

In operation, the positive plate is charged to about 4 kV storing approximately 21 m]. When it is desired , . to discharge this energy, a positive trigger pulse IS sent to the trigger electrode through the same cable that s~p­plies the charging voltage. The pulse is stepped up .wlth a pulse transformer just before it reaches the trIgger electrode (Fig. 4).

The secondary of the pulse transformer is across the minor gap, so no dc voltage appears on this gap. How­ever the minor gap does permit the initial part of the trig~er pulse to appear across the main gap. When the main gap breaks down, the trigger electrode beco~es

grounded, and the full charge voltage o~ the capaCltor appears across the minor gap. When the mmor gap breaks down the discharge current flows over the central elec­trode' and through the main gap to ground. It is the light in this main gap that is delivered to the film via the light pipes.

RESULTS

The light output rises from zero to crest in 10 nsec and lasts for 100 nsec. The 10-90% rise time is about 6 nsec. This produces such a sharp fiducial on the film that when the film is examined with an eye loupe, the indi~idual fibers of the bundle can be seen. The peak output is the same as the xenon lamp when co~pared with a S-4 photosurface. The lamp we were usmg, an EG&G FX-12-0.25,3 was flashed by discharging a 5 nF capacitor charged to 2.7 k V. More light could be obtain~d from the lamp with more energy input, however, not m the first 10 nsec of the flash. A comparison of the pub-

Rev. Sci. Instrum., Vol. 44, No. 11, November, 1973

1632

lished performance of several light sources with the LLL air gap is made in Table I.

Our results show that sharp, dark fiducials appear on Kodak Tri-X Estar AK, when developed for 5 min on DK-50, at camera speeds of 12 mm/Msec. A comparison of the fiducials is illustrated in Fig. 5. Note the improved leading edge in comparing these enlargements.

The jitter between a trigger pulse and the gap firing is less than 20 nsec. This is ten times better than the jitter achieved with the xenon system. A jitter of this magnitude means that the experimenter has an uncer­tainty in distance on the film of 0.25% at the highest camera speeds.

Six units have been fabricated and are in routine use supplanting the xenon lamps even at the slower streaking speeds. However, the experimenter is no longer limited to the slower speeds by his requirement for accurate timing. There has been no noticeable degradation in performance after a "life" test of 14000 pulses. This is due to the materials used and the low energy storage.

ACKNOWLEDGMENTS

We thank Professor Harold Edgerton, Dr. Frank Frtingel, and M. E. March for their help and encouragement. Also we are indebted to Neil Gibson for the original geometry (the parallel plate and central electrode system) that per­mits an extremely fast spark needed in this application. Also, we are grateful to the many other people ~ho helped fabricate and evaluate this optical fiducial hght and its driver circuitry.

·This work performed under the auspices of the U. S. Atomic Energy Commission. . .

1M. E. March, United Aircraft Research Laboratones (pnvate communication, 1972). .

2F. B. A. Friingel, High Speed Pulse Technology (AcademIc, New York, 1965), Vol. 2, p. 56.

3Reference to a company or product name does not .imply ap~rova.l or recommendation of the product by the UniverSIty o.f CalifornIa or the U. S. Atomic Energy Commission to the exclUSIon of others that may be suitable.

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