Afterglow Tails and Stability of High-Density Nanosecond

Arc Channels

Heinz Fischer and Walter B. Rippel

Afterglow tails of high-density 10-nsec are channels in 1-atm air have been analyzed to <10-3 of peakintensity value, exceeding 500-nsec duration, by employing a gated Kerr cell which protects the photo-detector from saturation. Without such protection too high intensities exceeding a factor of 4 wereobserved. Low-intensity afterglow channels are photographed by superimposed multishot integration.This is possible because of the exceptional stability of such a particular plasma, which is demonstrated.Photoelectric and photographic decay functions agree. Spectral differences are observed and discussed.

Introduction

High-density nanosecond arc channels, as producedby novel plated coaxial capacitor transmission linesof very low inductance, -10- 9 H, were described inthe application of a nanosecond light source.' Theachievement of time-resolved spectral emission andabsorbance was reported. 2 Recently, such light pulseshave been applied to the study of the time responsesof photosensitive elements. Carrier lifetimes of theindium antimonide PEM detectors were estimatedfrom the decay function.'

A difficulty in the analysis of any such time-responsecurves, however, is the long decay "tails" of the lightpulses which are observed in the oscilloscope trace.These tails possibly are not true afterglows but mayresult from "lingering" electrons in the photodetectoror may stem from various types of distortions in themeasuring electric circuit.

This article deals with the study of the afterglowtail of a 10-nsec high-density arc channel in 1-atmair, produced under single pulse dc-breakdown condi-tions without a trigger. A gated Kerr cell preventssaturation of the photodetector.

Afterglows from plasma of such short duration havenot been studied so far to the best of our knowledge.Here the driving current pulse is short in comparisonto the afterglow period. This may allow observation

The authors are with the Air Force Cambridge ResearchLaboratories, Bedford, Massachusetts.

Received 8 August 1963.This paper was presented at the Sixth International Sym-

posium on Ionization Phenomena in Gases, Paris, France, 8 July1963.

of the undisturbed recombination processes, especiallybecause of the exceptional stability of this particulartype plasma which is demonstrated below. Theshort duration of the phenomenon under observation,which approaches the "response time" of the measuringcomponents, on the other hand, requires considerableprecautions in the experiment and its evaluation. Theresults of this study show that previous observationswithout such precautions may be wrong by a consider-able factor.

Procedure

The Kerr cell* acts as a gate and is normally closed(see Fig. 1). It opens only during the afterglow periodat various times for 20 nsec. During this time inter-val the radiation flux of the afterglow is received eitherby a photodiode ITT-FW-114 or a photomultiplierRCA-1P28. The gating module* of the Kerr cellis triggered by the pulse of the discharge directly. Thiscauses a minimum initial delay of the Kerr cell openingof approximately 60 nsec with respect to the beginningof the light pulse. The time jitter is -10-9 sec. Addi-tional and variable delay of the gate opening is ob-tained by using delay cables of various lengths.

The intensity of the afterglow decreases with time;the signal from the photodetector, however, is kept toa constant level by combinations of neutral densityfilters. In other words, the photocurrents are nearlythe same for all afterglow observations. Afterglowarc images are also photographed through the gatedKerr cell by using a photographic camera instead of a

* Obtained from Electro-Optical Instruments, Inc., Monrovia,California.

June 1964 / Vol. 3, No. 6 / APPLIED OPTICS 769

Light Source

c = 690 pt- U=4.3 kv

Gap = .0 mm Kerr cellwith Polarizers

Neutral PhotoAbsorption Multiplier

TektronixScope585

Fig. 1. Kerr cell assembly.

Fig. 2. Photoelectric scope traces of 40 shots, superimposed, 10* nsec per division, repetition rate 40 sec-'.

photocathode. However, integration by superimposedmultishot exposure is required for this case.

The light source, an SM-21 with a capacitance of 690pF, a breakdown potential of U = 4.3 kV, and a gapof 1.0 mm shows an aperiodic damped discharge.The current rise of -2.2 nsec, a current half-widthof -4.2 nsec, and a tail at 10 nsec (indicated in Fig.5) is measured by a minute-size field probe. A pic-ture of the current pulse with better time resolution isnot shown because this measurement is only correct toa first approximation. Detailed knowledge of thecurrent pulse obviously is of secondary importance forthe deductions of this paper because of the short dura-tion of the pulse in comparison to the afterglow underobservation.

The constancy of the light amplitude is demonstratedin Fig. 2 by an oscilloscope trace showing 40 superim-

single shots

posed shots. The trace of this multishot picture has theappearance of a single-shot exposure as may be seen.The Tektronix 585 oscilloscope has a rise time of 4 nsecand is not able to show the fast -1.8-nsec rise ofthe light pulse, which had been measured by the EG& G traveling wave oscilloscope. The hump on the tailin Fig. 2 is not a part of the light pulse but an electricline reflection, which was not corrected in this par-ticular case. It demonstrates the well-known diTi-culties of the tail analysis. Further, the long tail isnot a true afterglow as will be shown in Fig. 5. Itmay be mentioned that the amplitude constancy ofafterglow signals as observed through the gated Kerrcell is still good when spectrally integrated but decreasesconsiderably when spectrally resolved.

The remarkable position stability of this high-densitynanosecond arc is demonstrated in Fig. 3. Threetime-integrated single shots at the left show a smallbend in the channel which apparently is the result of amicroscopic instability from electrode wear after -105shots. The arc diameter is approximately 0.1 mm.The multishot pictures, right, in comparison appear tobe nearly equally wide but straighter than singleshots, i.e., the bend is filled in. The anode is at thelower end of the picture. It is the side electrode of thegap.' Photographic densities for single and multishotexposures are kept equal by means of neutral densityfilters. The sharpness of the time-integrated arc im-ages has been previously related with magnetic pinchconditions as produced by the extremely high currentdensity of such channels.'

ResultsTime-resolved afterglow arc channels through a

20-nsec Kerr cell shutter are shown in Fig. 4. Theyare produced by superimposed multishot integrationand photographed on Tri-X Pan. The photographicdensities are kept constant by applying the proper

multishots

60 58 55

Fig. 3. Arc images, time-integrated, 40X enlarged, single and 60 shots superimposed.

770 APPLIED OPTICS / Vol. 3, No. 6 / June 1964

1 12 105 500 shots

75 nsec 127 nsec 260 nsee delay

Fig. 4. Afterglow arc channels through 20-nsec Kerr cell shutter, 55X enlarged.

number of shots (with no neutral filter). For com-parison there is one single shot, time-integrated arcchannel at the left, as observed through the open Kerrcell (with a neutral filter). During the afterglowthe channel remains sharp for at least 75 nsec after thecurrent, of 10-nsec duration, has ceased. During thistime interval the channel has widened to -0.3 mm at75 nsec, as may be seen. Afterwards, the afterglowdevelops a halo, but keeps a luminous sharp core of-0. 15 mm which appears to remain stationary. Mean-while the plasma climbs around the electrodes. In-creased luminosity at the anode indicates the presenceof an electron cloud and the beginning of slight elec-trode erosion, which is observed strongly in the sametype plasma of somewhat longer duration. Thereis no appearance of fluid instabilities in the arc imagesof Fig. 4. These also begin to show up with longercurrent pulses exceeding approximately 30 nsec.

Relative intensity markers are put on the photo-graphic film using a zirconium concentrated-arclamp as a standard. The pulse length is constant at1/loo sec, and the illumination is changed by combina-tions of neutral density filters. Thus relative radiativedensities of the afterglow images (Kerr cell transmit-tance from 4200A to 7000 A) were obtained -from thefilter factor vs photographic density curve. First-order reciprocity factors cancel. Second-order effectsas connected with the shot-repetition rate (intermit-tency) have to be considered, since multishots arebeing used for producing the integrated arc images.

The relative radiation flux of the entire afterglow arcimage is obtained by integration of the radiative densi-ties which are determined from photographic densities.

The results are shown in Fig. 5 and compared withphotoelectric measurements. Photoelectric and photo-graphic results agree reasonably well. The lower pointsof the photographic curve probably are caused by theintermittency effect.

The uppermost curve in Fig. 5 on the other handresults when the photodetector is not protected by theKerr cell, shutter during the light peak. This curveis obtained by taking out neutral density filters whenmeasuring in the tail, thus raising the signal through

0-t

I6f100 20t 300 400 500

Nanoseconds

Fig. 5. Afterglow tails, SM-21, by photoelectric and photo-graphic methods.

June 1964 / Vol. 3, No. 6 / APPLIED OPTICS 771

* Photodiode FW 114, Tektronix 519 overloadedPhotomultiplier 1P28, Tektronix 585

Photomultiplier IP28, Tektronix 585 through2t s Kerro Radiation flux, photographic Kodak Tri-X Pas cell shutter

Current Pulse _ _|( linear ) b

11� - I . - -

§ l

A-3

* *

#

0

2200 2500 3000 4000 5000 AI I I I I I I I I . , , , I l l I , ,I I ... Ii ,

Fig. 6. Time-integrated spectrum, SM-13A (C =

Wavelength

575 pF, U = 5.3 kV, 1=atm air). (Zeiss uv spectrograph Q24, Kodak spectroscopicplate 103a-F.) * = N+ lines.

u overloaded

I throughl 20ns

Kerr cell shutter

tOO 20 300 400 50

Nan secends

Fig. 7. Spectral differences in afterglow tails SM-21.(Gaertner monochromator L234-150,

photomultiplier 1P28, Tektronix oscilloscope 585.)

increased photocurrent. Also, the oscilloscope amplifica-tion was increased by a factor of 5. Agreement be-tween photodiode and multiplier appears to indicatethat the deviation of this "naive" curve from the "true"afterglow may stem from "lingering" electrons (spacecharge, delayed electrons) in the first stage of thephotodetector to a large extent. After 500 nsec thisdeviation amounts to a factor of about 4.

The time-resolved spectrum at current maximum pre-dominantly shows a continuum increasing in intensityby a factor of about 2 from 5500 A to 2500 A, withwide N+ and some 0+ lines superimposed. The pho-tometer curve of the photographed time-integratedspectrum (see Fig. 6) on thelother hand demonstratesseveral narrow and intense metal lines. Here theuv rise of the time-integrated intensity is disguisedby a combination of changing spectral dispersion ofthe spectrometer and emulsion sensitivity. SM-13Acompares with SM-21 in electric data and spectralemission.

It may be noted in Fig. 7 that the continuum at 5300A, predominantly free-free electron emission, decaysrapidly at the beginning of the afterglow period indicat-ing a rapid decrease of the electron temperature. Incomparison the decay of the N line at 5005 A rep-resenting the ion (electron) density is much slower.

Decay functions may have been affected by inade-quate spectral resolution of our Gaertner spectrometer.Spectral measurements in Fig. 7 without Kerr cellprotection (overloaded) show the same deviation fromthe "true" afterglow as was already demonstratedin Fig. 5 for the nonspectrally resolved light.

References1. H. Fischer, J. Opt. Soc. Am. 51, 543 (1961).2. H. Fischer, J. Opt. Soc. Am. 52, 605 (1962); H. Fischer and

W. B. Rflppel, J. Opt. Soc. Am. 53, 516 (1963).3. H. Fischer and P. von Thuena, Appl. Opt. 1, 373 (1962).

NOVEMBER 1964

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HIGH SPEED PHOTOGRAPHY

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