Radiance Saturation and Afterglow Emission of High Pressure20-nsec Spark Channels in Argon and Helium

Heinz Fischer nd Wilhelm Schwdnzer

Time functions of the radiance (ITT FW-114, spectral response S-4) were observed, as well as theradiant intensity of high pressure spark channels in argon and helium with pressures between 1 atm and40 atm, and a constant gap of 0.65 mm. The radiance, as well as radiant intensity, first increases ex-ponentially with pressure and breakdown voltage; later, the radiance curves bend into a saturation pla-teau in both gases. On the other hand, the radiant intensity does not reach saturation, but shifts itsmaximum up to 50 nsec after the 8-nsec current maximum. The shift of the radiant intensity and sat-uration are related and result from the increasing opacity of the channel. The observed 1.65 ratio of thesaturation radiances of helium and argon agrees with values reported in microsecond spark channelswhich have a diameter approximately six times wider.

IntroductionThere is a constant desire for pulse light sources of

higher intensity and shorter duration for multipurposeapplications. When using time integrating photo-graphic processes, the problem arises that the illumina-tion I X t might become too small for adequate photo-graphic exposure. Thus, for photographic purposes,the demand for extremely large light amplitudes be-comes a must in the case of pulse light sources in thenanosecond range. On the other hand, electronicimage intensifier cameras are more sensitive; however,inadequate image resolution might become the limitingfactor.

The production of low amplitude light pulses in thesubnanosecond range by means of the spark dischargesappears to present no serious problem,', 2 and the pulseshape may easily be recorded, since subnanosecondphotomultiplier and faster oscilloscopes are now com-mercially available. It appears that within a propergas atmosphere, a low level light pulse will follow theexciting electric spark pulse rather closely. With in-creasing electric amplitude, the light amplitude willincrease; however, the light pulse will widen because ofthe increasing amount of energy being stored within thespark channel. It will take a certain amount of timefor this energy-mainly ionization or excitation-to betransferred into emitted light by means of processes ofinternal pumping.

Our present study is concerned with the influence ofhigh gas pressures upon amplitude and pulse shape of

The authors are with the Angewandte Physik, TechnischeHochschule, Darmstadt, Germany.

Received 12 August 1968.

nanosecond light pulses as produced by a spark dischargeof very small inductance.' This light source is knownas the Nanolite and is being used for the photography ofnanosecond events such as fast moving particles ofmicroscopic size.4 5

In this report, it is shown that the radiance B in-creases in argon and helium with increasing gas pres-sure, but reaches a saturation plateau which apparentlyrepresents an upper limit to be reached within thisgeometry. The radiant intensity I = B X a (a =radiant area), on the other hand, also increases withp; (u), however, becomes wider by a considerablefactor when B saturation conditions are approached.This means that a large light amplitude and a narrowpulse shape will be a matter of compromise.

Apparatus and MeasurementsFigure 1 shows the schematic picture of a Nanolite

light source. Adapted is a high pressure chamber witha quartz or lithium fluoride window for vacuum uv work,allowing the operation of the spark gap at an el>vatedgas pressure in various gas atmospheres.

Electric data of the Nanolite used in the describedtests were C = 3.7 nF and L = 2.1 nH. The currentpulse in 1-atm air reached its first maximum after 7.5nsec, reversed after 15 nsec, had a 40% overswing, andtapered off to zero after approximately 30 nsec. Forthe energy input to the gap, only the first half-cycle issignificant, as may be assumed.

Relative amplitude and time functions of radianceB and radiant intensity I in argon and helium wereobserved within the range from 1 atm to 50 atm.Breakdowns were between 1.5 kV and 5 kV. The gapwas fired by slow charging up to breakdown voltage;however, there appeared to be substantial overvoltage

March 1969 / Vol. 8, No. 3 / APPLIED OPTICS 697

1

7

56

Fig. 1. Schematic design of Nanolite light source: 1-high pressure chamber; 2-inner conductor; 3-insulation; 4-outer conduc-tor; 5-housing; 6-HV connector; 7-quartz window; 8-spark gap.

and an equivalent related increased light output whenthe capacity of the lamp was charged up within a nano-second time interval.

ResultsFigure 2 and 3 show time functions of the radiance B

and radiant intensity I = B X a in argon with differentgas pressures p. The related breakdown values u areindicated in both figures. Figures 4 and 5 showtime functions of B and I in helium, also with p, (u) asthe parameter. Figure 6 compares the maximumradiances as a function of p in argon and helium.

Radiance in Argon

Discussion(1) The radiance Bmax increases exponentially with

p, (u) (see Fig. 6), but reaches a maximum saturationvalue which cannot be raised by increasing p, (u). Thismaximum saturation value in helium is higher by afactor 1.65 than that in argon. This ratio equalsroughly that of the ionization potentials of both gases;it also agrees with the resonance line potentials. Themaximum of the radiance in helium is close in time tothe first current maximum. The pulse width of B in-creases with p, (u), as is shown in Figs. 2 and 4. Inhelium, the pulse is narrower than in argon. Especially

Fig. 2. Time functions of the radiance B in argon. Fig. 4. Time function of radiance B in helium.

Radiant Intensity in Argon

[re u.] i lt

=_~~~~~~~~~~~a_ .~~~~~~~~~~a|- 20 ns - t i - 20 ns

Fig. 5. Time function of radiant intensity in helium.

698 APPLIED OPTICS / Vol. 8, No. 3 / March 1969

Fig. 3. Time function of radiant intensity I in argon.

Fig. 6. Maximum radiances as a function of p in argon andhelium.

in argon, the half-widths of the radiation pulses aremuch wider than the current pulse, which is 15 nseconly; the Bmax in argon appears to shift to later times.

(2) The maximum radiant intensities I in argon andhelium also increase exponentially with p, (u), bendlater to a smaller increase, but do not reach saturationwithin the range of observation. Also, the intensitypulse becomes much wider with increasing p, (u). Inaddition, in argon the maximum of I shifts to consid-erably later times than the current maximum; I pulsesin argon also are much wider that those in helium.

(3) Saturation of the radiance B in high pressuremicrosecond spark channels were previously observedby several authors6 -8 and were found to be connectedwith the increasing opacity of the spark channel, whichis approaching a surface radiator within the visible andnear uv region.

The diameters of these longer duration channelswere considerably wider (approximately 1 mm) in com-parison with the present Nanolite channels (approx-imately 0.3 mm). Gap, pressure, and breakdown volt-age in both cases were roughly the same. As a con-sequence, we would expect that shorter time spark chan-nels with smaller diameters would also demonstratesimilar B saturation phenomena at comparable gap, p,

and u values. Also the pulse shape of the radiant in-tensity I should follow the same relationship.

This relative independence of the saturation, (i.e.,opacity) upon the channel diameter would suggest theassumption that the number of radiating and absorbingparticles as seen across the spark channel does notchange strongly during the early stages of the expansionof the arc channel.

(4) From the observations in Figs. 2-5, it appearsobvious that the B saturation, pulse widening, andtime shift of the I maximum are related phenomena.Whereas the B saturation is related to increasingopacity, the pulse widening of I results from the relatedenergy storage within the channel. Under such con-ditions, the I pulse shape may become largely inde-pendent of the shape of the feeding electric pulse.Secondary collision processes of internal pumping willtransfer this stored energy-mainly ionization and ex-citation-into radiation emitted from the expandingspark channel.

(5) Radiance saturation is only reached with ex-tremely high radiation densities. With u = 2.6 kV(argon) the maximum current is approximately 3500A and the current density 5 X 106 A/cm 2 . The maxi-mum electric power input is estimated at P : 2.5 MW,and the radiative power would be in the order of 0.25-0.5 X 106 W.

The authors gratefully acknowledge the generoussupport by the Air Force Cambridge Research Lab-oratories, Bedford, Massachusetts, through their Eurc-pean Office in Brussels.

References1. J. Yguarabide, Rev. Sci. Instrum. 36, 1734 (1965).2. Q. A. Kerns, F. A. Kirsten, and G. C. Cox, Rev. Sci. Instrum.

30, 31 (1959).3. H. Fischer, J. Opt. Soc. Amer. 51, 543 (1961).4. H. Fischer and A. Fritzsche, Chem. Ing. Tech. 34, 118 (1962).5. W. G. Clay, R. E. Slattery, A. P. Ferdinand, and C. R. Kil-

cline, AIAA J. 364 (1967).6. H. Fischer in Proceedings of the Conference on Extremely High

Temperatures (John Wiley & Sons, New York, 1958), p. 11.7. M. P. Vanyukov and A. A. Mak, Usp. Fiz. Nauk 66, 301

(1958). [Sov. Phys.-Usp. 1, 137 (1958).]8. M. P. Vanyukov and A. A. Mak in Proceedings of the Fifth

International Congress on High Speed Photography (SMPTE,New York, 1962), pp. 41-45.

Books continued from page 696

Optical Properties of Ions in Crystals. Edited by H. M.CROSSWHITE and H. W. Moos. Interscience Publishers, J.Wiley & Sons, 1967. 552 pp. $8.50.

The development of new trends in optics, which is connectedprimarily with the invention of solid state lasers, stimulated-an extremely rapid and wide investigation of materials consistingof transparent matrices activated by the ions of rare earths andof transition metals. The spectroscopic and luminescent studiesof such systems were the main topics of a number of conferencesheld in many countries. The book under review is a collectionof papers presented at a conference held at The Johns HopkinsUniversity in September 1966 under the sponsorship of the

Office of Naval Research. The conference was initiated by thelate G. H. Dieke-a well-known pioneer of this field in theU.S.A.-and the book is dedicated to his memory.

The topics of the forty papers published in the book embraceall the main spectroscopic and luminescent aspects-of the problem:the study of the energy levels of ions in crystals and of theprocesses which occur at their optical excitation (relaxation,energy migration). It is a pity that the crystallochemicalaspects bound intimately with the spectroscopic ones did notfind their due place in this book.

An extensive review paper by H. M. Crosswhite and H. W.Moos on Crystal Spectroscopy at The Johns Hopkins Universityopens the volume. These investigations, which evolved underDieke's guidance, are distinguished by a high perfection of

March 1969 / Vol. 8, No. 3 / APPLIED OPTICS 699

. rel u ] *-. - He

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experimental technique and by a wide range of investigatedsystems, the crystals activated by rare earths taking first place.

The other papers on crystals activated by rare earths whichreveal the characteristic linear spectra are collected in thefirst part of the book. The papers on the general theory ofcrystalline field (a paper by G. Burns and J. D. Axe on CovalentBonding Effects may be mentioned among them) are accompaniedhere by the experimental investigations of individual systemsperformed at a high theoretical level, which is characteristicof all good work in this field. One of the papers in this part(J. L. Mertz and P. S. Pershan) deals with the important problemof valency states of activating ions and of their change underthe action of irradiation.

The second part of the book emphasizes similar theoreticaland experimental investigation of the systems activated thistime by transition metal ions, the ruby crystals first of all. Thepapers on ruby show a considerable progress in the study ofthis basic material of quantum electronics. The spectroscopicproperties of Cr3+ ion in other matrices were studied as well.

Considerable attention is paid to the problem of ion inter-action in magnetically ordered systems of transition metal ions,such as Mn, Fe, and Ni. The papers on the spectroscopicmanifestation of these cooperative effects are collected in thethird part of the book. A set of interesting papers on exciton-magnon effects in optical spectra is presented here as well assome papers on magnetically ordered systems including rareearth ions.

The fourth section of the book deals with the spectroscopicmanifestations of lattice vibrations of activated crystals. Animportant problem of the interaction between electronic statesof an activator ion and local as well as crystalline vibrations istreated here both theoretically and experimentally.

Finally, the fifth part of the book, entitled Energy Transfer,is dedicated to the processes in the excited states of activatedcrystals. The interactions which lead to the transfer of energybetween ions are treated here as well as the relaxation processesinvolving both radiative and nonradiative deactivation of theexcited states.

In an appendix, the abstracts of twenty papers are includedwhich were submitted to the conference but were not preparedin full for the publication.

The book as a whole gives a sufficiently full account of thevariety and high scientific level of work in the important fieldof solid state spectroscopy. Many of the papers in thisvolume have been already published in periodicals; neverthelessthe publication of them in a separate volume will be undoubtedlyuseful both for those who would like to become acquaintedwith the latest investigations and for those beginners whoneed the review literature to help them enter the field.

It will be of some interest for American readers to learn thata collection of papers read at a similar symposium held inFebruary 1965 in Moscow was published in 1966 by NaukaPress under the title of Spectroscopy of Activated Crystals. Theproceedings of the Second Symposium on the spectroscopy ofactivated crystals (Kharkov, October 1967) will be issued in thenear future. These books together with that reviewed here,which combine the latest information inherent in the originalpapers with the encyclopaedism of a monograph, will be valuablebooks of reference for those who are interested in spectroscopyof the solid state.

P. P. FEOFILOV

Gas Lasers; B C. G. B. GARRETT. McGraw-Hill BookCompany, New York, 1967. 144 pp. $10.95.

This is an excellent, well-written, brief summary of the physicsand selected elements of the technology of gas lasers. The text

is primarily directed toward a reader with some a priori knowledgeof the laser field as evidenced by a tendency of the author topull formulas out of the air; however, an ample supply of ap-propriate references are provided for those interested in details.

The book is divided into four sections. The first traces thehistory of lasers from the early ideas on negative absorption ofTolman and Fabrikant and then goes into a short but concisereview of spectroscopy pertinent to lasers. The section isconcluded with a discussion of optical cavities and their inter-action with media exhibiting gain.

The second chapter outlines the physics of gas discharges andthe excitation processes. The coverage of this material is insome respects superficial; however, a more adequate treatmentencompassing the extent of current knowledge on the subjectcould easily exceed the size of the entire volume. In the preface,the author attempts to excuse this superficiality with the some-what unconvincing argument that a complete understandingdoes not exist at the present time and furthermore if it did, itwould be of trivial interest to a physicist. Unfortunately, itis just the appreciation of some of these details that has led tosome of the significant breakthroughs in gas laser technology.

The third section on design parameters contributes little tothe text except perhaps to give a reader who has not had theopportunity to play with a gas laser a feeling for how such adevice is constructed. As noted earlier, however, this is not atext to be recommended for the novice.

The concluding chapter discusses output characteristics in-cluding spatial intensity distributions, phase and amplitudefluctuations, mode-coupling effects, and a discussion of Q-switching.

In summary, this text, or perhaps it should more appropriatelybe called a monograph, provides a highly readable review touchingon almost all important aspects of gas lasers and will providethose interested in the field with a valuable addition to theirlibrary. Perhaps the chief complaint that should be raisedis that this little book bears a big book price tag.

FRED W. QUELLE

IVES MEDAL

Edward U. Condon University of Colorado (left) after he hadreceived the Ives Medal from then OSA President A. Francis

Turner Bausch & Lomb in October 1968.

700 APPLIED OPTICS / Vol. 8, No. 3 / March 1969