JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Capacitor Discharge through a Capillary Used as a Spectroscopic Source

Po LEE;Physics Department, Saint Peter's College, Jersey City, New Jersey 07306

(Revision received 11 February 1965)

A spectroscopic source was developed by using a capacitor discharge through a quartz or aluminumcapillary tube. It furnished a constant-intensity emission band, a line spectrum, and a continuum fromthe visible to the vacuum-ultraviolet region. The technique of operating the source and the plasma tempera-ture in the capillary are discussed.

I. INTRODUCTION

TrHE discharge plasma in a quartz capillary emitsa continuous spectrum under a high-current dis-

charge and emits a line spectrum or bands under amoderate or weak discharge. The continuous spectrum,known as the Lyman continuum, has been used as asource for absorption spectroscopy' and the line spec-trum or bands have been utilized for investigation ofthe absorption in gases in the vacuum-ultravioletregion.2 The short-wavelength limit of the Lyman con-tinuum was observed by Rathenan3 to be 270 A and aline near the short-wavelength limit of a pure linespectrum was identified as 2X Oiv 192. Since thepressure of the carrier gas in a source tube can be aslow as 30 /u and the intensity of emission is fairlyconstant, it is a suitable source for spectroscopic in-vestigation. However, the life of the quartz capillaryis short under the heavy Lyman discharge. When pro-ducing the line spectrum of high intensity, the quartzcapillary cannot withstand high-current heating with-out deformation. Weissler el al.

4 used a porcelain or aceramic capillary with a thick wall in order to prolongthe life of the capillary tube. To eliminate this problem,we used an aluminum capillary tube in place of a tubeof quartz or other insulator. This paper describes along-life tube, its features, experimental techniques forgeneration of a discharge plasma throughout the lengthof an aluminum capillary, and adjustment of the plasmatemperature in order to emit the constant, intenselight of many desired spectra.

II. EXPERIMENTAL TECHNIQUES AND RESULTS

1. The Source Tube

The source tube, shown in Fig. 1, is demountable.The aluminum cylinder M can be replaced by a quartzor other insulator capillary. We chose the geometry ofthe electrodes such that the spark breakdown potentialbetween the electrodes through the capillary is lowerthan the breakdown potential between one electrodeand the end of the aluminum cylinder. The solid line

* The aid of the NSF is gratefully acknowledged.'R. E. Worley, Rev. Sci. Instr. 13, 967 (1942).2 P. Lee and G. L. Weissler, J. Opt. Soc. Am. 42, 80 (1952).

G. Rathenan, Z. Phys. 87, 32 (1934).G. L. Weissler, J. A. Samson, M. Ogawa, and G. R. Cook,

J. Opt. Soc. Am. 49, 338 (1959).

in Fig. 2 shows a Paschen curve of a source tube;circles represent spark breakdown completely throughthe capillary, squares partially through, and trianglesno spark discharge at all through the capillary. Thedotted curve shows the breakdown potential betweenthe end of the cylinder and the anode. The correctvalues of the carrier-gas pressure and the applied po-tential to operate the source tube should be chosen inthe shaded area indicated in Fig. 2.

We set the source tube in a pumping system andintroduced the carrier gas into the tube, which waspumped out continuously, through a variable leak.We used condensers ranging in value from 25 y/L/F to35 4F. When using a low-capacitance condenser (25yuF to 0.004 pF), we employed a 30 mA, 15 kV trans-former or a 200 mA, 20 kV rectifier for charging. Afixed spark gap, through which compressed air wasblown to increase the rate of deionization of the gap,was placed in series with the source tube in order toprevent a glow discharge. When a medium-sized con-denser (from 0.01 to 0.125 OF) was used, we chargedit with a dc power supply and discharged it throughthe source tube with a rotating spark gap. When usinga large-capacity condenser (1 ,uF or above), we chargedit with a dc power supply through a resistance anddischarged it directly through the source tube. Theinitial voltage of the discharge condenser was deter-mined by the breakdown potential of the source tubeand the rate of repetition of the sparks was controlledby the RC value of the charging circuit.

We measured the current through the capillary tubewith a small, rectangular loop, magnetically coupled tothe capillary tube or to the wire in the discharge circuit,and connected to a Tektronics 545A oscilloscope. Thevoltage across the source tube was measured by acapacity-balanced resistance divider. The emission fromthe back window of the source tube was examined by

FIG. 1. Aluminumcapillary source tube.WI, quartz window;G, glass waterjacket; M, alumi-num capillary tube;C, cathode; A, an-ode; Co, cooling coil;P, glass wool; Q,quartz guide; h,viewing hole; S, glassspacer.

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VOLUME 55, NUMBER 7 JULY 1965

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FIG. 2. Paschen curves. Region III, no discharge through thecapillary; region II, discharge partially through the capillary;region I, when the initial potential of the capacitor lies in theshaded area, the discharge passes completely through. Carriergas, air; 3-mm diam and 3-cm-long capillary; distance betweenanode and capillary end, 1 cm.

means of a small quartz spectrograph with a sodiumsalicylate coated 1P21 photomultiplier tube. The sametype photomultiplier was placed in a vacuum spectro-graph and connected to an RC integrating circuit tomeasure the intensity. Care was taken to minimize theinductance and resistance of the wire connections ofthe discharge circuit.

2. Emission Spectrum from the Source Tube

The spectrum emitted from a given source tubedepends upon the pressure and composition of thecarrier gas as well as the current density through thecapillary. For peak current densities above 15 000A/cm2 , the source tube emitted an intense line spec-trum (see Fig. 3) of multiply ionized atoms super-

FIG. 3. Emission lines from ionized oxygen.

imposed on a continuous spectrum from the visible tothe vacuum-ultraviolet region. The intensity of thecontinuous spectrum increased as the current densityincreased (Fig. 4). It did not depend upon the pressure

1930 A 1860 A

FIG. 4. Continuous spectrum from a spark discharge throughan aluminum capillary. The absorption bands are due to the 02in the air between the source and the photographic plate.

or the nature of the carrier gas. 02, N2, air, H2, Ar, andHe were used in the experiment. For peak currentdensities less than 10 000 A/cm 2, a pure line spectrumor a molecular spectrum was recorded (Fig. 5). By

1100 A Hi 1215l l

MKIIN NI[1111' LH[E[iFIG. 5. Molecular spectrum of hydrogen.

choosing the gas pressure, the capacitance, and theinitial voltage of the condenser, we could control thesource tube to emit the various desired spectra. Theexperimental results shown in Table I illustrate thisfeature.

3. Current and Equivalent Resistance

Curves in Fig. 6 illustrate the typical time variationof the current and of the light from the source tube.Under a constant initial voltage and constant pressureof the carrier gas, traces in the oscilloscope were verysteady and reproducible. The light pulses from succes-sive sparks remain constant in intensity. The currenttrace was in the form of a damped sine wave, like adischarge current from a condenser through an induct-ance and a constant resistance. Its frequency, dampingfactor, and peak current depend upon several factorssuch as: (1) the geometry of the source tube, the

TABLE I. Spectrum and operating conditions.

Diameter of Pressure of Capacity of Maximumcapillary carrier gas condenser Initial voltage intensity in

(mm) OA) (HF) (V) Spectrum the spectrogram

2 200 0.01 1500 Molecular bands of N 2and 02 down to 1000 A

2 200 0.01 1500-3000 Ni, Oi down to 1000A

2.5 200 0.01 2350-4100 toCIII, Niii down to 500 A near Nii 1085

2 50 0.02 3000-4000 Oiv, Niv to near Oiii 833

2.5 50 0.02 4500-6500 Ov, Nv down to 192 k near Oiii 508(Fig. 3)

Oiv 554

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CAPILLARY SPECTROSCOPIC SOURCE 785

nature of the carrier gas, and its pressure; (2) size andtype of capacitor; (3) inductance and resistance in thedischarge circuit; (4) initial voltage to which the ca-pacitor is charged; (5) gap and electrode material ofthe series spark gap, if any.

The equivalent resistance of the source tube can beestimated from the logarithmic decrement of the currenttrace or from the ratio of the current and voltage acrossthe tube. Table II lists some results under the condi-tions that the source tube emitted (I) the continuousspectrum, (II) the line spectrum of multiply ionizedatoms, and (III) the molecular spectrum.

4. Light Duration and Intensity

The light traces shown in Fig. 6 illustrate typicaltime variations of two spectral lines. The intensityrapidly increased and reached its peak within a fractionof one microsecond, while the discharge current wasat its first peak, and then gradually decreased withsome fluctuations exactly coinciding with fluctuationsof the current. After the current ceased, the tracefollowed a smooth decay curve. Define "duration" asthe time between the 1/3 peak-light points on therising and falling portions of the light trace. For a givensource tube operated under a given condition, eachindividual spectral line had its own duration which was,in general, of the same order of magnitude as thecurrent pulse, from 0.1 to 20 Msec.

In doing photometric work with the source tube, weusually detect the light with either a photographicplate or a photomultiplier with an RC integratingcircuit. The response is proportional to the product ofthe integrated light per pulse and the number ofsparks per second. The rate of repetition of sparksdepends upon the size of the condenser, the resistance

Tgme (miFboseconds)

FIG. 6. Time variations of current and intensity.C= 0.02 IF, Vo=9000 V, P= 190 , (air).

in the charging circuit, and the power output of the dcpower supply. The factors that limit the number ofsparks per second are the heat accumulation in thecapillary and the rate of deionization in the sourcetube. We solved the problem of deionization by usinga fixed or rotating spark gap in series with the sourcetube to prevent a glow discharge. A rotating spark gapprovided a perfectly deionized gap at the rate of 30,60, or 120 sparks/sec. Using compressed air to blowthe arc out of a fixed spark gap, we obtained a sparkdischarge through the source tube at the rate of 1.5X 105 sparks/sec.

Since there is no window to separate the source tubeand the vacuum,?spectrograph, it might be difficult tomaintain the constant pressure in the source tube incertain experiments. It is of interest to examine theconstancy of the integrated intensity due to a smallvariation of the pressure. Curves I, II, and III in Fig.7 represent the variation of integrated intensities of

TABLE II. Emission spectrum and equivalent resistance.

Carrier gasr, (mm) and pressure

I (a)

(b)

(c)

1.15 (A)f

1.25 (A)f

1.0 (Q)g

H2 (3 50p)air (50u)

air (40A)

II (a) 1.25 (A)f H2 (8 0 0,U)

(b) 1.0 (Q)g air (170p)

(c) 1.0 (Q)g air (40,u)

Cb (uF) VoC(kV) pd(A/cms) R.& (i2) Spectrum

17.5 3.2 68 000 0.11 Continuous (Fig. 4)3.0 6.0 45 000 ... Continuous0.5 5.4 54 000 0.18 Continuous

0.125 5.0 8000 0.32 Impurity lines Oi toOiv and Ni to Niv

0.02 7.0 7100 1.0 Oi to Oiii andNi to Niii

0.01 9.0 8500 1.0 OI to Oiv andNi to Niv

III (a) 1.25 (A)f H 2 (500p)

(b) 1.0 (Q)g H2 (lmm)

0.004 0.50

0.00005 1.0

200 3.9 Impurity N2 , 02 bands,Oi, Ni lines

150 ... H 2 bands (Fig. 5)

a r, radius of capillary.b C, capacitance of capacitor.e Vs, initial voltage.d p, peak current density.I R,, equivalent resistanceI A, aluminum capillary.6 Q, quartz capillary.

July 1965

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FIG. 7. Integrated intensity and pressure

spectral lines Oii 796, Oiii 833, and Oiv 790 i

pressures. There is an optimum condition atintegrated intensities of certain spectral lirconstant for a pressure variation of 50,u.

III. DISCUSSION

The change of the previously described spethe shift of the maximum intensity in the slled us to believe that the mean energy of th

in the plasma can be easily changed by v

operating conditions. If the plasma in threaches a stationary state during emissiorcan speak of the electron temperature or ioiture. There is a definite relationship betweetrum and the equivalent resistance. Both theof the plasma and the spectrum dependplasma temperature. Although the specific ris extremely complicated in this case, theresistance as well as the particular spectrunare determined by the temperature. Theequivalent resistance can be used as a reliaL

the specific spectrum that is produced.As an illustration, consider a discharge in'

an aluminum capillary. We observed intenElines such as Oiv and Niv superimposed c

tinuous spectrum from the plasma in th(Since the first ionization potential of NiII a47.40 and 54.87, respectively, and the iontential of H2 is 15.37 V, and in order that tl

have sufficient energy to excite emittersthose spectra, we assign 50 eV for the plasnr

ture inside the capillary and 15 eV outside. ]of positive hydrogen ions/cm 3 or electrons,

culated from the pressure to be 1.25X1016/

Spitzer's formula,5 the resistivity q we obtain

ij= 6.53X 10(1ogA/TV) Q cm,

where logA is constant, chosen to be 10 in this case, and

T the temperature is 'K. The resistance calculatedfrom this formula and the dimensions of the sourcetube with the assigned temperatures is 0.090 S2; the

X x resistance measured from the damping constant of theX current trace is 0.11 Q.

The low-pressure spark discharge in the source tubeis a complicated phenomenon. The plasma is probablyheated initially by a kind of low-pressure shock wavewith the temperature sustained over a longer periodby Joule's heating effect. For the high-current dis-

160 180 charge, as in case I(a) of Table II, the pinch effectprobably comes into play. It is difficult to calculate the

plasma temperature from the dimensions of the capil-lary and other parameters, and the problem is aggra-

Lt different vated when the materials from the wall contaminatewhich the the discharge plasma. But, from our experiences (e.g.,

ies remain Table I), we conclude that the plasma temperature inthe capillary varies inversely as the diameter of thecapillary and the pressure of the carrier gas, and variesdirectly as the current density. As a working rule foroperating a given source tube at a given pressure, we

ectrum and can choose a condenser of suitable size and, using the)ectrogram current trace as a guide, adjust the applied voltage V0

e electrons to the condenser to obtain a suitable plasma tempera-arying the ture in order to emit the desired spectrum.e capillary In conclusion, (1) the aluminum capillary can with-L, then we stand high-current heating without deformation; (2)a tempera- the aluminum capillary does not expand during a heavya the spec- Lyman discharge [e.g., in the case of the dischargeresistivity in Table II-I(a), after a thousand sparks there was noupon the measurable change in the diameter of the capillary];

elationship (3) the source tube is useful for study of molecularequivalent spectra with high-energy excitation and line spectra of

m produced multiply ionized atoms; (4) unlike the Lyman con-refore, the tinuum for the quartz capillary, the intensity of theole guide to superimposed oxygen lines in the continuum from the

aluminum capillary tube is much lower and more

H2 through stabilized. Therefore, it is good for work in absorption;e impurity spectroscopy.in the con- ACKNOWLEDGMENTS

capillary.nd Oiii are The author is indebted to Professor G. L. Weissler of

ization po- the University of Southern California for his help and

ie electrons encouragement to start the experimental work in his

to produce laboratory and also to Professor C. C. Galvin for his

La tempera- help in setting up the equipment for experimentalThe number study at St. Peter's College.

/cm3 is cal- L. Spitzer, Jr. Physics of Fully lonized Gases (Interscience

'cm3 . Using Publishers, John Wiley & Sons, Inc., New York, 1956), p. 84.

786

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