Journal of the

OPTICALOf

SOCIETYAMERICA

VOLUME 46, NUMBER 2 FEBRUARY, 1956

Anode Temperatures and Characteristics of the dc Arc in Noble Gases*t

BERT L. VALLEE AND MILTON R. BAKERBiophysics Research Laboratory, Department of Medicine, Harvard Medical School and

Peter Bent Brigham Hospital, Boston, Massachusetts(Received July 22, 1955)

The construction of a gas-tight arcing chamber and continuously recording optical pyrometer is described.The system is reproducible. The current and voltage of the dc carbon arc in noble gases show a sensitiveand systematic dependence on the atmospheres employed. A small admixture of a foreign gas, such as air,markedly alters the characteristics of the arc. The arc voltage for fixed current decreases in the order He,Ne, A, and Kr. Thus, the power input decreases in the same order. The anode temperature is linearlyrelated to the power input. The behavior of the cross sections of the gases to electrons in the energy range ofthe Ramsauer-Townsend effect is suggested as a major contributor to the observed behavior of the dc arcin noble gases.

PREVIOUS efforts have been concerned with theeffect of noble gases and their mixtures with 02

and C 2 on the dc arc as used in spectrochemistry.The significant findings of this work may be sum-marized as follows:

1. In argon and helium the over-all background isreduced.

2. In helium and particularly in argon certain spec-tral lines are selectively enhanced. These propertieshave been utilized pragmatically in carbon arc spectro-chemistry.1-4 No adequate theoretical explanation hasbeen proposed for the effects observed under theseconditions.

3. The volatilization rates of elements in directcurrent arc sources surrounded by these gases ismarkedly altered from their behavior in air. In argonparticularly, the volatilization time is markedly pro-longed, and it has been postulated that the decrease inthe rates of volatilization of the elements results in

* These studies were supported by a grant-in-aid from the Re-search Corporation, New York, and The Rockefeller Foundation,New York.

t A preliminary account of this work has been rendered: B. L.Vallee and M. R. Baker, J. Opt. Soc. Am. 43, 817 (1953).

1 Vallee, Reimer, and Loofbourow, J. Opt. Soc. Am. 40, 751(1950).

2 B. L. Vallee and R. W. Peattie, Anal. Chem. 24, 434 (1952).3B. L. Vallee and S. J. Adelstein, J. Opt. Soc. Am. 42, 295

(1952).4 R. E. Thiers, Appl. Spectroscopy 7, 157 (1953).

greater efficiency of excitation of atoms liberated intothe arc stream.

Preliminary experiments with a disappearing-filamentoptical pyrometer and the graphite dc arc in air, helium,and argon, and in the presence of spectrochemicalsamples in the anode crater, showed promise of relatingsome of the phenomena observed in the system to thetemperature of the electrodes.'

The present report is concerned with measurementsof apparent blackbody temperatures obtained on thegraphite arc electrodes in noble gases in the absence ofsamples. Data obtained in the presence of the alkalimetals and alkaline earths are reported elsewhere.6

INSTRUMENTAL AND EXPERIMENTAL

Disappearing-filament optical pyrometers do notpermit continuously recorded observations. They aresubject to considerable error when used by differentindividuals and when used by the same individual atdifferent times. The time intervals between readingsare unduly prolonged and measurements are uncertainin time.

Therefore, it was decided to design and employ acontinuously recording pyrometer to be described belowand illustrated in Fig. 1.

6 R. Prugh, Bachelor's thesis, M.I.T. 1952.6 B. L. Vallee and R. E. Thiers, J. Opt. Soc. Am. 46, 83 (1956).

77

B. L. VALLEE AND M. R. BAKER

SOpsreen is O jBt I|OLOMETE2 TEMPERRTURE

ARCUCAMBER en RECORDEIZ

0-20, o VTGE,ommeer i .

I' ARC POWER- ~~~~~~~~SUPPLY

I110,220 V.DC.

gas tank

FIG. 1. Continuous recording system for determining arc electrodetemperatures and associated parameters.

A. Pyrometer

An image of the glowing carbon, the (blackbody)temperature of which is to be measured, is projected bymeans of a lens onto a screen provided with a slit. Thisslit is followed by a second lens which condenses theimage of the narrow segment of the carbon, visiblethrough the slit, onto the sensitive element, a thermistorbead. The thermistor bead, consisting of semiconductingmetallic oxides in a glass envelope, has a high negativetemperature coefficient of resistivity. Two beads, ontoone of which the radiation is incident and another,which is kept in the dark, are balanced against oneanother in a bridge circuit. The potential difference,caused by the unbalancing of the bridge through theheating of one of the thermistors by radiation, is re-corded on a potentiometer-type millivoltmeter.

The two thermistors are mounted in a wooden boxof min. redwood, measuring 2 in.X2 in.X4 in. They areheld onto a Plexiglas sheet and electrical connectionsare made to them through silver conducting paint.tThe position of the thermistors is quite critical; there-fore, the plastic sheet is made to fit accurately into theslots on the interior of the box, which is accessiblethrough a removable sliding top. A felt contact preventsstray air currents and light from entering about theremovable top. The measured radiation enters the boxthrough a thin glass window.

Western electric type 23-A bead thermistors 0.016-in.o.d. (resistance at 200 -1500Q) were used, because oftheir small size and consequently low heat capacity.§Their response time was decreased by blackening withthinned lacquer. A pair of thermistors to be used shouldhave equal resistance at the current at which they are

t Conducting paint wag found superior for the electrical con-nections to the thermistor as the wire used in the thermistorssolders neither easily nor well.

§ A group of twelve 23-A thermistors had a mean resistance of16840 and a maximum variation from the mean of 160 atroom temperature.

to be operated because the resistance of a thermistor isa sensitive function of the applied current. Two ther-mistors of equal resistance at any one current may havedifferent resistances at any other current.

The voltage across the recorder is proportional tothe voltage across the bridge for a fixed change inresistance in one of the thermistors. However, theresistance of the thermistor, due to self-heating, dropsas the voltage increases. Therefore, a maximum sensi-tivity for a given small unbalance of the bridge exists.In the circuit used, maximum sensitivity was found tocorrespond to a current of 1.5 ma through the ther-mistors. To obtain stable operating conditions, thepyrometer was not run at maximum sensitivity, but ata current of about 1 ma, corresponding to an appliedvoltage of 3 volts. The sensitivity for this current isabout 90% of the maximum sensitivity. The sensitivityof the thermistors, and their individual response totemperature changes, demanded that the instrumentbe used at constant temperature (740 d41°F).

The deflection of the meter is not proportional tothe temperature of the carbon, since the energy radiatedvaries as the fourth power of the temperature. Thelowest range of the pyrometer with the present lenssystem is 800'C. At operating temperature, the timeresponse of the pyrometer is 0.5 second.

Calibration of the thermistor pyrometer was carriedout using a disappearing filament optical pyrometerilas a secondary standard and with a glowing carbonand/or a small filament lamp as a source.

The voltage and current across the arc were measuredby appropriately shunted recording millivoltmeters.Calibration was accomplished using battery voltagesand moving-coil ammeters over the range of values forwhich measurements were performed.

B. Arcing Chamber

The arcing chamber is cylindrical in shape, 4 in. indiameter, six inches long and made of nickel-platedbrass (Fig. 2(a) and (b)). The top of the chamber is aPyrex disk, which is easily removable; it seals onto asilicone rubber gasket by pressure exerted by fourthumbscrews set equidistantly around the top. Thebottom of the chamber is made of linen-base Bakeliteand is covered with a circular piece of stainless steelsheeting to protect its surface from heat. Four windowsmeasuring 4 in.XI in. in their effective dimensions, areset at 90° spacings about the side of the chamber.

The graphite electrodes are introduced into thechamber through two bearings set in the top andbottom of the chamber, and concentric with it. Theelectrodes are held in collets, which fit into water-cooled eledrode holders. A collet nut at the other endof the cylindrical electrode holders, tightens down ontoa silicone gasket and holds the electrodes tightly and

11 Leeds and Northrup, type 8622-c.

78 Vol. 46

February1956 ANODE TEMPERATURES OF DC ARC IN NOBLE GASES

concentrically. These two electrode holders then fit U 1500

precisely into the end bearings. ° ,400 HELIUM

The atmosphere within the chamber is controlled inthe following manner: through the collet nuts and the D 1300

hollow collets and through the three slits in each collet, S

gas is introduced or drawn out. One collet nut attaches A200 CURRENT 6 AMPS

to a gas supply, and the other to a vacuum pump, U 1100 NEON

all through appropriate valving. The chamber may be U evacuated, and then filled with gas, a procedure much o 1000 ARGON

KRYPTON

GAS OR VACUUM IN 3 4 5) TIME IN MINUTES

FIG. 3. Typical anode temperature records for an arc inKr, A, Ne, He, and still air at a current of 6 amperes and an arc

OUT COOLING gap of 6 mm.

WATER -I more economical than flushing. The atmoshere in theCOOLED chamber may be controlled under pressures varyingCOLLETASSEMBLY from 1 mm to 2 atmospheres. It withstands high tem-

- Idz * -peratures developing about an enclosed arc.The electrodes are fitted into the electrode holders,

through which the electrical connections are made. TheL I e 0 otop is sealed down using silicone vacuum grease on

QUARTZ the silicone rubber gaskets about the top of the chamber.W I N DOW I I O O The electrode holders are fitted into the top and bottom

u g C salon, bearings and respectively raised and lowered to al', l I properly centered position. The bearing surfaces and

I . , J n the outer surfaces of the electrode holders are coveredby a thin layer of silicone vacuum grease. After checking

COOLING all connections, including the gasketed surface under-OUT IWATER neath the collet nuts, which have been loosened when~.IN

mounting the electrode, the valves are closed to the gassupply and the chamber evacuated. The pressure is

OUT observed on a mercury manometer connected to the(a) chamber on the vacuum pump side of the gas con-

GAS OR VACUUM INLET nections. The valve to the vacuum pump is closed and,--I the gas introduced. The arc is struck by touching the

electrodes, and the gap is readily re-adjusted by meansof a magnified image on a calibrated scale.

SILICONE rCOLLET NUT EXPERIMENTALGASKET ~(\r iCOOLING Anode, (blackbody) temperature at a point 4.5 mm

OUTS- GIN from the tip, voltage drop, and current were measuredin air, helium, argon, neon, and krypton** at atmos-

WATER COOLEDCOLLET ASSEMBLY STEEL pheric pressure. Measurements were performed at arcSILICONE GASKETS currents between 3 and 15 amperes. The anode and

PYREX TOP cathode were separated by gaps of 4, 6, and 8 mm.Graphite rods were employed measuring 3 inches indiameter. No difference between spectroscopically pureand ordinary spectroscopic grade rods was noted.

16 COLLET RESULTS

ELECTRODE Figure 3 shows typical temperature records of indi-vidual experiments in air, helium, argon, neon, andkrypton at a current of 6 amperes and a gap of 6 mm.

(b) ¶ Dow-Corning silicone high vacuum grease.FIG. 2. (a) The arcing chamber. (b) Detail of water-cooled collet ** Courtesy Linde Air Products Company, Ticonderoga, New

assembly and top of arcing chamber. York.

79

B. L. VALLEE AND M. R. BAKER

I60orKRYPTON

\a

; t < ** HELIUM

1~ Z.' _ . - m...- 4..

ARGONebb

FIG. 4. Voltageacross arc gap versusarc current for anarc in Kr, A, and He.

C)0

zwi-

wLI-

L

0Q

al

0I-C-)MU-JLU

50 01-

14001-

1300F

1200

1100

1000

900

2 4 6 8 10 12 14 16

ARC CURRENT IN AMPS

46 8mm///

">A KRYPTON

100 200 300 400 500 600

POWER INPUT IN WATTS

(a)

Marked anode temperature differences are apparent inthe different gases. At these values of current andvoltage, the temperature in A is 965TC and in Kr is925C and is quite stable in both gases without fluctua-tions during the entire five minute period of arcing.Such stable records of temperature in argon and kryptoncould be obtained reproducibly. The temperature inneon averages 1080°. The record fluctuates in small,rounded oscillations quite typical in shape and heightof all arc discharges studied in neon. The temperaturein helium fluctuates over a total range of 750 about1360TC, this mean value being markedly above that ineither of the other two gases. The temperature in "still"air was found lower than in helium, as indicated on thegraph. Oscillations of frequencies and amplitudes,different from those in helium and neon, are charac-teristic. When a slow stream of air was used, thetemperature increased to 1450C.

The changes of temperature and voltage as a functionof the gap, amperage, and gas is shown in Figs. 4 and 5.The voltage-current curves are highly characteristic ofthe gases (Fig. 4). In helium, the best fit of the data isgiven bytt VH1= (1OG+40)/II, where the gap voltage,V, is in volts; G, the gap distance is in mm; and I, thearc current, is in amperes. By comparison, the voltagesin Kr and A are insensitive to variations of gap andcurrent. As a consequence, no equation can be givenfor VA and VK.

The anode temperature, T, as a function of wattsinput, P (P= VI), is also characteristic of the gas(Fig. 5). The relation between T and P is linear.tt Theslopes of these lines are higher in A than in He and Kr.The smaller the gap, the higher is the temperature fora given power input. In Kr, the slopes for gaps of 4, 6,and 8 mm are almost equal. In A, the slope for 6 mm

jt The solid lines fitted to the voltage versus current experi-mental points for He in Fig. 4 for 4, 6, and 8 mm gap are, respec-tively, V=79.29/10333, 101.6/1O3672, 118.9/0.-.

t$The lines were fitted to the experimental points by themethod of least squares.

c) lu0u0

Z 1500

z 1400I-1: 1300a]0..E 1200

W 1100a0a: 1000L-tL 900-JLU

0

zLa

i-

LUa-WUJI-Ld00

LU

Ld

I-

F

1g

4 68mm. ARGON

100 200 300 400 500 600

POWER INPUT IN WATTS(b)

/ '/*.

"I

6 84mm

I..

F-

16001-

1500 .

1400

1300-

12001-

11001-

10001-

900HELIUM

100 200 300 400 500 600

POWER INPUT IN WATTS

(c)

FIG. 5. (a) Anode temperature versus power input in Kr.(b) Anode temperature versus power input in A. (c) Anode tem-perature versus power input in He.

80

30

20

I0

80

70

60

50

40

30

20

10

0L

CD

0

('3

0

LU

0

Vol. 46

no . . . . . ..u-

s Ann.F

February1956 ANODE TEMPERATURES OF DC ARC IN NOBLE GASES

definitely exceeds that for 4 mm and 8 mm. In He, theslope is greatest for the narrowest gap, and least forthe widest gap.

The arcs in argon and krypton distinguished them-selves from those in helium and neon by their steadinessof localization, particularly on the anode. When thegap was lengthened in argon and krypton, the columbecame progressively thinner though the arc was notextinguished. The distance between electrodes could beincreased to as much as 50 mm. In all four gases, thevoltage increased as the gap was lengthened, until at acritical value, which increased in the order of helium,neon, argon, and krypton, it was extinguished.

The presence of as little as 0.5% of air altered thetemperature and voltage records of the discharge ingases markedly. This phenomenon was best discerniblein argon, krypton, and neon because of the superpositionof the characteristic temperature and voltage curvesobtained in air upon equally characteristic and differentrecords obtainable in the pure gases. The wandering ofthe anode spot was virtually absent in argon andkrypton, noticeable in neon, and marked in helium.

DISCUSSION

The reproducibility of temperature, voltages, andcurrent records is high. The apparent blackbody tem-perature of a filament lamp used as a standard sourcecould be reproduced to ±t200 C. The total estimatederror in recorded relative to true apparent blackbodytemperature is At50'C when the combined errors ofcalibration and instrumental variation are considered.The reproducibility in voltage and current recordings isestimated to be better than 42%. The error in adjust-ment of electrode gap is 410.2 mm. The uncertainty inthe position of the point, the temperature of which ismeasured, is :1:0.2 mm. These latter two factors becomeimportant in He, since in this gas, the voltage andcurrent are very sensitive to the width of the gap, andelectrode temperature gradients are high.

The wandering of the anode and/or cathode spot isa major cause of the fluctuations in the observed tem-perature, voltage, and current. The change in theeffective length of the arc column has two majoreffects: (1) fluctuations in voltage and current-morepronounced in He than in A and Kr; and (2) variationin the temperature at a fixed point on the anode byvirtue of motion of the hot anode spot and throughchanges in power input. A point, 4.5 mm from the anodetip, was chosen to minimize the effects of anode spotmigration.

The magnitude of migration of the anode spot maybe altered by changing the environment of the arccolumn. Magnetic fields and vortices of gas or water tostabilize the arc have been used toward this end.7-9

7 A. Guthrie and R. K. Wakerling (editors), The Characteristicsof Electric Discharges in Magnetic Fields (McGraw-Hill BookCompany, Inc., New York, 1949).

8 E. Traub, Ann. Physik 18, 169 (1935).9 H. Maecker, Z. Physik 129, 108 (1951).

Electrodes smaller in diameter than 3 inches as em-ployed in this study, have been used.6 This results inreduced spot wandering and the consequent experi-mental instability. The cylindrical symmetry of the arcchamber and internal geometry reduces the variationin temperature caused by changes in convection cur-rents, but no estimate of their magnitudes can be given.

The data presented here indicate a systematic se-quence in the behavior of the arc in different noble gasatmospheres. For a given current, the voltage acrossthe arc gap decreases from He to Ne to A to Kr. Thus,from a macroscopic view, the resistance of the arccolumn decreases for the different gases in the sameorder. The over-all behavior of the arc in A and Kr isvery similar. Anode temperatures and voltage vs currentrelationships in A and Kr are almost identical. Thecorresponding values and relationships in He are verydifferent from those in A and Kr. The behavior of thearc in Ne is intermediate between He and A- Kr. Thesedifferences may be explained qualitatively by thecorresponding macroscopic conductivity of the arccolumn, which is closely related to the total collisioncross section of the atoms of the atmosphere, presentedto the current carriers in the arc. In this discussion thedetailed behavior of the cathode and anode voltagedrop has been neglected.

The ease with which the current carriers may passthrough the gas is a highly important factor in determin-ing the behavior of the arc (see Maecker,10 page 310 ff.).Under the stated conditions of current and pressure, thevelocity distribution of the electrons, ions, atoms, andmolecules in the arc column is Maxwellian, i.e., theyare in thermal equilibrium. For the temperature of thearc plasma, about 6000'K, the mean energy of theseelectrons, the main current carriers, is approximately0.85 ev. The total collision cross sections for noblegases with electrons of incident energies of about 1 evshow a considerable deviation from their gas-kinetic

0

0U

U)0C-,

0

0

0

0I-

WGON

1 2 3 4 5 6 7 8

f ELECTRON ENERGY IN E.V

FIG. 6. Total collision cross section for Kr, A, Ne, and Heversus (incident electron energy)1.

'0 H. Maecker, Ergeb. exak. Naturw. 25, 293-358 (1952).

81

B. L. VALLEE AND M. R. BAKER

values (Ramsauer-Townsend effect)." He and Ne do notshow the pronounced minima in cross sections at 1 ev,observed with the heavier A and Kr. This variation ofcollision cross section for noble gases is shown in Fig. 6.The resistivity of the arc column is inversely propor-tional to the mobility of the current carriers. Equiva-lently, the voltage across the arc gap should increasewith the collision cross sections for electrons in noblegases. A voltage increase for a fixed current leads to ahigher power dissipation and higher environmentaltemperatures.

The cross sections in A and Kr are very close to eachother and low (-7rao2).§§ He is very much higher

11 H. S. W. Massey and E. H. S. Burhop, Electronic and IonicImpact Phenomnena (Oxford University Press, New York, 1952),Chapters 1, 2.

§§ ao-.53X10- 8 cm= radius of first Bohr orbit in H.

(-67rao2). Neon is intermediate (-27rao2) but closer toA and Kr than to He. The observed voltage-currentrelationships and anode temperatures when comparedfor A, Kr, Ne, and He follow this order as expected onthe basis of their cross sections.

Other differences in behavior of the arc in noble gases,such as the speed and frequency of motion of the anodespot cannot be accounted for and await a more detailedtreatment.

ACKNOWLEDGMENTS

The authors take pleasure in acknowledging thetechnical assistance of Mr. S. Neidleman and Mr. J.F. Munafo, the statistical calculations of Mr. A. F.Bartholomay, and the helpful advice of Dr. R. E. Thiers.

82 Vol. 46