The breakdown potentials have been measured for argon and helium discharges using three different cathode materials: aluminium, silver and magnesium. The measurements show that lower breakdown potentials are associated with lower work function of the cathode material. For the three different cathodes, the secondary ionization coefficients have been estimated using the measured values of the breakdown potentials and the first ionization coefficient, in the range 0.60 to 2.25 V/[Pa cm] (80 to 300 V/[torr cm]) for Ar discharge and in the range of 0.15 to 1.80 V/[Pa cm] (20 to 240 V/[torr cm]) for He discharge. The minimum breakdown potential has been found at (pd)min = 80 Pa cm (0.6 torr cm) in Ar discharge and at 530 Pa cm (4.0 torr cm) in He discharge.Mjerili smo probojne potencijale za izboj u argonu i heliju s trima katodnim materijalima: aluminijem, srebrom i magnezijem. Mjerenja pokazuju da su niži probojni naponi povezani s nižim radnim funkcijama. Odredili smo sekundarne ionizacijske koeficijente za te tri vrste katoda na osnovi izmjerenih probojnih napona i prvih ionizacijskih koeficijenata, u području 0.60 to 2.25 V/[Pa cm] za izboj u Ar i u području 0.15 to 1.80 V/[Pa cm] za izboj u He. Najniži probojni napon za izboj u Ar iznosi (pd)min = 80 Pa cm, a za izboj u He 530 Pa cm (4.0 torr cm)

Elakshar, F. Fahmy

Garamoon, A. AbuWali

Hassouba, M. Ali

English

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ISSN1330–0008CODENFIZAE4MEASUREMENTS OF THE BREAKDOWN POTENTIALS FOR DIFFERENTCATHODE MATERIALS IN THE TOWNSEND DISCHARGEMOHAMED ALI HASSOUBAa, F. FAHMY ELAKSHARb and A. ABUWALIGARAMOONbaPhysics Department, Faculty of Science, Benha University, EgyptE-mail address: hassouba@yahoo.combPhysics Department, Faculty of Science, Azhar University, EgyptReceived 22 October 2001; Revised manuscript received 9 October 2002Accepted 25 October 2002 Online 7 February 2003The breakdown potentials have been measured for argon and helium dischargesusing three different cathode materials: aluminium, silver and magnesium. Themeasurements show that lower breakdown potentials are associated with lowerwork function of the cathode material. For the three different cathodes, the sec-ondary ionization coefficients have been estimated using the measured values ofthe breakdown potentials and the first ionization coefficient, in the range 0.60 to2.25 V/[Pa cm] (80 to 300 V/[torr cm]) for Ar discharge and in the range of 0.15 to1.80 V/[Pa cm] (20 to 240 V/[torr cm]) for He discharge. The minimum breakdownpotential has been found at (pd)min = 80 Pa cm (0.6 torr cm) in Ar discharge andat 530 Pa cm (4.0 torr cm) in He discharge.PACS numbers: 51.50.+v, 52.80.Dy UDC 537.525, 531.742.34Keywords: discharge in gases, He, Ar, breakdown potentials, different cathode materials,secondary ionization coefficients1. IntroductionElectrical breakdown of gases is the transition from an insulator to a conductingstate and the minimum voltage at which this transition occurs is called the break-down voltage VBr. The physics of the electrical breakdown has a great significancebecause of its wide applications in electronics and technology [1].Low pressure electrical discharges are a very useful tool for understanding andpredicting the properties of discharges used for plasma processing, deposition of thinfilms by sputtering, plasma polymerization, etching, welding and cutting, plasmaspraying, plasma display panels and light sources [2].FIZIKA A 11 (2002) 2, 81–90 81hassouba et al.: measurements of the breakdown potentials . . .The interest in studying the properties of Townsend discharge is motivated bythe necessity of extending knowledge in the field of gas discharge physics and tohelp to solve practical problems associated with the use of this kind of discharge intechnical devices [3].The emission of secondary electrons as a result of collisions of ion beams withmetallic surfaces has been studied by many authors [4]. References [4] representonly a small part of recent work devoted to practical use of discharge in gases.The aim of the present work is to investigate and measure the breakdown po-tentials in Ar and He for three different cathode materials in the Townsend region.Also, to determine the generalized secondary ionization coefficient, ω/α, necessaryfor self-sustaining the gas discharges.2. Experimental setupThe discharge in He and Ar was studied in a Pyrex glass tube of 10 cm diameterand 40 cm long. The anode is an Al disk, while the cathode was made of threedifferent materials, Al, Mg and Ag. The diameter of the electrodes was 5 cm andthe cathode-to-anode separation was fixed at 1.0 cm.VAVariable dc supplyGas inletTo vacuum systemU.V. RadiationLimiting resistance1MΩFig. 1. The electric circuit for measurement of breakdown potential.By means of a rotary and a diffusion pump, the discharge system (Pyrex tube)could be evacuated down to about 1.3 mPa (10−5 torr). During all measurements,a continuous gas flow through the discharge vessel was maintained. High purityargon and helium gases were used as the working gases and they were fed to thedischarge tube through a needle valve. The pressures of the gases were varied from13 to 650 Pa (0.1 to 5 torr) and measured using a dial gauge (Edward Model CG3).82 FIZIKA A 11 (2002) 2, 81–90hassouba et al.: measurements of the breakdown potentials . . .The discharge tube is provided with a quartz window on its side so that UVradiation passing through it strikes the cathode at its center and so provides anefficient source of photoelectric current.Figure 1 shows the circuit used for the determination of the breakdown potential.The external resistance RL was used to limit the discharge current, to protect thecurrent meter and to ensure that the discharge would be limited to the Townsendregion and would not step up to the glow discharge region. The current flow inthrough the discharge tube was measured by a nanoampermeter (Mutlipreci ModelMN 5121) connected in series with the cathode.3. Results and discussion3.1. Measurements of the gas breakdown potentialThe gas breakdown voltage VBr was determined as a function of the productof the gas pressure p and the interelectrode distance d. The breakdown potentialshave been measured for Ar discharge in the range between pd 26 and 530 Pa cm (0.2to 4.0 torr cm) while for He discharge from 133 to 2400 Pa cm (1.0 to 18 [torr cm])for the three different cathodes.3.1.1. Argon breakdown potentialFigure 2 shows the Paschen curves for Ar gas using the three cathodes. The min-imum breakdown potential (VBr)min for the three cathodes occurs at the value of(pd)min = 80 Pa cm (0.6 torr cm). The minimum breakdown potential (VBr)min wasfound to depend upon the type of the cathode material. This can be related to thedifference in the work function of the different cathodes. On the left-hand side ofthe Paschen curve, VBr decreases when increasing pd which can be attributed to theincreases in the collision frequency, hence the increase of the number of collisionsbetween electrons and neutral atoms. However, on the right-hand side, the break-down voltage increases gradually when increasing pd, which can be attributed tothe decrease in the ionization cross-section. Therefore, electrons need more energyto break down the discharge gap, resulting in an increase of VBr [5].Mari [6] measured breakdown potentials in Ar discharge using Ag cathode. Theminimum breakdown voltage (VBr)min was found to be 170 V at (pd)min = 163Pa cm (0.7 torr cm), while in the present work (VBr)min was found to be 213 V at(pd)min = 80 Pa cm (0.6 torr cm) for the same type of cathode.3.1.2. Helium breakdown potentialFigure 3 shows the typical Paschen curves for He discharge using the three cath-odes. The minimum breakdown potential, (VBr)min, for the three cathodes, occursat a value of (pd)min = 530 Pa cm (4 torr cm). (VBr)min has been found for Hedischarge at 168, 162 and 152 V for Ag, Mg and Al cathodes, respectively. One canFIZIKA A 11 (2002) 2, 81–90 83hassouba et al.: measurements of the breakdown potentials . . .Fig. 2. Breakdown voltage vs. pd for Ar discharge.Fig. 3. Breakdown voltage vs. pd for He discharge.notice that the minimum breakdown potential increases for the larger work func-tions of the cathode materials.Table 1 shows the values of (VBr)min in Ar and He discharges and the workfunctions of the used cathodes for comparison.84 FIZIKA A 11 (2002) 2, 81–90hassouba et al.: measurements of the breakdown potentials . . .TABLE 1. The work function φ and the minimum breakdown potential (VBr)minfor different cathode type and different gases.Cathode material φ (eV) (VBr)min (Volt) (VBr)min (Volt)[Ar gas] [He gas]Silver (Ag) 4.26 – 4.74 213 168Magnesium (Mg) 3.66 192 162Aluminium (Al) 3.6 182 152Figure 4 shows the Paschen curves for Ar and He gases, using Mg cathode. It isclear that the minimum breakdown voltage (VBr)min of He is lower than that of Ar,in spite of the higher ionization potential of He (24.5 eV) than that of Ar (15.67eV). This can be attributed to the higher efficiency of the secondary ionizationprocesses in He discharge as compared to Ar discharge. Hagstrom [7] determinedFig. 4. Breakdown voltage vs. pd for Ar and He gases by using Mg cathode.the effect of bombarding clean metal with positive ions of rare gases. He reportedthat the efficiency of the process increases by decreasing the ion mass, increasingthe ionization potential of the gas and decreasing the work function of the cathodematerial. He concluded that the secondary Townsend ionization coefficient γi of Heis higher that that of Ar, hence, lower breakdown potentials occur in He dischargerather than in Ar discharge. Figure 4 also shows that (VBr)min of Ar occurs at(pd)min of 80 Pa cm (0.6 torr cm), while that for He occurs at (pd)min of 530 Pa cm(4 torr cm). Sinceλe−n =1NnQi, (1)FIZIKA A 11 (2002) 2, 81–90 85hassouba et al.: measurements of the breakdown potentials . . .andνe−n = NnQive , (2)where λe−n is the mean free path, νe−n the collision frequency (number of collisionper sec), Nn the density of the gas atoms, ve the velocity of electrons and Qi is theionization cross-section.At the same E/p, the ionization cross-section of He [Qi (He)] is lower than thatof Ar [Qi (Ar)] by a factor of ten [8]. Hence, the mean free path for He [λe−n(He)] islarger than that of Ar [λe−n(Ar)] at the same E/p by nearly ten times. Therefore,the probability of ionizing He gas is expected to be lower than that of Ar gas bythe same order of magnitude. This can explain the higher values of (pd)min in Hethan in Ar even though VBr in He is lower than that in Ar.Table 2 shows the values of mean free path λe−n and the collision frequencyνe−n of the two gases at different values of E/p.TABLE 2. Mean free path and the collision frequency at different E/p for Heand Ar gases. (1 torr corresponds to 133.3 Pa.)He gas Ar gasE/p p λe−n νe−n p λe−n νe−n(V/[torr cm]) (torr) (cm) ×105s−1 (torr) (cm) ×106s−150 3 0.23 30 *** *** ***100 1.8 0.39 46 2.2 3.9 × 10−2 46150 1.6 0.440 45 1.4 4.4 × 10−2 45200 1 0.704 31 0.9 7.9 × 10−2 27250 *** *** *** 0.5 0.14 16It is clear from Table 2 that λe−n(Ar) is lower than λe−n(He) by a factor of tenand νe−n(Ar) is larger than νe−n(He) by the same factor at any value of E/p.3.2. Secondary ionization coefficientsThe number of secondary electrons emitted in the discharge by the varioussecondary electron processes per ion pair produced in the discharge is known asthe total secondary Townsend electron coefficient, ω/α. Values of the secondaryionization coefficient ω/α are estimated using the following equation [9]ωα=1eηVBr − 1 . (3)In the case of large ηVBr, i.e., exp(ηVBr) ≥ 1, this equation can be written asωα=1eηVBr. (4)86 FIZIKA A 11 (2002) 2, 81–90hassouba et al.: measurements of the breakdown potentials . . .ω/α at a certain value of E/p is determined using the values of η (η is the firstTownsend ionization coefficient). Values of η were taken from the results of Kirthouf[10]. VBr at a certain E/p was obtained from the present results for the Paschencurves, by dividing VBr with the corresponding value of pd.3.2.1. Secondary emission in argonFigure 5 shows curves of ω/α as functions of E/p in argon gas using Ag, Mg andAl cathodes. The three curves have the same general behaviour. At low valuesof E/p, ω/α increases rapidly to a maximum value, then decreases very slowly,followed by an slow increase. Also, Fig. 5 confirms that the cathodes, which havea lower work function (e.g., Al), are associated with a higher secondary ionizationcoefficient, while Ag, which has the highest work function, has lower secondaryionization coefficient.Fig. 5. Relation between the secondary ionization coefficient ω/α and E/p for Argas.The secondary ionization coefficient curves can be subdivided into three regions.In the first region, E/p between 0.60 and 0.90 V/[Pa cm] (80 to 120 V/[torr cm]),ω/α rises sharply. This may be attributed to the photoelectric emission by photonbombardment of the cathode surface [11].In the second region, E/p between 0.90 and 1.50 V/[Pa cm] (120 to 200V/[torr cm]), ω/α decreases slowly when increasing E/p. This can be due to thedecrease in the photoelectric effect as a result of the decrease in the number ofexcited atoms in this region.In the third region, E/p above 0.90 V/[Pa cm] (120 V/[torr cm]), ω/α increaseswhen increasing E/p. This may be due to the increase in the contribution of ionsFIZIKA A 11 (2002) 2, 81–90 87hassouba et al.: measurements of the breakdown potentials . . .where the dominant secondary process is mainly due to the impact of ions on thecathode at high E/p [11].Phelps and Petrović [5] showed that, as E/p increases in the range from 0.26to 7.50 V/[Pa cm] (35 to 1000 V/[torr cm]), the photoelectric effect decreases, thecontribution of the ion bombardment increases, while metastable states have a smallinfluence in this range of E/p. These results are in agreement with the conclusionof the present work, where at low E/p the photoelectric emission from the cathodeis dominant, while at high E/p, the ion bombardment is dominating as a secondaryelectron process.3.2.2. Secondary emission in heliumFigure 6 shows the values of ω/α for helium gas, using Mg, Al and Ag cathodes.Values of ω/α are deduced from the measured breakdown potential of the Paschencurves, which were measured in the pressure range of 133 to 2400 Pa (1 to 18 torr).From the same figure, it is also clear that ω/α decreases sharply for Al, Mg, andAg cathodes in the narrow range of E/p from 0.15 to 0.60 V/[Pa cm] (20 to 80V/[torr cm]), and then it tends to be nearly stable in the range of E/p above 0.60V/[Pa cm] (80 V/[torr cm]).In the range of E/p below 0.60 V/[Pa cm] (80 V/[torr cm]), it is well known thatthe secondary electron emission from the cathode is mainly due to the incidence ofeither photons and/or metastable excited atoms onto the cathode [5].However, in the range of E/p above 0.60 V/[Pa cm] (80 V/[torr cm]), ω/α isnearly constant for each cathode material. Identification of the secondary processesFig. 6. Relation between the secondary ionization coefficient ω/α and E/p for Hegas.88 FIZIKA A 11 (2002) 2, 81–90hassouba et al.: measurements of the breakdown potentials . . .in that region is possible by the technique of formative time lag [9]. However, it iswell known that the secondary emission by the positive ions is the most probableprocess in that region due to the values of ω/α which are nearly constant.Figure 7 shows the values of ω/α for the two gases (Ar and He) as functionsof E/p for the Mg cathode. It is clear that values of ω/α for He gas are higherthan those of Ar. This may be related (as mentioned above) to the fact that theefficiency of electron emission by the incidence of the gas ions onto the cathodeincreases whem using a smaller ion mass, i.e., γHe > γAr [7]. This proves that,the positive-ion contribution to ω/α is very significant as a secondary ionizationprocess from the cathode.Fig. 7. Relation between the secondary ionization coefficient ω/α and E/p for Arand He gas using Mg cathode.4. ConclusionsThe breakdown potentials have been measured for Ar and He discharges usingthree different cathode materials: aluminium, silver and magnesium. The minimumbreakdown potential for the three different cathodes occurs at the value of pdminequal to 80 Pa cm (0.6 torr cm) for Ar discharge and at pdmin 530 Pa cm (4 torr cm)for He discharge. It is concluded that the minimum breakdown potential increaseswith the increase of the work function of the cathode materials.From the Paschen curves for Ar and He gases, it is clear that the minimumbreakdown voltage of He gas is lower than that of Ar gas. This can be attributed tothe higher efficiency of secondary ionization processes in He discharge rather thanin Ar discharge.The secondary ionization coefficients, (ω/α), have been estimated using theFIZIKA A 11 (2002) 2, 81–90 89hassouba et al.: measurements of the breakdown potentials . . .measured values of the breakdown potentials and the first ionization coefficients,for the three cathodes. The values were made in the range of E/p from 0.60 to 2.25V/[Pa cm] (80 to 300 V/[torr cm]) for the Ar discharge and in the range 0.15 to 1.80V/[Pa cm] (20 to 240 V/[torr cm]) for the He discharge. It is concluded that thevalues of (ω/α) for He gas are higher than for Ar gas. Various secondary processesof the discharge for the three cathode materials are discussed.AcknowledgementsThe authors are very grateful to A. M. Bakry for general experiment support.References[1] V. Lj. Marković, S. R. Gocić and M. K. Radović, Eur. Phys. J.: Appl. Phys. 6 (1999)303.[2] A. Bogaerts and R. Gijbels, J. Appl. Phys. 79 (1996) 1279.[3] B.G. Salamov, S. Bykakkas, M. Ozer and K. Colokoglu, Eur. Phys. J.: Appl. Phys. 2(1998) 275.[4] See for example A. Rostagni, Z. Phys. 88 (1934) 55; H. C. Hayden and N. G. Utterbach,Phys. Rev. A 135 (1964) 1575. K. H. Krebs, Fortschr. d. Phys. 16 (1968) 419; K. H.Krebs, Vacuum 33 (1983) 555. Yu. P. Raizer, High Temp. 24 (1983) 984.[5] A. V. Phelps and Z. Lj. Petrović, Plasma Source Sci. Technol. 8 (1999) R21.[6] T. Mari, 9th ICPG (1974) p. 85.[7] H. D. Hagstrom, Phys. Rev. 104 (1956) 309.[8] H. S. W. Massey, E. H. S. Burhop and H. B. Gilbody, Electronic and Ionic ImpactPhenomena, Oxford, Claredon Press (1969).[9] F. Llewellyn-Jones, Ionization and Breakdown in Gases, London, Methuen Co. (1956).[10] A. A. Kruithof and F. M. Penning, Physica 4 (1937) 430.[11] G. Auday, Ph. Guillot, J. Galy and H. Brunet, J. Appl. Phys. 83 (1998) 5917.MJERENJE PROBOJNIH NAPONA ZA VIŠE KATODNIH MATERIJALA UTOWNSENDOVOM IZBOJUMjerili smo probojne potencijale za izboj u argonu i heliju s trima katodnim materi-jalima: aluminijem, srebrom i magnezijem. Mjerenja pokazuju da su niži probojninaponi povezani s nižim radnim funkcijama. Odredili smo sekundarne ionizacijskekoeficijente za te tri vrste katoda na osnovi izmjerenih probojnih napona i prvihionizacijskih koeficijenata, u području 0.60 to 2.25 V/[Pa cm] za izboj u Ar i upodručju 0.15 to 1.80 V/[Pa cm] za izboj u He. Najniži probojni napon za izboj uAr iznosi (pd)min = 80 Pa cm, a za izboj u He 530 Pa cm (4.0 torr cm).90 FIZIKA A 11 (2002) 2, 81–90

Mjerenje probojnih napona za više katodnih materijala u Townsendovom izboju

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