Separation of vacuum and gas breakdown processes in argon and their influence on electrical breakdown time delay

Journal Pre-proof Separation of vacuum and gas breakdown processes in argon and their influence on electrical breakdown time delay Milić Pejović, Momčilo Pejović, Čedomir Belić, Koviljka Stanković PII:

S0042-207X(19)32405-4

DOI:

https://doi.org/10.1016/j.vacuum.2019.109151

Reference:

VAC 109151

To appear in:

Vacuum

Received Date: 16 June 2019 Revised Date:

20 December 2019

Accepted Date: 21 December 2019

Please cite this article as: Pejović M, Pejović M, Belić Č, Stanković K, Separation of vacuum and gas breakdown processes in argon and their influence on electrical breakdown time delay, Vacuum, https:// doi.org/10.1016/j.vacuum.2019.109151. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Separation of vacuum and gas breakdown processes in argon and their influence on electrical breakdown time delay

Milić Pejović1, Momčilo Pejović1, Čedomir Belić2 and Koviljka Stanković2 1

Universty of Niš, Faculty of Electronic Engineering, Aleksandra Medvedeva 14, 18000 Niš, Serbia 2 University of Belgrade, Faculty of Electrical Engineering, Bulevar kralja Aleksandra 73, 11000 Belgrade, Serbia E-mail: [email protected]

Abstract Research presented in this paper is dealing with the determination of static breakdown voltage U s in an argon-filled tube at 0.5 mbar pressure, for different inter-electrode gaps d with the aim of separating vacuum and gas breakdown processes contribution in electrical breakdown time delay. The U s determination was performed using data on the dynamic breakdown voltage experimental data obtained for different voltage increase rates. It was shown that up to d ≈ 9 mm estimated value of U s is relatively small change corresponds to breakdown which is vacuum-like, where as for d > 9 mm, breakdown takes place as in normal gas discharge. For d ≈ 11 mm , U s has the minimal value, when gas ionization process induced by secondary electron emission from the cathode is maximal. The measurement of electrical breakdown time delay t d was performed for three different values of inter-electrode gap: 0.1, 4 and 11 mm. On the basis of memory curve (mean value of electrical breakdown time delay t d as a function of relaxation time) behavior and statistical analysis of td data, mechanisms which make main impact to breakdown initiation were estimated for different values τ . Also, the impact of additional electron yield originating from UV radiation with wavelengths 302, 312.5 and 313.1 nm on the memory curve were investigated.

Keywords: Breakdown voltage, electrical breakdown time delay, relaxation time, vacuum and gas discharge processes, memory effect 1. Introduction

Electrical breakdown is commonly defined as rapid gas transition from high resistive state from about 1014 Ωm to conductive state with resistance of about 10 3 Ω m in glow discharge. Electrical breakdown has been a point of interest of various investigations for several decades. Main fields of interest are for example, electromagnetic-pulse generation, direct-energy weapons, gas-filled switchers in lasers, gas-filled surge arresters and Geiger-Muller counter [1-8]. Electrical discharge processes leading to electrical breakdown can be separated into primary and secondary group. Primary are collisions leading to ionization and recombination. These processes determine the electron density during electrical discharge. Secondary discharge processes provide the self-sustenance of electrical discharge. If secondary processes are dominant in gas, breakdown occurs through the streamer process, however if these processes are dominant on electrodes of gas-filled tube (two electrode systems filled with the gas) breakdown occur through Townsend process [9, 10]. Thus, streamer breakdown process is dominant for higher gas pressures and higher inter-electrode gaps, while Townsend discharge process is dominant in the case of lower pressures and smaller inter-electrode gaps [11-13]. DC value of breakdown voltage depends on the gas pressure p and inter-electrode gap d product (pd). This dependence is called Paschen’s law and it is of an asymmetrical U-curve shape having the minimum (Paschen’s minimum) determined by gas type [14, 15]. At the Paschen’s minimum ionization effect is maximal and gas breakdown takes place through Townsend process [10, 16]. For very small pd values, vacuum breakdown process occur when the breakdown may take place only through the Townsend process in metal vapors of the electrode material [17]. In order to provide sufficient amount of metal vapors, it is necessary to drive one of electrodes into the state of thermal instability. The energy necessary for appearance of thermal instability on one of the electrodes can be delivered by the emission process, by accelerated electrode material micro particles or through avalanche effect in the adsorbed residual gas layer on the electrode [12, 18]. It should be emphasized that there is no distinct boundary between breakdown induced by gas process and breakdown induced by vacuum process, but there is a boundary area. Within this area, breakdown may take place through either gas or vacuum process or through a combination of the two. Electrical breakdown in gases is of a stochastic nature and therefore statistical methods are applied for its analysis [19-23]. Such methods are most commonly used for analysis of breakdown voltage and electrical breakdown time delay. Breakdown voltage value depends on applied voltage waveform. If the rising voltage rate of change is much slower than the rate of the elementary processes of gas breakdown voltage is a deterministic quantity and it is called static breakdown voltage. If the change in rising voltage is comparable to the rate of elementary processes of the gas discharge, breakdown voltage is a stochastic quantity and it is called impulse or dynamic breakdown voltage. Breakdown voltage value also depends on gas type and pd product. In the presence of electron emission from the metal cathode (Townsend’s process), breakdown

voltage value depends on electrode material, through the parameters such as work function, thermal conductivity and melting point [10]. Electrical breakdown time delay measurements are based on the fact that transition of gases into conducting state is not instantaneous. Namely, electrical breakdown always appears with certain delay when voltage higher than static breakdown voltage is applied on gas-filled tube electrodes [23]. The time interval between the moment of voltage application on a gas-filled tube electrodes and the moment when electrical breakdown occur represent electrical breakdown time delay t d . t d consists of two parts, statistical time delay ts and formative time t f , ( t d = t s + t f ) [11]. t d depends on applied voltage, cathode material work function, discharge current and the time between two successive voltage pulses, τ , when voltage on the electrodes equals zero ( τ is called relaxation time) [20]. It was also shown [24] that t d value decreases during tube irradiation by gamma and UV rays. The aim of this paper is to estimate static breakdown voltage U s on the basis of experimental data of dynamic breakdown voltage Ub as a function of voltage increase rate for different inter-electrode gaps d. Using U s = f (d ) dependence separation of vacuum and gas breakdown process was performed. Possible processes in afterglow responsible for breakdown initiation with respect to relaxation time were discussed on the basis of memory curves and statistical analysis of electrical breakdown time delay.

2. Experimental details 2.1. Argon-filled tube The measurements of dynamic breakdown voltage and electrical breakdown time delay were performed on argon-filled tube with adjustable inter-electrode gap, which could be varied from 0 to 15 mm using permanent magnet from outside (Fig. 1). Zero interelectrode gap was established by ohmic resistance measurements. The electrodes are rounded and polished to high gleam copper rods with 99.998% purity and 10 mm in diameter. The tube glass container radius was 25 mm and it made from 8245 Schott technical glass. Molybdenum wires with 2 mm in diameter were used as electrode carriers because the mean linear thermal expansion coefficient for Schott technical glass and molybdenum is approximately the same. Before argon was admitted, the tube was baked out at 350 oC and pumped down to a pressure of 10-7 mbar. During these processes the cathode was cleaned by sputtering with glow current of 0.5 mA in order to remove the monolayer of gases remained on the cathode surface after manufacturing and polishing. After that the tube was filled with argon 99.998% purity (impurities in argon were CO2 < 1 ppm, O2 < 3 ppm, THC < 0.5 ppm, nitrogen < 4 ppm and water < 5 ppm) at a pressure of 0.5 mbar (the accuracy of measuring the argon pressure was ± 0.001 mbar) and sealed.

Fig. 1. Argon-filled tube with one fixed electrode and the other movable electrode. 1fixed electrode, 2- movable electrode and 3- rotation shaft from iron. 2.2. Estimation of the static breakdown voltage In order to estimate static breakdown voltage, measurements of dynamic DC breakdown voltage were performed for voltage increase rate interval from 1 to 5 V/s. Three consecutive measurement cycles of dynamic breakdown voltage U b are presented in Fig. 2. In order to minimize the time required for breakdown voltage measurement, the initial voltage applied to the tube U k is not zero, but arbitrarily chosen value much smaller than expected value of Ub. Voltage applied on argon-filled tube electrodes is being increased in steps U p which duration is also predefined and fixed t p . The voltage increase rate can be refereed as k = U p t p . Dynamic breakdown voltage is the voltage value for which current through the tube starts to rapidly increase. After the breakdown, glow current of I g = 0.5 mA is forced through the argon-filled tube for the time

tg = 1 s (glow time). This value of glow current is not yet sufficient to produce the cathode spots that erode the cathode surface and value of t g is long enough for stationary glow condition to be established after breakdown. After glow time the argon-filled tube is disconnected from the voltage for the period of relaxation time τ = 150 s . It can be seen from the figure that, due to breakdown stochastic nature, dynamic breakdown voltage values, U b1 , U b2 , … are different. For U b measurements modified version of system described in [25] was used. U

Up tp Uk

U b3

U b2

U b1

t

t

t

Fig. 2. Measuring cycles: U b1 , U b 2 ,... values of dynamic breakdown voltage; U k starting voltage; U p voltage step, t p time interval between two successive voltage step,

τ relaxation time. The static breakdown voltage U s , as a deterministic quantity, cannot be preciously determined by the method of voltage increase rate because in that case voltage increase rate should be very small (theoretically zero). The value of U s is estimated by fitting of U b = f (k ) dependence and finding the intersection of fitting line with U b axis (for k = 0). U b represents mean value of dynamic breakdown voltage U b . Total of 100 data of U b were measured for every single k value and mean value U b was determined. As mentioned above, dynamic breakdown voltage values were obtained for voltage increase rate k from 1 to 5 V/s. Values of k were obtained for Up = 0.1 V, while tp values were 0.1, 0.2, 0.3, 0.4 and 0.5 s. Fig. 3 represents results of U b = f (k ) dependence with appropriate standard deviation for inter-electrode gap d = 0.1 mm. As it can be seen, experimental data can be described with linear regression, which intersection with U b axis correspond to U s = 592.6 V . U s values for other inter electrode gaps were estimated for the same k interval, as in Fig. 3. It was shown that for all of the values of inter electrode gaps there is linear dependence between U b and k. U b (V) 660

U b = 6.2 × k + 592.6

640

620

600

580

U S » 592.6 V

560

540 1

2

3

4

5

k (V/s)

Fig. 3. Mean value of dynamic breakdown voltage U b as a function of voltage increase rate k for inter-electrode gap d = 0.1 mm. 2.3.Determination of the electrical breakdown time delay

The measurement cycles of electrical breakdown time delay t d for applied voltage pulses U w > U s is presented in Fig. 4. In the figure τ represents relaxation time, Ig = 0.5 mA is glow current flowing through the tube after breakdown for the time tg = 1 s. Voltage pulse during measurements was U w = 1.15U s . 15% higher voltage than Us on the tube was sufficient enough to assure electrical breakdown. As it can be seen, values of electrical breakdown time delay t d 1 , t d 2 ,... are also different since the electrical breakdown processes has a stochastic nature. t d experimental data were obtained using the enhanced version of data acquisition system described in [26]. This system is designed in such a manner that voltage pulse rise time is about 20 ns. Such value is for about two orders of magnitude smaller than shortest td value obtained in measurements, and it can be concluded that the influence of voltage pulse rise time to electrical breakdown time delay measurements can be neglected.

Uw I g

td

1

τ

tg

td

2

tg

τ

Fig. 4. Measuring cycles: t d 1 , t d 2 ..., values of electrical breakdown time delay; t g discharge time; τ relaxation time; U w voltage pulse; I g discharge current. The t d values were obtained for the cases when argon-filled tube was protected from the light and in the presence of UV radiation originating from mercury lamp quceksilver/mercury (Hg100). The emission spectrum of this lamp is presented in Fig. 5 and obtained with Avantes spectrometer Avaspec 3648 which has a useable range from 200 to 850 nm. Our earlier investigations regarding Schott 8245 technical glass have shown that transparency equals zero for wavelengths below 300 nm [27].

Relative intensity

Hg100 lamp emission spectrum

366.3 nm 313.1 nm 312.5 nm 302 nm

l (nm)

Fig. 5. Emission spectrum of Hg100 lamp.

In order to determine reliable distributions of td experimental data for different τ values, series of 1000 measurements were used. Mean value of electrical breakdown time delay, t d as a function of relaxation time τ (memory curve) was determined from set of 100 td measurements for τ value ranging from 1 µs up to the τ value when memory curve reaches saturation.

3. Results and discussion 3.1. Breakdown voltage vs electrode gap Static breakdown voltage U s as a function of inter electrode gap d is displayed in Fig. 6. 640

620

US (V)

600

Vacuum breakdown Gas breakdown

580

560

540

520 0.1

1

10

d(mm)

Fig 6. Static breakdown voltage U s as a function of inter-electrode gap d. As it can be seen in inter-electrode gap d interval from 0.1 to 9 mm values of U s insignificantly changes. In d interval from 9 to 11 mm Us value rapidly decreases with the increase in inter-electrode gap d. Minimal value appears for d = 11 mm, followed by increase in U s for higher values of d . Minimum of these curve correspond to (pd)min = 0.54 mbar cm and Us = 530 V. Different data for (pd)min can be found in literature. In [9] it can be found that (pd)min = 0.9 mbar cm and value U min = 250 V. These data is given for argon-filled tube with plan-parallel electrodes (cathode made from copper). Papers [12,28] show Paschen’s curves for Rogovsky type electrodes and electrodes of cylindrical shape. It was shown that (pd)min = 1 mbar cm was for Rogovsky type electrodes, while (pd)min = 0.47 mbar cm for cylindrical shape electrodes. In both cases was U min = 350 V. The difference in (pd)min values in our experiment could be due to constant pressure and variable inter-electrode gap, while in [9,12,28] was the opposite case. Higher values of static breakdown voltage in our experiment might be due to

electrode shape and determination method as well as relatively large value of τ ( τ =150 s). It should be pointed out that the shape of curve presented in Fig. 6 is similar to the curve for DC breakdown voltage as a function of argon, nitrogen and air pressure in ring assembly geometry [13]. The authors in [13] concluded that vacuum breakdown mechanism occurs for pressures from 2 × 10 −5 to 10 −3 mbar what is manifested through small variation in breakdown voltage with the increase in pressure, while gas breakdown process is dominant at high pressures . In transition zone between 10 −3 and 10 −2 mbar both breakdown processes can be present. As it can be seen from Fig. 6, static breakdown voltage Us up to d = 9 mm insignificantly changes with the increase in inter-electrode gap, corresponding to vacuum-like breakdown. For d > 9 mm breakdown takes place as normal type breakdown. It should be pointed out that the curve shape in Fig. 6 is similar to the curve for nitrogen filled tube given in [29]. Also, our results for U s = f ( pd ) dependence (p = 0.7 mbar) for air-filled tube have shown that up to pd = 0.2 mbar cm Us is approximately constant, as a consequence of vacuum breakdown process [23].

3.2. Distribution of electrical breakdown time delay

3.2.1. Distribution of statistical time delay As it was mentioned in Section 1, electrical breakdown time delay t d represent sum of statistical time delay ts and formative time t f ( t d = t s + t f ). ts represents the time elapsed from the moment of U w > U s application on the tube up to the appearance of initial electron in inter-electrode gap, while tf is the time needed for ionization to become intense enough to initiate breakdown [20]. It was shown that experimental data for ts can be described by Laue distribution [30]: n(t ) = exp(−YPt ), N

(1)

where N is the total number of ts experimental data obtained for the same parameters (applied voltage U w , relaxation time τ , glow current I g , glow time tg ), n(t) is the number of ts larger than actual t, Y is the number of electrons generated in the inter electrode gap per time unit, known as electron yield and P is the probability that an initial electron created in inter-electrode gap, leads to breakdown initiation. It is assumed that appearance of initial electron in inter-electrode gap is Poisson random process. In Eq. (1)

YP = 1 / t s , where t s is the mean value of statistical time delay. Standard deviation σ in the case of large number of experimental data is σ = t s [20].

In the case when formative time cannot be neglected Eq. (1) can be written as: n(t ) = exp[ −(t − t f )]YP N

(2)

and it is valid when t f = const . As it can be seen from Eqs. (1) and (2) there is linear dependence between ln( n (t ) / N ) and t, known as Lauegram. Figs. 7, 8 and 9 present Lauegrams of t d experimental data obtained for inter electrode gaps of 0.1, 4 and 11 mm, respectively. Experimental data were obtained for U w = 1.15U s , I g = 0.5 mA , tg = 1 s and τ values of 30 and 70 ms. Since experimental data are very well fitted by straight line (correlation coefficients R 2 range from 0.98 to 0.99) it can be concluded that Laue’s distribution holds. The estimation of t s can be obtained from the slope of straight line, while t f can be estimated as the intersection of this line with ln( n (t ) / N ) axis corresponding to zero what proves ts >> tf , so that t d ≈ ts . With the increase in τ , the slope of straight line decreases, i.e. t s value increases.

é n(t ) ù ln ê ë N úû

1

t = 30 ms t = 70 ms

0 -1 -2 -3 -4 -5 -6 -7 4

0

1x10

4

2x10

4

4

3x10

4

4x10

4

5x10

4

6x10

7x10

td (ms)

Fig . 7. Lauegrams of 1000 t d data for inter-electrode gap d = 0.1 mm and values of relaxation time τ =30 ms and τ =70 ms. é n(t ) ù ln ê ë N úû

1

t = 30 ms t = 70 ms

0 -1 -2 -3 -4 -5 -6 -7 0

4

1x10

4

4

2x10

4

3x10

4x10

4

5x10

td (ms)

Fig . 8. Lauegrams of 1000 t d data for inter-electrode gap d = 4 mm and values of relaxation time τ =30 ms and τ =70 ms. é n(t ) ù ln ê ú ë N û

1

t = 30 ms t = 70 ms

0 -1 -2 -3 -4 -5 -6 -7 0.0

3

5.0x10

4

1.0x10

4

1.5x10

4

2.0x10

4

2.5x10

4

3.0x10

td (ms)

Fig. 9. Lauegrams of 1000 t d data for inter-electrode gap d = 11 mm and values of relaxation time τ =30 ms and τ =70 ms. As it can be seen from these figures Laue’s distribution appropriatelly describes experimental td data for τ = 30 ms and τ = 70 ms regardless if the breakdown is initiated

by vacuum (d = 0.1 mm and d = 4 mm) or gas (d = 11 mm) breakdown processes. Only difference is in td values which are longer for the case of vacuum than gas discharge processes.

3.2.2. Distribution of formative time Earlier investigations [31] have shown that tf experimental data can be very well described using Gaussian density distribution function: f (t f ) =

exp[−(t f − t f ) 2 / 2σ f 2 ] 2 πσ f

,

(3)

where t f is the mean value of formative time and σ f is the standard deviation of t f . Figs. 10, 11 and 12 presents the histograms of relative frequency of t d data for inter electrode gaps 0.1, 4 and 11 mm, respectively. Experimental data were obtained for U w = 1.15U s , I g = 0.5 mA , tg = 1 s and τ = 70 µs . The solid lines represent Gaussian density distribution function (Eq. (3)) and corresponding parameters are also given in the figures. As it can be seen this distribution satisfactory describe t d data, what further

Histogram, distribution density function (arbitrary units)

proves that ts << t f , i.e. t d ≈ t f .

300

f (td ) = 11 .9 +

æ td - 20.3 ö ÷ 0 .2 ø

2

-2×ç 70.8 ×e è 0.2 × π/2

250

200

150

100

50

0 19.9

20.0

20.1

20.2

20.3

20.4

20.5

td (ms)

Fig. 10. Histogram and Gaussian distribution function of 1000 t d data for inter-electrode gap d = 0.1 mm and relaxation time τ =70 ms.

Histogram, distribution density function (arbitrary units)

250

f (td ) = 1.2 +

æ td - 22.5 ö ÷ 0 .3 ø

2

-2×ç 101.2 ×e è 0.3 × π/2

200

150

100

50

0 22.0

22.2

22.4

22.6

22.8

td (ms)

Fig. 11. Histogram and Gaussian distribution function of 1000 t d data for inter-electrode gap d = 4 mm and relaxation time τ =70 ms.

Histogram, distribution density function (arbitrary units)

500

f (td ) = 26.4 +

æ td -15.1 ö ÷ 1 .1 ø

2

-2×ç 580.4 ×e è 1.1 × π/2

400

300

200

100

0 13.0

13.5

14.0

14.5

15.0

15.5

16.0

td (ms)

Fig. 12. Histogram and Gaussian distribution function of 1000 t d data for inter-electrode gap d = 11 mm and relaxation time τ =70 ms. 3.3.

Memory curves

During breakdown and latter discharge certain concentration of positive ions and electrically neutral active particles is formed in argon-filled tube. After the discharge ceases (relaxation period) positive ions recombine and neutral active particles recombine and de-excite (volume recombination and de-excitation on container wall and electrodes). These processes are not sudden, and some time is required for them to cease, i.e. to form a full gas relaxation. If the voltage U w > U s is applied to the tube electrodes before full gas relaxation period, positive ions and some neutral active particles can release secondary electrons from the cathode (secondary electron emission, SEE), which can initiate breakdown, what further makes significant impact on td value. Our earlier investigations have shown that presence of these particles after discharge ceases can be

followed by measurements of electrical breakdown time delay t d for different values of relaxation time τ and analysis of memory curve [20,23]. In this paper such analysis was performed for inter electrode gaps of 0.1 and 4 mm when vacuum breakdown process is dominant as well as for inter electrode gap of 11 mm, which is at the minimum of U s = f (d ) curve (see Fig. 6) when gas ionization effect induced by SEE is maximal. Figs. 13, 14 and 15 present memory curves for inter electrode gaps 0.1, 4 and 11 mm, respectively. The memory curves are obtained for U w = 1.15U s , I g = 0.5 mA and

tg = 1 s . In the same figures presents standard deviation of electrical breakdown time delay σ as a function of relaxation time τ . It can be observed that every memory curve contain three distinct areas. The first area is characterized with relatively small increase in t d with the increase in τ . This area for inter electrode gaps of 0.1 mm and 4 mm is in τ interval from 1 ms to 7 ms, while for inter-electrode gap 11 mm this τ interval ranges from 1 ms to 15 ms. The characteristic of this area is that standard deviation of electrical breakdown time delay σ is for about two orders of magnitude smaller than t d . Since

σ = t s and t d = t s + tf it can be concluded that for these τ intervals t s << tf ; t d ≈ tf . The relatively small change in t d value in these intervals is due to considerable concentration in positive ions created during previous breakdown and discharge. The main reactions responsible for positive ions creation in argon are [32-34]: Ar + e → Ar + + e,

(4)

Ar M + e → Ar + + e,

(5)

Ar M + Ar M → Ar + + Ar + e,

(6)

Ar M + Ar M → Ar2+ + e,

(7)

where Ar is the argon atom in ground state, Ar M denote argon metastable atom, Ar + and Ar2+ denote argon positive atomic and molecular ions, respectively. It has been shown [35] that molecular ions concentration comprises 30% of atomic ion concentration. It can be emphasized that concentration of positive ions originating from ionization of nitrogen molecules as impurities in argon could be neglected due to their low concentration with respect to concentration of argon atoms. If the voltage is applied on the electrodes of argon-filled tube during this period of time, the dominant role in breakdown initiation is played by SEE process induced by positive ions. Due to high drift velocity these ions reach cathode almost instantly upon application of the voltage U w .

t d , σ (ms)10

7

td σ d = 0.1 mm US = 593 V UW = 1.15US

6

10

5

10

4

10

3

10

2

10

1

10

0

10

-1

10

-3

10

-2

10

-1

0

10

1

10

2

10

3

10

4

10

5

10

10

τ (ms)

Fig. 13. Mean value of electrical breakdown time delay t d and standard deviation of electrical breakdown time delay σ as a function of relaxation time τ (memory curve ) for inter-electrode gap d = 0.1 mm (vacuum breakdown). The data was obtained for applied voltage U w = 1.15U s , glow current I g = 0.5 mA and glow time t g = 1 s.

t d , σ (ms)10

6

td σ d = 4 mm US = 595 V UW = 1.15US

5

10

4

10

3

10

2

10

1

10

0

10

-1

10

-3

10

-2

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

τ (ms)

Fig. 14. Mean value of electrical breakdown time delay t d

and standard deviation of

electrical breakdown time delay σ as a function of relaxation time τ (memory curve) for inter-electrode gap d = 4 mm (vacuum breakdown). The data was obtained for applied voltage U w = 1.15U s , glow current I g = 0.5 mA and glow time t g = 1 s. t d , σ (ms) 10

6

td σ d = 11 mm US = 536 V UW = 1.15US

5

10

4

10

3

10

2

10

1

10

0

10

-1

10

-3

10

-2

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

τ (ms)

Fig. 15. Mean value of electrical breakdown time delay t d and standard deviation of electrical breakdown time delay σ as a function of relaxation time τ (memory curve) for

inter-electrode gap d = 11 mm (gas breakdown). The data was obtained for applied voltage U w = 1.15U s , glow I g = 0.5 mA and glow time t g = 1 s. The ions drift velocity vd can be estimated using expression [36]: vd =

eλ U w , Md v

(8)

where e is the elementary charge, λ is the mean free path, λ = kT /( 2πD 2 p) (k is the Boltzmann’s constant, T = 300 K is the gas temperature, D is the ion diameter and p is the pressure), M is the ion mass, d is the inter-electrode gap and v = 2kT / M is the mean thermal velocity. Estimated value for vd for argon ions for our experimental conditions is about 4 ⋅10 6 cm/s and for two orders of magnitude higher than v . It should be pointed out that beside positive ions SEE process can be initiated by some electrically neutral active particles also formed during breakdown and discharge. However, due to their electrical neutrality the probability for their impact with the cathode is much smaller than the probability for positive ions. Because t d value insignificantly changes for d = 0.1 and 4 mm up to τ = 7ms, while for d = 11 mm such behavior can be seen up to τ = 15 ms it can be concluded that positive ions full recombination time is shorter when vacuum discharge process is dominant. This is probably a consequence that fewer positive ions are created in vacuum breakdown compared to gas breakdown. Furthermore with a narrow inter-electrode gap the charged particles escape to the electrodes intensity and with a lesser rate to the tube walls. Increasing the inter-electrode gap lowers the so-called diffusion length of the discharge chamber, and the losses of the charged particles to the walls decrease. Therefore the time of plasma decay also increases for large inter-electrode gap values. The second area in Figs 13, 14 and 15 is characterized in rapid increase in t d with the increase in τ . It appears in the τ interval from 7 ms up to 150 s for inter electrode gaps 0.1 and 4 mm, while for inter-electrode gap 11 mm this τ interval is from 15 ms up to 30 s. Such a rapid increase in t d value is a consequence of positive ions full recombination so that the dominant impact to SEE process is due to electrically neutral active particles formed during previous breakdown and discharge. It is assumed that recombination/de-excitation time of these particles is considerably longer than positive ions recombination time. On the other hand, due to their electrical neutrality the probability for their collision with the cathode is considerably smaller than the probability for collision of positive ions with the cathode when voltage U w > U s is applied on argonfilled tube electrodes.

If the argon could be released from all of impurities, during breakdown and discharge, beside positive ions, only metastable atoms would be created. They have sufficient energy to initiate SEE process in collision with the cathode. During breakdown and discharge

3

P 0 and 3 P 2 metastable states form, with radiative lifetimes of 44.9 and

55.9 s, respectively, with energy sufficient enough to induce SEE process [37]. Our earlier analysis of rapid increase in t d with the increase in τ was based on assumption that metastable atoms are responsible for this process [38]. The dominant production process of metastable atoms Ar M is induced by electron impact excitation from the Ar atom in ground state [39]: Ar + e → Ar M + e,

(9)

On the other hand the processes of ArM loss are electron impact quenching, quenching by collision with argon ground state and diffusion [39,40]: Ar M + e → Ar* + e,

(10)

Ar M + e → Ar* + Ar,

(11)

Ar M → wall,

(12)

where Ar * represents resonant excited argon atom. Because of that non-radiative lifetime of metastable atoms is very short (several hundred of microseconds) [41]). Because of that these particles cannot be responsible for SEE process in the area of rapid increase in t d (Figs. 13, 14 and 15). Electrically neutral active particles, which can induce SEE process in the area of t d rapid increase, originate probably from argon impurities (see Section 2). Most probably those are N2 molecules with the concentration < 4 ppm. This assumption is based on the appearance of Lewis-Rayleingh yellow reddish for a long period after the discharge in nitrogen and this process arises from recombination of N(4S) ground state atoms [42]. These atoms are created during breakdown and discharge in collisions

between electrons and ground states nitrogen molecules N 2 (X 1 Σ +g ,ν = 0) [43,44]: N 2 (X1Σ g+ ,ν = 0) + e → N( 4 S) + N( 4 S) + e

(13)

N 2 (X1Σ g+ , ν = 0) + e → N( 4 S) + N( 4 D) + e

(14)

where N( 2 D) is the metastable state nitrogen atom. N( 4 S) atoms are also formed during relaxation period in mutual collisions of metastable and highly excited vibration level of nitrogen molecules also formed during previous breakdown and late discharge [45]. However the probability for this process is small due to the low concentration of nitrogen molecules as impurities in argon. Recombination time of N(4S) atoms is much longer than positive ions recombination time [46] and their recombination on the cathode lead to SEE process [47]. Also, these atoms are very effectively recombined on glass container surface of the tube, which is much larger than electrode surface (glass container surface area is significantly larger than the electrodes surface). Because of that, with the increase in τ the probability for SEE decreases what further increases t d . Memory curve investigation for xenon [48], neon [41] and krypton [42] with nitrogen as impurity have shown that there is also area of rapid increase in t d with the increase in τ . It is assumed that such an increase due to the presence of N(4S) atoms. When gas breakdown process is dominant, rapid increase in t d occur up to τ = 30 s (Fig. 15) and it is probably due to the presence of only N(4S) atoms in afterglow. In the case of vacuum breakdown process rapid increase in t d appears up to τ = 150 s (Figs. 13 and 14). Beside N(4S) atoms, there are additional long living electrically neutral active particles in afterglow which could initiate SEE process. These particles probably originated from metal vapors of the electrode material (they could perhaps be sputered particles or condensed clusters). The third area in Figs. 13, 14 and 15 is characterized by insignificant variations in t d values with the increase in τ (memory curve saturation). In this area the concentration of electrically neutral active particles which can initiate SEE is significantly reduced, so free electrons in inter-electrode gap originate only from cosmic ray and natural

radioactivity. Since the flux of these radiations insignificantly changes, approximately constant and it can be assumed that full gas relaxation took place.

t d is

Figs. 16 and 17 present memory curves for inter electrode gaps of 4 and 11 mm, respectively for the case when argon-filled tube was not irradiated and when it was irradiated by Hg100 lamp. The data was obtained for applied voltage U w = 1.15U s , glow current I g = 0.5 mA and glow time t g = 1 s. It can be seen that this light has a significant influence on memory curve in area of rapid increase in t d and in area of its saturation. Such tendency cannot be seen for smaller values of time delay because in that area positive ions are still dominant in breakdown initiation. The decrease t d in these areas during the tube irradiation is a consequence of additional electron yield originating from some of wavelengths which can penetrate through the glass container of the tube and induce SEE by photoelectric effect process. Wavelengths which can probably induce SEE

process are 302, 312.5 and 313.1 nm with corresponding energies of 4.10, 3.96, and 3.97 eV (see Fig. 5). It is known that for SEE process it is necessary that photon to have energy hν > Wi , where Wi is the cathode material work function. Pure copper work function is 4.5 eV [49], what proves that photons with these energies cannot induce SEE process. However, since copper electrodes have purity of 99.998 and impurities are always present on their surfaces as well as oxides remained during their production, it can be assumed that there is reduction in their work function, so the photons of previously mentioned energies can initiate SEE process. Moreover, electron yield induced by electrically neutral active particles is smaller than electron yield induced by UV radiation. Also, flux originating from UV radiation is bigger than flux originating from cosmic ray and natural radioactivity, what is manifested in memory curve saturation for shorter τ values. t d (ms)

t d without UV radiation t d with UV radiation d = 4 mm US = 595 V UW = 1.15US

6

10

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-2

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τ (ms)

Fig. 16. Mean value of electrical breakdown time delay t d as a function of relaxation time τ (memory curve) for inter-electrode gap d = 4 mm (vacuum breakdown) and in the case when tube was not exposed to radiation and exposed to UV radiation from Hg100 lamp. The data was obtained for applied voltage U w = 1.15U s , glow current I g = 0.5 mA and glow time t g = 1 s. t d (ms) 6

10

t d without UV radiation t d with UV radiation d = 11 mm US = 536 V UW = 1.15US

5

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τ (ms)

Fig. 17. Mean value of electrical breakdown time delay t d as a function of relaxation time τ (memory curve) for inter-electrode gap d = 11 mm (gas breakdown) and in the

case when tube was not exposed to radiation and exposed to UV radiation from Hg100 lamp.

4. Conclusion On the basis of static breakdown voltage Us data as a function of inter-electrode gap d the separation of vacuum and gas breakdown processes was performed for argon-filled tube at 0.5 mbar pressure. It was concluded that in d interval from 0.1 to 9 mm vacuum breakdown process is probably dominant. For d > 9 mm gas breakdown processes is dominant, for d = 11 mm gas ionization process is maximal. It was also shown that memory curve shape is similar for vacuum and gas breakdown process, i.e. there is area of relatively small change in t d for small τ values, area of rapid increase in t d with the increase in τ and area of memory curve saturation. On the basis of statistical analysis of electrical breakdown time delay td, it was concluded that the first area originates due to positive ions in relaxation period created during previous breakdown and discharge. High velocity of these ions leads to SEE process almost instantly upon the application of voltage U w > 1.15U s . Recombination time of these ions for vacuum breakdown process (d = 0.1 mm and d = 4 mm) is about 7 ms, while for gas breakdown process (d = 11 mm) is about 15 ms. In the case of positive ions presence in afterglow statistical time delay ts is much smaller than formative time tf, so it can be concluded that t d ≈ t f . Area of rapid increase in t d for higher τ values is due to the fact that dominant role in SEE process is taken over electrically neutral active particles also formed during previous breakdown and discharge. We assume that for gas breakdown ionization process (d = 11 mm) SEE process is induced only by N(4S) atoms formed during discharge by dissociation of N2 molecules present in argon as impurities. Their recombination time is about 30 s. In the case of vacuum breakdown process (d = 0.1 mm and d = 4 mm), rapid increase in t d is up to τ = 150 s, what proves that beside N(4S) atoms other long living electrically neutral active particles originating from electrodes metal vapors can initiate SEE process. Statistical analysis has shown that in the area of rapid increase in t d with the increase in τ , ts >> tf, so t d ≈ ts . After full recombination and de-excitation of electrically neutral active particles in afterglow, electrical breakdown is initiated only by cosmic rays and natural radioactivity. This is manifested in memory curve saturation. It was also shown that UV radiation presence of wavelengths slightly longer than 300 nm makes significant impact on t d values when dominant role in SEE process is played by electrically neutral active particles as well as in the case when dominant role in breakdown initiation is taken over by cosmic rays and natural radioactivity.

Finally, we would like to emphasize that results presented in this paper could be interesting for gas and vacuum electronic components manufacturers. Moreover, the knowledge of relaxation processes after the breakdown and discharge ceases in insulating gas of gas-filled switchers and gas-filled surge arresters are very important for their reliable operation.

Acknowledgements This work is supported by Ministry of Education, Science and Technology Development of Republic of Serbia under contract no. 171007.

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• • • •

Static breakdown voltage was estimated on the basis of measured dynamic voltage. Separation of vacuum and gas breakdown processes was performed. Memory curves were measured for argon filled tube. Processes induced by breakdown in argon were discussed.

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All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

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Author’s name Dr Milić Pejović Dr Momčilo Pejović Mr Čedomir Belić Dr Koviljka Stanković

Affiliation University of Niš, Facuty of Electronic engineering University of Niš, Facuty of Electronic engineering University of Belgrade, Facuty of Electrical engineering University of Belgrade, Facuty of Electrical engineering