lecture#14 high vacuum advance h v engg

"Prof Dr. Suhail Aftab Qureshi" 1

High Voltage Engineering

LECTURE-10

Prof Dr. Suhail A. Qureshi.Elect. Engg. Deptt, UET, Lahore.

High Vacuum

(Electrical Properties)


1. Introduction.2. What is Vacuum?3. Mean Free Path.4. Pre-breakdown Conduction.5. Factor Affecting the Breakdown Voltage.

5.1 Electrode Separation.5.2 Electrode Effects (Conditioning)5.3 Material and Surface Condition of

Electrodes.5.4 Surface Contamination.5.5 Area and Configuration.

HIGH VACUUM (Electrical Properties)


5.6 Temperature.5.7 Frequency of Applied Voltage.5.8 Pressure Effects.5.9 Time lags.

6. Breakdown hypothesis.

6.1 Particle Exchange Hypothesis. 6.1.1 Positive ion Hypothesis. 6.1.2 Positive ion-Negative ion HyPothesis.



6.2 Electron Beam Hypothesis. 6.2.1 Anode Heating Hypothesis 6.2.2 Cathode Heating Hypothesis 6.3 Clump Hypothesis.

7. Conclusions.

7.1 Overall Conclusions



1. INTRODUCTION

The idea of using Vacuum as an insulation is very old and the reasoning behind this idea is self-evident. If the transport of electricity depends on the transit of charged particles (electrons, positive ions, etc.) then the absence of any such particles that is an absolute Vacuum, should produce the perfect insulation.

ELECTRICAL PROPERTIES OF HIGH VACUUM


1. INTRODUCTION

In practice, however, the existence of metallic and insulating surface within the vacuum and the presence of absorbed gases and oil vapours, complicate the issue so that even in a Vacuum a sufficiently high applied voltage will cause a break down with the abrupt dissipation of the stored energy across the gap in a luminous arc.



1. INTRODUCTION

Recently there has been an increasing interest in the electrical properties of high vacuum and its use as insulation. This has arisen because in addition to being used in devices such as high powered vacuum switches, electronic valves microwave tubes, photocells, controlled nuclear fission devices, low-loss high frequency capacitors, and electrostatic voltmeters, gyroscopes and bearings, there is now the requirement for high-voltage apparatus, such as electrostatic generators, to operate in outer space and to make use of the natural vacuum environment as dielectric.



1. INTRODUCTION

Compare now the case of two electrodes separates by vacuum with that in which the inter - electrode gap contains a gas. If the two metal electrodes are separated by say air, at atmospheric pressure, then any electrons moving between them cannot travel far without hitting a gas atom.

If the electric field is sufficiently strong an electron will have enough energy to ionize an atom on collision. An electron avalanche is then produced by electron liberated from gas molecules.



A Vacuum system which is used to create vacuum is a system in which the pressure is maintained at a value much below the atmospheric pressure. In vacuum much below the atmospheric pressure. In Vacuum systems the pressure is always measured in terms of millimeters of mercy, where one standard atmosphere is equal to 760 millimeters of mercury has been standardized as “Torr” by the international vacuum Society, where one millimetre of mercury is taken as equal to one Torr. Vacuum may be classified may be classified as

2. WHAT IS VACUUM?


High Vacuum : 1 x 10-3 to 1 x10-6 Torr Verty High Vacuum : 1 x 10-6 to 1x 10-8Torr Ultra high Vacuum : 1 x 10-9 Torr and below.

For electrical insulation purposes, the range of vacuum generally used is the “high vacuum”, in the pressure range of 10-3 Torr to 10-6 Torr.

2. WHAT IS VACUUM?


The distance an electron can typically travel without colliding with another particle is defined as the mean-free path. In a vacuum better than 10-4 torr ( 1 torr = 1 mm Hg). There are less than 3xl012 molecules/ cm3 and the length of the mean-free path is of the order of metres. Thus when the electrodes are separated by say 1 cm in such a vacuum, an electron can cross the gap without any collision taking place.

3. MEAN FREE PATH


In this case, therefore, the initial stage of breakdown cannot be due to formation of electron avalanches, that is multiplication of charged particles by collision in the space between the electrodes is now insufficient to create a self sustaining discharge. If however, a gas could forms in the vacuum gap then usual kind of gas break down can take place.

3. MEAN FREE PATH


Since the breakdown is always preceded by the flow of a measurable conduction current between the electrodes, a large number of workers have investigated the nature and origin of this current in order to give further insight in to the sequence of current that finally lead to breakdown.

When the voltage across a small gap is sufficiently increased a relatively steady current begins to flow. With larger gap spacing, small pulse of charge (micro-discharges) of the order of microcoulombs for milliseconds duration occur. Such micro discharges can exist either when no steady current flows or superimposed on the steady current.

4. PRE-BREAK DOWN CONDUCTION


However, with further increase in the voltage the effect of the micro-discharges eventually gives rise to a steady current.

The main mechanism that can give rise to electron emission from metallic surface in vacuum are;

1) Thermionic emission.2) Field assisted thermionic emission.

(Schotty emission)3) Field or cold emission.

In small gaps, the steady current is mainly produced by electron emission.

4. PRE-BREAK DOWN CONDUCTION


Before reviewing the various hypothesis put forward to explain the mechanism of the initiation of electrical breakdown in vacuum gap; we will consider briefly the factor found by experiment, to affect the breakdown voltage of a vacuum gap.

Firstly however, it is necessary to indicate exactly what is meant by the term breakdown voltage.

5. FACTORS AFFECTING THE BREAK DOWN VOLTAGE


Ideally the breakdown Voltage of high vacuum is defined as the voltage which, when increased by small amount, will cause the breakdown of vacuum gap that has held that voltage for an infinite time.



The easiest parameter to vary and therefore, investigate is that of electrode separation and its effect on the breakdown voltage of a vacuum gap has been known for many years.


5.1. ELECTRODE SEPARATION



Fig: 1.



Breakdown Voltage of small vacuum gaps.

For Vacuum gaps less than about 1 mm in length the breakdown voltage has been shown to be approximately proportional to length, keeping all other parameters constant.

Therefore,V=Kd for d < 1 mm.




Where "V" is the breakdown voltage, "K" is constant and "d" is the gap length.

For such small gaps the breakdown stress in relatively high being of the order of 106 V/cm. So breakdown mechanism is stress dependent so that field emission of electrons probably plays an important role in the breakdown process.




However as the distance between a pair of plane-parallel electrodes in Vacuum is increased beyond 1mm the breakdown voltage does not increase at an equal rate and so the apparent breakdown stress for longer gaps is much reduced, being about 104 V/cm at 100 mm.

Here stress means the voltage required to cause the break down divided by the distance between the electrodes.





Fig: 2.



Even for longer gaps there still appeared to be a simple relationship between the gap length and the breakdown voltage.

VE = K1 d>I-mm

E is gross surface gradient and K1 a constant the value of which depends on the material and surface condition of the electrodes.




Conditioning:- As mentioned earlier, if vacuum gap is continually sparking over the breakdown voltage increases until it reaches a plateau that is the gap becomes conditioned.

5.2. ELECTRODES EFFECTS



Fig: 3

5.2 ELECTRODES EFFECTS



The method of conditioning that has been found to give the most consistent results is to clean the electrodes by means of hydrogen gas discharge.

Other methods of conditioning consist of allowing the pre breakdown currents in the gap to flow for sometime, on to heat the electrodes, in the vacuum, to high temperature. For hydrogen conditioned electrodes the number of sparks required to reach a plateau is about 10; otherwise it can be as many as 10,000.

5.2 ELECTRODES EFFECTS



The electrode surfaces form the physical boundaries between which the breakdown finally takes place, so it not surprising to find that the breakdown strength of a given size of gap is strongly dependent on the material of electrodes.

Table showing variation in voltage held across a 1 mm vacuum gap with electrode material polished and spark conditioned electrodes.

5.3 MATERIAL AND SURFACE CONDITIONS OF ELECTRODES



Electrode Material

Voltage insulated across

1mm gap (kV)

Steel 122

Nickel 96

Aluminum 41

Copper 37




Due to the difference in actual electrode materials used by different investigators, for example there are many different compositions of stainless steels and copper, however the above order should not be taken as a standard.




Fig: 4.

5.3 MATERIAL AND SURFACE CONDITiONS OF ELECTRODES


Fig: 5



Evidence as to the effect of surface finish is still contradictory. Extensive and polishing apparently gives only small changes in breakdown voltage, but in general the smoother the surface the greater the breakdown voltage.




Direct studies of clean surfaces under normal vacuum conditions are complicated by the fact that oxidation takes place rapidly, that organic vapours are generally present which may give rise to carbon on the electrodes and also that the action of polishing is to form a fudge of oxide, metal and abrasives. The growth of a molecular layer of oxide on copper at a pressure of 10-4 mm of Hg of Oxygen molecules to cover a surface is 0.06 sec at pressure of 10-5 mm Hg of Oxygen.

5.4 SURFACE CONTAMINATION



Therefore in vacuum apparatus it can be assumed that the electrode surfaces are heavily oxidized if oil diffusion pumps rubber O-rings and Vacuum grease are used in the vacuum system, that the electrode support a layer of organic contamination. Such contamination can lower the breakdown strength by as much as 20 per cent.




Even in completely sealed-off baked out systems the electrodes surfaces can still carry some contamination. It has been found that when glass is heated to its working temperature, such as when scaling electrodes in to a closed cell, fluxes are vaporized from the glass and are re-deposited on the cool inner surfaces in the form of spherical particles of the order of 10-6 to 10-4 cm diameter.




Thus any electrode surface in a sealed system might carry such particles or contamination resulting from such particles, which contain sodium, potassium, and boron as well as traces of aluminum, and silicon. The presence of such contamination in the test cell reduces the breakdown voltage sometimes by as much as 50% of the clean electrode value.




Increasing the electrode area makes it more difficult to maintain a given breakdown value. For example electrode of 20 cm2 can hold 40 KV across a 1 mm gap, others of the same material of dia 1000 cm2 can only hold 25 KV.

5.5 AREA AND CONFIGURATION



5.5. AREA AND CONFIGURATION

Fig: 6



5.5. AREA AND CONFIGURATION

Fig: 7


For Nickel and iron electrodes the strength remain unchanged for temperatures as high as 500C0 thus leading to the conclusion that gases, and vapours of organic compounds absorbed on the surface of the electrodes did not substantially influence the occurrence of breakdown.

Further heating, above 500C0, of nickel on iron cathodes increased the breakdown strength whilst heating of the anodes above 500C0 decreased the strength.

Recently several investigations have shown that cooling the electrodes to liquid nitrogen temperature increases the breakdown voltage.

5.6 TEMPERATURE


Many measurements have been made using different types of voltage, namely impulse direct and alternating (50 cps) voltages, it is difficult to compare them due to various experiments, using different techniques and electrode materials.

However, it was thought that a given gap should stand a higher impulse then alternating and a higher alternating than direct voltage.

5.7 FREQUENCY OF APPLIED VOLTAGE



Fig: 8



It has been shown that for a. small gap up to 2mm there is no dependence of breakdown voltage on frequency in the range of 60 cps to 45 M cps.

A special case of vacuum breakdown can occur if the electrode surfaces are such than the secondary emission coefficient for electrons is greater that unity. In these circumstances, above a certain frequency, multipactoring can arise y which process an electron can be accelerated across the gap to arrive at the opposite electrode to release secondary electrons just as the field is zero.




Here electrodes are then accelerates back to the first electrode where the process is repeated so that the number of electrons will increase rapidly and breakdown can occur.




The vacuum breakdown region was defined earlier as that region in which the breakdown voltage became independent of the nature of pressure of the gas between the electrode. This is certainly the case of vacuum gaps of less than I mm. For example in a gap of 0.4 nun varying the pressure from 4 x 10-7 to 1.4 x 10-4 torr gave no change in the breakdown voltage.

5.8 PRESSURE EFFECTS



5.8 PRESSURE EFFECTS

Fig: 9


For larger gaps however, there is a definite pressure effect using a 0.16 cm diameter sphere opposite a plane cathode an a gap of 20 cm, the result obtained in shape of graph shown in Fig:9.

The breakdown voltage was essentially constant for pressures less than 5x10-6 torr, but rose with increasing pressure to a maximum at 5x10-4 torr with further increase in pressure the voltage fell sharply and a continuous dark discharge then took place.

5.8. PRESSURE EFFECTS


Under steady-state conditions vacuum breakdown is often quite unheralded (Out of blue / surprise) and sporadic (Scattered / Occasional) so that a given pair of electrodes may support a constant voltage level for hours, or even days and then suddenly breakdown will occur. There have as yet been no complete investigations of this statistical time lag.

On the other hand the formative time delay in the breakdown of a vacuum gap has been studied and has been found to increase with electrodes spacing and decrease with increasing voltage. This time delay is of the order of 10-8 sec, for gaps of about 1 mm.

5.9 TIME LAGS


During the last fifty years many different hypothesis have been put forward in attempts to explain quantitatively the mechanism initiating electrical breakdown in vacuum. These hypothesis can be split into three categories:

a. Those postulating an interchange of elementary articles (i.e electrons, positive ions, negative

ions etc.) due to secondary emission processes. Breakdown is assumed to occur when the interchange becomes cumulative.

6. BREAKDOWN HYPOTHESIS


b. Those postulating discrete beams of electrons from small areas on asperities (roughness) on the cathode, bombarding areas on the anode. Bombardment effects at the anode or resistive heating effects at the cathode are assumed to cause localized rise in temperature sufficient to cause the release of gas or vapours in which low-pressure gas breakdowns can then take place.



c. Those postulating the transport across the inter-electrode gap of aggregates of material or "clumps“ (Shapeless Pieces); the impact of such clumps on the opposite electrode giving rise to heating and hence to the release of vapours and therefore breakdown.



6.1.1 Positive ion hypothesis

The hypothesis was first proposed in 1933 to explain certain conduction and breakdown phenomena in an accelerator tube. Experimental evidence was taken to indicate that the conduction was due to ionization at the electrode surface as a consequence of the impact of ions, electrons and photons.

6.1 PARTICLE EXCHANGE HYPOTHESIS



An electron, by chance present in the gap, would be accelerated by voltage and would impinge on the anode where it would produce positive ions and photons. The positive ions in turn would be accelerated and back to the cathode where they and the photons would cause secondary electrons to be emitted. These would then be accelerated to the anode, releasing further positive ions and photons.




Fig: 10




LetA=Average number of positive ions produced by one

electron.

B=Average number of Secondary electrons produced by one of these positive ions.

C=Average number of photons produced by one electron.

D=Average number of secondary electrons produced by a photon.




When the coefficients for the production of particles exceeds unity, a runaway process, that is breakdown, takes place. Thus when (AB + CD) ≥1

The coefficients were found to depend primarily on the total gap voltage, the field at the electrode sun faces, and the nature of the metal and its contaminants.



6.1.2 Positive ion - Negative ion hypothesis

Positive ion Hypothesis, suggested by Mckibben and Boyer in 1951, where negative ions are substituted for electrons in the postulated chain reaction. Breakdown then occur when

(AB + GH) ≥ 1

Where

G = average No of -ive ions produced by one +ive ion.H = average No of Positive ions produced by one -ive ion.Produced by one negative ion.



6.2.1 Anode Heating Hypothesis

Gaps of less than 2mm and postulated that electrons, produced at small emitting micro projections on the cathode, bombard the anode causing a local rise in temperature and release of gases and vapours.

6.2 Electron Beam Hypothesis (Field emission Mechanism Hypothesis)



Additional electrons ionize the atoms of gas and produce positive ions. At the cathode the effect of these ion is two-fold; increased primary electron emission due to the space charge formation and hence local field enhancement and the production of secondary emission of electrons by bombardment of the surface. The process continues until sufficient gas is generated to low pressure gas discharge.

6.2 Electron Beam Hypothesis (Field emission Mechanism Hypothesis)



6.2 Electron Beam Hypothesis

Fig:10 Electron Beam anode heating mechanism of Vacuum breakdown.

Cat

hod

e

An

ode


6.2.2 Cathode Heating Hypothesis

Using very clean, electrolytically etched cathodes very high static vacuum and pulse voltage technique it was shown that voltage was increased between sharply pointed cathode and a concave hemispherical anode, in a modified field emission projection microscope, the field current density from the point' was consistent with the value predicting by the wave mechanical field-emission theory, where space charge effects were taken into account upto the time when the observations were terminated by an explosive arc.




The arc formation involved both high fields and high temperatures of the cathode paid and high temperatures at the cathode point and the breakdown was shown to result from resistive heating of the point.





Fig:11



It was postulated that near the breakdown voltage sharp points on the cathode are responsible for the pre-breakdown current, which is emitted according to the field emission relationship. This current produces resistive heating at the tip of a point and when a critical current density is reached, the tip melts or explodes, thus initiating the vacuum arc. That is, the initiation of breakdown depends only on the condition at and properties of the cathode surface.



As long ago as 1931 premature vacuum breakdown were explained on the basis of loosely adhering charged, aggregates (clumps) being torn from the anode or cathode by electrostatic forces, flying to the cathode by electrostatic forces, flying to the cathode or anode and causing emission.

6.3 CLUMP HYPOTHESIS


Detected from the cathode surface and is accelerated across the cap

Impact of ht clump on the anode gives out a cloud of metal Vapour.



6.3 Clump Hypothesis

Fig: 12 Clump loosely


Fig: 13

6.3 Clump Hypothesis

Detected from the cathode surface and is accelerated across the cap

Impact of ht clump on the anode gives out a cloud of metal Vapour.


Another scientist postulated more detailed analysis of the clump type mechanism and this postulation was somewhat different mechanism of the breakdown.

He assumed that even for small voltages particles begin to be detached from both electrodes. When the voltage is sufficiently high they collide with the opposite electrodes and adhere to surface and only become free again on the application of higher stress. When the kinetic energy of a micro partice at the moment of its collision with an electrode sufficiently large, the particle vaporizes to a gas cloud in which the, discharge commences. Thus the Slivkov theory postulate the evaporation of the particle and not of the electrode.



Visual observation were made on clump movements by using the light from an impulse lamp synchronized with the applied voltage impulses. By depositing particles on each electrode in turn and measuring time lags to breakdown it was found that the breakdown was brought about by +ive charge particles hitting the cathode. On breakdown the particle did not evaporate completely but joined on to the electrode and could be drawn out again by the application of a large field.



It had been suggested that clumps may originate either from material removed from the surface of electrode, or from loosely adhering particles left on them during the polishing and cleaning process. One of the main objections to a clump-type mechanism has been that the electrostatic forces acting prior to breakdown appears to be insufficient to pull off pieces of the parent electrode material and that loose aggregates would not exist on smoothly polished clean electrodes.



The electrostatic stress required to remove such inclusions would only be that required to overcome the low cohesive forces usually existing between inclusion-metal interfaces which is considerably less than the ultimate tensile strength of the metal itself.



7. Conclusions

Whist the large amount of work carried out in recent years understanding of the pre-breakdown Phenomena in vacuum, so far no single hypothesis is capable of explaining all the available experimental measurements and observations.

Vacuum Breakdown


7. Conclusions

However, as corroborating (authenticate) experimental evidence exists for all the postulated hypothesis for the breakdown mechanism, they appear to be more of a complementary nature rather than a contradictory one.The pre-breakdown and breakdown mechanisms seem to be in fluenced to a large extent by the experimental conditions.

Vacuum Breakdown


7. Conclusions

The most significant of these are;

Residual pressure in the gap, Length of gap Geometry and material of electrodes Homogeneity and treatment of their surfaces Presence of extraneous particles or contamination on

the surface of the electrodes or walls of the container.

Vacuum Breakdown


7. Conclusions

The correct choice of the electrode material, the use of thin insulating coating glass cathodes, or an increase of pressure in long gaps, can improve the voltage holding of a vacuum gap whilst an increase of electrode area or the presence of particles (Clumps) tends to have an opposite effect.

There is evidence to indicate that with increasing gap lengths transition from one type of breakdown mechanism to another take place at some critical electrode separation.

Vacuum Breakdown


7. Conclusions

For combined vacuum-solid insulation arrangement there in definite evidence to indicate that breakdown is initiated by phenomena occurring at the junction between the cathode and the insulator. Significant improvement in voltage holding can be achieved in practice by ensuring intimate contact between the cathode surface and the insulator, proper choice of the geometrical shape and electrical properties of the insulator, and by coating surface with a film giving low surface resistivity.

Vacuum Breakdown


1) So far no single hypothesis is capabie of explaining all the available experimental measurement and observations. However experimental deviances exists for all the postulated hypothesis for B.D. mechanism, they appear, they appear to be more of a complementary nature rather than a contradict tory one.

Vacuum Breakdown

7.1 Overall Conclusion


2) The prebreakdown and Breakdown mechanim seems to be influenced to great extent by the experimental conditions.

3) The most significant parameters effecting B.D process in vacuum are;

Vacuum Breakdown



I. Residual pressure in the gap.II. Length of the gap.III. Geometry and material of Electrodes.IV. Homogeneity and Treatment of their surfacesV. Presence of extraneous particales or

contamination, On the surface of electrode or walls of the container.

VI. Material and gerometry of vacuum solid insulation arrangement.

VII. Coating surface near the joint with a film giving low resistivity surface.

Vacuum Breakdown


lecture#14 high vacuum advance h v engg

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