Vacuum Breakdown Theories

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Vacuum Breakdown Theories

  1. Particle Exchange Mechanism
  2. Field Emission Mechanism
  3. Clump Theory

1. Particle Exchange Mechanism (PEM)

Particle Exchange Mechanism
Figure A
  • Assumption : Under the action of the high electric field a charge particle would be emitted from one electrode, and when this charge particle impinge on the other electrode, it liberates oppositely charged particles due ionization of absorbed gases. The applied voltage causes acceleration of these particles. These come back to the first electrode where they release more particles thanoriginal type of particles. If this process is cumulative, a chain reaction occurs leading to the breakdown of the gap.
  • In particle exchange mechanism the electrons, positive ions photos and absorbed gases at the electrode surfaces play a role.
  • An electron in the vacuum gap is accelerated towards the anode, these create an impact on the anode and due to impact, positive ions and C photons are released. The positive ions are then accelerated towards the cathode. When they make an impact on cathode, each positive ion liberates B electrons and each photon liberates D electrons as shown in Figure A.
  • The breakdown will occur if the coefficient of production of secondary electrons exceeds unity . i.e. (AB – CD) > 1
  • This theory was modified by Trump and Van de Graff. They gave consideration for the presence of negative ions and the criterion for breakdown was then modified to, (AB – EF) > 1 
  • Where A and B are same as before and E and F represent coefficients for negative and positive ion liberation by positive and negative ions.
  • Experimental results showed that values of product EF were close enough to ONE for stainless steel, Al, Cu and electrodes to make this mechanism applicable to voltage above 250 kV .

2. Field Emission Theory Field

  1. Anode Heating Mechanisms
  2. Cathode Heating Mechanisms

1. Anode Heating Mechanisms

Anode Heating Mechanisms
Figure B
  • Assumption : Electrons are produced at small micro – projections on the cathode due to field emission causes bombardment at the anode. This causes local rise in temperature and release gases and vapors into the vacuum gap. These electrons ionize the atoms of the gas and produce positive ions. The positive ions arrive at the cathode, and due to bombardment on the surface secondary electrons are produced.
  • The process goes on until a sufficient number of electrons are produced .These electrons give rise to breakdown (as in case of low pressure Townsend type gas discharge).

2. Cathode Heating Mechanisms :

Cathode Heating Mechanisms
Figure C
  • Assumption : In the span of the breakdown voltage of gap, sharp points on the cathode surface are responsible for the pre-breakdown current. These currents are generated as per field emission process described below. The currents cause resistive heating at the tip of a point and when it reaches critical value, the tip melts and explodes, thus vacuum discharge is initiated.
  • This mechanism is called Field Emission as shown in Figure C. The initiation of breakdown thus depends on condition and properties of the cathode surface. Experimental results show that breakdown takes place when effective cathode electric field is of order of 106 to 107 V/cm.

3. Clump Theory Mechanism

  • Cranberg was the first to propose this theory. This theory has been developed on following assumptions :

  1. A clump (loosely bound particle) exists on one of the electrode surfaces.
  2. On application of HV, this particle is charged and subsequently gets detached from mother electrode Then it accelerates across the gap.
  3. The breakdown occurs because of a discharge in the vapour or gas released by the impact of the particle at the target electrode.
Clump Mechanism
Figure D

  • Cranberg initially made an assumption that breakdown takes place when the energy per unit area; delivered to the target electrode by a clump exceeds a value C, a constant, characteristic of given pair of electrodes. The quantity W = product of gap voltage (V) x charge density on the clump.
  • The charge density is proportional to the electric field E at the electrode of origin.

The criterion for breakdown is || VE – C’ ||

In case of parallel plane electrodes the field,
E = V / d, Where
d is distance between electrodes.
So , generalized criterion for breakdown is
|| V = V√cd ||
Where C is another constant involving C and electrode surface condition.
  • He stated that origin of clump was cathode and obtained a value for the constant C as 60 x 1010
    V2/cm (for iron particles).
  • The equation was modified as ||V = dα|| where α – 0.2 and 1.2 depending on gap length and electrode matter with a maximum at 0.6.After a single breakdown craters were observed on anode and melted regions on cathode.

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