Microwave Engineering Magnetrons - Microwave Engineering

What is Microwave Engineering Magnetrons?

Not like the tubes conversed so far, Magnetrons are the cross-field tubes in which the electric and magnetic fields cross i.e. run at right angles to each other. In TWT, it was detected that electrons when made to interrelate with RF, for a longer time, than in Klystron, caused in higher efficiency. The same technique is followed in Magnetrons.

Types of Magnetrons

There are three key types of Magnetrons.

Negative Resistance Type

  • The negative resistance between two anode segments, is used.
  • They have low efficiency.
  • They are used at low frequencies (< 500 MHz).

Cyclotron Frequency Magnetrons

  • The synchronism between the electric component and oscillating electrons is considered.

  • Useful for frequencies higher than 100MHz.

Travelling Wave or Cavity Type

  • The interaction between electrons and rotating EM field is taken into account.
  • High peak power oscillations are provided.
  • Useful in radar applications.

Cavity Magnetron

The Magnetron is called as Cavity Magnetron for the reason that the anode is prepared into resonant cavities and a permanent magnet is used to produce a strong magnetic field, where the action of both of these makes the device work.

Construction of Cavity Magnetron

A dense cylindrical cathode is existing at the center and a cylindrical block of copper, is fixed axially, which acts as an anode. This anode block is made of an amount of slots that acts as resonant anode cavities.

The gap present among the anode and cathode is called as Interaction space. The electric field is present radially while the magnetic field is present axially in the cavity magnetron. This magnetic field is shaped by a permanent magnet, which is located such that the magnetic lines are similar to cathode and vertical to the electric field existing between the anode and the cathode.

The resulting figures show the constructional particulars of a cavity magnetron and the magnetic lines of flux present, axially.
cavity_magnetron_and_magnetic_lines

8 cavities which are tightly coupled to each other are present in the Cavity Magnetron .An N-cavity magnetron has N modes of operations. These operations relay upon the rate of recurrence and the phase of oscillations. The total phase shift around the ring of these cavity resonators should be 2nπ where n is an integer.

If ϕv represents the relative phase change of the AC electric field across adjacent cavities, then

Equation

This is called as the Zero mode, because there will be no RF electric field between the anode and the cathode. This is also called as Fringing Field and this mode is not used in magnetrons.

Operation of Cavity Magnetron

We have different cases to consider when the Cavity Klystron is under operation. Let us go through them in detail.

Case 1

If the magnetic field is not present, i.e. B = 0, then the manners of electrons can be witnessed in the resulting figure. Bearing in mind an instance, where electron a directly goes to anode under radial electric force.
movement_of_electron_a

Case 2

If there is an growth in the magnetic field, a lateral force acts on the electrons. This can be detected in the resulting figure, bearing in mind electron b which takes a curved path, while both forces are acting on it.

movement_of_electron_b

Radius of this path is calculated as

R=mv/eB

It differs consistently with the velocity of the electron and it is inversely proportional to the magnetic field strength.

Case 3

If the magnetic field B is additionally enlarged, the electron follows a path such as the electron c, just grazing the anode surface and making the anode current zero. This is called as "Critical magnetic field" (Bc)(Bc), which is the cut-off magnetic field. Refer the following figure for better understanding.

movement_of_electron_c

Case 4

If the magnetic field is made greater than the critical field,

B>Bc

Then the electrons follow a path as electron d, where the electron jumps back to the cathode, without going to the anode. This causes "back heating" of the cathode. Refer the resulting figure.

movement_of_electron_d

This is attained by cutting off the electric supply when the oscillation starts. If this is continuous, the emitting competence of the cathode gets affected.

Operation of Cavity Magnetron with Active RF Field

We have talk over so far the operation of cavity magnetron where the RF field is absent in the cavities of the magnetron (static case). Let us now debate its operation when we have an active RF field.

As in TWT, let us take on that initial RF oscillations are present, due to some noise transient. The oscillations are continued by the operation of the device. There are three types of electrons produced in this procedure, whose actions are assumed as electrons a, b and c, in three different cases.

Case 1

When oscillations are existing, an electron a, slows down transferring energy to oscillate. Such electrons that transfer their energy to the oscillations are called as favored electrons. These electrons are responsible for bunching effect.

Case 2

In this case, another electron, say b, takes energy from the oscillations and increases its velocity. As and when this is done,

  • It bends more sharply.
  • It spends little time in interaction space.
  • It returns to the cathode.

These electrons are called as unfavored electrons. They don't participate in the bunching effect. Likewise, these electrons are harmful as they cause "back heating".

Case 3

In this case, electron c, which is emitted a little later, moves faster. It tries to catch up with electron a. The next emitted electron d, tries to step with a. As a result, the favored electrons a, c and d form electron bunches or electron clouds. It called as "Phase focusing effect".
This whole process is understood better by taking a look at the resulting figure.


phase_focusing_effect

Figure A displays the electron actions in different circumstances while figure B shows the electron clouds formed. These electron clouds happen while the device is in operation. The charges existent on the internal surface of these anode segments follow the oscillations in the cavities. This generates an electric field rotating clockwise, which can be really seen while performing a practical experiment.

Even though the electric field is rotating, the magnetic flux lines are designed in parallel to the cathode, under whose combined effect, the electron groups are formed with four spokes, directed in regular intervals, to the nearest positive anode segment, in spiral trajectories.

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