Assertion (A): TWT uses a focussing mechanism to prevent the electron beam from spreading.

Reason (R): In a TWT the electron beam has to travel a much longer distance than in klystron.

A Klystron is a vacuum tube used for generation/amplification of microwaves.

An electron beam is produced by oxide coated indirectly heated cathode and is focussed and accelerated by focussing electrode.

This beam is transmitted through a glass tube. The input cavity where the beam enters the glass tube is called buncher.

As electrons move ahead they see an accelerating field for half cycle and retarding field for the other half cycle.

Therefore, some electrons are accelerated and some are retarded. This process is called velocity modulation.

The velocity modulation causes bunching of electrons. This bunching effect converts velocity modulation into density modulation of beam.

The input is fed at buncher cavity and output is taken at catcher cavity.

In a two cavity klystron only buncher and catcher cavity are used. In multi cavity klystron one or more intermediate cavities are also used.

The features of a multicavity klystron are :

1. Frequency range - 0.25 GHz to 100 GHz

2. Power output - 10 kW to several hundred kW

3. Power gain - 60 dB (nominal value)

4. Efficiency - about 40%.

A multicavity klystron is used in UHF TV transmitters, Radar transmitter and satellite communication.

They are used in all the devices.

and is independent of line length.

Reflection coefficient

Charging current and MVAR are proportional to frequency.

X_C should be at least ten times greater than R. Therefore for 50 ? resistor at 3 GHz, C should be less than 0.1 pF.

In a klystron the resonant structure limits the bandwidth.

A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave.

The velocity of wave propagation along the helix structure is less than velocity of light.

The beam and wave travel along the structure at the same speed.

Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave.

Therefore the signal gets strengthened and amplified output is delivered at the other end of tube.

The main features of TWT are :

1. Frequency range - 0.5 GHz to 90 GHz

2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz

3. Efficiency - about 5 to 20%

4. Noise - about 5 dB for low power TWT 25 dB for high power TWT

TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc.

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