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Travelling Wave Tube Amplifier

Travelling Wave Tube Amplifier

Travelling Wave Tube Amplifier

Travelling wave tube amplifier is basically an amplifier, which makes use of a distributed interaction between travelling wave and electron beam. It is necessary for interaction that they are both travelling in the same direction with the same velocity. The interaction between the RF field and moving electrons will tack place only when the velocity of RF fields is retarded by some means. This is achieved by the slow-wave structure.

The invention of Travelling Wave Tube

The TWTA was invented in 1944 by Kompfner, when he felt that in two-cavity klystron, full energy of the electron is not getting transferred to the microwave signal in the cavity for amplification, due to interaction in electron beam and RF field being only in the cavity. In TWT the cavity is not there, and continuous interaction between electron beam and RF field is there, by making their velocities some. This is by slowing down RF wave field velocity from c to cp/πD, in the axial direction, by making it to pass through a helical path. The microwave RF signal is pumped through coaxial cable, with its central cable connected to the helical wire through which the current moves with the velocity of light (c).

This current causes an RF field inside the helical wire, and an electron beam is passed through the axis of the helix. This electron beam interacts continuously over the length of the helix (around 12″ or so, instead of just 1″ or so within a resonant cavity as in two-cavity klystron) and transfers energy to the RF field and hence to the beam current of helix. This causes amplification of RF signal when it reaches the other end of output. From the figure, we see that if the electron beam and RF field wave of helix both reach A to B together, then time of movement of both is same (tp = tbeam)

where  p = pitch, i.e., distance between two rings of helix,

D = its diameter, Φ = pitch angle,

c = velocity of light = 3 x 10 cm/s, and  πD >> p.

The phase of velocity of the wave vp is the velocity of electricity beam vbeam that is

Construction of TWT

Image of Travelling Wave Tube Amplifier
Travelling Wave Tube Amplifier

The Physical construction of a TWT is given in Figure. The electron gun is just like that in klystron, and the electron beam is regulated by a control anode so as to pass through the centre of the long helix. An axial magnetic focusing field prevents the beam from spreading and guides it through the centre of the helix. Finally, the electrons are collected by the concave collector plate. When the RF signal propagates through the wire of the helix, it produces an RF electric field along the centre of the helix. When the velocity of the electron beam is close to the velocity of this axial RF field, then due to the interaction between them, the electron beam delivers energy to the RF wave of the helix.

This leads to more and more amplification of the RF field and the wave of the helix, as it keeps moving along its length, with the axial velocity of VP = pc/(πD). This phase velocity is helix geometry dependent and therefore a constant. Therefore, a TWT can be used over a wide range of frequencies.

Velocity modulation and bunching of electron beam take place along the axis of the helix. Those electrons which move along with the positive cycle of the RF field get accelerated, while those electrons moving along the negative cycle of the RF field get decelerated. Thus, some electrons are moving slower, while the electron behind might be faster, and this leads to some of them catching them up, causing bunch formation. When this bunch encounters the retarding field, it delivers energy to the wave resulting in amplification.

Velocity modulation and bunching of TWT
Velocity modulation and bunching

Here, the RF field causes velocity modulation, which in turn amplifies the RF field/signal, leading to regenerative amplification of each other, as we move along the axis. For better operation, electron beam velocity v0 is kept slightly greater than the RF field wave velocity vp, as more electrons face the decelerating field and give energy to the field (i.e., its amplification).

Operation of TWT

The applied input (RF) signal propagates around the turns of the helix and produces an electric field at the center of the helix. The input signal i.e. the signal to the amplified propagates with the velocity of light as compared to the axial electric field of RF signal. Which travels with velocity of light multiplied by the ratio of helix pitch to helix circumference. Thus helix acts as a slow wave structure, used to slow down, the velocity of RF signal. When the velocities of both the signal, approximately become the same, interaction takes place in such a way that on an average the electron beam delivers energy to the RF wave on the helix.

Thus due to this energy transfer, the RF signal grows and amplified output is available at the TWT output. The axial velocity VP is given as VP = VC (pitch/2πr). Here r is the radius of the helix, which is constant over a range of frequencies. The electrons entering the helix at the accelerating field are accelerated and those at the retarding filed are de-accelerated. As the electrons travel along the helix, they bunch at the collector end. The bunching shifts the phase by π/2. Each electron in the bunch experience a strong retarding field. Then the microwave energy of the electrons is delivered by the electrons bunch to the wave on the helix, and results in amplification of input signal.

Performance Characteristic Range of TWT

  1. Collector beam voltage: 1–10 kV
  2. Beam current: 10–100 mA
  3. Frequency: 5–100 GHz
  4. Cut-off power output: 5 MW (10–40 GHz) to 300 kW (3 GHz)
  5. Efficiency: 5–20%
  6. Bandwidth: Being a non-resonant device, large bandwidth ±30% can be there; e.g., a typical TWTA can give 35 ± 3 dB gain from 2 to 4 GHz.
  7. Life: 50,000 h, much larger than other tubes
  8. Noise: 5 dB (low-power and lower-frequency TWT), 15 dB (high-power and high-frequency TWT)

Application of TWTA

  • In the medium- and high-power satellite transponder, because of its very long life.
  • It is used in wideband communication links.
  • In CW-RADAR and RADAR jamming on land, aeroplane (air), and ship (water).
  • At very higher power and wide tunable [by beam voltage] device.

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