What is a Klystron Tube?
A Klystron (also known as a Klystron Tube or Klystron Amplifier) is a vacuum tube that is used to oscillate and amplify microwave frequency signals. It was invented by American electrical engineers Russell and Sigurd Varian.
A klystron uses the kinetic energy of an electron beam. Generally, Low-power klystrons are used as oscillators and high-power klystrons are used as output tubes in UHF.
There are two configurations for a low-powered klystron. One is a low-power microwave oscillator (Reflex Klystron) and the second is a low-power microwave amplifier (Two Cavity Klystron or Multi Cavity Klystron).
What is a Reflex Klystron Oscillator?
Before answering this question, we need to know how the oscillations are generated. To generate the oscillations, we need to give positive feedback from the output to input. With constraint that the loop gain is unity.
For a klystron, the oscillations will generate if a part of the output is used as feedback to the input cavity and keep the loop gain magnitude unity. The phase shift of the feedback path is one cycle (2π) or multiple cycles (multiple of 2π).
Construction of Reflex Klystron
The electron beam is injected from the cathode. Then there is an anode, known as focusing anode or accelerating anode. This anode is used to narrow down the electron beam. The anode is connected with the positive polarity of the DC voltage source.
The reflex klystron has only one cavity, which is placed next to the anode. This cavity works as a buncher cavity for forward-moving electrons and catcher cavity for backward moving electrons.
The velocity and current modulation take place in the cavity gap. The gap is equal to the distance ‘d’.
The repeller plate is connected with the negative polarity of the voltage source Vr.
Working Principle of Reflex Klystron
Reflex Klystron works on the principle of velocity and current modulation.
The electron beam is injected from the cathode. The electron beam passes through the accelerating anode. The electron moves in the tube with uniform velocity until it reaches the cavity.
The velocity of electrons is modulated in the cavity gap and these electrons try to reach the repeller.
The repeller is connected with the negative polarity of a voltage source. Hence, because of the same polarity, it opposes the force of electrons.
The kinetic energy of electrons decreases in the repeller space and at some point, it will be zero. After that, the electron pulls back to the cavity. And in the return journey, all electrons bunched at the one point.
There will be current modulation due to the bunch formation. The energy of electrons are converted in the form of RF and RF output is taken from the cavity. For maximum efficiency of the klystron, the bunching of the electron must take place in the center of the cavity gap.
How do electrons moves in the klystron tube?
From the electron gun (cathode), the electron beam is injected in the tube. These electrons move towards the anode with uniform velocity. Then electrons pass through the cavity gap. The velocity of electrons varies according to the cavity gap voltage.
If the cavity gap voltage is positive, the electron will be accelerated and if the cavity gap voltage is negative, the electron will be decelerated. If the voltage is zero, the velocity of electrons will not change.
When electrons leave from the cavity gap, all electrons have different velocities and these electrons will travel in the repeller space.
These electrons travel the distance according to the velocity. Higher the velocity, the electron will travel more distance and lower the velocity, the electron will travel less distance in the repeller space.
All these electrons will return to the cavity and bunched at the center of the cavity gap. The energy of electrons transferred from the cavity is known as the RF output.
Apple-gate diagram is a graph between the distance from the cavity gap and the time taken by the electron in the repeller space.
Different electrons follow different paths depending upon their velocities. The velocity of electrons depends on the cavity gap voltage.
Let’s take the example of three electrons. The reference electron (e0) enters the cavity gap when the cavity gap voltage is zero. Hence, the velocity will not change. It travels L0 distance in the repeller space and pulls back to the cavity. Because of the repeller plate is highly negative and it will oppose the kinetic energy of an electron.
The electron enters before e0, this electron is known as an early electron (ee). This electron enters the cavity gap when the cavity gap voltage is positive. Therefore, the electron velocity will increase. It will travel Le distance and pull back to the cavity.
The electron enters after e0, this electron is known as a late electron (el). This electron enters the cavity gap when the cavity gap voltage is negative. Therefore, the electron velocity will decrease. It will travel Ll distance and pull back to the cavity.
The graph below helps explain this process:
The total time needs from the cavity gap to repeller space and repeller space to cavity gap are the same for all electrons. And that time is Td.
At time Td all electrons bunched at the center of cavity gap. And this is how the bunching process occurs in the klystron tube.
Applications of Reflex Klystron
The applications of a Reflex Klystron include:
- Radio and RADAR receiver
- A signal source in microwave generators
- Frequency modulated oscillator in portable microwave links
- Pump oscillator for parametric amplifiers
- Local oscillator in microwave receivers
Two Cavity Klystron
The working principle of Two Cavity Klystron is the same as the Reflex Klystron. The construction diagram of two-cavity klystron is as shown in the below figure.
As the name suggests, there are two cavities; the first cavity is a buncher cavity or input cavity and another cavity is the catcher cavity or output cavity. Electrons injected from the cathode and reach with uniform velocity till the buncher cavity.
The input RF signal is given at the input cavity and output signal collected from the output cavity. There is a gap in both cavities, these gaps are known as microwave interaction region.
In the first cavity, the velocity of electrons modulated by the input RF signal present in the first cavity. This called velocity modulation.
It formulates the bunching of electrons. And passing through the catcher cavity. The current modulation takes place in the catcher cavity.
After passing the second cavity, all electrons lose their kinetic energy to the microwave fields. They will be collected by collectors because of the reduction of velocity.
Klystron vs Magnetron
The Klystron is a vacuum tube which used as an oscillator and amplifier of microwave signals. The magnetron is different from the klystron tube. The magnetron used only as an oscillator.
In a klystron, the electron is injected normally from the cathode. But in the case of magnetron, the electrons are forcefully injected.
In a klystron, the electrons move linearly in the tube and in the magnetron, the electrons follow a spiral path from cathode to anode.
The klystron is used in TV transmitters, RADARs and particle accelerators. It is also used as a high power, narrowband stable amplifier. The magnetron used in microwave ovens, operating at 2.45 GHz. It is also used for RF heating, operating at 900 MHz or 2.45 GHz.
- Frequency of operation: 1 to 200 GHz
- Bandwidth: +/- 30 MHz
- Power Output: 10 mW to 2.5 W
- Practical Efficiency: 10-20%
- Theoretical Efficiency: 22.78%
- Tuning Range: 5 GHz @ 2W, 30 GHz @ 10mW
- Frequency Range: 500 MHz to 12 GHz
- Power: 600W @ 2.45 GHz
- Peak Power: 40MW with DC voltage of 50 kV @10 GHz
- Average Power: 800kW
- Duty Cycle: 0.1%
- Efficiency: 40 to 70 %
Difference Between Two Cavity Klystron and Reflex Klystron
In two-cavity klystron, the buncher cavity and catcher cavity is different. But in reflex klystron, there is only one cavity. And this cavity works as a buncher cavity as well as a catcher cavity.
In two-cavity klystron, the collector used to collect the electron. Whereas in the reflex klystron, the repeller plate used in place of collector and used to repel the electron beam.
The main purpose of Two cavity klystron is to amplify the microwave signal. The main purpose of the reflex klystron is to oscillate the microwave signal.