Points of Discussion
- Introduction to Microwave Tubes and Klystron (What is Klystron?)
- Klystron Amplifier (Operation of Klystron)
- Reflex klystron and Working
- Optical Klystron
- Two Cavity Klystron
- Klystron vs Magnetron
Introduction to Microwave Tubes and Klystron
Microwave Tubes: Microwave tubes are devices which generate microwaves. They are the electron guns which produces linear beam tubes.
Microwave tubes are generally divided into categories on the type of electron beam-field interaction. The types are –
- Linear beam or “O” type
- Crossed-field or “M” type
Linear-beam: In this type of tube, the electron beam traverses through the tube’s length, and it is parallel to the electric field.
Crossed-field: In this type of tube, the focusing field is perpendicular to the accelerating electric field.
Microwave tubes can also be classified into amplifiers or oscillators.
Klystron: Klystron is a type of microwave tubes which can amplify the higher range of frequencies, especially from Radio Frequencies to Ultra High frequencies. Klystrons can also be used as Oscillator.
In an amplifier, the electron beam is sent through two or more resonant cavities. The very first cavity receives the RF input and bunches it into high- and low-density regions to modulate the signal. The bunched beam then goes to the next cavity, which accentuates the bunching effect. In the following or final cavity, the RF’s power is extracted at a highly amplified level.
The two cavities generate about 20 dB of gain, and using four cavities may produce up to 80-90 dB of gain. Klystron amplifiers can peak powers in the range of megawatt. It has power conversion efficiencies of about 30% to 50%.
Operation of Klystron Amplifier
Klystron amplifiers amplify the Rf signal. It converts the kinetic energy of the signal in a DC electron beam into the RF power. Inside a vacuum, an electron gun emits a beam of electrons, and the high-voltage electrodes accelerate the electron beam.
Then, an input cavity resonator accepts the beam. Here some series of operation occurs. At first, the input cavity is fed with RF energy. It creates standing waves. The standing wave further produces oscillating voltages which function on the beam of an electron. The electric field bunched the electrons.
Every bunch enters into the output cavity when the electric field decelerates the beam by opposing the electron’s motion. That is how the conversion of kinetic energy to the potential energy of the electrons occurs.
Reflex klystron and Working of Reflex klystron
Reflex Klystron: Reflex klystron is a klystron with a single-cavity which acts as an oscillator by using a reflector electrode next to the cavity to deliver positive feedback through the electron beam. Reflex klystrons can be tuned mechanically to adjust the cavity size.
A reflex klystron is often called “Sutton Tube” after the name of scientist Robert Sutton, one of the Reflex klystron inventors. It is a low power klystron with applications as a local oscillator in some of the radar receivers.
Working of Reflex Klystron
In a reflex klystron, the electron beam is passed through the only cavity present in the klystron. After the pass, they get reflected by a reflector of a negatively charged electrode. They make another pass through the cavity. Then they are collected. When the electron beam has their first pass, they are velocity modulated. The electron bunches are formed inside the drift space of the reflector electrode and the cavity.
The reflector voltage is tuned to ensure the maximum branching. The electron beam gets reflected by the reflector and re-enters into the cavity. The maximum branching makes sure that the maximum amount of energy is transferred from the beam of an electron to the radio frequency oscillation. The electronic range of tuning of a reflex klystron is typically referred to as the change in frequency between two half PowerPoint.
Applications of Reflex Klystron
Some of the reflex klystrons are listed below.
- One of the significant applications of reflex klystrons is in Radio and RADAR systems as the receivers.
- They are also used as signal generators.
- Reflex klystrons can be used as Frequency modulators.
- Also, they can be used as pump oscillator and local oscillators.
Nowadays, most of the applications of reflex klystron has been replaced by semiconductor technologies.
Gyroklystron is one of the types of microwave amplifier whose working is almost the same as of a klystron. But for a Gyroklystron, unlike a klystron, the bunching of an electron is not axial. Instead, the modulation forces change the cyclotron frequency, and thus the azimuthal part of the motion creates the phase branching.
At the last or the output cavity, the received electrons transfer their energies to the cavity electric field, and the amplified RF signal can be coupled off from the cavity. The cavity structure of a Gyroklystron is cylindrical or coaxial. The main advantage of a Gyroklystron over a normal klystron is that a Gyroklystron is capable of delivering high power at high frequencies which is very difficult for a typical klystron.
Optical klystrons are the devices where the method of amplification inside is the same as of a klystron. The experiments are done primarily on lasers at optical frequencies, and they are known as Free Electron Laser. These types of devices use ‘undulators’ in the place of microwave cavities.
Two Cavity Klystron
Two cavity klystron is the simplest type of klystron available. As the name suggests, this type of klystron has two microwave cavities. They are known as ‘catcher’ and ‘buncher’. If the two cavities klystron is used as an amplifier, the buncher receives the weak microwave signal and couples out from the catcher, and it gets amplified.
Working of a Two Cavity Klystron
In this klystron, there is an electron gun which generates electrons. An anode is placed at a certain distance from them. Electron gets attracted by the anode and passes through them with high positive potential. An external magnetic field, outside the tubes, produces a longitudinal magnetic field along the beam axis. It helps to stop the beam from the spreading.
The electron beam first goes through the ‘buncher’ cavity. There are grids on both sides of the cavity. The electron beam produces excitation to the standing wave oscillations, which further causes an oscillating AC potential across the grids. The field’s direction varies two times for a single cycle. Electrons enter the cavity when the entrance grid is negative and exits when the exit grid is positive. The field affects the motion as it accelerates them. After the change of direction of the field, the motion of the electrons gets decelerated.
After the ‘buncher’ cavity there, coms the drift space’. The bunching of electrons occurs here as the accelerated electrons get bunched with the decelerated electrons. The length is made precisely so that the maximum branching occurs.
Then comes the ‘catcher’ cavity. It has similar grids on each side. The grids
absorbs the energy from the electron beams. Like the ‘buncher’ here, the electron moves due to the electric field’s change of direction and thus the electrons work. Here the kinetic energy produced by their movement is converted into potential energies. The amplitude of the oscillating electric field is increased to do so. That is how the signal of the ‘buncher’ cavity is get amplified in the ‘catcher’ cavity. Specified types of waveguides and transmission lines are used to couple out from the catcher cavity.
Klystron vs Magnetron (Difference between the Klystron and Magnetron)
To find out the differences between the Klystron and Magnetron, we have to know about the Magnetron.
Magnetron: Magnetron is a type of vacuum tube which generates signals of the microwave frequency range, with the help of interactions of a magnetic field and electron beams.
|Points of Discussion||Klystron||Magnetron|
|Definition||Klystron is a type of microwave tubes which can amplify the higher range of frequencies, especially from Radio Frequencies to Ultra High frequencies.||The magnetron is a type of vacuum tube which generates signals of the microwave frequency range, with the help of interactions of a magnetic field and electron beams.|
|Frequency of operation||The operating frequency range for Klystron is 1 GHz to 25 GHz.||Working frequency range is 500 MHz to 12 GHz.|
|Efficiency||The efficiency is around 10% to 20%.||The efficiency of the magnetron is relatively high, and it is around 40% to 70%.|
|Output Power||Output power ranges between 1 milli-watt to 2.5 watts.||Output power ranges between 2 mW to 250kW.|
|Injection of Electrons||Electrons usually are injected from outside.||Electrons are injected forcefully from the outside.|
|Traversing path of the Electrons||Electrons traverse linearly along the axis.||Electrons traverse spirally along the axis.|
|Usability||Can be used as an amplifier as well as an oscillator.||Can be used as Oscillator only.|
|Applications||Klystrons are used in RADARS, like particle accelerators, transmitters, etc.||Magnetrons are used in many types of home appliances, including microwave ovens, special heaters.|
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