Derive the velocity modulation process in two-cavity klystron amplifier. Also find its efficiency.
Answers
Introduction to the Two-Cavity Klystron Amplifier
one year ago by Mark Mitchell
Klystron amplifiers are used in a variety of industries, including satellite systems, television broadcasting, radar, particle accelerators, and in the medical field. In this article, we'll learn about the two-cavity klystron's unique build and the concept of electron bunching.
Klystron amplifiers are used in a variety of industries, including satellite systems, television broadcasting, radar, particle accelerators, and in the medical field. In this article, we'll learn about the two-cavity klystron's unique build and the concept of electron bunching.
The klystron is a device for amplifying microwave frequency signals that achieve high levels of power gain by applying vacuum tube principles and the concept of “electron bunching”. Klystrons are used in satellite systems, television broadcast, and radar, as well as particle accelerators and medicine.
The klystron was invented by the brothers Russell and Sigurd Varian at Stanford University. Their prototype was completed and demonstrated successfully on August 30, 1937.
Klystrons can be used in the UHF region (300 MHz to 3 GHz) up to 400 GHz. There are several flavors of klystron amplifiers. One major type is the reflex klystron, which is used primarily as an oscillator.
For this article, however, we will focus on another popular type: the two-cavity klystron.
Principles of Two-Cavity Klystrons
Two-Cavity Klystron Geometry
The two-cavity klystron utilizes an electron source (heater), an anode, and a cathode like a conventional vacuum tube. It also utilizes a collector element at the end of the electron stream. The heater boils off electrons when heated and the electrons are ejected from the cathode and accelerate towards the anode due to the high dc potential between the two elements. A focused beam of electrons is thus produced.
In the case of the two-cavity klystron, the electron beam passes through a central hole in the first toroid-shaped cavity and through a similar second cavity, terminating at the collector.
On each side of the cavity hole is a grid that the electrons pass through. It is the interaction of the cavities with the beam that provides the high levels of amplification that the device can produce.
Figure 2. Layout of klystron tube
Cavity
Perhaps we can digress a moment to discuss the cavity used in the buncher and the catcher. The cavity in this story is a toroid-shaped object with the following cross section:
Figure 3. 3a) Resonant cavity; 3b) Equivalent in pseudo electrical form; 3c) Equivalent circuit; 3d) Frequency response.
This can also be shown as a resonant tank circuit with the parallel region the capacitor and the circular part a single turn inductor as shown in Figure 2b and 2c.
The cavity can be made to resonate at a narrow frequency range (Figure 2d), defined by its geometry, of course. The central part of the structure acts like a capacitor with a hole in it which is where the electron beam can pass through. This capacitor and thus the charge applied to anything passing through the central hole will flip charge at the resonant frequency.
From an electrical perspective, the capacitance and inductance define the electrical resonant frequency of the structure. An exciting signal is fed into the resonator externally via a coax connection shown at the top of Figure 2a. This coax connection excites the cavity at the resonant frequency.
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