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Two Cavity Klystrons

  1. Jun 2, 2014 #1
    I am reading Two cavity Klystrons but not able to understand at all. I have a lot of questions about its working.

    Q1. Firstly, as we know that Klystrons are used as amplifiers for microwave frequency and conventional vaccum tubes can't be used due to interelectrode capacitance and lead inductance effect,but though microwave signals can be amplified by Klystrons is there any limitation to the Amplitude of the input signal to the Buncher Cavity?

    Q2.An electron beam is generated due to DC voltage and this beam gets velocity modulated in buncher cavity due to which bunching takes place and energy is supplied to electron in buncher cavity by the input Microwave signal in terms of velocity(i hope so as kinetic energy) and this energy is lost in catcher cavity , but why is it lost there ? I mean how and why does it happens?
  2. jcsd
  3. Jun 2, 2014 #2


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    "Lost " in this context is not a good word

    The energy in the modulated electron beam is absorbed by the pickup loop in the catcher cavity and "sent" to the RF output connector

  4. Jun 3, 2014 #3


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    I am assuming that you are referring to the sort of two cavity klystron that is used in the sound carrier amplifiers for UHF TV etc.. (I believe there are a number of different klystron designs and this is the type I worked on - way back.)
    The KE supplied to the beam (velocity mod, due to the alternating PD across the input cavity gap), translates into PE (density mod / bunching) during the drift. The input cavity is designed and tuned to produce a large RF voltage across the gap to maximise the velocity mod). The secret is to have the drift length between cavities just right or the beam will become velocity modulated again. I think you can look upon the different (velocity / density) modulations are equivalent to different impedances as the waves move down the tube.
    For energy (power, actually) to couple off the density modulated beam into the output cavity, there needs to be resonance. The electron density fluctuations induce a (magnified) alternating potential across the output gap and what happens, basically, is that the RF energy on the beam is matched (as with a resonant transformer) to the output loop in the cavity and into the feeder to the Load / Antenna. The impedance presented across the gap is made 'just right' so that the RF beam power is 'stripped off'. Without the output cavity being matched, the RF energy will just dissipate in the collector.
    Imo it's a really smart system and, although it's not a very linear amplifier (good for FM sound!), it can have a very high gain for a thermionic power device with not too poor efficiency (70% is not bad).
    Bandwidth is not too good but extra cavities (staggered tuning) can improve that. The way the parasitic cavities work is a bit more difficult to understand with waving arms as they're dealing with a beam that has a mixture of velocity and density modulation - brain hurts now.

    The Maths of what happens to turn the Velocity Mod to Density Mod is lumpy and the solutions give the output powers of the fundamental and higher harmonics are Bessel Functions. (I haven't touched this stuff since the 70s but it did leave a lasting impression on me!
  5. Jun 4, 2014 #4
    Does this mean that the electron bunches entering catcher cavity which induces alternating field, enters the catcher cavity for the cycle which opposes the motion of these bunches due to which these electron bunches loses kinetic energy since velocity decreases and the lost energy is gained by the alternating field which has the same frequency as that of the input RF signal(since it is induced by bunches of electrons). Now this RF signal(Catcher cavity) has a very large energy as compared to RF energy at buncher cavity and since this signal have a frequency equal to resonant frequency of the cavity at which its impedance is small the RF energy can be coupled out if the load is matched to the cavity.
  6. Jun 4, 2014 #5


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    I, too, find it hard to figure what is happening to the energy during the process. The share of power (RF / DC) changes as with any amplifier. With a triode, there is straight density mod by the grid and the beam power increases as it travels to the Anode. I varies at the (low potential) grid and VI increases on the way.

    In a Klystron, the DC power is already on the beam and the current variation increases during the drift. Your description of the effect of the induced PD across the output gap seems a good one. The cavity resonance presents the beam with a load on which the bunches do work. So that couples power where it's needed. If ever a tuned circuit or even a synchronous motor takes power into a load, the same thing is at work. It's all down to the relative phases between source and load, to decide which is, in fact, which. Those two cavities work in a totally opposite fashion- which allows one to put power in and the other to take power out.
    Sorry if that all rambles a bit but my message is valid I think.
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