Answer:
The power gain of a two cavity klystron amplifier is about 30 dB.
In order to achieve higher overall gain, one way is to connect several two-cavity tubes in cascade, feeding the output to each of the tubes to the input of the following one. Besides using the multistage techniques, the tube manufacturers have designed and produced multi cavity klystron to sere the high-gain requirement.
In a multi cavity klystron each of the intermediate cavities, placed at a distance of the bunching parameter X of 1.841 away from the previous cavity, acts as a buncher with the passing electron beam inducing a more enhanced RF voltage than the previous cavity, which in turn sets up an increased velocity modulation.