josephkk writes:

Since then, I hear that high power klystrons have been used as
amplifiers in radar systems, and I believe there's a still newer tube
type which is used in the latest gear.

I believe you are talking about a thing called a gyrotron. Useful at 100
GHz to 300 GHz in different models and uses.

Could be; the word seems vaguely familiar. But I'm surprised about the
frequencies. Had thought it was for much lower frequencies than that.
300 GHz sounds as though it's well on the way towards infra-red frequencies.

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