RF tube with photocathode, photoinjector

[artis]

https://dspace.mit.edu/bitstream/handle/1721.1/95319/97ja001_full.pdf;sequence=1

In most (all?) RF tubes from klystron to magnetron to TWT and I assume also the gyrotron they use a DC accelerating field and a thermionic cathode with constant emission. The RF signal then gets introduced and it interacts with the beam creating bunching and subsequent RF energy amplification in the output cavities.

Have there been attempts like in the link I posted at creating a photocathode for a klystron or other RF tubes in general?
From what I know it seems to me that a photocathode would be superior in that the input signal would be applied to the cathode instead of having a input cavity. The RF simply modulates the light source that shines onto the cathode in the presence of an DC electric field.
The added bonus that the cathode itself doesn't take energy to heat but only as much energy as the light source requires.

So I think I'm asking can you create a powerful RF amplifier tube with a somewhat similar structure to that of a photomultiplier tube whereby you have a photoinjector cathode , some dynodes for electron multiplication and eventually an output either via a cavity or by other means?

 

[Klystron]

Hi Artis, thanks for posting this paper.

Studying a 17 Ghz klystron requires adaptation in my understanding. Read about, yes. UV lasers, yes. Have seen Bragg filters used to restrict cavity resonance but MIT seems to have reduced impedance matching issues. Forty meter run between devices impressive. I shall try to finish paper today and respond about your ideas.

 

[artis]

Hi Artis, thanks for posting this paper.

Studying a 17 Ghz klystron requires adaptation in my understanding. Read about, yes. UV lasers, yes. Have seen Bragg filters used to restrict cavity resonance but MIT seems to have reduced impedance matching issues. Forty meter run between devices impressive. I shall try to finish paper today and respond about your ideas.

Hey, glad you found it interesting, so did I.
Well the paper was meant more of a example rather than the specific method in mind. The difference between what they made in the paper and what I'm asking is that they used a photocathode but combined that with a RF cavity and used the gyrotron for excitation if I understood correctly. My idea was to do without a "input" cavity and simply have a photocathode electron source with some current amplification down the line which can then be used either directly with an output cavity as is done in a klystron or in some other way.

 

[tech99]

Photo emission is very weak and cannot provide sufficient beam current for a power tube.
In addition to velocity modulated tubes, such as the klystron and TWT, we can of course use a vacuum tube as a straightforward amplifier at lower frequencies. There was also a high gain electron multiplier tube with a thermionic cathode during WW2 but it fell out of favour.

 

[artis]

Photo emission is very weak and cannot provide sufficient beam current for a power tube.

Yes, but the beam current can be amplified in steps as is done in a photomultiplier tube, or would it still be too small for a power tube , or possibly introduce some secondary effects that would diminish the quality of the beam and therefore the output signal?

 

[tech99]

Yes, but the beam current can be amplified in steps as is done in a photomultiplier tube, or would it still be too small for a power tube , or possibly introduce some secondary effects that would diminish the quality of the beam and therefore the output signal?

You are quite correct that the dynodes contribute to beam current, but it has never been used for a power device to my knowledge. I think secondary emission involves a lot of heat dissipation, so maybe it is inefficient for high power operation, or maybe there is too much heating of the dynodes.

 

[artis]

I shall try to finish paper today and respond about your ideas.

Hey Klystron, so have you had the time to look at the paper, any thoughts about it?

 

[Klystron]

Hey Klystron, so have you had the time to look at the paper, any thoughts about it?

HI Artis, read the paper and each comment. Coming from a background of air traffic control ATC and firecontrol AAA radars, the power levels, frequencies, and synchronization intervals in this paper astound me. I understand the laser provides tighter pulse edges and improved sync.

In order to synchronize these time scales, the laser is used as the master trigger. The laser produces two timing signals, lamp and Q-switch.

If that is your goal, the results appear promising. If your idea using photomultipliers is to augment peak output power aside from crisper faster synchronization, that premise remains unclear from these results.

My principal area in RF focused on receivers, local oscillators and system synchronization. Most everyone else wanted to work on transmitter designs. Receivers take barely detectible pulse returns and clean and amplify the signal portion. I have used masers to amplify weak signals including routing return frequencies back to the transmitter section to compensate for frequency drift. IOW maximizing output into useful band.

The experiments in this paper seem similar but using lasers at much higher frequencies to match the overall higher frequencies of their rig. Judging from the distribution of the components, timing must be a major concern. At those pulse widths and distance between components, speed of light lag becomes a factor in synchronization.

Addendum: outside my field, so I searched PF threads for 'Q switch' in the title receiving back much potential information.