Testing the basic concept

    To get the for factor as small as possible, the only way to go, as far as I've seen or read is by using a pyroelectric crystal. This has the advantage of taking up a few cubic centimeters of space and uses voltages somewhere between 5 and 12 V. Perfect!

   Ok, so what is a pyroelectric crystlal, I hear you ask? Well, it's a crystal (most common ones are Lithium Niobate or Lithium Tantalate) that when subjected to a temperature differential, charges inside the crystal matrix start wandering about and elctrons wil be emmited from the Z-plane of the crystal matrix. In other words, you  heat up the crystal from the bottom, for example, electrons gather to the top of the crystal and  will be emitted outwards. Without any additional acceleration voltage.

If you don't believe me, (and it's understandable, because I didn't until I tried it) here's a scope shot of a quick experiment I did.

   The set-up is pretty simple. I took a slither of Lithium Niobate crystal, hooked one end to the ground of a scope and I left the the top of it floating near the positive of the scope's input (just a straight BNC cable, attenuation X1).

Whenever the crystal  was exposed toa  temperature differential, electrons started spewing from it and would appear on the scope as soem very narrow pulses. The slither was heated using a regulat heat gun, set to about 100 degrees C. A few 2 second shots of hot air on the crystal generated the scope shot you see.

Now, please take into account that what I've done here is just proof of concept (for myself  at least) to see if I have something tangible here.

  As you can see from the scope shot, a  lot of pulses show up, so tht means that indeed, electrons do indeed escape from the crystal. No, these are not X-rays. At least not yet. These are just run of the mill electrons :D. For the actual X-rays, a few more things are needed.

   Also, bear in mind that electrons are only emitted when the crystal has a tempeerature differential across it. Once the crystal uniformly heats up, there are no more emmisions from it.

   Ok, so now that I have some proof that I'm not just chasing green unicorns, I can get on to bigger and better things.

    Let's get to work on those X-rays

   X-rays are actually generated when an electron collides with another material (i.e. the atoms inside that material). So, basically, if I take the slither of crystal and put it in a medium where electrons can travel unimpeeded (say, in a vacuum) then have those electrons slam into something, X-rays will be genrated.

Granted, those will be  some very low energy X-rays, but hey.... if you want to go handheld, it's better than lugging a whole grandma's basket filled with tubes and tranformers.

What I want to do next  is build a vacuum enclosure, stick the crystal in it , get some vaccum inside it then have a small window where X-rays can escape from.

   I figured that aluminium foil, copper foil and mica are some good candidates for the window material.

I know it doesn't look like much, but hopefully those fittings are going to be my vacuum chamber. Why go for those, instead of a glass envelope? because I wnt to experiment with this, meaning I might want to try different crystal orientations and different window materials, therefore I want something that seals adequately under vacuum and can be easily taken apart. Will it really work ? I don;t have a clue, but I hope it does. All I can do i just hope and speculate, as at the moment I don't have a vacuum pump.

Speaking of vacuum, there seems to be a sweet spot for the vacuum level for this thing. That seems to be somewhere between 1-10 Pa of pressure (or 10^-2 to 10^-3 Torr). I'll try to aim for anything under 5 Pa, but as I'm a noob regarding this kind of stuff, again, I can only hoep for the best right now.

  So, where can one get those crystals?

   The most expensive thing about this build is actually the crystal itself.  Somthing like a Z-cut Lithium Niobate slither 2mm x 2mm x 5mm is somewhere around 200 $. And it's not like you can just walk into your closest store and take one off the shelf.

  So, like any mad scientist, I had to do some research, to see where I can get one for cheap.

I scrounged the net for any information I could find, and only came across 2 other  sources attempting the same thing as me:

1. http://www.rapp-instruments.de/index5.htm

2. https://www.intechopen.com/books/scanning-electron-microscopy/palmtop-epma

 The most important information I came scross was that LiNbO3 or LiTaO3 are, or more accurately were used in some high bandwidth  SAW filters.

With this in hand, I searched the interweb high and low for any reference of a SAW filter that used these materials as a substrate. Also, very few manufacturers actually write in the datasheet the substrate material, so it was a very frustrating search.

  The reason they are not used today as a substrate i SAW filters is exactly the reason I wanted them in the first place: their pyroelectric properties. Meaning that for a few degrees of temperature change, theyradically changed their SAW characteristics.

Asking around on the EEVBlog forum, I got a hit on some old SAW filters. The company that manufactured them was C-Tech and the part number for the one I got was 312991. It was only about 20 $, which is perfect for a proof of concept experiment. They also seem to have a LiTaO3 substrate filter available. Jackpot!

  What I want to do isn't  by any chance new. mptek have the  Cool-X generatos, that works on exactly the same principle. Except they want to charge about 2500 $ for it. Naa, I'll pass.

   That's pretty much it for now. I got a Peltier module with which Im going to heat and cool the crystal and next step should be getting a vacuum pump that can do 5 Pa or less. Some eBay ones are advertised as beig capable of that, but if they really deliver... I have no idea.