This is an OK place to put this - we don't have a chemistry section because I'm afraid it would devolve into too much info about one of my hobbies - high explosives, and get us in trouble.
I have the licenses and approvals from BATF but...I'd like to keep them too.
Though an electro-chemistry thing might be good as we do a lot of that too, for physics. Plating and electro polishing are two things we do a lot of here, I'll see where that would fit.
Some things will just never fit into a simple category, and this might be one of those.
And an X ray tube is, after all, an electron accelerator. Looks like one with good focus too - I'd like to see the other end of that to see the design, as a side issue.
Lessee -- some of this would be guessing, so take it with a
grain of salt, OK?
Yes, some Al has Mn and Si and other junk (Cu) in the alloy, but I sort of doubt that's the purple. More likely it's an interference color from the different index of refraction of salt vs 1.0 and a particular layer thickness showing that. You might have created a vapor deposition device by accident. Likely.
6 watts in that tiny focus is one heck of an energy density to handle -- John F was getting a silicon substrate to glowing heat with a beam of only 15w and liquid nitrogen cooling on the backside in his recent movie here, which I can't find now - he might have removed it. Salt has lousy conductivity (both kinds) and that's going to be a problem. If you have a thick layer, it's worse for a couple reasons. One is that it's going to act like a little capacitor, charge up, then arc through -- and salt goes blooie. The other is that it's going to melt/vaporize and same result. A little more info on why you're using it might suggest a better compound to work with there. You might have to go to "thin target" in the sense that some electrons will make it through to the substrate and make X rays from that too - but all spread out in energy if the salt is any thickness (energy straggling in the electrons that do make it through) -- it's a hard problem! You might reduce this problem by going to a pulsed mode so it can re-freeze and lose its surface charge more gracefully. Again, a guess, but one based on a little bit of experience (but not tons).
You might be better off with a deliberate vapor depostion of salt, real thin, and perhaps on a different substrate, depending on which other X ray lines would or wouldn't pollute what you're trying to measure here. And/or a different target composition than NaCl. There are some compounds of either Na or Cl that might work better, depending one what you're after with this.
You might also, with that nice focus, put the salt into a cavity with a tiny hole to let the beam in, which would let you hammer the salt in bulk, but keep most of it inside the cavity. The fact that it's going to get real hot might make for line-spread (doppler) but...I don't know enough about what you're doing to say. If the cavity was something that let out the X rays well (like Be) then it might work out.
Remember, a 5w resistor is big and even in air it gets pretty hot at 5w -- here, well, it's not as good a thermal situation. Can you observe the target when in use and see if you don't have some incandescence on it? You should see that at the melting point of salt (about 800c). At that temperature, it's going to surely react with the Al and any alloying components. Salt is only held together by bonds of a few eV...and both sodium and chlorine will react nicely with Al at temperature.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.