# Lasers for plasma

Recently I had the same idea as given at A question about the properties of plasma and its potential use in recycling, or a similar one, rather. The basic idea is: could you turn stuff into plasma and sort the atoms by element? The answer appears to be roughly: yes, with difficulty. I have a few followup questions. Suppose you used pulsed lasers to focus on a point, to quickly turn that spot into a plasma. (That works, right?) You may suppose the presence of a vacuum, since I suspect that simplifies a number of things.

1. What kind of laser would be sufficient for that?
2. How much energy would be approximately optimal, in how much time and space? (You need a certain minimum energy per cm^2, I think, but my understanding is also that if you simply try to pour more and more energy into a spot at once, the plasma absorbs it and you don't do much more to the remaining material.)
3. How much does the material in question affect the required temperatures? (I haven't been able to find many numbers, such as the minimum plasma temperature of iron.)

Is a laser like this even approximately sufficient, or would it require a much higher powered laser? Ballpark estimates are ok; preferable would be links to commercial lasers that would work.

• There are likely better ion sources than a laser. A variety were developed in the 1940's and 1950's for Calutron-type applications. I suppose with a simple laser-generated plasma one could do a time-of-flight type separation, but that has a lower duty cycle than brute force magnetic separation. – Jon Custer Jul 9 at 21:34
• @JonCuster When you say "ion sources", you mean "ways of turning stuff into (non-molecular) ions"? The Calutron sounds pretty relevant; thanks. Re: magnetic separation, wouldn't that only work with things like iron, and not oxygen or aluminum, etc.? – Erhannis Jul 9 at 21:49
• No - read up on the Calutrons (see Wikipedia). Accelerate charged particles (ions) to a given energy, pass them through a magnetic field, and the separate out by their charge to mass ration ($q/m$). The Calutrons at Oak Ridge separated out the uranium isotopes ion-by-ion to make Little Boy. – Jon Custer Jul 9 at 21:52
• Oh, I see. I looked up "magnetic separation" and got basically "pulling iron out of stuff via magnetic fields". Thanks. – Erhannis Jul 9 at 22:00
• It would probably start burning stuff around the focal point and only turn a small amount into plasma, and the amount of energy lasers require is probably not a good environmental solution for use with recycling. – user47014 Jul 9 at 22:14

For ionization of your material, you need high field strength (high laser intensity). That is because in laser plasmas, usually strong field ionization, where the strong external field facilitate electron quantum tunneling out from the potential well of the atom, is the dominant ionization mechanism. So, as you say, you need to achieve a certain number of W/cm$$^2$$. Without plugging in the numbers in the formula to check, I would suspect that you would want to reach at least about $$10^{14}$$W/cm$$^2$$; you can see this paper (not open access) or this page where they talk about implementation of ionization into their code.

So then, is the laser you suggested enough? With 36 kW, you would need to focus down the laser to a spot size of about $$0.2$$ µm to reach an intensity of $$10^{14}$$W/cm$$^2$$, which is smaller than the wavelength of the laser and hence not possible. It would be more feasible to focus a laser to ~10 µm in size, but you could perhaps get a way with a bit lower laser intensity, which then would require around 100 kW of laser power to reach the desired ionizing field.

You are also right in that you can't just pour in more energy at the same spot if you want to ionize as much material as possible, since the ionization only happens where you point your laser.

As for what temperature you need, most elements, especially metals, will have an ionization energy of the order of 1$$-$$10 eV for the first electron, hydrogen famously has 13.6 eV (but that is rather high, since it only has one electron). Through the use of Boltzmann's constant $$k_B=8.6\times10^{-5}$$ eV/K, the ionization energies translates to temperatures of roughly 100000 K to have a fully ionized plasma (all atoms has lost one or more electrons, which would be required if you wanted to sort them).

By now I also feel compelled to point out (like in the answer to the other question) that this method would not be economical (either monetarily or energywise) for the purposes of recycling. Since it is much easier to move electrons around between atoms (chemistry) than to completely strip them off, chemical recycling is still much more effective than plasma recycling.

• The laser I've seen used for plasma research had 100 GW. Not continuously, but in 1 ns pulses, so each had just about 100 J. It used gas iodine medium and xenon lamps for charging. Each of the four stages had a rack of massive capacitors (I am not sure I remember it right, but might have been 1 mF each and there was tens of them) charged to 5 kV. This power then flashed the lamps and the oscillator was opened shortly to generate the pulse. Charging took several minutes and the medium was replaced for each shot, so it couldn't do more than one pulse per 10 minutes. Totally impractical. – Jan Hudec Jul 13 at 19:14
• Yes, infact I'm involved in some lase plasma research (that does however not mean that you should blindly take my word on whatever I say here), we are however somewhat involved with colaboration with a laser lab (llc.lu.se/research-fields/experimental-facilities/…) which operates at TW laser powers at 150 mJ energy and 10 Hz repetition rate. Still the energy efficiency of converting electricity to laser energy is very low. – Andréas Sundström Jul 13 at 19:25
• So that's even shorter pulses, though, in the fs range, right? I don't think that kind is much more energy efficient though. Basically no laser except semiconductor ones is, and semiconductor lasers don't scale to the required powers. – Jan Hudec Jul 13 at 19:39
• No, the fact that the pulses are in the fs regime does not effect electricity -> laser energy conversion efficiency, but the short pulses might actually somewhat improve the laser -> plasma conversion efficiency since more laser energy might go into ionization of a larger target volume instead of more heating of already created plasma. But the efficiency is only a side point I think. The cool thing is still that we have these kinds of laser systems so readily available. – Andréas Sundström Jul 13 at 19:55
• They are sure cool. But not practical for the purpose suggested in the question. – Jan Hudec Jul 13 at 20:21

EUV light sources for the lithography industry use kW CO2 lasers to turn tiny liquid tin droplets into a hot, highly ionized plasma. This plasma emits a lot of radiation, among which EUV light. This light has a wavelength of only 13.5 nm and an energy per photon of 92 eV/c2. It is collected and sent into a machine for imaging of very small structures used to produce silicon devices. See how this works at https://www.youtube.com/watch?v=iByn8l3f9pE

• Not an ideal answer, certainly - it only really answers half of #1, and no others. However, it's a good lead, at least, so I guess I'll take what I can get. – Erhannis Jul 11 at 15:21
• New answer more fully answers the question. But your answer is still a good example of a relevant technology, so +1. – Erhannis Jul 13 at 5:51
• The tools are operative in major fabs. It works already with a few kW pulsed power. – my2cts Jul 13 at 13:26