limits of laser ablation propulsion and surrounding questions

[gpeabody]

Hi folks,

I've been learning a bit about laser ablation propulsion, and I'm curious about the potential limits of it. My interest in laser ablation is quite new, so forgive me if my understanding is quite basic. To summarize, as I understand it, laser ablation propulsion is basically using intense light to heat up a medium and turn it into a hot gas or plasma, and extracting thrust as this heated medium flows away. Lab tests show Isp values ranging all the way up to ~10^7 s.

(see fig 6: https://www.psi.ch/lmx-interfaces/BooksEN/Claude_JPP_2010-1.pdf)

One thing I've noticed in my reading of the literature is that all of the laser ablation tests seem to be done with *extremely* intense laser pulses (>MW/cm^2 but lasting only for a short pulse), and laser wavelengths that are relatively long (photons with energies below the ionization energy). I believe this is sometimes called "multiphoton" ionization, and is due to the intense electric field from a coherent laser beam overwhelming the potential barrier that electrons see in an atom.

So my main question is, if you had access to lasers that operate at short wavelengths (above the ionization energy of the ablation medium), but with perhaps lower intensities, would laser ablation work similarly? Could you still in theory achieve the very high Isp's that's been reported with multiphoton ionization? I realize that the plasma reaction in laser ablation can be very complicated, and the strong electric field inherent in multiphoton ionization might play an important role...

For context, I'm thinking about this because I'm wondering if it might be possible and useful to do away with the laser entirely in some cases. For instance, preparing a spacecraft with a tank-full of ablation material, sending it on a highly elliptical orbit to get within fractions of an AU of the sun while protected by a sunshade, and then on closest approach removing the sunshade and exposing the ablation engine and letting the sun's light provide the ablation energy. At some level the simple act of sublimating and heating up an ablation material will provide *some* thrust, but would you be able to get any kind of impressive Isp with the correct material? Anything worth using for a quick interplanetary journey?

I realize this is different in a number of ways from typical laser ablation. The light power density is *much* lower (1.4 kW/cm^2 at 0.01 AU, and that's pushing the realistic closest approach), and the light waves aren't coherent, but the Sun does have a decent chunk of its spectrum in the UV which could provide classic photoelectric ionization.

Anyway, if you have any thoughts or discussion I would certainly be interested in hearing them.