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In a Bussard ramjet interstellar gas is collected using a large magnetic field to be fused and used as reaction mass. While a number of papers have explored the constraints on the magnetic field, what kind of fusion reactions would work, and whether drag renders the whole scheme inoperable I have not seen any mentions in the literature of the issue of ionizing the gas. In order to capture gas using the magnetic scoop neutral hydrogen atoms and molecules need to be ionized so the protons follow the field lines into the ramjet.

Fishback 1968 amusingly handwaves away the issue:

we simply shine a beam of light in the forward direction to ionize the medium ahead of the vehicle. This is done with very little cost as it takes only a few electron volts to ionize each atom while we get millions of electron volts from each nucleus that reacts.

This assumes that more than a photon in a million will hit an atom. Given that H$_2$ cross section for UV photons is about $10^{-17}$ cm$^2$ this sounds optimistic.

The ionization probability of a hydrogen atom is $\sim 1$ for 80-90 nm laser light with intensities $10^{11}$ to $10^{12}$ W/cm$^2$ (Brunetti et al. 2015). But shining that intensity ahead would both produce noticeable drag from the laser (!) and waste a lot of power since the mean free photon path would be about 0.1 parsec or so: most ionized atoms would drift far beyond the scoop. If the scoop has radius $r$ and the mean lateral velocity of atoms is $v_{atom}$ we only need to ensure that atoms within a distance $<r(v_{ship}/v_{atom})$ are ionized.

Presumably a pulsed laser would be able to achieve high intensities without wasting significant power. Has this question been studied in the literature?

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  • $\begingroup$ This is one of those cases where you multiply very small and very large numbers together and get surprising results. $\endgroup$ Oct 31, 2020 at 18:44
  • $\begingroup$ This is a hard problem due to nonlinearities; in addition to single photon and multi-photon ionization, at the highest intensities there are non-linear field-ionization effects; the electric field of the laser is so strong (think volts per angstrom) that it can ionize the atom by simply pulling off the electron unrelated to time-domain effects like resonance. $\endgroup$
    – uhoh
    Nov 1, 2020 at 0:07

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