Our recent discussion of deep space magsails propelled by neutral particle beams inspired a lot of comments and a round of comment response from author Jim Benford. For those just joining us, Benford had studied a magsail concept developed by Alan Mole and discussed by Dana Andrews, with findings that questioned whether interstellar applications were possible, though in-system work appeared to be. The key issue was the divergence of the beam, sharply reducing its effectiveness at the sail. Today we’ll wrap up the particle beam sail story for now, with Jim’s thoughts on the latest round of comments. The full paper on this work is headed for one of the journals for peer review there and eventual publication. I’ll be revisiting particle beam propulsion this fall, and of course the comments on the current articles remain open.

by James Benford


Eric Hughes wrote in the comments that my work had shown only that one method of neutralizing the neutral particle beam would produce divergence. Specifically, his comment read: “I think it’s important to recall that Benford’s article last Friday only addresses one class of methods for making a neutral particle beam. He acknowledges that himself in the last sentence of the article, when he speaks of “much more advanced beam divergence technology than we have today.”

Are there other methods of producing these beams that don’t produce divergence? Let me re-state my basic argument:

  • Accelerating low-energy particles in electromagnetic fields produces high-energy particle beams.
  • For those electromagnetic fields to interact with the particles, the particles must be charged. Only charged particles interact with electromagnetic fields.
  • Therefore, accelerating charged particles to high-energy to produce the final beam, which is then neutralized, produces neutral beams.
  • I showed that the neutralization process itself would produce an irreducible divergence. This applies to all methods for producing neutral beams.
  • The only possible exception would be to produce high-energy neutral particles by nuclear reactions. But nuclear reactions are not highly directional and won’t produce a narrowly collimated beam.
  • Consequently, the argument I made is quite general and fundamentally limits the properties of neutral beams.

On the other comments, these remarks: James Essig is certainly correct that the Sun provides plenty enough power for thrusters to maintain the Beamer in place. A more demanding problem is how to operate such powerful thrusters while not disturbing the microradian pointing of the beam. The beam has to stay on the sail for a long time and variations in the thrusters’ sideways motion could easily direct it away from the sail.

Electrostatic and magnetic forces never cancel no matter how relativistic the beam is; certainly they are far from cancellation for the example, where gamma is only 1.02.

Eniac hopes that gravity will provide a restoring force to the momentum of the beam generator. No such thing happens. Gravity is an attractive force. There will be a restoring force only in a potential well such as a Lagrange point, but these are noticeably weak and not up to the scale of these forces.

Eniac also writes: “Would the beam be dense enough to tear the field right off the loop and carry it away, leaving the craft behind? Yes, I think moving plasma does wreak havoc on fields that way.”

But the answer is no. The magnetic field won’t depart unless the current leaves the conductor. What does it flow in then?

The transform of the magnetic field to the moving frame of the beam is given by the product of gamma, beta and the field strength. My estimate is that ionization will be easy. Eniac’s 10 GV/m for ionization, when only 13 eV is needed, would mean that there would never be ionization in the universe, so this number is ridiculously far off.

Michael and others seem to think that the charged particles will not interact strongly if they are far apart. But they cannot be far apart and part of a beam going out to hit this 270 m sail. Divergence inevitably follows.