The electric sail, Debye shielding, and the ponderomotive force

[endlesslimitation]

BACKGROUND
The electric sail is a concept for an in-solar-system propulsion system which uses electric fields to capture momentum from the positive ions flowing radially outward in the solar wind (http://www.electric-sailing.fi/paper2.pdf). The simple design is to have long, positively charged wires sticking out, and any solar wind ions that pass by will be deflected or reflected, causing a force on the charged wires. Even though the solar wind pressure is extremely weak, the potential to use very low mass wires to capture a very large solar wind area, make it an interesting idea. The design referenced in the paper above uses 20 micron diameter wire - required to be so thin due to the incredibly low areal mass requirements.



Aside from wire mass, the major limiting factor on the mass to area ratio is the distance over which a charged wire can exert a force. In a normal plasma, free charge carriers will cluster around a point charge, screening out the force it exerts, a phenomenon known as Debye shielding. In this case, electrons from the solar wind will cluster around the positively charged wires and screen out their electric field on distances of order the Debye length, about 10 m in the solar wind. This all got me thinking, could it be possible somehow to expel electrons from the vicinity of the wires, and if so, how does Debye shielding function in the resulting non-neutral plasma?


PONDEROMOTIVE FORCE
What if, in addition to the static positively charged wires, we added an oscillating electric field to create a ponderomotive electric force? Briefly, if you have a point source rapidly oscillating between positive and negative charge states, a nearby test charge will alternately feel pushed away and pulled toward the source. It might seem like it should get pushed and pulled equally and therefore have no net force, but in fact, every time the 'push' portion of the cycle begins the test particle is a bit closer in than it is when the 'pull' portion begins. Therefore, averaged over cycles there is a net force moving the test charge from regions of strong electric field to weak. This is true regardless of the test charge's sign. The ponderomotive force is given by:
Fp=[q² / (4m w²)] grad(E²).
Where q is the particle charge, m is the mass, w is the oscillation angular frequency, and E is the amplitude of the electric field oscillation, and the direction of the force is always from high field to low field. It is worth noting that the m in the denominator makes it much easier to ponderomotively move electrons than ions. Also, unlike a static electric field, the ponderomotive force can penetrate a plasma without Debye shielding as long as it oscillates faster than the screening particles can respond (IE above the plasma frequency ~2e5 sec^-1 for the solar wind).

So how about we set up a grid of oscillating wires which evacuate electrons from a bubble in the vicinity of the wires, and then additionally have a static electric field grid which is no longer diminished by Debye screening.

I'm curious if such a thing could work in theory, or practice. Am I correct that if you evacuated the electrons from around the plane where the static field is generated, then Debye shielding would no longer occur (until you moved outside of that evacuated bubble)? You wouldn't need the bubble to be very thick, the idea would be to give it the same form factor as the electric sail, kilometers wide to present the greatest area to the solar wind but with negligible thickness. Whether it would ever be practical is another question :-p It may seem like we're just adding more wires and therefore mass, but the idea would be that each individual wire is able to access a much greater projected area of solar wind. It might even be possible to forego the physical oscillating grid of wires by brodcasting UHF radio waves in the region.

[edzieba]

The main question would be if the ponderomotive force is effective when you are faced not with a cloud of static (you know what I mean!) electrons, but a stream of electrons blasting towards you at 400km/s. And if your oscillating grid can do that effectively, the ponderomotive force itself is now your sail and the static electric field is basically no longer required.

[endlesslimitation]

I think the bulk 400 km/s flow is actually a minor adjustment. Solar wind electrons have a temperature of about 10 eV, corresponding to an rms thermal velocity of 2600 km/s. As large as that sounds, electrons are still pretty easy to push around - it would only take a 10 volt potential to do so.

if your oscillating grid can do that effectively, the ponderomotive force itself is now your sail and the static electric field is basically no longer required

Unfortunately, the momentum carried by electrons in the solar wind is a factor of m_p/m_e~1800 times smaller than the proton momentum. That's way too weak to make use of with any kind of sail that has finite mass.

[TrevorMonty]

I posted information recently about company working on this technology but can't remember under which thread or company's name.

Thrust is quite high especially conpared to solar sails. Direction can be change by controlling electrical field on different sides of sail. Won't work inside earth's magnetsphere, missions really need to start from earth escape.

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