[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: starship-design: Plasma power

From: "L. Parker" <lparker@cacaphony.net>

But thermal motion is very, very
> > slow as seen by
> > a beam of relativistic protons, so even the hot neutral gases
> > won't get far
> > from the central jet target zone without getting bashed.
> Not true, you are considering only thermal MOTION in an normal thermal
> regime. The thermal PRESSURE in a plasma is hundreds of orders of
> magnitude greater. Unless you can find a way to project the magnetic
> confinement beyond the physical scope of your nozzle, you will have to
> deal with thermal absorption in the nozzle itself.

Let's keep pulling ideas apart until we can find out what the objections
are. I used "nozzle" in quotes to refer to the magnetic field structure
which collimates the exiting plasma of the arcjet. Fields aren't damaged by
heat. The physical coil unit producing that field, I don't want heated,
because I would like it to stay at 20 Kelvins if I'm using liquid hydrogen
to keep its superconductivity, or 77 Kelvins if I'm using liquid nitrogen
for that. But I don't have to expose my coil to extreme heat, because a
solenoid coil of any diameter will give the same magnetic result. So I can
make my coil yards wide to reduce its exposure to the hot stuff, which the
magnetic field forming the pinch, or "nozzle", still squeezes just as hard.
Also in practice, I would use a thermal shield which doesn't block the
magnetism, to keep radiant heat off my superconducting coil.

Current designs
> such as VASIMR rely on extremely high density magnetic fields in the
> nozzle to keep the plasma from coming into contact with the nozzle,
> but radiated energy is still a problem. At these performance levels,
> radiated heat absorption is greater then what we are currently dealing
> with in CHEMICAL rocket engines. If the plasma were to be permitted to
> contact the nozzle it would melt practically instantly.

The thermal considerations of an electric arc are pretty plain, you have to
keep stuff away from it you don't want vaporized. Propulsion engineering
like we're doing here has to take this factor into account among the first

> What you are describing is actually a modified Daedalus engine,
> replacing the crude "throw the nuclear bomb out the back" concept with
> "throw the target for the nuclear reaction out the back."

Daedalus was the name, huh? Searching my memory, all I came up with was
something called Project Orion.

As was
> already pointed out,  this will only work if the reaction occurs
> within close proximity to the pusher plate of the vehicle.

Related to starship terms, a pusher plate does not have to be made of a
solid substance. It need only be a medium which can transmit force to the
ship. That widens the possibilities, most apparently in the case of magnetic
coupling to the ship. Everyone here will allow a Bussard ramjet to scoop up
the tenuous ionized hydrogen of interstellar space for its fuel, by the use
of a magnetic field many miles in diameter extended away from the solid
ship. I contend that use of a similar field will allow efficient use of
thermonuclear reactions ignited far behind the ship to help propel that

> drops off rapidly as the distance from the pusher plate increases.

Because the solid angle subtended by the small solid pusher plate grows
rapidly smaller. Intercepting the particulate fusion products with a very
large size magnetic field generated on the ship would enable the ship to
scavenge thrust from continuous fusion reaction produced well back in its

> VASIMR is a much more low tech concept that is capable of providing
> the same performance and is "tunable". There is currently research
> ongoing that is similar to your concept in that it is still a "throw
> the nuclear something out the back and detonate it" approach, but it
> is using antimatter as the activation, not an accelerator. This
> concept provides many orders of magnitude increase in performance, but
> is still far short of interstellar capability.
> Lee Parker

I'm quite willing to ignore antimatter research until somebody shows up with
some to sell me. Like zero-point energy, vacuum energy and fairy dust, it
doesn't matter whether I believe the theory or not, for I won't encounter
any, so I refuse to waste my time thinking about it. I design things to be
built with the most plentiful and abundant materials, to raise the
probability they will really be constructed sometime.

Anyhow, I still press for acceptance of fusion in the jet as a feasible
design concept. A population of  "slow" thermal nuclei in the plasma emitted
by the arcjet, when bathed in a high-brightness beam of relativistic
protons, will produce individual fusion reactions, unquestionably. The
number of such reactions is enhanced by the presence of a population of
nuclei with larger nuclear interaction cross section, measured in barns,
such as boron or carbon. These statements are true of individual nuclei,
without regard to the plasma composition or density: if those individual
nuclei are there, they will get hit and they will consequently undergo
individual fusion reactions. That means no special conditions are needed for
fusion to occur, fusion will absolutely occur.

Most of the confusion which has risen on this point has to do with the
criteria for self-sustaining fusion reactions to happen, in the proverbial
chain reaction. I have tried to get across the notion that a relativistic
particle beam makes fusion happen, whether or not the reaction may be
self-sustaining, whether or not a chain reaction occurs. Once that physical
fact is accepted, we can work on tuning up the conditions to increase the
rate of the reaction, and to harness its energy most efficiently for
propulsion. For example, here's the real trick. We can confine the jet to
increase its density, without melting any wires, by injecting electrons
around it in a spiral path.

The jet of our ship is composed partly of a proton beam. We have ionized
neutral hydrogen to get those protons, to feed into our linac. But we have
to shed an equal number of electrons, so our ship will remain in charge
balance. A cathode fires those electrons on a trajectory which nearly grazes
the proton beam at an angle, and those electrons will orbit the proton beam
in a helical path, by their electrostatic attraction to its positive charge.
So we have spiraling electrons, but as moving charges, spiraling electrons
are a solenoid, they are a magnet. We have built a magnetic field which
confines our jet, for some arbitrary distance back along our jet. Now we
have something to tune, now we can start to optimize our conditions to make
the most of our fusion. We have no coils to melt, no wires to burn, but we
have a magnet which is not sensitive to temperature, not affected in the
least by radiant heat. We have a magnet which is as strong as we want to
make it, and is as long as we want to make it, and is directly coupled to
the inertial environment of the ship, stiff as a steel beam. That's where
your fusion belongs.

Does that sound any better?

Johnny Thunderbird