H bombs

John K Clark (johnkc@well.com)
Sun, 1 Mar 1998 21:06:26 -0800 (PST)


On Sun, 1 Mar 1998 Eugene Leitl <eugene@liposome.genebee.msu.su> Wrote:

>From the very few things I remember from my Atkins, the energy
>radiated by a blackbody is proportional to T^4. However, the
>emission maximum gets shifted to short wavelengths with higher

That's right, the Black Body radiation is continuos but the peak wavelength
at temperature T is at [.290cm K]/T .

>which makes matter far from being a perfect reflector/absorber.
>So you've got quite a few mm of material suddenly heated,
>comparatively slowly exploding into space.

I don't see where you're going with this. If the target reflects a photon
then it will receive a jolt equal to twice the photon's momentum of hc/w,
h is Plank's constant c is the speed of light and w is the wavelength of the
photon. If the photon is absorbed then it will only get half as much momentum
but the energy absorbed will contribute to the rocket effect, exploding the
outer layer of the target outward and compressing the inner layer. Either
way if you're close to a H bomb so many photons will be encountered that
you'll feel a hell of a bang even in a vacuum.

>Moreover the effect is brief, the nuke fireball in space cooling
>very rapidly

Granted, but when you're talking about pressures comparable to those at the
center of the sun it wouldn't take long to shatter a target.

>Also, assuming wide habitat separation, on the average that means a
>few km between me and ground zero.

Ok, if it's a few kilometers away then there are things you could do to
protect yourself from a small H bomb, but even today ICBM's can achieve
better accuracy that that.

>Modern high-performance nukes, being high-tuned devices, do not age

True, that's mainly because of the tritium, H bombs don't need a lot of it
but they do need some and it's very expensive and has a half life of only
10 years.

Wolfkin rrandall6@juno.com On Sun, 1 Mar 1998 Wrote:

>Not true. :) Radiation pressure is not a significant compressant
>of the fusion fuel, though this was an interesting false lead for
>the journalist who almost went to jail (and whose name I cannot

It was Howard Morland and the trial was in 1979. Morland's lawyers were not
scientists but they were given security clearances so they could read secret
documents. To keep their client out of jail they said Morland did not reveal
anything of importance, and then they said the following:

" Essentially, the X rays produce a plasma of energized matter which pushes
on the fusion fuel tamper in much the same way that boiling water produces
steam which pushes the blades of a turbine. But Morland's discussion of
role of radiation coupling in the compression of fusion fuel is as
inaccurate as if he said that boiling water turns the blades of a turbine,
he leaves out the steam. [...] Morland's discussion of the role of
radiation pressure is entirely incorrect."

This was a very clever move, the Judge did not have access to secret documents
so he had no way to know the accuracy of this statement, and for obvious and
predictable reasons the prosecution refused to either confirm or deny it.
Reasonable doubt having been established Morland walked.

The exploding Styrofoam theory never smelled quite right to me, in addition
to the Rayleigh - Taylor instability problem at such huge pressures there was
another problem. The foam absorbs X rays and vaporizes explosively, but that
means the foam further from the fission trigger must receive weaker X rays
and explode with less power, this would compress the fusion cylinder unevenly
making a dud.

It's clear that foam is important in an H bomb but probably not for the reason
Morland's lawyers said. I don't know it for a fact but here is my theory.

As I said in my other post the compression of the U238 around the fusion
cylinder by the soft X ray photons would be even greater than that produced
by the huge radiation pressure because the soft X rays would vaporize the
outer layer of the cylinder which would rocket outward, and because momentum
is always conserved the inner parts of the cylinder must rocket inward.
The fact that Uranium atoms are very massive would add to this rocket effect
on the cylinder. Neutrons made by the fission trigger probably also
contribute to the pressure by bouncing off the wall of the Black Body
chamber which is made of lead, that heavy element reflects X rays as well as
anything (still only a few percent or less) and the remaining photons are
absorbed by the lead and heated to very high temperature and then re-radiated.

The inner wall of the Black Body chamber is lined with a plastic foam made of
light elements like Hydrogen and Carbon, because unlike the case of the
fusion cylinder the rocket effect is a bad thing here, using light elements
will minimize this and keep the chamber from blowing apart a little longer.
A heavy slow moving atom and a light fast moving atom may have the same
energy, but the light atom will have less momentum and so produce a smaller
rocket effect because energy is mv^2 while momentum is just mv.

The secondary use of the foam is probably what Morland's lawyers picked up on.
The light, fast moving debris from the foam will put a little more pressure
on the cylinder and do it when it can still do some good, heavy slow moving
Uranium would take too long, the reaction would be over by the time they got

You can't eliminate the rocket effect entirely and you still have the huge
radiation pressure to deal with, so you must make the Black Body chamber very
massive with lead to keep it intact for as long as possible. In general,
if you want a part not to be destroyed by a huge pressure spike and remain
intact for at least a few nanoseconds then coat it with a thick foam made of
light elements and make the part heavy.

>There is a thick cap of U-238 between the fission device and the
>fusion fuel, precisely to *prevent* the radiation from hitting the
>fusion fuel directly before compression can occur.

Correct, that's one reason the compression is so uniform. The cap is so
massive it survives long enough to shade the fusion cylinder for a while,
it makes a shadow, a vacuum in the high pressure photon gas. Nature abhors a
vacuum and demands it be filled.

John K Clark johnkc@well.com

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