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From: smb@ulysses.homer.nj.att.com (Steven M. Bellovin)
Newsgroups: sci.military
Subject: Re: Linear implosion in nuclear weapons?
Keywords: nuclear implosion fusion fission
Message-ID: <10744@cbnews.ATT.COM>
Date: 30 Oct 89 02:51:24 GMT
References: <10711@cbnews.ATT.COM>
Sender: military@cbnews.ATT.COM
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In article <10711@cbnews.ATT.COM>, dcn@cbnewsd.ATT.COM (david.c.newkirk) writes:
> I have seen a reference to a linear implosion technique using conventional
> explosives to trigger a fission or fusion explosion....  The basic idea
> is the same as spherical implosion, except in a cylindrical shape.

I've never heard of a linear implosion technique based on conventional
explosives (which isn't to say it doesn't exist, of course); however,
H-bombs do use a linear implosion as a crucial part of their design.
(The following explanation is taken from Richard Rhodes' ``The Making
of the Atomic Bomb'', which I highly commend to anyone interested in
the subject.)

If you use a straight-forward fission bomb to set off a thermonuclear
explosion, it doesn't work; the shockwave starts to ignite the fusion
reaction, but blows it apart too soon.  Instead, H-bombs depend on the
fact that X-rays from the fission explosion propagate much faster than
the shockwave.  Imagine the following configuration.  At one end of a
cylinder, you have a spherical implosion-based fission bomb.  The case
of the cylinder is made from U-238; a bit away from the fission
trigger, you have several cylindrical layers.  The outermost one is a
dense plastic foam; the next layer in is more U-238, followed by
lithium-6 deuteride, followed by a central core of plutonium.


			**
		       ****	<--- fission trigger
			**

		      ||||||
		      ||||||
		      ||||||
		      ||||||
		      ||||||
		      ||||||    <--- nested cylinders
		      ||||||
		      ||||||
		      ||||||
		      ||||||
		      ||||||
		      ||||||
		      ||||||

When the fission trigger detonates, it produces X-rays.  The outer
shell of U-238 helps reflect these back into the plastic foam.  The
foam is instantly heated by the X-rays; it immediately expands
explosively as a plasma.  This causes an implosion in the inner layers,
compressing the Li-6 D mixture and the Pu core.  Several things start
happening now.  The plutonium is compressed to critical mass as well,
causing another fission reaction.  The flux of neutrons from it (and
from the original trigger reaction) causes the Li-6 to produce tritium
(and, I assume helium, which is unimportant here); under the pressure
of the imploding plastic and exploding plutonium, the tritium and
deuterium fuse, liberating vast amounts of heat and more neutrons.  And
these neutrons in turn initiate more fission in the normally-inert
U-238.  The whole thing is known as a fission-fusion-fission bomb.

The idea of using the plastic foam in that way is apparently due to Ulam;
Teller's original designs just wouldn't work.  He did, however, come
up with the concept of the central plutonium core.

>From the picture in the book, the whole thing is shaped to fit very
well into a bomb.  The presence of more tritium would presumably speed
up the fusion reaction; modern weapons do in fact use tritium.  In
fact, some of them can be ``tuned''; by varying the amount of tritium
actually in the bomb, the yield can be varied in the field.


		--Steve Bellovin
		smb@ulysses.att.com
		att!ulysses!smb