Wow, so I met Howard Morland today. That was pretty awesome.
James Nagelberg wanted me to blog the panel on What the Future Holds for U.S. and Russian Nuclear Weapons. As I have no rational basis on which to make such decisions, I am pleased to do James’ bidding.
Sadly, no one mentioned extending START in any detail. However, I did learn a few interesting things, including:
- One can call the Czech Republic “Czechia.”
- The Polish national mission, according to Alexei Arbatov, is to annoy Russia.
- The Cold War is over. Kinda.
- Ambassador William J. Burns in Moscow is the son of one of the speakers, General William F. Burns.
- Yeltsin, “in a rare moment of sobriety” according to Roald Sagdeev, wanted to re-target Russia’s ICBMs, perhaps “at Mars.”
- Sagdeev, obviously, has a future in stand-up comedy if this whole disarmament thing doesn’t work out.
- Linton Brooks, who anticipates that 80-90 percent of the people in the room will disagree with his talk, is an optimist.
- Brooks, who believes the nonproliferation community “should be marching in the streets demanding” the RRW, should consider taking his act on the road with Sagdeev.
- Rose Gottemoeller is extremely gracious but firm chair, giving Bob McNamara the first comment after he wasn’t able to ask a question during a previous panel and cutting people off when necessary.
Actually, Brooks’ talk was really interesting—he made a nonproliferation case for the RRW, arguing that it would be the “nail in the coffin” for nuclear testing and that the chance the next Administration would secure ratification of the CTBT is “relatively large.” He also argued, of course, that the RRW could enable further reductions and preserve extended deterrence.
Brooks—who made a similar argument at a private dinner the New America Foundation hosted at Nora—is very persuasive, but to take the deal he proposes would require a leap of faith that I find very difficult. One must believe that the Bush Administration, or an ideologically inclined successor, would not simply use the RRW program to resume something like the Robust Nuclear Earth Penetrator and/or testing.
That’s a hard sell, at least to me, at least right now.
Still, his talk is one of the most interesting I’ve heard today.
Any information on what the current state of Russian retargetting is?
Morland may be a sinner or a saint, but he remains to this day completely screwed-up in his understanding of an H-Bomb.
He is fixated on styrofoam and misses the real concept.
Morland is close enough for anyone who couldn’t figure it out for themselves. He may not understand it, but he is simply pointing out that you have to use low-Z materials for ablators. High-Z materials like steel and uranium have too much X-ray stopping power and do not ablate well under the prevailing conditions.(see eq. 5.56 Sec. 8 Zel’dovich and Raizer) This is also why the radiation cases can be so thin around the secondary.
If there ever was a secret, it is long gone.
Thank Jeffrey.
I have to admit a personal interest in this panel since both Rose and General Burns are former professors of mine.
Arbatov, provocative as he was, made a good addition to the first day of the conference, since all of the other participants were so fixated on displaying their familiarity of the Nunn, Kissinger, et al op-ed.
Clinton promised us that ICBMs were no longer aimed at America’s children, and of course that is still true. =)
John wrote:
You are correct that his information provides a major insight into the concept, shaving who knows how many months to years of development for a vertical proliferator.
They would see the real process hidden in his error.
However, you have it totally reversed yourself about the process. The foam (a low-z material) DOES NOT participate in the ablation process. (It also does not provide the compression shock wave that Morland is convinced of).
Contrary to your statement it is BECAUSE the fusion capsule (or secondary) is shelled with a high-z material like lead or uranium that it absorbs the x-rays, ablates and forms the imploding rocket. The opaqueness is KEY to its working. The purpose of the channel filler (typically, but not necessarily, a carbon-hydrogen foam) is to vaporise to a plasma, becoming x-ray transparent in the process. This high pressure plasma suppresses the “boil-off” of x-ray opaque radiation case liner and the secondary pusher. This keeps the radiation channel open. Otherwise the x-rays would be blocked and the process fails.
Yale, I strongly disagree. Your explanation is the one that is typically given, but it appears to me to be incorrect.
KeV x-rays are absorbed in just a few microns in compressed uranium plasma. Even less than that as the stuff is ionizing up to equilibrium. And, the absorption is nonlinear in the density. So instead, the radiation would be absorbed in the evaporating cloud away from the surface. Some radiation will diffuse inward and electrons could carry some, but nearly all the heat will be rescattered back against the radiation case. If the case were 2 tons of Uranium, maybe it could take it, but it is steel and paper thin so it can’t take much of this rescattering. Anyway, from a thermodynamic point of view, it makes no sense to expand your ablation fuel before heating it.
On the other hand, wrap your uranium pusher with a calibrated layer of low-Z ablator, and it will soak up the x-rays like mad, which of course is the whole point, and you’ll be left with a symmetric and uniform impulse against the U pusher. Ideal gas pressure of KeV ionized carbon at solid density is 1000 Mbar, already too much, hence the foam.
Thermodynamic efficiency is high, sensitivity to nonuniform illumination is low, lifetime requirements for the radiation case are minimized.
Take a look for ‘can opener’ in :http://www.fas.org/sgp/eprint/w-88sand.htmand then send me your calculations if you still disagree.
What I found interesting is Brooks apparently is on the exact same talking notes as D’Agostino and Harvey. We should all be for the RRW since it will reduce the odds for testing… since the military is so fond of untested weapons.
John..
You are confusing the outer protective shell – the “Canned Subassembly” (CSA) with the actual secondary within:
Contained within is typically a molded plastic form lined with a thin membrane of U238 – the outer wall of the radiation case. Then a gap (or radiation channel) filled with a low-z foam (or “channel filler”)- to hold the inner parts in place and to provide the “boil-off” suppression.Then comes the pusher – composed of a high atomic weight (or high-z) material such as lead or uranium. Inside that is the fusion fuel.
Here is commentary by Sublette, discussing a paper on ICF where he describes the difference between non-weapon laser Inertial Confinement Fusion (ICF) targets (which do use plastic ablators) and nuclear weapon secondaries (which do not use plastic ablators) – I added text within curly brackets and added the emphasis:
Continuing my post from this morning (I am on my own clock now)…
If the CSA is defined for the modern arsenal (yeah, I am old) as the actual secondary assembly starting at the tamper then some estimates can be made.
Working off the back of an envelope, you get about 2×10 18 W/cm 2 radiation intensity on the secondary with something like an ablation of 2 grams per cm 2 each nsec.
With stainless steel, this would be maybe 2-3 millimeters per nanosecond.
When I have some time, I will do some legitimate calcs.
In any event, foam channel filler is not involved.
Is there anything to prevent Senator Biden from bringing the CTBT up for a vote right now in the Foreign Relations Committee, and passing it on to the Senate floor for ratification?
If you try to push on high-density matter with a low-density gas, the gas will move the other way and the dense matter will stay put.
Implosion occurs because the spherical or cylindrical secondary is suddenly heated uniformly from the outside to keV temperatures, which at solid densities means Gbar pressures, much higher than the Young’s modulus of the material. Of course it is high-Z matter pushing on high-Z matter. The low-density, low-Z matter cannot even “suppress boiloff” because the hot high-density, high-Z matter will just penetrate into it as if it weren’t there.
Calling this an “ablation rocket” is a bit confusing. If the process were much slower, all of the secondary would “ablate” i.e. boil off and float away, and there would be no implosion. Ablation as such can thus be viewed as a pressure loss mechanism. However, the momentum of implosion must be balanced by a momentum of explosion, since the material is not effectively solid. Thus, an outer layer of high-density matter of the secondary must explode. Since the exploding outer layer can rapidly expand, while the velocity of implosion may be much less, momentum balance requires only a thin layer of high-density matter to explode, thus justifying the term “ablation.”
I may not have been clear enough. The “boiloff” suppression is not for the secondary pusher. It is primarily for the outer lining of the radiation channel, slowing things down enough that the channel is not blocked.
The suppression is not central to the inner lining, where the absorbed x-rays do the desired ablation( which is quite brief – a rocket with a very short burn)
High-Z materials like steel and uranium have too much X-ray stopping power and do not ablate well under the prevailing conditions.(see eq. 5.56 Sec. 8 Zel’dovich and Raizer)….Your explanation is the one that is typically given, but it appears to me to be incorrect.
KeV x-rays are absorbed in just a few microns in compressed uranium plasma. Even less than that as the stuff is ionizing up to equilibrium. And, the absorption is nonlinear in the density. So instead, the radiation would be absorbed in the evaporating cloud away from the surface. Some radiation will diffuse inward and electrons could carry some, but nearly all the heat will be rescattered back against the radiation case. If the case were 2 tons of Uranium, maybe it could take it, but it is steel and paper thin so it can’t take much of this rescattering. Anyway, from a thermodynamic point of view, it makes no sense to expand your ablation fuel before heating it.
On the other hand, wrap your uranium pusher with a calibrated layer of low-Z ablator, and it will soak up the x-rays like mad, which of course is the whole point
Make that Eq. 5.56 in Vol. I, Section V.8 in Zel’dovich and Raizer. This equation gives the Planckian mean free path in a multiply-ionized plasma, though Eq. 5.55, which gives the Rosseland (weighted transport) MFP is more relevant here. Taking T to be 3 keV (15 kT of energy distributed over the total volume of the W80 case and a 40 kg primary), and an ionization state of 75 in uranium at STP density, this gives 0.01 cm.
This is just a starting point for a heating analysis, though, since it is just one component of the coefficient of radiation thermal conductivity. Radiation diffusion (heat conduction) carries energy much farther than the MFP. The diffusion approximation requires, for example, that the temperature gradient be small, that is, the heat flow is many MFPs from any abrupt termperature change. The conduction wave front, where the temperature drops from the equilibrium value to near zero, by itself is a few MFPs thick.
A convenient formula to estimate radiation penetration depth in cold uranium is given in LAMS-364 (“Penetration Of A Radiation Wave Into Uranium”):d (cm) = 0.1 T^3 (t)^(1/2)where the temp T in in keV and t is in microseconds.
For T = 3 keV, and t = 0.1 microsec we get 0.85 cm. This is a fairly large amount of ablation – 40 kg off of a 20 cm sphere. Since the absorption coefficient varies as 1/N^2 (N=the particle density) the ablating material’s conductivity will increase quickly as it expands, correspondingly reducing its contribution to blocking the heat flow.
Now,there are big differences between 80 ton Ivy Mike and a W80, so when we talk about “the H-bomb” (or TN weapons, to use a more wonky phrase) we need to abstract away the particular variations of specific designs. Thus the thin (in spots) uranium radiation case reported for the W76 implies other techniques at work than merely a heavy high-Z can holding radiation so a high-Z tamper ablates away, but this does not invalidate the general idea of a relatively opaque material being used as the ablator.
I concur that graded, layered ablators are very likely in modern compact light weight warheads—without abandoning the idea that relatively high-Z opaque materials (not low-Z transparent ones) are the chief ablating layers in general.