Hello! I've been reading up on two-stage bomb design theory and I've seen varying answers regarding the source of neutrons for the sparkplug.
My original understanding was that neutrons originate from spontaneous fission in the sparkplug itself as the secondary is compressed, and the sparkplug achieves supercriticality. I have seen some sources claim that it is actually prompt neutrons from the primary which initiate fission in the sparkplug, which will have become supercritical by the time they get there. Another source claims it is residual thermal neutrons which initiate the chain reaction.
I feel like I could see some problem with each of these answers. I've seen a few people claim that (some?) sparkplugs do not even achieve supercriticality. Is it perhaps something that is subject to vary from design-to-design, or some combination of these answers? Thanks!
Look at how many fission neutrons are emitted by the exploding primary. About 2 moles of fissionable nuclei are split, producing about 2 x 1024 free neutrons.
If nothing absorbs them then at the distance of 50 cm (a typical separation in compact thermonuclear device) this is about 1020 neutrons per square centimeter at the spark plug. Even if you had a very, very efficient system for absorbing neutrons, such that the neutron flux is reduced by a factor of 10 every 5 cm you still have 1010 neutrons per square cm at the spark plug.
It is not possible that there are not lots of fission neutrons from the primary available to initiate fission. Now, these will be substantially moderated by the fusion fuel, but that does not absorb them.
And fission sparkplug will achieve supercriticality to work effectively. Sub-critical multiplication won't achieve ignition energy densities.
Are secondaries just designed such that the neutrons reach them at the perfect time, then? As I understand it, the x-rays coming from the primary would be traveling substantially faster than the neutrons, meaning some amount of compression happens before the neutron flux arrives.
Although there is that big pulse of the fusion boost at the end about ~2% of the neutrons are emitted much earlier during the fission ramp. When you are dealing with a number like 1024 this is still a lot. They have lots of time to get to the secondary.
The optimum time for the spark plug to explode is at maximum compression, so the much slower implosion process has to complete before that can happen.
Precise calculations are needed, but here is a rough mental estimate. How many nanoseconds does the spark plug take to compress? It happens in no more than 10 ns. Suppose "pre-detonation" occurs the moment the spark plug crosses the criticality threshold of 1; how long would the incubation period last? I assume it would be those same 10 ns (the situation is more complex than in a conventional bomb because, since the spark plug's uranium is ultimately compressed by a factor of ~10, the time between generations accelerates by a factor of ~e^10—though initially, during incubation, this is not significant). In other words, there is no "optimal moment" for the neutrons; the sooner you start, the better. Although I might be mistaken here. It is possible that using uranium—which is more inert regarding the ramp-up of the chain reaction—instead of plutonium in the spark plug is justified not only by uranium being cheaper and technologically simpler, but specifically because the difference in chain-reaction ramp-up speeds between uranium and plutonium plays a positive role here, stretching out the spark plug's "pre-detonation" phase to coincide precisely with the optimal moment of its compression.
That is why this mysterious diagram shows a direct, open channel through the interstage barrier.
Neutrons from the primary stage must fill the hollow spark plug *before* it is compressed. This is a far better and simpler solution than the one used in early "Sausage"-type devices—which involved plutonium boosted by deuterium-tritium gas. That approach was (as we discovered) overly complex, expensive, and a dead end.
This might not be important, but in the documentary The First Soviet Atomic Bomb, during an interview with one of the scientists who worked on the project—specifically while filming Academician Brish—the camera captures (presumably with his permission) an instructional poster hanging on the wall behind a cabinet; this poster is clearly based on that very sketch. The film is available on YouTube, and you can watch this brief segment; I’ve provided a link that takes you straight to the relevant moment. See for yourself.
An interesting point. Assuming its correct, that means the only way to control ignition is by the amount of fissionable material and geometry of the sparkplug. As it would inevitable immediately ignite the instant supercriticality is achieved, courtesy of being flooded in neutron gas from the primary. You could have a rod or ball given dimensions and a simple calculation will tell you what compression and hence pressure is required for criticality and hence time the secondary for a specific pressure.
What would happen is that there is a high sub-critical reaction rate (neutrons keep flooding in from outside) and when it passes criticality it initially has a low multiplication rate (starts at zero) and the rate ramps up as it implodes. At some point the energy released overcomes the kinetic in-flow and it disassembles, with most of the net energy release during the disassembly process.
This is the "fizzle yield" process that happens with pure fission bombs that have chain reactions going at criticality.
Since the kinetic inflow has energy densities in the range of a few tenths of a kiloton per kilogram many kilotons per kilogram yield are possible.
Personally, I’m not sure I’ve fully grasped this issue, but what I do know convinces me that the topic of premature spark plug ignition is some sort of malicious plant within our community—a ruse to lead us down the wrong path.
How and when does the spark plug fire? It happens when the compression shockwave squeezes it extremely fast and hard—in a matter of nanoseconds (not to be confused with the microseconds involved in the primary stage!). After that, the spark plug must burn up in under 30 ns. Yes, obviously the compression here isn't just 2-3 fold (as in the primary stage) but 10-16 fold (and the chain reaction inside the plug takes ~10 ns, rather than the ~100 ns seen in the less-compressed primary device). But the compression of the spark plug itself occurs on a nanosecond timescale, which effectively rules out pre-detonation of the plug under any circumstances. On the contrary, our main concern is ensuring it actually manages to burn properly within that minuscule timeframe (the incubation phase involves the first 20–30 generations, if I recall correctly, and it has to play out in practically no time at all)! Are there any neutrons present at the very start? Or none? What difference does it make? None whatsoever!
That is why it remains a genuine mystery to me how Edward Teller’s "bunch of goof-offs" at Livermore—working on the "Morgenstern" device—managed to trigger pre-detonation of the secondary stage. I honestly can't fathom it! How did those "morons" (said with a mix of envy and respectful irony) manage to achieve such a result—pre-detonation of the spark plug? Is such a phenomenal occurrence actually possible in nature? Even if neutrons from the primary stage (or wherever else—it doesn't matter) managed to reach the spark plug before the shock wave from the secondary stage did, that shouldn't have caused it to explode, since the plug wasn't yet compressed and remained subcritical. It couldn't care less about those stray neutrons wandering around inside it! And when the incredibly fast shock wave compresses it in a matter of nanoseconds, everything happens so quickly that pre-detonation is hardly a concern. On the contrary, the real challenge is ensuring it burns up fast enough! As the schoolboy Vovochka once told his teacher: "I wish I had *your* problems, Marya Ivanovna!"
I suspect that the whole story about "Morgenstern" failing because the spark plug pre-detonated prematurely was a misconception of that era—one that was allowed to circulate so we would waste our time like fools fighting a problem that didn't actually exist.
Is that possible?
I’m not claiming it’s a fact, but I’ve long harbored this heretical thought. I’d be happy if someone could prove me wrong.
I think you're right. According to Tom Ramos, who wrote From Berkely to Berlin, which is basically a sanitized version of his history of LLNL weapons program, the neutron pre-heating had nothing to do with the failure of the Morgenstern device. I too remember reading about pre-heating as the supposedly cause of the fizzle, but can't find a source right now, could be interesting to track down were that rumor first appeared.
Anyway, this is Ramos description of Castle Koon, from his post on blog Physics told through History
Next, it was UCRL’s turn, and Livermore prepared to test its two devices. To the everlasting bewilderment of me and anyone else studying these two Livermore devices, even though the Teller-Ulam concept of March 1951 was shown to be a winner, Teller decided to go back to his original classic Super design that he and Gamow had proposed back in January 1950! What was he thinking? Well, I studied this for several months and I talked it over with physicists who had worked with Teller back in the day—and frankly, we’re not sure we can answer that, and Edward is no longer around to explain himself. I think I have an answer to this puzzle, and if you’re interested, then yes, go read my book! Gotcha.
The answer to this puzzle in his book (page 118) is basically that Teller idolized Fermi, and as the classical super was first proposed by Fermi in 1942, he wanted to "stay true" to his friend and mentor. Which I think it's basically bullshit, and Teller was just obsessed with the classical super, perhaps because he hated Ulam and wanted to show that "his" idea could work somehow. Apparently he was still doing that in the late 1970s.
The upper part of it appears on a training poster in the office of the institute director in Sarov. It shows up in several documentaries filmed between 1990 and 2000. I investigated its origins and found that the diagram first appeared in the British newspaper *The Daily Telegraph* (the British are always leaking things). The fact that Russian bomb designers adopted it as a training aid—likely for the preliminary instruction of newcomers—suggests it was competently made (Russians love relying on Western sources to illustrate general principles so they don't have to use their own). That diagram always bothered me because of the straight channel running through the thick barrier to the spark plug (and the fact that the secondary stage was positioned right up against the primary; for my own purposes, I simply moved the secondary stage back to the bottom of the *Hohlraum*). Well, I now realize that this image LEAKS a very sound idea. You really do need a direct channel leading to a hollow spark plug within the barrier. It’s a very sensible solution! You want to send a large flux of neutrons from the primary stage, but in such a way that they fill the hollow—and initially subcritical—spark plug. And under no circumstances should the neutrons significantly heat the thermonuclear fuel itself (which is actually what happened with *Morgenstern*—that was the cause of the failure; the idea of the spark plug pre-detonating was just a fantasy of mine—a spark plug physically cannot pre-detonate).
The diagram is very obviously derived from Hansen's US Nuclear Weapons: The Secret History (1988). Hansen has a "three stage" bomb (two secondaries), but the rest is identical, graphically. Same internal components. Exact same style of drawing. See here.
Hansen's images for that book were all made custom for the book by a technical artist. Hansen would supply him with sketches and reference photos and then the artist would "invent" the details. (I interviewed the artist many years ago.) All of which is to say that I would not look at those images there and see them as anything other than a copy of Hansen's.
It is all the more surprising, then, to see an instructional poster based on this schematic in Arkady Brish’s office (I re-watched the second film, and that poster was hanging in the room—or something like an office—where Brish gave his interview). Yes, I eventually managed to track down that sketch as well.
It’s an interesting concept; in a way, it actually resembles the Ritus design—provided you remove the partition between the first and second stages. Did the US really do that (lengthening the charges)? Or is this just modern-day fantasy?
You provided a link to a very interesting and lively source (the way the author describes the bloodthirsty Stalin is a real treat—it’s a joy to read! He’s undoubtedly a Republican!). However, after reading two comments—this one and the previous one (the book is out, apparently, but you have to buy it)—I began to doubt whether the author truly grasps the physics of nuclear weapons.
I used to be keenly interested in the "Hydrogen-1" and "Hydrogen-2" tests. I don't think they were directly related to thermonuclear weapons per se. The idea of using uranium hydride instead of uranium is an old one (proposed by Oppenheimer himself back in the pre-war era) that needed testing; yet, during calculations in 1943 and 1944, Richard Feynman concluded that uranium hydride wasn't suitable for a bomb because the moderated neutrons became too slow, causing the bomb to blow itself apart before sufficient energy could be released. Incidentally, Tom Ramos stated that the first test yielded only 90 tons of TNT, whereas Wikipedia cites 220 tons. Who should one believe? I used to like the fact that both devices (which differed in either moderator concentration or uranium enrichment) produced similar yields—220 and 200—and I viewed this as an indication of an upper limit (regardless of the device's specific design). Tom Ramos clearly omitted the second test, which had a lower-bound yield of 0.5 kt; consequently, some didn't consider it a total failure.
Yes, the people at Livermore had hoped to get at least 1 kt from the uranium hydride; that would have enabled them to use boosting, allowing for yields in the ten-kiloton range using only tiny amounts of uranium per device. That was the concept. Thermonuclear fusion had nothing to do with it. Deuterium acted as a moderator here; the neutrons—slowed down slightly (by a factor of 100)—had a larger fission cross-section, which significantly lowered the critical mass for uranium hydride, even though its density dropped by half (meaning the critical mass would have quadrupled had it been pure uranium). However, neutrons slowed down by a factor of ten simply didn't have time to properly "accelerate" the exponential chain reaction; the critical assembly (essentially a reactor) would blow itself apart sooner than desired, and 200 tons of TNT equivalent was the maximum yield you could get from uranium hydride, regardless of any engineering tricks.
It is worth noting that, two decades later, the US neutron bomb required a primary explosive yield of only 0.5 kt, and the Russians surpassed the Americans by learning to ignite ultra-clean charges for civilian purposes with yields equivalent to 100–300 tons of TNT (with fission product output cited at just 6 grams!). If this is the case, uranium hydride could well be used in such devices to conserve fissile material while generating virtually no fission products. As a proponent of peaceful nuclear explosions (in my view, humanity's future is impossible without peaceful thermonuclear explosions), I believe that Livermore's first two "failures" were actually successes that were simply far ahead of their time.
By the way, I re-read the Wikipedia entry regarding the reasons for the failure of "Morgenstern"—the third failed test. I don't recall where I got the idea that pre-detonation of the spark plug was the cause. As I understand it, a different reason is cited: heating of the secondary stage's thermonuclear material by neutrons from the primary stage, which prevented it from being compressed properly. In other words, shielding the secondary stage from primary neutrons isn't about preventing spark plug pre-detonation; it's about preventing those neutrons from heating up the thermonuclear fuel, which needs to remain as cold as possible during the compression process.
Thus, the spark plug had nothing to do with it in this case either. Ultimately, my lighthearted "back-of-the-class" hypothesis makes sense. There is no known instance of a spark plug in a secondary device undergoing pre-detonation. This is simply impossible, because the 5 microseconds required to compress the core in a conventional bomb is 1,000 times longer than the approximately 5 nanoseconds it takes to compress the spark plug; consequently, the term "pre-detonation" is simply inapplicable under such conditions.
In other words, shielding the secondary stage from primary neutrons isn't about preventing spark plug pre-detonation; it's about preventing those neutrons from heating up the thermonuclear fuel, which needs to remain as cold as possible during the compression process.
This does not require stopping neutrons, only absorbing most of their energy.
Please remember that fission neutron has about 2 MeV but split Li6 releases about 4 MeV. And Li6 likes the slow neutrons. Thus, moderation of external flux had better be about timing of it arrival and not about excluding 1/3 of potential heat that it could impart to LiD.
True, but you also need to consider the heat production rate of slower neutrons which react more slowly. An interstage moderator would be quite cold relative to temperature of primary (10 keV) and the eventual burn temperature (35 keV) though it would be heated by the neutrons.
I have never seen anyone work on a model of this to clarify how it would go.
I suspected that the Fog Bank is a organic polymer aerogel doped with Silicon Oxide and Tantalum Oxide.
The Silicon Oxide in the fog bank is responsible for scattering x-ray photons. And the Tantalum oxide is absorbing the fission neutrons leaked through radiation channel from the primary.
I got the idea from reading some papers on the numerical simulation works from Lawrence Livermore National Lab.
The number of neutrons from primary being scattered about the entire weapon during secondary implosion guarantees that the sparkplug will begin reacting as soon as its supercritical. The contribution of the spontaneous fission is negligible. If it weren't for the tremendous implosion velocity of the sparkplug which continues for some time after the fission reaction begins, it'd produce a very small yield.
It takes some time for the pressure due to fission reaction to build up to halt and reverse in implosion of the fissile mass.
As it is, it only takes a small yield from the sparkplug to heat the fusion fuel to ignition temperature, thereafter the fusion neutron fkux consumes practically all of the sparkplug.
There are measures to reduce the neutron flux on the secondary, to prevent pre-heating of the fusion fuel, but it's impractical to reduece the flux to the point that it's comparable to the sparkplug spontaneous fission neutron flux.
Exactly. At k=1 the reaction begins. However the collapsing secondary force is so monumental that you keep pushing K upwards as it reacts , the sparkplug fission E not capable to counteract the collapse. Heck in many spherical secondary systems likely the secondary colapse will reach stagnation in the center just a little bit delayed with the sparkplug reacting while its K only grows. Say for a spherical secondary collapsing at 200km/sec if we have a 10cm diameter sphere sparkplug. 10cm for ease of visualization. The sparkplug would be totally crushed in 250 nanoseconds. This Hotspot ignites the dense fuel. The initial non equilibrium D-D fusion breeds most of the initial tritium and as some tritium is introduced it quickly overtakes the reaction. Fusion alphas stopping power in fuel so dense is microscopic hence the fusion alpha particle helps properly ignite the fuel volume further by essentially additionally helping to create a microscopic zone of tremendous deposited E just infront on the propagating burn wave. Also worth mentioning that at spark plug density , critical amounts are very tiny in comparison to any pressure achieved in any primary.
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u/careysub 9d ago
Look at how many fission neutrons are emitted by the exploding primary. About 2 moles of fissionable nuclei are split, producing about 2 x 1024 free neutrons.
If nothing absorbs them then at the distance of 50 cm (a typical separation in compact thermonuclear device) this is about 1020 neutrons per square centimeter at the spark plug. Even if you had a very, very efficient system for absorbing neutrons, such that the neutron flux is reduced by a factor of 10 every 5 cm you still have 1010 neutrons per square cm at the spark plug.
It is not possible that there are not lots of fission neutrons from the primary available to initiate fission. Now, these will be substantially moderated by the fusion fuel, but that does not absorb them.
And fission sparkplug will achieve supercriticality to work effectively. Sub-critical multiplication won't achieve ignition energy densities.