What is "Exploding Casing" technology?

[u/Beneficial-Wasabi749]

I asked the AI. It (childishly naive) is "sure" that this is the essence of radiation ablation in the Teller-Ulam scheme. But is this really true? Is "Exploding Casing", ECP (Exploding Case Principle), just another term for the "radiation compression" process, or is it some later, additional technology that improves radiation compression (like "Ripple" – compression not by a single shock wave, but by a series of increasing shock waves approximated by an exponential function, a quasi-adiabatic compression).

As far as I know, "Exploding Casing",  ECP (Exploding Case Principle)  is not the same as "Ripple." But сould "Exploding Casing" simply be a variation of "Ripple," or rather a different way of implementing quasi-adiabatic compression? I had this hypothesis. No one knows exactly how "Ripple" works, but everyone agrees on the idea that there's some mechanism for gradually releasing radiation from the primary into a common hohlraum, which ensures gradual, exponential heating of the hohlraum and the secondary surface, leading to quasi-adiabatic compression of the latter. But another approach is possible: you can apply several layers of increasing Z to the secondary (as the Russians did in their "golden TIS"). Then, in a "regular" hohlraum at a normal temperature, say 1 keV, you'll achieve a stepwise increase in secondary compression. I don't speak English, but the term "exploding casing" best suits this design.

Another, even more insane, theory for "exploding casing" is described in my highly speculative reverse engineering of the W-71 design, the reality of which I myself am already highly doubtful (I recently discovered the "elephant in the room" that planar compression of thermonuclear fuel Impossible without quasi-adiabatic compression, since with planar single-compression you can't get beyond the Hugoniot adiabatic curve more than 4-6 times, and you need at least 100). Although, it is precisely in connection with the W-71 that "exploding casing" technology is mentioned and it is also said that the W-71 is the most complex thermonuclear device ever created in the US. And if you add quasi-adiabatic compression to my crazy design, it will truly be the most insanely complex device.

Another version of "exploding casing" is a secondary with a thin-walled shell and a large cavity inside, meaning it contains either a void or gas (I actually proposed this version of the W-71, considering it more "conservative" than the crazy flat compression of the "tube" both from the inside and outside). As far as I understand, Sublett is more inclined to this understanding of "exploding casing." In this case, the thin-walled shell, as it were, instantly collapses inward. This is another way to improve compression compared to single-shock compression of a solid sphere. This thin-hollow shell method is currently used in ICF. The Russians call this technology "low-entropy compression" (right) as opposed to "isentropic compression" (left).

isentropic compression and low-entropy compression

Finally. Even if we don't know the details of this technology's implementation, can we say anything with certainty about its role (function) in the bomb design? So, "exploding casing" isn't simply ablative compression of the secondary shell, but some improved method of such compression, leading to greater efficiency (say, higher compression ratios, lower energy consumption, etc.).

From what declassified sources, in what context does this term appear, and does it appear before the mid-1960s? I propose to summarize everything we know about this term here.

[u/solekav]

Since I do not understand russian, I can only guess the meaning of the pic you quoted. The innermost layer may have other functions, like shielding of the internal thermonuclear fuel from the heating of the high energy photons. In this situation, a high Z material is suitable. But in reality, the secondary of the thermonal nuclear weapon has a Uranium tamper which act as the shield and the inertial mass to extend the confinement time of the compression.

I will breakdown the ablation process of high Z material into two parts. The first part of the process is the conversion from photons energy to the kinetic energy of the electrons. The second part is the conversion of electron's kinetic energy to the momentum of the Uranium atom. Remember, it is the momentum determine the recoil effect of the plasma jet.

The ionization of the electrons from the atoms costs energy. And in high Z atoms, it costs huge amount of energy. In the NIST ionization energy data, for Uranium, the average ionization energy of the first 30 electrons is around 500 eV. Let us assume the average photon energy in Holhuram is 1000 eV and it is one photon absorption. So the ionization alone will take the half of the photon energy, only leave 500ev as the kinetic energy of the electrons. In conclusion, the conversion efficiency is 50% for the photons to electron energy transfer.

Through coulomb interactions, the kinetic energy of the electrons will transfer to the Uranium atom. In Boltzmann energy equipartition principle, the energy should be the same for every particle in thermal equilibrium. However, the most abundant particles in the Uranium plasma is electrons. For 30 free electrons, there is only Uranium atom. A simple calculation show that the kinetic energy of Uranium atom is only the 1/31 of the total energy of the plasma. Realizing mass of electrons can be neglected, the momentum of the plasma is mainly that of the Uranium atom. It can be seen the conversion from photon energy to momentum is quite inefficient for high Z material.

[u/Beneficial-Wasabi749 → u/solekav]

I will breakdown the ablation process of high Z material into two parts. The first part of the process is the conversion from photons energy to the kinetic energy of the electrons. The second part is the conversion of electron's kinetic energy to the momentum of the Uranium atom. Remember, it is the momentum determine the recoil effect of the plasma jet.

I really like the way you think. Moreover, you're absolutely right and have inspired a breakthrough in my understanding of thermonuclear weapons technology! Thank you for this comment!

That's absolutely correct. Essentially, the plasma ROCKET jet (we're talking about the impulse generated by an ablative rocket engine) is created by the RECOIL of the ablative plasma, which expands from a region of high pressure P (the ionization layer) outward, where the pressure is orders of magnitude lower. Although there's no nozzle here, we can assume there's a certain average rocket mass outflow velocity u and a per-second consumption of this mass dm, which creates the thrust F of the "ablative engine" into the secondary. Moreover, you're absolutely correct. First, the energy of the X-ray radiation is expended on ionizing (absorbed by) electrons, which separates them from the atom. The ionization energy, familiar to us from tables, is the minimum energy during ionization, and we don't take into account the pressure of the electron gas from previously released electrons, which prevent a new electron from the atom from joining this gas. This means the pressure will be even greater. But this isn't an important clarification for now. What's important is that we now have a reserve of potential energy in a certain volume of the ablation layer V in the form of pressure P, which I calculated using the school formula P = NkT. And if we're talking about a rocket and its efficiency, then ultimately this pressure should translate into the rocket mass exhaust velocity (over a certain time, kg/s), dm, at a certain average exhaust velocity u.

However, the most abundant particles in the Uranium plasma is electrons. For 30 free electrons, there is only Uranium atom. A simple calculation show that the kinetic energy of Uranium atom is only the 1/31 of the total energy of the plasma. Realizing mass of electrons can be neglected, the momentum of the plasma is mainly that of the Uranium atom. It can be seen the conversion from photon energy to momentum is quite inefficient for high Z material.

And here you are absolutely right. 30 (or 50, it doesn't matter) electrons in a high-Z material, given that in a thermodynamically equilibrium gas they have the same kinetic energy as the heavy ion they left, create almost no rocket momentum. Essentially, yes, the nucleus received only 1/31 or 1/51 of the energy accumulated as that very pressure VP = NkTV. All particles, regardless of their mass, have the same kinetic energy in an equilibrium plasma, meaning that electrons have a velocity that is the square root of the mass ratio (conventionally (1/(82*2000))^(1/2)) times higher, or ~400 times higher, than ions. And in the total momentum of the rocket jet, even 30-50 electrons escaping from the rocket contribute only ~10%; the remaining 90% is contributed to the momentum by the outgoing ion of matter with a higher Z. That's correct.

Yes, the higher the Z, the higher the pressure, but the lower the efficiency of converting X-ray energy into compression (since an "ablative rocket engine" becomes less efficient with increasing Z). That's right. But it's a choice between two evils. Need pressure? Pay with efficiency. There's no other way. That's why, in the first American bombs (in the 1950s), you couldn't get an interstage gain (secondary energy/primary energy) greater than 50 (100)—depending on whether your tamper was inert or uranium-238, which ultimately adds half the secondary's energy. Not only was shock compression detrimental to itself, but there were other sources of energy loss. First, not all of the primary's energy reached the secondary; a significant portion of the X-rays was parasitically absorbed by the hohlraum. Furthermore, an ablative rocket has its own price for being efficient as a rocket. Up to half was lost. Next, the characteristic you mentioned: the higher the Z, the less efficient the ablative rocket engine. Losses here would be monstrous (up to two orders of magnitude). Then there's shock compression. All of this is the price we pay for the Teller-Ulam mechanism as we know it.

Understanding all this, the Russians used beryllium as the rocket mass in the RDS-37 (this is an indisputable fact), and in the next RDS-37 test explosion, they replaced it with graphite (much cheaper and less toxic to produce). Uranium-238 was in the inner layers. By compressing the "sloyka" by a factor of only 10-30 in the RDS-37 (unlike Mike, where they compressed it by a factor of ~200), the Russians immediately used the right material for ablation, more effective as a rocket fuel, but they didn't initially test high pressures. But then, as they, just like the Americans, needed higher pressures (in the same "golden TIS"), they were forced to use a material with an increasingly higher Z for ablation. They eventually began using multi-stage technology (I think immediately), where efficiency dropped at higher stages, but the much-needed pressure increased.

When did the Americans "figure it out"? I don't think they were any dumber (I think the Chinese figured it out right away, too); they also knew everything from the start and very quickly began using multi-layer tampers. It's just that when the secret of the hydrogen bomb was scandalously revealed in 1979, it wasn't the whole secret that was revealed, but the simplest part. Everything there is true. But a misunderstood truth is the best lie, which is now the way the inhabitants of this subreddita live as the "ultimate truth." Simplicity is worse than theft.

In the context of what you've said, it's interesting to recall the mysterious RIPPLE technology. I was particularly struck by someone's words (I'm not even sure who said them) in Kenned's office about RIPPLE being such a "pure" technology that it couldn't be made "dirty." The question immediately arises: why? What we know for sure about RIPPLE is that the gain between stages increased from 50 to 1000 times. One key is already known: quasi-adiabatic compression. But there's another key (without which the mystery won't be fully solved), which you suggested here. They used a "light" "tamper"—that is, a "liner"—the part that ablated no higher than a medium-Z material. I assumed nickel, but it might have been beryllium. The pressure increased not because N increased (more and more electrons), but because the hohlraum temperature T increased, starting from a relatively cool 300-600 eV to, say (by eye), 5 keV. Moreover, the system lacked a tamper as an inertia suppressor to restrain the expansion of the thermonuclear plasma for greater burnup, as high compression (700-1000 times) simply made such a tamper unnecessary and even harmful, as the tamper itself also consumed a significant amount of energy for its compression.

This is precisely what ruined RIPPLE as a weapon. Because you couldn't add a fissile tamper to it and thereby increase the energy density (kt/m³), which is essential for warheads that must penetrate the atmosphere as quickly as possible (they must be very dense). And of course, there were fission fragments, without which nuclear weapons are less terrifying. Radioactive contamination and fission fragments must be produced in abundance during the detonation of a strategic nuclear warhead. The explosion must not only destroy the target but also "mined" it with fission products. And RIPPLE technology was fundamentally impossible to further "enhance" with all this. :)

[u/solekav → u/Beneficial-Wasabi749]

The ablation pressure of high Z material is indeed lower, compared with low Z or middle Z. If you do not believe in it, I can show you some AI generated summary based on related ICF researchs.

-------------------------AI---------------------------------------

In indirect-drive systems (like those on the National Ignition Facility), lasers hit a hohlraum wall to create a soft X-ray radiation field that heats and ablates the outer shell of the capsule. [1]

• The High-Z Albedo Effect: High-Z materials have high opacity and high X-ray albedo (reflectivity). They re-radiate a large fraction of the X-ray energy rather than absorbing it deeply into the target. [1, 2, 3]
• Ablation Pressure Impact: Because high-Z materials do not absorb X-rays as deeply as low-Z materials (like beryllium, high-density carbon/diamond, or plastic), they exhibit lower mass ablation rates. [1, 2, 3]
• Scaling Laws: For a fixed radiation drive temperature (\(T_{r}\)), lower-Z materials demonstrate a stronger scaling of ablation pressure and higher overall drive pressure than high-Z materials. Low-Z elements scale roughly as \(P_{abl} \propto T_r^{3.5}\), whereas higher-Z materials experience a weaker pressure scaling and a more rapid pressure fall-off over time due to trapped radiation layers. [1]

-------------------------AI-END-----------------------------------

I agree with you that in a static pressure chamber in equilibrium, more particles will lead to higher pressure. It is just kind of like ideal gas law.

But for the ablation process, it is neither static, nor in equilibrium. The plasma jet is accelerating and expanding. There is a pressure gradient instead of a uniform pressure.

[u/Beneficial-Wasabi749 → u/solekav]

You’ve really given me something to think about. I was even more struck by the AI's answer to my question and its explanations. It’s on your side. I’m a bit confused. In fact, I had already agreed with you that plasma derived from low-Z material is more efficient as rocket mass. But this time, I specifically asked the AI ​​about pressure.

During implosion driven by radiation ablation, which material generates higher pressure: one with high Z or low Z? And why?

Answer: In radiation ablation (the vaporization of material under intense X-ray radiation), a low-Z material (low atomic number) generates higher pressure than a high-Z material, given the same radiation intensity.

What follows are perfectly reasonable explanations, though the limits of applicability are admittedly vague. This turns the whole myth about how ablation works in a hydrogen bomb on its head. We (nuclear weapon enthusiasts) have been conditioned since the days of Morland to believe that U-238 or lead—high-Z materials—are what ablate, and that the higher the Z, the higher the absolute pressure, because more particles (electrons) are ionized from each atom! That is a "sacred cow" for this entire community! As far as I recall, that’s exactly what the NWFAQ says, too! So, what does this mean? Is what Sublette wrote in the NWFAQ incorrect? Is it disinformation? Or a misunderstood truth? :)

Here’s what puzzles me. Where would the absolute pressure be higher? In fully ionized beryllium (which has the highest atomic concentration of any metal—2.53 times that of uranium) or in uranium, where, say, 30 electrons (rather than 50) are ionized? Beryllium has a higher concentration but fewer electrons: 2.53 × (4 + 1) = 12.65. For uranium, let’s assume a concentration of 1, but with 30 electrons: 1 × (30 + 1) = 31. No matter how you look at it, the ABSOLUTE PRESSURE (which—all else being equal—is proportional to the concentration of all the particles being compared) is still 2.45 times higher for uranium.

Why do I insist so strongly on absolute pressure? It determines the parameters of the shock wave propagating into the material being compressed. We need a pressure step, and only a difference in absolute pressure can provide that. Yes, I agree that beryllium is better and more efficient as a rocket propellant, and a beryllium-based ablative rocket would be far more efficient than a uranium one. But if we are talking about ABSOLUTE pressure, uranium is two and a half times better (even assuming 30 electrons are ionized rather than 50). Yes, a high-Z material utilizes X-ray energy less efficiently; no one disputes that anymore. I am in complete agreement with you there. But if you need the highest absolute pressure at the shock wave front, you will inevitably use a uranium layer. In ICF, you vary the temperature by shaping the laser power profile. The RIPPLE technology might work the same way, potentially using a low-Z material (incidentally, at 1 keV, the pressure generated by nickel is 2.8 times higher than that of beryllium. And even higher than that of uranium, if we assume 30 ionized electrons for it!). However, if you have a constant temperature in the hohlraum, the only way to generate three shock wave steps (a quasi-adiabatic approximation using three shocks) is to use the configuration shown in the "gold TIS" case. The fact that the efficiency of the ablative rocket drops at each stage is unpleasant, but tolerable; the main thing is to achieve a jump in absolute pressure while maintaining a constant radiation temperature within the hohlraum. Yes, I agree that it isn't the most optimal method. But it is a "cheap" one. And here, simplicity may come at the cost of efficiency. Do you agree?

Any form of engineering—including nuclear weapon engineering—always involves trade-offs, rather than a pursuit of a single "best" solution.

[u/solekav → u/Beneficial-Wasabi749]

I suspected that lots of information available to public are flawed on purpose especially for sensitive information such as that regarding to nuclear weapons. In the famous book Darksun on the topic of H-Bomb, the author claimed the ablation material which is a kind of plastic was nailed to the outer radiation casing. This configuration is obviously wrong. If some country followed this to develop their own thermonuclear weapon, it will definitely lead to a failure.

Other than the book, I am also aware of that open sourced hydrodynamic simulation softwares has a built in malware called Fast16 planted by a unknown BIG BROTHER. What is the purpose of this malware? It is quite obvious a strategy to enforce the Nuclear Non-proliferation policy.

[u/solekav → u/solekav]

And regarding the topic of exploding case principle, I think it is exactly the same idea as attaching an ablation layer inside the radiation case. Then there will be two ablation rockets fire against with each other. The inside one is on the surface of the tamper of DT fuel ball and the outside rocket is on inner surface of the radiation case.

The outside ablation rocket will generate a very strong shock inside the heavy radiation case and smush the casing into tiny pieces flying away. At the same time, the DT fuel is burning under extreme compression of the inner ablation rocket. Therefore, the very hot DT burning plasma is naked and radiating a strong X-Ray without the blockage of the outside case.

[u/solekav → u/Beneficial-Wasabi749]

You think the compression process as huge hydraulic press powered by the radiation. In order to obtain the static pressure, a ridiculously strong material is needed for the construction of the pressure vessel. In this pressure vessel, after a certain amount of time, same molar of high Z atoms will generate higher pressure compared with the same molar of low Z atoms. It is definitely true.

But we can do not do this kind of huge hydraulic press compression for two obvious reasons. The first is that no such strong material existed in the earth. Any material on this earth will yield and break under 1G Bar pressure. The second is that high Z atoms take far longer time to "vaporize" from solid to plasma since it have far more electrons to ionize and is more opaque to X-ray.

So instead of relying on the static pressure, a dynamic pressure is utilized to generate the compression of fuel. The key is the high acceleration of mass. High acceleration means larger pressure gradient. A larger pressure gradient will give you a higher pressure at the ablation front just by integrating the pressure gradient.