For the Russians, the idea of quasi-adiabatic compression of the secondary module of a two-stage device was obvious from the very beginning of their work on "atomic compression," at least since 1955.
In the 2010 book "РЕШАЮЩИЙ ШАГ К МИРУ ВОДОРОДНАЯ БОМБА С АТОМНЫМ ОБЖАТИЕМ РДС-37" (A DECISIVE STEP TOWARDS PEACE: THE RDS-37 ATOMIC-COMPRESSIONED HYDROGEN BOMB), among numerous excerpts from declassified documents, I came across a document that, in my opinion, has sensational content. Below I'll provide a direct link to the relevant page in this book, available from the Rosatom library, but since links to the .ru domain are unacceptable here, it's replaced with asterisks:
https://elib.biblioatom.**/text/andryushin_reshayuschiy-shag-k-miru_2010/p63/
First, I quote in Russian:
Записка В. И. Ритуса
"О некоторых возможность применения малого термоядерного заряда.
Для атомного обжатия большого количества легкого вещества, по-видимому, целесообразно использовать в качестве обжимающего изделия систем из обычного изделия и маленького термоядерного заряда, так что вся система в целом будет выглядеть, как на рис. 11. < ... >
После взрыва "1" происходит AO термоядерных зарядов "2" и "3".< ... >
Такое растущее со временем давление приводит, как известно, к значительно лучшему обжатию изделия "3", чем давление, спадающее со временем.
Ниже приводятся некоторые расчёты обжатия и КПД конкретного малого термоядерного заряда."
Следует подчеркнуть, что в 1955 г., кроме разработки РДС-37, велась масштабная работа по другим направления. План работ на 1955 г. определял разработку 5 ядерных зарядов, 8 ядерных боеприпасов.
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Now the English translation:
Note by V. I. Ritus
"On some possibilities of using a small thermonuclear charge.
For the atomic compression of a large amount of light matter, it is apparently advisable to use a system consisting of a conventional charge and a small thermonuclear charge as the compression element, so that the entire system will look like Fig. 11. < ... >
After explosion "1," AO of thermonuclear charges "2" and "3" occurs. < ... >
Such increasing pressure over time, as is known, leads to significantly Better compression of the "3" product than pressure decaying over time.
Below are some calculations of the compression and efficiency of a specific small thermonuclear charge."
It should be emphasized that in 1955, in addition to the development of the RDS-37, extensive work was underway in other areas. The work plan for 1955 called for the development of five nuclear charges and eight nuclear weapons.
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Of course, Figure 11 from Ritus's report is missing from the book, but it doesn't take a rocket scientist to reconstruct it with 80% certainty. I've done so on the left in the collage attached in the title.
The topmost Figure A shows the structure of the device in its original state. Within a single radiation enclosure (the hohlraum), separated by membrane partitions and neutron filters (the purpose of which is to prevent neutrons and direct rays from propagating from stage to stage), are the primary unit 1 and two secondary units: the small thermonuclear unit 2 and the large thermonuclear unit 3.
In Figure B, we see the detonation of the primary unit. The hohlraum is filled with X-rays, which causes the simultaneous compression of both thermonuclear units, the small 2 and the large 3. This is shown by the transparent red arrows.
Figure C shows that, because small fusion node 2 is smaller, it fires earlier and now, instead of being an energy consumer in the hohlraum, becomes a new, more powerful X-ray source than the primary node. Thus, the still-contracting second, larger fusion node 3 begins to contract even more, as indicated by the solid red arrows.
Figure D shows that large secondary fusion node 3 has now also exploded.
The phrasing in the original 1955 source is striking: "Such increasing pressure over time, as is known, leads to significantly better compression of the '3' product than pressure decaying over time." I mean "as is known." From the very beginning of their atomic compression research, the Russians were well aware that compression with a single shock wave was ineffective, and this solution was proposed precisely to increase compression efficiency, that is, to approximate the compression cvazi-adiabatic with two shock waves. In 1966, Zeldovich's famous book was published, clearly demonstrating and stating that three such correctly sent shock waves, one after the other, are sufficient to approximate adiabatic compression with the required degree of accuracy. I've included the relevant figure from Zeldovich's textbook in the collage, although it's already well known here.
And finally, why did the Russians so easily declassify and publish a description of this idea? Because, while fundamentally correct, it was subsequently deemed a dead end, not the best solution (it's worth revealing). A far better solution to this same problem was quickly discovered, and we see it in 1962, in the no less famous "Golden TIS" (I also include it in the collage), in which the USSR first achieved ignition of a thermonuclear stage without a spark plug. Note the three-layer outer shell made of low-, medium-, and high-Z materials. This is essentially a three-stage ablative "rocket" that sequentially creates three compression shock waves with a constant radiation source in the hohlraum. That is, unlike RIPPLE technology and Ritus' proposal, they did not control (stepwise increase) the temperature in the hohlraum using any intermediate devices. They simply deposited three layers of different Z values on the secondary module. As a result, the same radiation from a single primary, with an equilibrium temperature of hohlraumet T (it's even stated to be ~1 keV), created three pressure steps:
P(i, T) =N(i, T)kT = N[Z(i),T]kT ; i =1,2,3
Therefore, for us, reconstructors of early Soviet thermonuclear bomb designs, an additional problem arises. When did the Russians begin using this more complex compression? Starting with which devices? Before Khrushchev's moratorium or after? I'm inclined to think that after the moratorium, simply because they had so many more pressing problems before the moratorium, that even with every opportunity to test the idea, they put it aside and probably thought it through thoroughly during the testing break. And then the question arises: was this multilayer compression already used in the AN-602 design, in "Kuzkina Mama"? If so, this would completely resolve the confusion surrounding how this device could have achieved 50% burnup in the final stage. Let me remind you of the crux of the matter. The dimensions of the "Главный Кенр" "main core" (jokingly nicknamed "The Death of Capitalism" back in RDS-37), a 1.7-meter-diameter sphere, when completely filled with lithium deuteride at its normal density (820 kg/m³), simply do not allow for a lower burnup percentage (given the Lidochka's calorific value of 50 kt/kg). If we don't accept the 50% burnup as fact (reducing it to the usual 25-30%), we would have to invent absolutely insane designs for this device with two or more "main components," which would completely contradict everything we've pieced together from countless Russian memoirs.