The successful development of a flexible, multi-megaton hydrogen bomb design (the Teller-Ulam design, as it became later known) was not the end of a road of technical inquiry, but the beginning of one. The scientists, like the military, were interested in exploiting all of its possibilities. OK, you could make extraordinarily large weapons (that needed to be lugged to a target in a massive airplane) — could you take the same design and scale it down so it could fit as the payload on a long-distance missile? Could you tailor it so that it had more, or less, radioactive contamination? Could you make it fit into a very compact shape? Could you make it get a bigger bang with the same amount of fuel, or the same sized bang with less fuel? These are just in the area of optimization, but these took up a lot of effort after the initial invention was completed.

There were also different sorts of weapons pushes going on at the same time, like figuring out how to make fission bombs considerably smaller, for use as tactical nuclear weapons. That is an entirely different direction to come the problem from, but also involved considerable scientific and engineering difficulties.

And the invention of both fission and thermonuclear weapons did not mean that the underlying phenomena were completely understood. So there was still plenty of fundamental research to be done on, say, the behavior of metals under extreme temperatures and pressure, the shaping of radiation through various media, and other matters that were implicated in advanced weapons designing. The initial hydrogen bombs (the Ivy Mike and Operation Castle designs) were largely conservative designs (with a few exceptions, like Castle Koon, which was a failure) meant to prove that it was doable; they did not exhaust the possibilities, nor did they indicate the extremes to which it could be taken if you really understood the underlying phenomena well (like the ultra-compact thermonuclear weapons that were developed in the early 1960s).

They were also intensely interested in the effects of these new, larger weapons. What happens when you set them off in the upper atmosphere? How do they interfere with radar? What knowledge about how they work could you use to defeat an enemy's defenses, or to construct your own defenses?

We might also add to the list of things the scientists were interested in, "unusual uses for the weapons," which did preoccupy some of them: things like Project Plowshares, in which they sought to look into whether the weapons could be used to dig great holes or canals, or the frack natural gas, or other such projects.

Which is only to say that the mere invention was again, only the beginning. There were several decades of work left in really exploiting the low-hanging (and perhaps not as low-hanging) fruit that resulted from the new invention, as there always is; the first prototype is always a crude thing compared to its later developments.

By the 1970s, however, they had more or less run out of "new ideas you can use for making thermonuclear weapons." There is only so much optimization that you can do, only so many new ideas to exploit practically. After this point, as one weapons designer described it, working on weapons was like "polishing a turd" — you were just scraping for an additional partial percentage of optimization, or a tiny bit less uncertainty, or aspects that were more engineering than science (like improving the safety of the weapons — an important area that was disturbingly neglected for a long time). But the possibilities of fundamentally new ideas became constrained. This does not mean that the weapons designers hung up their hats — there is always plenty of work to do when it comes to maintenance and iteration and designing a specific warhead for a specific application — but it does mean that some of the more imaginative ones tended to drift into more tangential areas (such as the nuclear-pumped X-ray laser, and other "out there" missile defense encouraged by Reagan's Strategic Defense Initiative, or even more benign areas that have overlapping skill sets like controlled nuclear fusion and cosmology).

It's hard to recommend a single source for post-1952 weapons developments, but Eric Schlosser's Command and Control has some of this. Hugh Gusterson's Nuclear Rites is the source of the quote about polishing the turd, and is a very careful anthropology of nuclear weapons designers (mostly in the 1980s-1990s, but he provides historical context as well).