I see that the W66 was much wider in diameter than a HARM, so in my fictional universe I guess my engineers will be hard at work designing a 10" diameter fission-fusion-fission device.
There are implosion fission devices narrower than the one the W66 apparently uses for its primary. But I don't know if there are any narrow enough to let you get a multi-stage nuke on a HARM missile.
The thinnest staged warhead (of a typical design, where the fission primary is compressed by implosion) that I know of is the W80 that we use on our cruise missiles. It is listed as being 11.8 inches wide. I really doubt you can make a regular staged nuke any narrower than that.
There are however thinner devices that don't use traditional implosion in the fission primary. We use them in atomic artillery shells.
Instead of a traditional implosion where a shock wave converges on the plutonium from all directions and compresses it to a greater than normal density, the plutonium starts off shaped like a football (American football) and the explosive pressure reshapes it into a sphere. There is less surface area on a sphere than there is on a football shape, so fewer neutrons escape once the plutonium is spherical. Retaining these extra neutrons makes the core supercritical and causes an explosion.
They try to help this out a little bit by having voids in the plutonium that are collapsed by the explosive pressure, and by having the plutonium alloy be compressed into a denser arrangement of atoms, but the plutonium still remains at normal everyday density (whereas in a real implosion the converging shockwave would squeeze the plutonium into a much higher density than normal). Remaining uncompressed at normal density means you need a greater amount of plutonium in order to achieve a supercritical mass and explode.
You can read a little about this technique in section "220.127.116.11.2 Linear Implosion" here:
And also the section on "Two-point linear implosion" here:
These devices weigh more for a given yield than a traditional nuke (that relies on implosion for the fission primary) would weigh. A staged version would probably be too heavy for a HARM missile to carry.
Note the W79 artillery fired neutron bomb. The secondary achieves only 0.3 kt of fusion. It weighs 200 pounds, already too heavy for the HARM even with such a tiny yield.
To get within the size and weight limits of 10 inches diameter and
150 pounds, you will probably have to use the technology of our artillery shells, but not
make it a multi-stage device.
Staging really adds a lot of weight to a nuke, because you need a heavy shell to protect the fusion fuel from being destroyed by the explosion that is compressing it, and another heavy shell to surround the explosion to hold the energy in so that the fuel gets compressed. Without staging, a weapon can be made much lighter.
You can still get a respectable yield even without it being a multi-stage device if you use deuterium/tritium boosting to increase the yield. And the lower efficiency of the weapon, requiring a greater amount of plutonium to reach criticality, actually helps you here. In general once half the atoms in the fissile material have been split, the fission fragments start impeding the chain reaction enough to bring it to a halt. If you have more plutonium in your device, that means you can split more atoms before you reach 50%. You might even want to have your engineers surround the plutonium with a neutron absorber instead of a neutron reflector in order to maximize the amount of plutonium in the device.
Note section "4.2.4 High Yield Weapons"
"In very large fission bombs (hundreds of kilotons) the major disadvantage of HEU, its lower maximum alpha, disappears. This is because the race between the exponential growth in energy release and the disassembly of the core stops being the limiting factor in efficiency. Instead the problem of dilution of the fissile material by the fission products comes into play as the limiting factor. This limits efficiency to a maximum of about 50%."
Such a device should be able to produce a yield of 100 kt if you provide enough deuterium/tritium gas to boost it that high.
I base my 100 kt estimate on the following:
Section "18.104.22.168 Minimum Size"
suggests that it will require about 13 kg of uncompressed unreflected plutonium to produce a significant explosion:
"Since the critical mass for alpha-phase plutonium is 10.5 kg, and an additional 20-25% of mass is needed to make a significant explosion, this implies 13 kg or so."
If you boost 13 kg of plutonium all the way to the point where 50% of the plutonium atoms have been split, that means you've split 6.5 kg of plutonium.
This page gives the energy content of plutonium fission as 17.3 kt/kg:
6.5 x 17.3 gives you a theoretical maximum yield of 112.45 kt.
Boosting a weapon from a very low yield all the way to 100kt will require much more tritium than is normally used in a weapon, but it is doable.
Before France developed staged thermonuclear weapons, they once had warheads that used U-235 (which has a much larger critical mass than plutonium, so they could split many more atoms before reaching 50%) that they boosted all the way to a half-megaton yield: