A shaped-charge-pumped particle beam looks like this, in reverse. The second point of concern is preventing the particles from damaging the particle accelerator. This can be remedied by building the accelerator as a series of widely spaced loops of wire acting as electromagnets. The particle beam is focused in stages, narrowing after each loop.
The optimal Casaba Howitzer configuration for this weapon is a fusion device that is built to maximize particle velocity. 10000km/s (3% of the speed of light) may be achieved. This is much slower than an electromagnetically-accelerated particle beam weapon, but it has the advantage of requiring little to no external power (the electromagnets can be fed by the heat they receive from the nuclear detonation), massing much less than a regular particle accelerator and able to extend the range of small nuclear pulse weapons to useful distances (in the thousands of kilometers).
Integrating the Nuclear Lance into your settingEdit
The Casaba Howitzer is best used as an 'early technology' science fiction setting. When space exploration is still new, and opponents start out in the same orbit, the short-ranged but powerful nuclear shaped charges available are extremely effective.
It can be mounted on modern-technology missiles to allow them to be effective regardless of the impact velocity, alternatively, missiles will accelerate to low velocities then expend the majority of their dV in evasive maneuvers. It will more likely be used by the most technologically advanced nation to greater effectiveness, as the technology is far from well understood even 58 years after its conception.
When the technology becomes widespread, such as following its development in nuclear pulse propulsion, it will still be the favorite of nations with greater access to fissile materials. While a fusion device allows greater yields, and would be better for propulsion, a Casaba Howitzer weapon does not benefit from the 1000km/s particle velocities. Easy to detonate fission charges are easier to handle and use.
They will however fall out of favor as lasers extend the range of combat beyond even their reach, into the tens of thousands of kilometers. More efficient missile propulsion, or the development of cold stealth technology, might change the battlefield even further.
However, as developments in propulsion continue, newer, simpler methods of detonating thermonuclear devices might become commonplace. Antimatter-catalyzed fusion or supercapacitors powering Z-pinch devices might allow Casaba Howitzers to return to the battlefield as cheap anti-missile defenses, free from the requirement of fissile materials.
Throughout history, however popular or effective they are, Casaba Howitzers will force states to carefully watch who and where to fissile materials are sold. Just 2 kilograms of uranium can be converted into a several kiloton-yield weapon, easily hidden in a civilian cargo-bay or remote satellite, and used to destroy an expensive warship in an instant.