AS the title says, Neutron and Proton bombardment as a WMD and how would it work?
I've tried researching it but i can't find concrete answers but it is viable as a WMD.
What would necessitate its use in ground and orbital warfare?
The Neutron Bomb — or the so called the "Enhanced Radiation Weapon" in its euphemized form — is what you are looking for.
What would necessitate it? If you need to exterminate the people without doing (too much) damage to the infrastructure or polluting the countryside with nasty fallout. Imagine for instance a "low grade" zombie apocalypse... the population has gotten infected with a pathogen of some sort that does not outright kill them — they go on living (without that brain-eating malarkey) — but that is hugely dangerous to your general interests. Imagine for something instance that causes irrevocable and wide-spread sterility. You need to euthenise a large number of people to prevent the spread.... but you want to repopulate the cities soon enough. That would call for a weapon like that.
(NOTE: I got so wrapped up researching particle beam weapons that I forgot about the WMD part of the question. Sorry! I'm going to leave it anyway because I think the information is applicable.)
You're asking about a particle beam weapon. Much of the material I'm getting for this question is from a 1984 Air University Review article entitled "Introducing the Particle-Beam Weapon" by Dr. Richard M. Roberds.
Here's a tidbit.
The mechanism by which a particle beam destroys a target is a depositing of beam energy into the material of the target, which might be any material object. As the particles of the beam collide with the atoms, protons, and electrons of the material composing the target, the energy of the particles in the beam is passed on to the atoms of the target much like a cue ball breaks apart a racked group of billiard balls. The result is that the target is heated rapidly to very high temperatures--which is exactly the effect that one observes in an explosion. Thus, a particle beam of sufficient energy can destroy a target by exploding it (although that is not the only means of destruction).
Let's address this part, what makes a good weapon?
What would necessitate its use in ground and orbital warfare?
Like any other weapon, it either has to be better than what we have now (the M16 replaced the M14), or address a new defensive technique (HEAT to deal with increased tank armor), or deal with a unique situation (F-117 used stealth to bypass enemy air defenses in the first Iraq War). It also has to be durable and cost effective.
This explains why, for example, military rifle technology hasn't changed much since the 1950s. There might be slightly better ways to kill a lot of people than bullets (specifically Spitzer bullets), but nothing so much better to replace them.
What are particle beams good at? They project a lot of energy in a straight line very fast. They don't use ammunition.
What are particle beams bad at? They require a tremendous amount of energy. They require a complicated (and probably delicate) particle accelerator. They can only fire at targets in line-of-sight (LOS).
When do we currently use bulky, high energy, LOS weapons? 1) Tanks. 2) Shooting down missiles.
SDI conducted the Beam Experiments Aboard a Rocket or BEAR particle beam experiment, so at least they thought it might be good at shooting down nuclear missiles. It would be competing with various lasers and railguns.
As for it's use as an anti-tank weapon I see two primary advantages over conventional weapons. First, a speed of light projectile makes aiming much, much simpler. Tanks are already really good at aiming, so there would have to be some sort of fundamental change in how targets are behaving for this to make a difference.
Second, modern armor is designed to defeat conventional kinetic penetrators and HEAT. It's possible they are vulnerable to particle weapons. If so, it would give an army a great advantage to field particle weapons, at least until armor technology adapts.
What has to happen to make particle beams a viable weapon? Dr. Roberds has a section on this "Development Areas for PBWs". Each of these would have to be reliable, rugged, affordable, maintainable, and transportable.
Neutron bombs have been discussed at considerable length, and the only ting that needs to be clarified is that most conceptual or real neutron bombs were tactical weapons for attacking things like columns of enemy tanks and armoured fighting vehicles without vaporizing large tracts of the Inner German Border during a putative Warsaw Pact invasion of Europe during the Cold War. This means the amount of damage through both neutron radiation and nuclear effects (blast, thermal pulse, fallout) would be focused on a relatively small area, which is not really a WMD by definition.
Now I'm not clear on what sort of mechanism is used to drive the preferential formation of neutrons in a nuclear weapon, but I suspect that after a certain size, the nuclear effects will overwhelm the neutron cascade effect. In other words, creating a megaton sized neutron bomb will not be much different than firing a regular megaton sized H bomb.
Proton beams fired from space might actually be usable as a WMD through indirect means. The protons moving through the atmosphere at near light speeds will ionize the various molecules that make up the air and possibly create a cone of radioactive particles and ions that will strike the ground. Depending on the size and power of the beam, and the orbital parameters, this cone of radiation could be energetic enough to deposit lethal or crippling doses of radiation to unshielded people on the ground. Proton beams are used as a form of radiation treatment to cure cancer, so the beam itself should also have a negative effect (although it will scatter in the atmosphere and be far less concentrated).
For more on how charged particle beams in general work, read the Atomic Rockets website "Conventional Weapons" section: http://www.projectrho.com/public_html/rocket/spacegunconvent.php
For proton bombardment, the things that comes to mind are particle accelerators and bombs, for neutron bombardment it would be bombs. Here's why:
Protons are basically hydrogen cores and as such are charged particles. They can thus be influenced by electrical/electromagnetic fields. Particle accelerators require immense amounts of power to work in addition to the hydrogen necessary to produce the protons for the beam. Based on what we currently know of Particle Accelerators, we would need a power level an order of magnitude higher than the current ones, while using slightly lower speeds - I believe you dont need near-c protons for them to be deadly.
However, we need a certain volume of fire to achieve the required saturation, as current accelerators only produce a very narrow beam, whereas for the use as a WMD we need a wide beam with sufficient lethality for it to be viable as a weapon.
Depending on the availability of power and hydrogen fuel, I would rather use a LASER or MASER, even though the atmosphere makes using them difficult due to dissipation. For them to work you only need power and no extra fuel. A facility with such a device installed is probably large and needs a whole space station to work as well as an appropriate infrastructure to supply the fuel and energy carrier (if you'd use a reactor to produce the power)
Why not a bomb? There seems to be no way to produce free protons using nuclear decay /chain reactions. You could use some method of ionizing hydrogen, but then again, you wouldn't be able to accelerate them enough to be of concern
Neutron bombs are not a new concept, as we already know how to build such weapons. They are proposed as a weapon that kills life but largely lets infrastructure as it is. Keep in mind though that even though its primary purpose is to be a radiation weapon, it is still a bomb with considerable power!
You'd use small tactical fusion bombs to produce large amounts of neutron radiation, where you can control the area and density of neutron radiation by detonating the bomb in different heights. You can use a delivery method of your choice, but satellite-dropped glider bombs should be best for this weapon. The satellites themselves would be rather small.
However, a Neutron Bomb is only viable as a weapon against civilian population, as modern tanks seem to be largely shielded against any type of radiation, thus making the Neutron Bomb a WMD, but not a weapon of war.
Why not a particle beam? Since neutrons aren't electrically charged (nor do they have any potential differences as in virtual charges), they cannot be influenced by electromagnetic fields, making particle accelerators unsuitable to produce a coherent beam, even though it is possible to directly produce such particles in such a device.
I'd say as a weapon of war, both proton beams and neutron bombs are completely useless. Spaceships and stations already need to be strongly shielded against solar radiation, and there is no reason why spaceships would not follow the same path as ground-based tanks and ships already do today and get shielded against weapons of this type: a pure radiation weapon is not feasible.
The only way I can think of a use for an enhanced radiation weapon like neutron bombs would be if you wanted to kill large amounts of civilian population or infantry in open field for whatever reason, on ground that is.
Given the current development of warfare technology, I don't think you'll ever find the latter much, as I'd expect the Infantry to be Mechanized, which would mean that most of the time, the infantry will be shielded by the vehicles they are in.
PArticle Accelerators suffer from the same drawbacks as Lasers when used in an Orbital-To-Ground role, as any atmosphere will scatter and absorb much of the energy you pump into it without achieving anything.
The easiest and most effective weapon in any of the scenarios would be a mass driver, particularly rail guns. If you'd really need a particle weapon, it'd be a heavy element ion beam, using elements at least as "heavy" as iron for it. Otherwise, if a radiation weapon is needed, use a MASER (aka. Microwave Laser).
If you'd really need a WMD, use mass drivers. With sufficient speed, any sufficiently large projectile can have enough energy to rival nuclear weapons without irradiating the area. Also, they'd be almost impossible to intercept. Fusion and Fission weapons as well as Anti-Matter weapons, unless achieving a direct or near-direct hit, are only usable as a way for electronic warfare in space, blinding or burning out sensors. Obviously, all of these would be feasible as a WMD in Orbital-To-Ground warfare, although I question the military use of WMDs in general, especially in an offensive role (why are you even attacking, if you dont gain anything from it. Using WMDs instead of precision weaponry only destroys what you could have conquered).