Particle Beam Weapons

Particle beam weapons fire a stream of relativistic atoms or subatomic particles, causing damage both through direct heating and severe radiation exposure.  They require long and bulky accelerators, limiting their use to large vehicles or fixed installations.  The particle beams pose a radiation hazard to all bionts and non-radiation hardened electronics in the vicinity of the beam's point of incidence and, in atmosphere, near the path of the beam itself.

Categories of Particle Beam

Electron Beam
Proton Beam
Exotic Particle Weapons

Electron beams are used in atmosphere where radiation weapons are needed in an atmosphere.  Highly relativistic electrons can penetrate relatively far through air, and by heating the air they pass through to a partial vacuum the leading edge of the beam can increase the range of the electrons that follow.  The high currents cause the beam to self-pinch, reducing beam spread due to scattering off air molecules.  Nevertheless, the beam constantly loses energy as it ionizes air molecules in the beam.  This limits the range to a few hundred meters or a couple kilometers at best in earth-like atmospheres.  At high altitudes or in thinner atmospheres, an electron beam's range is greatly extended.

An electron beam in air looks like a geometrically straight bolt of blue-white lightning, surrounded by a blue nimbus of Cerenkov radiation due to the electrons scattered from the primary beam.
Scattered electrons and bremsstrahlung x-rays create a high radiation environment both near the beam path and in the vicinity of the point of incidence of the beam.

Electron beam weapons have a minimum length of over a meter.  These minimal electron accelerators have a range of around 200 meters in air at the density of sea level on earth.  Longer accelerators can produce higher energy electrons that can penetrate further through air.  The upper limit is around one to two kilometers, for accelerators in excess of ten meters in length.  Electron accelerators are typically long, linear structures.

Electron beams are easily steered with magnets.  This enables the beam to be rapidly redirected without turning the entire accelerator.

In the vacuum of space, the highly charged electrons repel each other and the beam quickly loses focus.  In addition, the electrons are deflected by planetary magnetic fields and the magnetic fields in the solar wind, causing their paths to veer erratically.

Proton Beam
Proton beams are used for combat in vacuum.  Individual protons are accelerated to ultra-relativistic velocities.  As the beam exits the accelerator, it is neutralized by injecting an electron beam to cancel the charges.  This prevents self repulsion from defocussing the beam and keeps the beam from veering in ambient magnetic fields.

The primary limit to the range of a proton beam is the thermal velocity of the protons.  Neutralization of the beam unavoidably heats the beam due to the energy of recombination with the electrons.  After exiting the accelerator, they begin to drift apart at roughly 15 km/s.  The higher the proton energy, the farther the travel in the time it takes the beam to disperse.

Proton beam accelerators are typically circular tracks several hundred meters to several tens of kilometers in diameter.  Even the largest proton accelerators do not give their protons enough energy to rival x-ray lasers in range, and thus x-ray lasers dominate for deep space combat.  Proton beams are typically employed in craft designed for combat in planetary orbit, and find use in blockades and operations to achieve orbital superiority prior to a ground assault.

like electron beams, proton beams can be steered with magnets prior to neutralization.  In addition, the beam can be emitted from several ports along the ring diameter, allowing rapid retargeting.

The relativistic protons in these beams can be extremely penetrating, typically punching through a meter or so of solid or liquid matter before disintegrating into a shower of radiation, which itself can penetrate many more meters of solid or liquid matter.  These "cascade" radiation showers produce an extremely high radiation environment which will sterilize the area of all biological life and destroy unhardened electronics.  The only defense against a proton beam is thick layers of inert shielding, or using only radiation hardened control systems.  Proton rich shielding is most effective on a per-mass basis.

In an atmosphere, proton beams lose energy through ionization and direct collisions with the nuclei of air atoms, limiting their range to a few hundred meters in earth-like atmospheres.  While this is comparable to the range of electron beams in air, an electron beam accelerator is much more compact.

Exotic particle weapons
There are controversial reports that transapient intelligences have used neutron and muon beams in conflicts.  A neutron beam weapon, if it could be made, would pass through several tens of centimeters of solid matter with little interaction, but would rapidly interact with any material containing hydrogen (including water, wax, oil, and biological tissue), heating the hydrogenated material in the beam path.  A neutron beam would also cause residual radioactivity where it struck heavy elements.  There is little perceived benefit to a neutron beam over a proton beam, as the ranges in air and penetrating power would be about the same, unless the beam generator could be more compact or the thermal spread of the neutrons was lower (thus enabling a longer range in space).

Muon beams would be able to punch through kilometers of air, producing a very long range radiation weapon for use in an atmosphere.  Muons are short lived particles, however, and would decay within several tens of kilometers in any environment, effectively preventing their use in space combat.

Modosophont technology can create uncollimated beams of neutrons or muons at low efficiency, and commonly use such beams for research, but there is no known method to produce highly collimated, efficient beams suitable for use as weapons.

It is also rumored that high transapients have efficient plasma accelerators that allow significantly more compact proton and electron beam weapons.  Alternately, they may employ methods to cool the proton beam after neutralization, allowing significantly longer ranges. Again, while modosophonts have inefficient and poorly collimated plasma accelerators (such as wakefield accelerators) and have hypothesized about using laser cooling to reduce the spread of neutralized proton beams, there are no known methods of achieving these results.