or Direct Energy Weapons are the second type of the three typically used large scale weapons systems used in space battles (the other two being Kinetic Kill Cannons and Missiles).
A long time ago lasers and particle beam cannons were considered to be the weapon system of the future, superior to conventional ballistic guns and only limited by their great need for energy. Soon after mankind settled the solar system DEWs proved to be rather useless for most combat applications. The main reason was that all energy weapons suffer heavily from diffraction. Especially lasers depend on focusing their energy on very small spots to melt through ship armor and damage interior systems. Unfortunately the laws of physics cause laser beams to widen dramatically on longer distances and to lose their damaging capacity, while still consuming enormous amounts of energy.
Lasers are notoriously low efficiency compared to projectile weapons. But that’s not the only issue. When comparing hypervelocity projectile impact research with laser ablation research, one discovers a stark contrast in their efficacy. Laser ablation is simply less effective at causing damage than projectile impacts. Whereas hypervelocity projectiles cause spallations and cave in armor effectively, laser ablation is poor, with energy wasted to vaporization, radiation, and heat conduction to surrounding armor. On the other hand, at very close ranges, where diffraction is not an issue, lasers outperform projectiles easily. Unfortunately, nothing aside from missiles will likely ever get that close, and even then, they will likely be within close focus ranges for milliseconds at most.
Lasers still useful at long ranges, though. Lasers fill a very specific niche in space warfare, and that is of precision destruction of weakly armored systems at long distances. Lasers are very good at melting down exposed enemy weapons, knocking out their rocket exhaust nozzles, and most importantly, killing drones. While many missiles have very few weak points, and can shrug off laser damage with thick plating, drones have exposed weapons and radiators, which makes them very vulnerable to lasers.
Lasers are also perfect for Point Defense Systems (PDS or CIWS) as they can easily create beams very fastly at almost any direction to blind or damage incoming missiles. While not strong enough to deviate a KKC shot or stop an armored missile, they can effectively shoot down smaller and lighter missiles used in volleys or against drone swarms.
Laser weapons require huge amounts of energy and large batteries to store that energy. While the actual weapon is rather simple and small, not even requiring barrels, the supporting parts of the system inside the ship hull are heavy, large and fragile. Laser weapons are rather cheap systems as they don’t suffer from the high maintenance cost of KKC weapons or the initial build costs for missiles.
“Say you have an ultraviolet (20 nanometer) laser cannon with a 3.2 meter lens. Your hapless target spacecraft is at a range of 12,900 kilometers (12,900,000 meters). The Beam Radius equation says that the beam radius at the target will be about 4 centimeters (0.04 meters), so the beam will be irradiating about 50 cm2of the target’s skin (area of circle with radius of 4 centimeters). If the hapless target spacecraft had a hull of steel armor, the armor has a heat of vaporization of about 60 kiloJoules/cm3. Say the armor is 12.5 cm thick. So for the laser cannon to punch a hole in the armor it will have to remove about 625 cm3 of steel (volume of cylinder with radius of 4 cm and height of 12.5 cm). 625 * 60 = 37,500 kiloJoules. If the laser pulse is one second, this means the beam requires a power level of 37,500 watts or 38 megawatts at the target. In practice, a series of small pulses might be more efficient, causing a shattering effect and driving chips of armor out of the hole, which of course requires less energy than actually vaporizing the armor.” (from http://www.projectrho.com)
Particle Beam Cannons
A variation of directed energy weapons is the particle beam cannon. These systems accelerate particles to nearly the speed of light. Particle beams have a advantage over lasers in that the particles have more impact damage on the target than the massless photons of a laser beam (well, photons have no rest mass at least. The light pressure exerted by a laser beam pales into insignificance compared to the impact of a particle beam). There is better penetration as well, with the penetration climbing rapidly as the energy per particle increases. Particle beams deposit their energy up to several centimeters into the target, compared to the surface deposit done by lasers.
They have a disadvantage of possessing a much shorter range. The beam tends to expand the further it travels, reducing the damage density (“electrostatic bloom”). This is because all the particles in the beam have the same charge, and like charges repel. Self-repulsion severely limits the density of the beam, and thus its power.
They also can be deflected by charged fields, unlike lasers. Whether the fields are natural ones around planets or artificial defense fields around spacecraft, the same fields used to accelerate the particles in the weapon can be used to fend them off.
Particle beams can be generated by linear accelerators or circular accelerators (AKA “cyclotrons”). Circular accelerators are more compact, but require massive magnets to bend the beam into a circle. This is a liability on a spacecraft where every gram counts. Linear accelerators do not require such magnets, but they can be inconveniently long. Another challenge of producing a viable particle beam weapon is that the accelerator requires both high current and high energy. We are talking current on the order of thousand of amperes and energy on the order of gigawatts. About 1e11 to 1e12 watts over a period of 100 nanoseconds. The short time scale probably means quick power from a slowly charged capacitor bank.