dfacto wrote:Mjolnir wrote:I haven't suggested anything remotely like firing solitary high-energy particles, I was in fact very specifically describing the very opposite...low particle energies compared to research/medical accelerators, and very high beam currents.
Just so I'm sure I understand you here: low energy, lots of particles? And when you say low energy you mean high speed, but low mass, so low KE per particle?
Exactly. Still highly relativistic, but that's not at all hard to achieve with particle accelerators.
Relativistic kinetic energy: E = m*c^2/sqrt(1 - (v/c)^2)
Proton mass: 938 MeV/c^2
WolframAlpha:
http://www.wolframalpha.com/input/?i=%2 ... 2%29*c%5E2
0.99c: 5.7 GeV/proton
0.9c: 1.21 GeV/proton
0.5c: 145.1 MeV/proton
dfacto wrote:Out of curiosity, what's the formula/formulas used here? Not to be a doubter, but you're throwing numbers around with no references.
Power is just energy (work) over time: P = dW/dt
Imparting the same energy over half the time means double the power. And time in the weapon:
t is time time spent in the gun, a is the acceleration, v is the end velocity, D is the length of the gun.
D = 0.5*a*t^2, a = v/t
Some algebra gets you:
2*D/v = t
So time scales proportionally to barrel length and inversely proportional to exit velocity. Given the same projectile energy and the same efficiency at converting energy into kinetic energy of the projectile, peak power scales inversely with weapon length and in direct proportion to the exit velocity. There's no simple equations for modeling losses, but generally the faster things happen, the more you leave the system ringing after a shot with excess energy that needs to be absorbed by the cooling systems...plus the more you lose to EM radiation.
Railgun projectiles don't stay in the gun for long...about 10 microseconds was quoted for the Navy railguns. Making the weapon longer helps, but also produces longer conductors that slow pulse rise times and increase losses, plus experience more severe stresses during firing...also makes the thing harder to aim. Long weapons are probably going to be something more like a coilgun, more closely related to a linear synchronous motor than to a railgun.
dfacto wrote:So you would want something with ~15,000 km/s speed and energy of 110TJ/kg (
http://en.wikipedia.org/wiki/Alpha_decay)? That would require a shot containing grams of material. How does this compare to conventional linear accelerator injections, and what does that mean for energy requirements? Additionally, how would you deal with the acceleration itself? You can't have an emitter as a source because you will need very large amounts of particles all traveling in a tight bundle, and you can't have a kilometers long accelerator either. Would you use a cyclotron for initial acceleration before injecting the shot into a linear accelerator (barrel)?
For Outsider Terrans, I'm thinking about something with shots in the milligrams to grams range, not too far off from a baby version of the Loroi and Umiak plasma weapons. It's vastly greater than what's typically used in research accelerators, but the focus with research accelerators has been to achieve vastly greater particle energies, not beam currents. There will likely be plenty of issues with scaling beam intensity up, but not as severe as for scaling railgun velocities up...at the worst, you'll have to gang up multiple beam lines in parallel.
Huge beam intensities simply haven't been a particularly great design goal for research accelerators...about the only thing it would be useful for would be a space weapon without a space warship to mount it on, though some applicable work has been done for things like ion beam and electron beam milling/drilling machines. Looking at some numbers, I underestimated the energy of low energy alpha radiation, but the 0.9-0.99c range I was thinking of is still very far away from being so energetic that it passes through the target without depositing most of its energy. The impact characteristics *would* be different, there would be less momentum for the amount of energy, but such a weapon should still be able to deal large amounts of damage.
I don't envision any cyclotron, just a straight linear accelerator...more straightforward to scale up in beam intensity, and it's what existing particle beam milling machines use. And probably with a particle source more like those used in plasma drives in development now, such as VASIMR.
dfacto wrote: This is another advantage of particle beams...the lower flight time makes it easier to actually hit the target.
Indeed. But remind me why you wouldn't just create a more powerful laser and tag ships multiple light seconds away with ease?
Well, lasers are generally even less efficient and more difficult to scale up in power. And in fact, the ones most likely to be used as really high power (MW-class) weaponry are free electron lasers, which use a high-power particle accelerator (generally a linac) to produce the laser beam.
http://www.onr.navy.mil/Media-Center/Pr ... stone.aspx
http://www.onr.navy.mil/Media-Center/Fa ... Laser.aspx