From: daellis94@gmail.com   
      
   On Monday, June 18, 2018 at 1:52:47 PM UTC-4, nu...@bid.nes wrote:   
   > On Friday, June 8, 2018 at 7:38:08 AM UTC-7, David Ellis wrote:   
   >    
   > (snip)   
   >    
   > > The big problem I still have is whether or not it is even remotely feasible   
   > > to suggest that I could take a laser and arbitrarily compress a quarter of   
   > > a second of average power into a microsecond pulse.  I feel like I would   
   run   
   > > into some physics or engineering bottlenecks that I just don't know about.   
   >    
   >   Compressing laser pulses usually involves a secondary amplifying medium   
   like the old standard neodymium glass discs used in the Shiva/Nova systems.   
   They have a tendency to get hot of course because they can't be 100%   
   efficient, and the more you time-   
   compress a laser pulse the faster the temperature of the medium rises,   
   increasing the danger of damaging the medium. The COIL laser you mentioned   
   would be useless as it's IR unless you also used a frequency doubler or   
   quadrupler which will also get hot.    
   All of this steals beam energy obviously cranking up your basic laser   
   power-handling and -production requirements.   
   >    
   >   You might as well start with the most efficient laser sources which are   
   diodes (I've seen up to 70% joules electrical in/joules of light out quoted).   
   They are way easier to get short pulses out of than chemical lasers without   
   using secondary active    
   media. We have UV laser diodes now (blu-ray) that can produce picosecond   
   pulses (at low power), and there's no fundamental reason they can't be scaled   
   way up to get weapon-grade power-per-pulse levels by making them broader   
   rather than longer, or just    
   using a whole bunch of them in flat arrays.   
   >    
   > Since they inherently have high beam divergence you'll need a focusing   
   system, and since lenses tend to be massive I'd consider an active mirror   
   which will permit rapid refocusing plus handle beam steering, and maybe a   
   final exit window to protect the    
   mirror (phase-locking diode lasers is tricky and phase-shifting them for beam   
   steering is even trickier- I don't know of it being done with more than two   
   diodes). The window will need to be something like calcium or magnesium   
   fluoride because the most    
   popular UV-transparent material, quartz, gets too absorbent at such short   
   wavelengths.   
   >    
   >   That 70% leaves you with almost half the energy you put on target   
   loitering around in the diode and its support structure. You say you want to   
   dump ~30 MW into a target all at once, that means you need to dissipate 15 MW   
   of heat out of the laser, _   
   per shot_. Four shots per second averages to 60 MW of cooling you're gonna   
   need.   
   >    
   >   I'm also wondering why you assume iron targets- missiles these days are   
   built of lighter weight aerospace alloys and composites, but then most of them   
   are still good UV absorbers.   
   >    
   >   Anyway, figure out the energy density you want on the target per pulse and   
   how often you want to hit it (joules per area per second) to damage it so that   
   when it hits it won't go BANG (hopefully this won;t be trial and error) and   
   work backwards    
   through your system to see how much power you're asking a diode system to   
   handle per square meter of emitter.   
   >    
   >    
   >   Mark L. Fergerson   
      
   Thanks, Mark.     
      
   I was working on the assumption that the lasers might be free-electron lasers,   
   based mostly on mention of the FEL having a theoretical efficiency of up to   
   65% (beam power vs electrical input) according to Atomic Rockets.     
      
   Aside from that, AR also made it seem like FELs might have fairly good beam   
   quality compared to other laser types, which would also be important in a beam   
   weapon from what I can tell.     
      
   Either way, be it diode or free-electron laser, I figure each pulse would see   
   the beam having an instantaneous power of over one terawatt, assuming 5   
   megawatts (I mostly just used the YAL-1's COIL as an example for what sort of   
   beam power I might want)    
   of average power being emitted as a beam.  This is based on the four pulses   
   per second (making 1.125 megajoules per pulse), and microsecond pulses, which   
   means the beam power DURING each pulse would be 1.125 mega-megawatts, which   
   is, of course, in    
   terawatts.     
      
   From Atomic Rockets, I get the impression that a laser's beam quality will   
   suffer as it heats up more and more, meaning a laser of a given mass that just   
   has more and more watts pumped into it will function more poorly until, at   
   some point, it melts and    
   ceases to function at all.  Obviously, a terawatt is an enormous amount of   
   power, even if it is only sustained very briefly and thus yields a total   
   energy output that is much more reasonable.     
      
   That leads me to the trouble of how to DELIVER that kind of power to the   
   laser, since I imagine I need something to deliver, say, a terawatt (really,   
   more like two terawatts if we count for efficiency) of electricity to the FEL   
   or to the diode, and of    
   how to decide whether or not it would make sense for a laser roughly sized for   
   a 5 MW beam output would believably survive ACTUAL power outputs in the   
   terawatt range for brief moments each second.     
      
   As for the steel/iron assumption, I just used that as an arbitrary material   
   that is a familiar armor-type material in the present day.  I'm used to   
   thinking of weapon performance in terms of how it can perform against steel   
   targets, so as a stand-in    
   meant to help me gauge how much firepower I am dealing with, I decided to use   
   steel for my thought experiment.     
      
   Of course, the missiles that would be targeted by my lasers would probably   
   never have any steel in their space-frames.  It would all be aluminum or,   
   perhaps even more like, some kind of fancy carbon-composites.     
      
   --David Ellis   
      
   --- SoupGate-Win32 v1.05   
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