From: nuny@bid.nes   
      
   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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)