In the book "The Space Elevator", the author proposed a laser-powered, plasma-ion thruster onboard the initial craft to get it to its unraveling point at GEO.  I don't have the book in front of me, but I think the Isp for that system was ~40,000. 

This proof-of-concept was demonstrated today of a Photonic Laser Thruster:
http://www.baeinstitute.com/pr1.html

The idea is to provide thrust on a mirror from lasers, which (if kept well focused enough) can be bounced back and forth ~3,000 times to amplify the thrust.  The demonstration (made from off-the-shelf parts, apparently) produced 35 uN of force.  Theoretically these drives have an Isp of 40 million.

My question: could this technology scale up to a level where it could be used to ferry the ribbon seed-craft into GEO?  Some contemplation on materials over at: http://advancednano.blogspot.com/2007/02/photonic-laser-propulsion.html
suggests that the best mirrors (20,000 reflections) would yield 130 mN per kilowatt laser power (0.13 N/kw).  With very powerful, ground-based lasers (like the hundred MW beasts used later to power the lifters) this could translate into significant thrust. 

I don't have a copy of the book handy, but it has info for: 1) laser power, 2) initial launch vehicle weight, 3) thrust from proposed plasma-ion engines.  If someone has the book, I would appreciate that info, to calculate if this is sufficient or better than the proposed design. 


note: this would also displace some (but not all) of the thruster fuel mass, though it would add mass for the mirror.  The thrusters might have to remain on the ship, in order to make the transition into an equatorial orbit from a polar one; though I suppose it's possible that the angle of the driving laser hitting the satellite could be used to change the orbital direction. 

cheers, Hans

Force = mass * acceleration
F = m a

The acceleration to rise has to be stronger than gravity say 1.1 g = 1.1 * 9.81
Mass of climber say 20,000 kg

Required force F = 20000 * 1.1 * 9.81 = 215,820 newtons or 216 kN

The climber on an Edwards type ribbon is travening much less than orbital speed, so graviy at 300 kilometers altitude is still about 0.99g The climber must overcome significant gravity until it reaches an altitude of about 35,000 kilometers.
Some beam spreading occurs, so a beam which has traveled a million kilometers total over many reflections requires a prohibitably large mirror at both ends, especially if the mirror aiming is typically in error by one second of arc. At visable light wavelengths, the mirors also require extreem surface precision to avoid scattering most of the light out of the beam. That is not to say the concept is entirely worthless, but we will be disappointed if we expect a big improvemement.  Neil