Laser-induced electrical discharge Attracting lightning with an ion beam has been done in a lab but how strong of a laser is needed to reach the clouds to redirect the lightning bolt?

 A: I can talk about the discharge experiment since I am not aware of any experiment to trigger real lightning with lasers. Note Discharge in lab is different than the lightning however the effort of all experiments is towards taming the lightning.
Laser filaments are low density plasma channels (order is $10^{16}cm^{-3}$). This channel could be achieved by focusing a several mili-Joules pulses of 800nm and hundreds of femtosecond duration into air. A variety of energies and pulse durations work as well, however the plasma density remains the same. In laser physics this orders of energies are a lot and you can do many cool stuff with them.
Some technical stuff for future lightning experiments are:
Since the ionization draws energy from the pulse, so, couple of meters after the focusing lens, the pulse no longer has enough energy to ionize the medium. For such short and strong pulses even without a lens the beam starts to collapse on its axis of propagation as if there is a lens. This effect is called Kerr Lensing which depends on the intensity of the beam and causes your beam to focus before the clouds and hence the ionization starts too early before reaching the desired altitude and as a result the ion channel is established prematurely. In order to avoid this unwanted lensing effect you can make your beam bigger and focus it with a very long focal length lens.
