Launch | Keeping beam pointed on meter-scale lightsail
There are a number of effects that make this task difficult. These include beam instabilities, laser mode issues, differential forces on the sail, differential heating of the sail, and instabilities in the atmosphere induced by the energy of the beam.
The above challenges can be mitigated by spinning the sail, and by shaping both the sail and the beam. Feedback from the sail to the array helps, but the short time of flight requires a self-stabilizing system. The firing time of the beamer is on the order of 10 minutes. During this time the Earth rotates, requiring some beam agility on the order of 2 degrees. Coarse sub-element pointing and fine pointing via phase corrections at the array system can provide beam agility on the order of 30 degrees in order to accommodate various targets.
One promising approach is to shape the sail so that its position on the beam is stable— i.e. the spinning sail itself experiences torques and forces that restore its position and orientation as low-frequency pointing errors move the beam away from the sail’s centroid. High-frequency jitter degrades the overall power imparted to the sail, but the sail’s dynamics limit its susceptibility to such disturbances above a certain bandwidth.
Since a phased array would be used to form the spot, the beam profile could be shaped to maximize the sail’s ability to maintain its own position on the beam without active feedback control.
- Hughes, G.B., Lubin, P., Griswold, J., Bozinni, D., O’Neill, H., Meinhold, P., Suen, J., Bible, J., Riley, J., Johansson, I., Pryor, M. and Kangas, M. “Optical modeling for a laser phased-array directed energy system,” Nanophotonics and Macrophotonics for Space Environments VIII, edited by Edward W. Taylor, David A. Cardimona, Proc. of SPIE Vol. 9226 (2014)