Structure: (4) Powering Climbers: Both power and energy are significant issues for climbersthe climbers need to gain a large amount of potential energy as quickly as possible to clear the cable for the next payload. Various methods have been proposed to get that energy to the climber: Transfer the energy to the climber through wireless energy transfer while it is climbing. Transfer the energy to the climber through some material structure while it is climbing. Store the energy in the climber before it starts - requires an extremely high specific energy such as nuclear energy. Solar power - power compared to the weight of panels limits the speed of climb. Wireless energy transfer such as laser power beaming is currently considered the most likely method. Using megawatt powered free electron or solid state lasers in combination with adaptive mirrors approximately 10 m (33 ft) wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency. For climber designs powered by power beaming, this efficiency is an important design goal. Unused energy must be re-radiated away with heat-dissipation systems, which add to weight. Yoshio Aoki, a professor of precision machinery engineering at Nihon University and director of the Japan Space Elevator Association, suggested including a second cable and using the conductivity of carbon nanotubes to provide power. Various mechanical means of applying power have also been proposed; such as moving, looped or vibrating cables