Electrical Engineer Required for a small task

main idea of work is Capacitor charge in lower part of a sinus voltage. I need an idea of how to charge a capacitor when the voltage is in decay on a sinus voltage wave form (only the positive side). I have attached an image, see the blue part of this image when voltage reaches its positive peak and starts decaying to zero. I need to conserve that energy somewhere, look at the blue side of the image. I can't do it in a capacitor. It is produced by magnet over coil. PS: Its just a bad drawing, nothing is child proof, I need an idea of how to conserve that ( blue on the image ) energy somehow(eg. condenser, battery, etc) or at least much of it. you should have an oscilloscope to show me the demonstration. I would need for you to produce a sine wave like from a generator (coil + magnet) and save the "going down sine" energy somehow, is this making any sense?