1. In a medium saucepan over medium heat, combine sugar, orange juice, butter, cinnamon stick, nutmeg, and salt. Stir to combine and bring to a boil, then reduce to a simmer and cook until it has thickened slightly, about 10 minutes. When reduced, remove and discard cinnamon stick.
2. As the syrup thickens, prepare sweet potatoes: Peel and slice into 1/2" rounds, then layer in greased foil tray. Pour thickened syrup over potatoes and cover tray with foil.
3. Preheat smoker to 400F. Smoke for 30 minutes, then remove foil and continue to smoke for another 50 minutes to 1 hour. Approximately every 15 minutes, use a spoon or turkey baster to baste the surface of the sweet potatoes with the syrup.
4. Remove from smoker and allow to rest for about 15 minutes before serving.