In a large pot over medium heat, sauté the onions and green peppers for about 5 minutes.  Now, add the ground turkey, salt, chipotle powder, cumin, chili powder and brown sugar.  Cook until the turkey is cooked through…use a large spoon to crumble the turkey as it cooks.  Add the sweet potatoes and cook for another 5 minutes.  Now, add the black beans, tomatoes, diced chilis and chicken stock.  Mix everything together- bring to a boil, and then reduce to a simmer and cover.  Cook for about 40 minutes- until the sweet potato is cooked through.