Combustion requires fuel (wood), heat (element), and oxygen... You could change the wood and possibly cut down on the combustion, but the reality is you want it to smoke and and wood that starts smoking is at risk of combustion if it's hot enough and has enough oxygen. You can ramp up the heat over time as suggested by Jim to try to prevent the combustion. Once the smoker is full of smoke it will have less oxygen and the heat can go up some more. Finally, you can restrict airflow/oxygen to the wood which is what the foil does. The best way to do that is to restrict the access inside the smoke box by reducing the number and size of holes through which oxygen can be introduced.
On a side note, here's a shot of some of my wood after a smoke from a some time ago...