How to increase distance between Nodes in ForceDirectedLayout

I use ForceDirected Layout for my diagram but after diagram loaded nodes are too close. (See the pic)
I tried to move nodes manualy after load but When i expand a node nodes went to first situation
How can i increase distance between parent and child nodes?

You probably need to adjust the forces between the nodes/links. Try changing the values for ForceDirectedLayout.defaultElectricalCharge and ForceDirectedLayout.defaultSpringLength.

You may also be interested in the interactive Force Directed Layout sample to see how changing the forces affect the graph, along with the API page.