Physical origin for differences in electrical tree structures

Computation of breakdown structures from the discharge-avalanche model have shown that in spite of the generation of lateral damage an unbranched runaway is produced unless the local field fluctuates in time as the tree propagates, in which case a fractal branched tree is produced. Here we show that the tree structure produced may range from branch to bush depending upon the way in which space charge modifies the local fields, and relate the time dependence of the modification factors to the discharge activity during propagation.