Electron density at 1.2 ns when the ion strikes at x = 8 μ m with (a) Φ p = 0 , (b) Φ p = 1 × 10 12 cm − 2 , (c) Φ p = 1 × 10 13 cm − 2 , and (d) Φ p = 5 × 10 13 cm − 2 .