659872.fig.007a
(a) First-order excited resonance
659872.fig.007b
(b) Second-order excited resonance
659872.fig.007c
(c) Combination resonance of difference types
659872.fig.007d
(d) Combination resonance of sum types
Figure 7: Instability boundaries for various applied electric fields: curve a,  kV/mm;  curve b,  kV/mm;  curve c,  kV/mm;  curve d,  kV/mm;  curve e,  kV/mm.