(a) First-order excited resonance

(b) Second-order excited resonance

(c) Combination resonance of difference types

(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.