Vd = -0.4Vth Vdi = 0.4Vth

In this case, type 2 double layer is not clearly formed in comparison with the

previous case (a type 1 weak double layer is found near x=370).  A type 2 weak

 double layer, which has small amplitude of  electrostatic potential, exists 

near x=250.  Because the drift velocity of hot ions is too large to give energy

 to ion acoustic waves, the growth of solitary wave is not so large.  Furthermore,

 many hot ions are not reflected by this solitary wave because of the small size

 of the potential barrier. Type 2 weak double layers are not formed, if there are not

 plenty of hot ions resonant with ion acoustic waves.