“…[24], a parameter t R is used to refer the repetition time of the release of the thermal wave. It is believed that when t R is small, the background level of VLD is generally higher because the frequent release of the thermal wave in He II will cause the VLD does not have enough time to recover to the previous level, thus the next evolution of VLD will start from a relatively higher level; and vice versa, t R = 5 s. The initial VLD of is 1 · 10 6 cm À2 , the same as that used in [24].…”