The Effect of Seed Electrons on the Repeatability of APPJ Propagation

APPJs are typically composed of bullet-like plasma volumes travelling with velocities of $10 ^{4-6}\mathrm {m} /\mathrm {s}$, which is close to the propagation velocity of traditional streamers. However, in contrast to the traditional streamers, which propagate in a stochastic manner, plasma bullets have been found to exhibit high repeatability, i.e., propagating the same distance after the same delay time. To understand the high repeatability of the plasma bullets, lots of work has been carried out recently. According to all the studies reported, it is believed that the repeatability of the plasma bullets might be related to background ionization level which is reflected by the density of seed electrons $( n_{seed})$in the gas channel. However, it is not known how high the seed electrons $n_{seed}$is needed for the plasma bullets to propagate in repeatable mode. Besides, when the plasma bullets exhibit random behavior, there are two potential reasons that could result in the unrepeatability, one is the randomness of the ignition and another one is the inconsistence of the propagation speed. This is not clear either.