Research on space charge packet migration combining the electron beam irradiation technique and real-time space charge distribution measurement by piezo-PWP method

The space charge packet is a special sort of space charge phenomenon that is characterized by the migration of aggregated space charge in the form of a packet within the sample. Currently, most of the experimental and simulation studies on the generation and migration of space charge packets are focused on the space charge packets with positive polarity in polyethylene and its cross-linking products, while the characteristics of the space charge packets with negative polarity are yet to be studied. This paper presents an integrated experimental system for the researches on space charge packet with negative polarity, which enables experimental studies combining electron beam irradiation technique and real-time space charge distribution measurement. The beam flux for electron beam irradiation is controlled by a metal grid with an optical fiber-electric relay and the space charge distribution measurement is performed by the piezo- pressure wave propagation method. The system achieves a withstand voltage value of 17 kV and a measuring resolution of 25 ns for space charge distribution measurement and is suitable for experimental studies of various material samples with different thickness ranges under different electric fields. In this paper, the migration behavior of space charge packets with negative polarity in polypropylene (PP) and polymethyl methacrylate (PMMA) samples under different applied electric field (15-20-25-30 kV/mm) were studied using the experimental system. The relationship between the migration properties of carriers with negative polarity (electrons) and the electric field can be extracted from the experimental results. The "negative differential mobility" phenomenon was found for both materials, i.e., the migration rate decreases with the increase of the electric field. The threshold electric field for the "negative differential mobility" phenomenon of electrons in PP samples is about 26.0 kV/mm while the threshold electric field for the "negative differential mobility" phenomenon of electrons in PMMA samples is 19.5 kV/mm and the phenomenon disappears at an electric field of 27.5 kV/mm. The electric field where the "negative differential mobility" phenomenon of electrons appears and disappears within different materials can be extracted using the experimental system proposed in this paper.