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Highlights • Both of DBD plasma deposition and direct fluorination can suppress the surface charge accumulation and increase the vacuum flashover withstanding voltage. • Due to the introduction of shallow traps and polar hydrophilic groups in the reaction process of the DBD plasma deposition and oxyfluorination, the surface conductivity and the decay rate of surface charges are increased. • The degradation of the surface modification effect is attributed to the unstable polar hydrophilic groups. Abstract Polystyrene (PS) as an industrialized insulating material is widely used in the pulsed power system, due to the good mechanical and electrical properties. However, the surface flashover of dielectric material limits the development of pulse power technology seriously. It is believed that the surface charge dynamics are related to the surface flashover closely. Therefore, in this paper, several surface modifications including plasma deposition assisted by dielectric barrier discharge (DBD) and direct fluorination (with/without O 2) are utilized to enhance the surface electrical properties of PS. The surface charge dynamics characteristics before and after surface modifications are investigated. The experimental results show the decays of surface charges for DBD plasma deposition and oxyfluorination become faster significantly. The surface potential of PS after DBD plasma deposition and oxyfluorination decreases to around 0 V over 100 s. The FTIR spectra indicates that a large amount of polar hydrophilic groups are introduced on the surface of PS samples by DBD plasma deposition and oxyfluorination, which are conducive to the dissipation of surface charges. The decay rate of surface charge after fluorination treatment without O 2 is also increased, which is associated with C F and COOH groups in the fluorination layer. Due to the rapid reduction of accumulated charges, the flashover voltage of all modified samples are increased by more than 30%. Furthermore, the stability of surface modification layer is compared. It is found that the stability of fluorination is better than oxyfluorination and DBD plasma deposition. [ABSTRACT FROM AUTHOR]

• Using APPJ surface modification, a TiC x O y N z film can be deposited on PS. • The surface withstanding voltage of modified samples are improved by 50%. • The maximum SEY of sample with TiC x O y N z film is approximately decreased to 1.5. Surface flashover at the solid dielectric/vacuum interface is a fundamental physical phenomenon, which restricts the development of pulsed power system and high voltage electrical equipment. The initiation and developing processes of surface flashover at the solid dielectric are associated with electrode-dielectric interface, dielectric surface and other external factors. Especially, the surface charges transport properties that related with surface flashover developing process are determined by the physicochemical of the dielectric surface. Therefore, the surface insulating performance can be enhanced by deposited with appropriate functional films. TiN film has a low secondary electron yield (SEY), which can help to suppress the secondary electron multipactor effect and suppress the surface flashover. In this paper, a preparation method of film contained TiC x O y N z using atmospheric pressure plasma jet (APPJ) is proposed. The experimental devices include AC power supply, plasma jet chamber and gas/precursor system. The AC power supply has a power of 1000 VA and frequency of 20 kHz. The precursor tetraethyl titanate (TTEO) is pumped into plasma jet chamber by argon gas and the working gas is nitrogen. The experimental results show that the maximum SEY of sample with TiC x O y N z film is approximately decreased to 1.5. The flashover voltage of sample with TiC x O y N z film is increased obviously. The morphology of the dielectric surface indicates that the rough surface is introduced after modification. Also, the physical processes of film deposition are investigated by the emission spectroscopic analysis. [ABSTRACT FROM AUTHOR]