The influence of electrode surface structure on behavior of plasma in high-power electron beam diode was investigated using a high-speed intensified sCMOS camera. In high-power electron beam cathodes, explosive electron emission (EEE) is typically utilized to generate electron beams, where plasma formation is essential. Plasma in high-power diodes serves as an electron source but also contributes to gap closure, making its behavior critical to understand. This study compared planar and groove-type cathodes, revealing that although the difference in the field enhancement factor between these two cathode surfaces was 2.3-fold, the difference in the threshold electric field for EEE was lower than expected. This discrepancy was attributed to ion bombardment on the cathode surface, primarily caused by ions generated near the anode edge. Since these ions were predominantly produced at the anode edge, where the field enhancement factor was high, plasma formation was also localized on the corresponding regions of the cathode surface. The localized plasma formation led to concentrated power consumption and expansion, ultimately resulting in gap closure. In contrast, groove cathodes exhibited uniform plasma expansion without gap closure, suggesting that grooves can enhance diode performance by promoting uniform plasma formation.