Coaxial dielectric barrier discharge（DBD） is very important in environmental protection and energy fields. The electrode structure is one of the key factors to influence the discharge characteristics of the coaxial DBD reactor. We developed a coaxial double dielectric barrier reactor driven by a nanosecond（ns） pulsed power supply

and investigated the influence of outer electrode length on characteristics of coaxial DBD. The electrical

optical

and temperature diagnoses methods were adopted to analyze the discharge characteristics. The electrical parameters of the coaxial DBD were obtained using an equivalent electrical model. The power deposition and energy efficiency with various electrode structures were further calculated. Moreover

the discharge uniformity

discharge power and energy efficiency with various electrode structures were studied

and the operation temperature and heat loss of the coaxial DBD reactor were analyzed by a heat conduction model. The results show that ns pulsed coaxial DBDs with various outer electrode lengths are uniform and the effect of the outer electrode length on the discharge uniformity is negligible. With the increase of the outer electrode length

the energy efficiency and the average discharge power across the air gap increase accordingly. With the increase of operation time

the operation temperature increases first and reaches a saturated value after 900 s operation. At 230 mm outer electrode length and 24 kV applied voltage

the energy efficiency of the coaxial DBD is 71.3% and the average discharge power across the air gap is 34.3 W. The corresponding operation temperature of the coaxial DBD reaches 80.1 ℃after 900 s operation and the temperature of the inner dielectric barrier is 135.3 ℃. Analysis with the heat conduction model shows that the energy loss of the reactor presents the heat loss. The operation temperature and the temperature of the inner dielectric barrier increase with the increase of the outer electrode length. The heat loss rate decreases with the outer electrode length.