Axial motion of collector plasma in a relativistic backward wave oscillator

作者：Xiao Renzhen^*; Chen Changhua; Deng Yuqun; Cao Yibing; Sun Jun; Li Jiawei

来源：Physics of Plasmas, 2016, 23(6): 063114.

DOI：10.1063/1.4953915

In this paper, it is proposed that plasma formed at the collector may drift back to the cathode and cause pulse shortening of the relativistic backward wave oscillator. Theoretical analysis shows that the axial drift velocity of plasma ions can be up to 5 mm/ns due to the presence of space charge potential provided by an intense relativistic electron beam. Particle-in-cell simulations indicate that the plasma electrons are initially trapped around the collector surface. With the accumulation of the plasma ions, a large electrostatic field forms and drives the plasma electrons to overcome the space charge potential and enter the beam-wave interaction region along the magnetic field lines. As a result, the beam current modulation is disturbed and the output microwave power falls rapidly. The plasma ions move in the beam-wave interaction region with an average axial velocity of 5-8 mm/ns. After the plasma ions reach the diode region, the emitted current at the cathode rises due to the charge neutralizations by the ions. The impedance collapse leads to further decrease of the microwave power. In experiments, when the diode voltage and beam current were 850 kV and 9.2 kA, and the collector radius was 2.15 cm, the output microwave power was 2.4GW with a pulse width of less than 20 ns. The ion drift velocity was estimated to be about 5mm/ns. After an improved collector with 3.35 cm radius was adopted, the pulse width was prolonged to more than 30 ns. Published by AIP Publishing.

• 出版日期2016-6
• 单位西北核技术研究所