Theoretical analysis and numerical simulations on a plasma relativistic microwave noise amplifier

The physical mechanism and output properties of the plasma relativistic microwave noise amplifier (PRNA) is studied numerically using the all electromagnetic PIC(Particle-in-Cell) code. Firstly, the dispersion relations between the operating mode and the slow space charge wave of relativistic electron beam without coupling are simulated and analyzed. Simulation results show that both the plasma density n_p and radial thickness Δr_p affect the dispersion characteristics markedly and the frequency at the beam-wave resonant point increases with the increasing of them. The beam voltage and current also affect the resonant frequency, but the effect is relatively slight. Secondly, variations of the linear growth rates and the bandwidth are then evaluated using the linear theory. Calculations show that the PRNA has the virtue of wideband output. Its bandwidth can reach GHz level. By adjusting the plasma parameters n_p and Δr_p, the relativistic electron beam voltage and current, the operating frequency can be tuned over a wide frequency range. Therefore the PRNA also has virtue of fine frequency tunability. Based on the above calculation results, the whole PIC simulations of the PRNA are then carried out to verify the virtues of wideband microwave output and frequency tunability. The basic features of the field distributions of the operating during the evolution and out coupling processes are given. The bunching and energy release process of relativistic electron beam are also plotted. Simulations show that with the plasma density of 1.4×10¹⁹/m³, the beam voltage and current of 500kV and 2kA and the applied magnetic field of 2.0 Tesla, 200MW output microwave with efficiency about 20% can be obtained. The frequency range is from about 7.0GHz to 9.0GHz, the band width reaches 2GHz. And the output mode is the TEM mode of the coaxial waveguide. Both n_p and Δr_p affect the dispersion relations markedly and the output frequency increases clearly with the increasing of n_pand Δr_p. The beam voltage and current affect the output frequency relative slightly and the gap distance between the plasma and electron beams has almost no effect on the output frequency. The research results will provide useful references for the further design of the PRNA.