Theoretical and experimental investigation of heat dissipation performance of graphite collectors with different thermal conductivities

Background With the advancement of repetitive frequency relativistic backward wave oscillators (RBWOs) in both pulse power and average power, electron beam collectors, as critical components of RBWOs, face significant thermal management challenges. Among candidate materials such as stainless steel, titanium alloy, and graphite, graphite has emerged as the preferred collector material owing to its exceptional combination of low density, elevated melting point, superior thermal conductivity, and remarkable resistance to electron bombardment.

Purpose This study aims to reveal the mechanisms by which differences in thermal conductivity influence the temperature evolution of the collector, the heat dissipation efficiency of the system, and the resultant microwave output waveform.

Methods Three high-purity graphite specimens with different thermal conductivities were selected. CFD simulations were conducted to investigate the influences of thermal conductivity and heat flux density on the internal and external surface temperatures of the collector. Subsequently, high-power microwave output experiments were performed on collectors fabricated from the three graphite materials.

Results The simulation results revealed that at a heat flux density of 660 W·cm⁻², the inner surface temperature of the graphite collector was approximately half that of stainless steel and titanium alloy collectors, and remained well below its sublimation point. Furthermore, higher thermal conductivity of graphite led to lower inner surface temperatures. As the heat flux increased from 500 W·cm⁻² to 800 W·cm⁻², both inner and outer surface temperatures rose, with the rate of temperature rise remaining nearly constant, indicating a linear temperature response. Experimental results from high-power microwave output tests demonstrated a significant correlation between collector material thermal conductivity and microwave waveform quality. Statistical analysis over 300 pulses revealed that the flat-top ripple coefficients for graphite 1#, 2#, and 3# collectors were 8.36%, 4.68%, and 3.59%, respectively.

Conclusions This indicated that improved thermal conductivity significantly reduced ripple coefficient and enhanced waveform flatness, highlighting the critical role of efficient thermal management in achieving high-quality high-power microwave output.