Research And Application Of Carbon Fiber Velvet Cathode

It has been shown that, in comparison with conventional velvet, carbon fiber velvetcathode is more practical in such applications as high-power microwave generation andpumping of gaseous lasers for low out-gassing and good ablation resistance. In thisdissertation, the related aspects of carbon fiber velvet have been studied systematicallyand profoundly, such as material fabrication, establishment of the platform for opticaldiagnostics of cathode plasma, property investigations and applications. These effortsare of great theoretical and realistic significance for both basic and applied research ofthe carbon fiber velvet. The detailed contents include the followings:1. Fabrication of the carbon fiber velvetThe dynamics of the conductive fibers in electrostatic field were studiedtheoretically. The method based on the surface modification was proposed for thecarbon fiber dispersion in circumstances of gas. The N2/SF6gas mixture was filled intothe electrostatic flocking box to avoid the short circuit between electrodes. The velvetconsists of numerous2.0±0.5-mm-long carbon fibers aligned perpendicularly to thesubstrate. The fiber packing fraction is about5%.2. Establishment of the platform for optical diagnostics of cathode plasmaAs the basis of the platform, the pulse power modulator could provide an electricalpulse with voltage of several hundred kV and pulse width of110ns. The diode currentup to kA level could be extracted. The trigger signal for the camera was picked up afterthe main switch of the modulator, which could avoid the disadvantageous influence ofthe jitter time (ns~μs) caused by the breakdown of the gas gap. Then the samplednegative electrical pulse was converted into a TTL signal via a processor within5ns.The jitter time was less than1.5ns. And this was taken as the synchronization time base.The synchronization scheme relying mainly on electrical pulse delay supplemented bylight delay was employed to make sure that the camera could work synchronously withlight production and transportation from the cathode plasma within the time scale ofnanosecond. The shielding and filtering methods were used to make sure that thecamera could work properly and steadily within5meters of the modulator. At last, thecathode plasma imaing experiments were carried out on this platform. The temporal andspatial resolution was10ns and0.167mm/pixel, respectively.3. Research of the carbon fiber velvet properties(1) Research of the cathode edge effect. It was shown that the cathode edge effectcould be eliminated by using the screening ring. Based on this, the diode structure wassettled.(2) Comparison of the cathode plasma characteristics. It was shown that the carbonfiber velvet turned on at the electric field of7±1kV/cm, which was nearly one third of that of the conventional velvet. The long-term electrical stability of the diode could beimproved by using carbon fiber velvet cathode. The2-D expansion velocity of thecathode plasma was investigated by utilizing the digital image processing methods.During the rise time of the current pulse the plasma could cover the entire surface of thecarbon fiber velvet cathode in a shorter time. The value of the axial velocitycomponents of the carbon velvet cathode plasma decreased from2.3cm/μs to1.3cm/μsduring the current flattop. By contrast, the value of the axial velocity components of theconventional velvet cathode plasma decreased from7.3cm/μs to5.3cm/μs. Moreover,the waveform distortion in the latter part of the current could be avoided by using thecarbon fiber velvet cathode.(3) Comparison of the effects of the cathode and anode materials on the outgassingof the diode. When the current density was less than230A/cm2, the amounts of gasesgenerated in the diode could be reduced by using the carbon fiber velvet cathode. Thedense passive and anodized coating could be created on the anode surface. And thiscould reduce the amounts of gases generated in the diode.(4) Comparison of the pulse-to-pulse instability in the rep-rate mode. When thecurrent density was less than100A/cm2, the instability was about half as many as that ofthe conventional velvet at20Hz rep-rate. When the current density was60~70A/cm2,the carbon velvet cathode was more robust at the same rep-rate.(5) Research of the anode splash on the erosion of the velvet cathode. Based on thetemporal variations and spatial distributions of power density of the electron flow, theprevious model put forward by Mesyats was modified. The thermal regime of the anodewas studied theoretically. It was shown that, for the power density of108W/cm2,conditions for intense erosion of the anode material were created. The anode splashdestroyed the surface morphology of the velvet, resulting in the carbonization anddetachment of the fibers. Moreover, this decreased the electrical stability of the diode.4. Application of the carbon fiber velvet(1) Application of the carbon fiber velvet in MILO. The conventional and carbonfiber velvet cathode were studied utilizing a stainless steel MILO. It was shown that notonly the higher harmonic of the output microwave was suppressed but also the peakpower was increased when the carbon fiber velvet was used in MILO with diode voltageof600kV and beam current of50kA.(2) Application of the carbon fiber velvet in microwave stealth and stray-lightsuppression. It was shown that the reflectance values of the carbon fiber velvet withfiber packing fraction of5%was-4.8~-5.0dB in the frequency range of9.2~11.8GHz.The surface of the carbon fiber velvet with fiber packing fraction of2%yieldedreflectance values of less than1.4%for ultraviolet, visible, and near-infrared light.