| Popis: |
Heat removal in metal structures is often the limiting factor in high power, high frequency microwave tube devices. As higher critical heat flux cooling techniques are found, the operating power of the microwave device may be increased if other limiting factors are not of importance. Larger output powers may be realized for brief periods in time at the expense of operating in a pulsed power and/or pulsed cooling mode. During these brief periods, temporary temperature fluctuations on the walls of the microwave tube will be realized. It is hypothesized that the temperature fluctuations and to a much lesser extent temperature gradients may influence secondary electron emission resulting from primary electrons colliding with the microwave wall. Low energy primary electrons, that penetrate the microwave wall, experience the collective effects of the electrons (plasmons) in the conduction band of the metal. It is anticipated that the surface and bulk temperature of the metal wall influences the ability for plasmons to interact with the primary electron in the scattering process. Further, aging effects resulting from heat stress cycling of the metal cavity may alter the state of the cavity walls leading to changes in the secondary electron emission properties. Pulsed, low energy (1 keV), low beam currents are used to examine collectively the change in the spatial distribution profile of secondary electrons as the sample cools without significantly modifying or stressing the surface of the sample under test. SEE distribution profiles will be presented for a metal target at room temperature and in transient cooling mode with peak temperatures on the order of 390° K. |
