Stimulated Electron Desorption Studies from Microwave Vacuum Electronics / High Power Microwave Materials

Secondary electron emission (SEE) can potentially lead to beam instabilities, material degradation, beam closure (shorting or pulse shortening), and R.F. breakdown. Under certain conditions, SEE may be desired as a source of electrons for a high power microwave device. Material surface physics, morphology, and possibly the state of the material play a role in the emission process. This DEPSCoR research effort examined secondary electron emission from thin films of potential interest for anode application in high power microwave devices and SEE from a warm metal material. The study ultimately led to a strong AFRL collaboration (Materials and Manufacturing Directorate at Wright-Patterson Air Force Research Laboratory). Absolute and comparative studies were performed with a number of results. Strong evidence suggests that backscattered electrons as a whole have a preferred scattering direction with a distribution that is dependent on the type of surface treatment. Therefore, it may be possible to segregate backscattered electrons from true secondary electrons allowing for optimal anode designs in managing energetic and non energetic electrons. Secondary electron emission was studied from an aged, buffered chemical polished niobium sample during transient cooling from a measurable ~360 K temperature to near room temperature. It was observed that: 1) The “True” secondary electron distribution (as we define it) tends to exhibit a shifted Gaussian-like spatial distribution for the surface grade under test; 2) There are indications that a metal structure undergoing transient cooling exhibits some small overall average tendencies on the scattering process; 3) Although not conclusive, the change in the mean number of scattered electrons seems to increase on average as the metal sample cools; and 4) The emitted backscattered electron cluster appears to be scattered normal to the sample surface within the temperature range considered (~360 K to 293 K). This appears to agree with past cold study findings (>50 K). The AFRL/UNLV collaboration examined three potential anode coatings (TiN, AlTiN, TiCN) on a polished, oxygen free copper substrate, one titanium-on-copper laminate, and a coating free, UNLV polished, copper wedge with noticeable grain imperfections. Based on single, pulsed, emission tests noticeable improvements in secondary electron emission count were observed with the coated copper substrates. Although not conclusive, there is evidence that secondary electron emission hardening based on a single location studies illuminated with up to fifty short duration, low energy, low current electrons does not take place in all films. It is observed that the secondary electron emission count and distribution are dependent on the beam’s angle relative to the surface of the film. Currently, morphology and molecular studies of the different films are being conducted at AFRL to this end.