Electron GunThermalDesign, Analysis andExperimental Validation

While electrical requirements typically determine the geometry andmaterials usedinanelectron gun,the thermal-mechanical attributes ofthegunareoften equally asimportant. Temperatures inside typical gunscanreach wellinexcess of1000°C, whichpresents asignificant challenge forthemechanical designer. Thegoalofthis project wastodevelop athermal design methodology for electron gunsusingmodemsimulation tools.This consists ofcreating adetailed finite element modelwhich wouldbeable topredict gunthermal characteristics such asheater powerrequired toproduce adesired cathode temperature, warm-upcharacteristics of thegun, temperature distribution ofinternal components, and thermal expansion ofcritical guncomponents. This methodology isnowcurrently being usedatL-3Electron Devices foralltypes ofelectron guns, including diode guns, gridded guns, adjustable perveance mini-tube guns, andlarge, high-power klystron guns. A test vehicle waschosen todevelop andvalidate this thermal design methodology. Thevehicle wasahigh powerelectron gunbeingdesigned toproduce ahigh perveance, annular electron beamwitha cathode temperature ofaround 1100°C.Thisparticular electron gundesign presented a veryunique setofthermal constraints. Theannular beamrequires aninner focus electrode which, along withtheouterfocuselectrode, mustbelimited toa temperature belowtheelectron emission temperature. Inaddition, support ceramics atthe backofthegunmustremainbelowthemelting temperature ofthehighvoltage encapsulation that insulates thegunleads. Thegunalso needs toberigid enough tomeetsthevibration specification. Inaddition, thecathode hasawarm-up timerequirement whichhasto bemet.Therefore, atransient analysis wasrequired to verify thewarm-up time. A modelofthegunwascreated andanalyzed with COSMOS/M(also knownasGeoStar). COSMOS/Misa package offinite element modeling software capable of running thermal, structural, andvibration analysis. The non-linear capabilities ofthesimulator allow fortransient analysis andtemperature dependent material properties. In addition, thermal results canbeusedtogenerate athermal expansion andthermal stress profile within thegun. Inthecaseofthisanalysis, a quarter symmetric modelwasusedtofully define thegeometry while keeping thenumberofelements toaminimum.The modelisshowninFigure 1below.