Electron gun design for traveling wave tubes (TWTs) using a field emitter array (FEA) cathode

We first built two traveling wave tubes (TWTs) using a Spindt-type field emitter array (FEA) cathode. The first FEA-TWT achieved excellent RF performances. However its beam transmission ratio was only 82%, in contrast with more than 99% of a conventional TWT. To improve the beam transmission ratio and other characteristics, we re-designed the FEA electron gun and fabricated a second experimental FEA-TWT. At the current less than 20 mA, the beam transmission ratio of the second TWT was improved as high as 98%. However, when the current was more than 20 mA, space charge effect became significant and beam transmission ratio was rapidly degraded. The test results indicated that the design criteria for FEA electron guns needed to take into consideration the strong influence of space charge and how a good beam transmission ratio could be obtained.