[en] Measurements are made of surface doses necessary to initiate an anode plasma by electron bombardment of Ta, Ti, and C anodes for coaxial geometries characteristic of high-power electron-beam diodes. Measured lower and upper bounds of doses necessary to form an anode plasma are 54 +- 7 to 139 +- 16 J/g in Ta, 214 +- 23 to 294 +- 71 J/g in Ti, and 316 +- 33 to 494 +- 52 J/g in C. Within these bounds, probable values for the threshold are given under specific assumptions. The measurements are consistent with a thermal desorption model for plasma formation. 41 refs., 20 figs., 3 tabs