In order to produce high brightness beam at energies below 100 eV, the effects of high deceleration voltage in a triode extraction system and plasma potential have been studied. Although the application of high deceleration voltage caused beam spreading, beam brightness was increased at least up to accel–decel ratio of 80. It is found that the reduction of sheath potential of the source plasma caused the increase in beam brightness under high deceleration voltage condition. In particular, it is desirable to reduce sheath potential at the energy below 50 eV because this potential is comparable to external applied acceleration voltage. ©1996 American Institute of Physics.