Patent ID: 7402779

Claim:
An effusion cell comprising: a crucible having lateral and end walls that form a cavity; said cavity having a longitudinal axis, a closed end and an open end; said lateral wall having a base longitudinal region adjacent said closed end, a tip longitudinal region adjacent said open end, and a central longitudinal region between said base and tip regions; first heater filaments extending substantially along said axis and being located laterally adjacent said tip region; second heater filaments extending substantially along said axis and being located laterally adjacent said central region; and third heater filaments extending substantially along said axis and being located laterally adjacent all of said regions; wherein the radial density of the number of said heater filaments is lower adjacent said base region than adjacent said tip or central regions, and further including a multiplicity of dielectric annular spacers located around said lateral wall and distributed along said longitudinal axis; each of said spacers having a first and second ring of holes, said first ring being located closer to said axis than said second ring, wherein said first filaments pass through the holes of said first ring in a first subset of said spacers, said second filaments pass through the holes of said first ring in a second subset of said spacers, and said third filaments pass through the holes of said second ring.