Abstract
Electrostatic oscillations of spheroidal non-neutral electron plasmas are experimentally investigated with the multiring electrode trap which provides an electrostatic quadrupole potential with a long axial extent and enables confinement of plasmas with large aspect ratios. The frequencies of axial modes l = 2 and 3 of trapped plasmoids shifts upwards from those predicted by Dubin's cold plasma theory for the free boundary case, mainly due to the image charges induced on the conducting electrodes. These frequencies continue to increase as the plasma temperature is increased from 0.03 eV to 1.5 eV by rf heating. This temperature dependence can be interpreted as a change in the dielectric tensor caused by the temperature increase.