Electrostatic modes in an electron plasma confined in the Penning‐Malmberg trap ELTRAP (Physics Department of Milan University) are investigated, analyzing the electric signals on the electrodes, and using the CCD diagnostics. In different experimental conditions, a well‐defined mode (possibly them= 1 diocotron mode) is found, with a frequency proportional to (Q/B) (Qis the total charge,Bthe magnetic field strength). The amplitude starts to increase at the beginning of the hold phase of the cycle, reaches a maximum, and then decreases to the background noise level. The decrease of the amplitude corresponds to a decrease of the frequency, indicating plasma loss at the wall. A fast rise of the potential barrier easily excites the mode. Measurements performed with increasing ramp times show that the growth rate decreases, and at a long enough ramp time the instability does not arise. © 2003 American Institute of Physics