Electrostatic modes in an electron plasma confined in the Penning-Malmberg trap EL-TRAP (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 the m = 1 diocotron mode) is found, with a frequency proportional to (Q/B) (Q is the total charge, B the 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.
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