  The presence of charged particles in an ion trap modifies the harmonic
trapping potential in which they are trapped, leading to observed shifts in
secular frequency as well as appearance of collective oscillation. In a linear
trap geometry, both of these effects have been observed under different
trapping conditions using narrow non-linear resonance and external excitation.
The observations have been modeled with minimal fitting parameter showing good
agreement with results obtained. The space charge in our experiment plays an
important role in terms of criticality of the onset of collective oscillation.
