We model the trapping of plasma $e^-$ within the density structures excited
by a propagating energy source ($\beta_{S}\simeq1$) in a rising plasma density
gradient. Rising density gradient leads to spatially contiguous coupled
up-chirped plasmons ($d{\omega^2_{pe}(x)}/{dx}>0$). Therefore phase mixing
between plasmons can lead to trapping until the plasmon field is high enough
such that $e^-$ trajectories returning towards a longer wavelength see a
trapping potential. Rising plasma density gradients are ubiquitous for
confining the plasma within sources at the vacuum-plasma interfaces. Therefore
trapping of plasma-$e^-$ in a rising ramp is important for acceleration
diagnostics and to understand the energy dissipation from the excited plasmon
train \cite{LTE-2013}. Down-ramp in density \cite{density-transition-2001} has
been used for plasma-$e^-$ trapping within the first bucket behind the driver.
Here, in rising density gradient the trapping does not occur in the first
plasmon bucket but in subsequent plasmon buckets behind the driver. Trapping
reduces the Hamiltonian of each bucket where $e^-$ are trapped, so it is a
wakefield-decay probe. Preliminary computational results for beam and
laser-driven wakefield are shown.