Abstract: Clusters of galaxies are embedded in a hot (T~10^7 K), low-density halo of gas called the intracluster medium (ICM). In some clusters, the ICM hosts low-frequency (MHz–GHz) radio synchrotron emission associated with radio galaxies, jets, and cluster merger-induced shocks and turbulence. The radio emission comes from GeV cosmic ray electrons (CRe). Some CRe may be "fossils", i.e., previously-accelerated CRe that then radiated and cooled to long-lived MeV energies. Fossil CRe are invisible until some perturbation, like adiabatic compression or turbulence, re-accelerates the CRe back to GeV energies. I will present a recent study of CRe re-acceleration via magnetic pumping, a mechanism in which small-scale (nanoparsec) plasma waves coupled to large-scale (kiloparsec to Megaparsec) motions can efficiently energize CRe. In 1D kinetic plasma simulations, we show that ion cyclotron wave scattering with background compression leads to resonant CRe gaining ~10-30% of their initial energy in one compress/expand cycle, assuming adiabatic expansion without further scattering. I will comment briefly on the applicability and limitations of this re-acceleration mechanism, and on prospects for future work.