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Recent experimental studies have shown that the presence of plasma in the high-power microwave sources using intense charged beams, when properly introduced, may enhance the efficiency of the source. These findings have been confirmed by numerical simulations for some high-power microwave sources. The enhancement has been partly attributed to the neutralizing effect of the positively charged plasma on lowering the disruptive space charge effects of the electron beam used to generate the RF, and partly to a modification of the dispersion relation leading to higher group velocities for the traveling radio frequency (RF) waves. Mode selection may also be favorably affected by the presence of plasma in the source. We present the results of numerical simulations carried out to study the effect of plasma on an acceletron device. Both neutral as well as positively charged plasmas have been explored. The acceletron device is run in transverse magnetic TM001 mode and generates a 3.2-GHz steady oscillation without any competing modes. The effect of the plasma at several different densities has been explored. Our results indicate a significant enhancement of the acceletron operation due to the plasma. Other plasma densities are being studied. The PIC code used in our simulations was the MRC 2 1/2-dimensional code MAGIC.