Recently, a periodically layered metal-dielectric structure was introduced as a new type of tunable radiation source by Adamo et al. [Phys. Rev. Lett. 103, 113901 (2009); J. Opt. 12, 024012 (2010)]. It is anticipated that the proposed structure forms an important part of future THz systems. In this paper, the effect of plasma, as an extra controlling parameter, on this radiation source has been investigated. The results of the numerical calculations show that the presence of plasma can lead to a significant increase in the output power, and therefore, the device can be tuned over a wide frequency range by changing the plasma density.