A plasma generator includes an outer electrode that encloses a first inner electrode and a second inner electrode. The first inner electrode includes a plurality of protrusions that extend towards the outer electrode. A voltage signal can be applied across the outer electrode and the first inner electrode to excite gas injected into gaps between the protrusions and the outer electrode. Plasma is generated surrounding the protrusions. The second inner electrode is at a downstream location of the excited gas relative to the first inner electrode. The second inner electrode forms a second gap with the outer electrode. A voltage signal can be applied across the second inner electrode and the outer electrode, further exciting the gas to generate second plasma at the second gap. The second plasma is spread evenly across the second inner electrode and the outer electrode.