How fast the drive field oscillates decides who can follow it. A microsecond high-voltage pulse hits the gas as a discrete event — a burst of transient discharge, then silence. At RF (13.56 MHz) the field reverses tens of millions of times a second: electrons surf it, ions feel only the average, and a steady glow forms behind thin space-charge sheaths. At microwave (2.45 GHz) even the coupling changes character — power arrives through a resonant cavity or waveguide, usually with no driven electrode immersed in the plasma. Switch sources and compare the drive waveform (top) with the discharge it produces (bottom).
Schematic character sketches, not calibrated waveforms: pulse widths, cycle counts, sheath thicknesses, and the cavity field are stylized for visibility. The physics they illustrate is real — pulsed drive delivers energy in discrete transient events, RF sustains a quasi-steady glow behind sheaths because ions follow only the averaged field (the vignette shows the capacitive CCP form; inductive ICP sources couple through a coil instead), and microwave power couples through a resonant structure, usually with no driven electrode immersed. The ACS Material commercial line covers the kHz-AC and microsecond-pulse territory; RF and microwave are shown for orientation.