A periodic slow wave structure is really just a metal pipe which has "washers" spaced inside the length of it. Electromagnetic waves travel down the pipe and are slowed by the washers. This arrangement is a rudimentary linear accelerator, for accelerating particles to higher energy using (usually) microwave frequency RF power.
linacman may be right about the pipe with washers, but a periodic slow wave structure is more general that that specific example.
Basically, a slow wave structure can be thought of as any number of periodic elements. For example, the study of waves within crystals gives rise to slow wave structures. In this case, the one-dimensional crystal lattice (atoms spaced a certain distance apart in a straight line) can be represented by a number of linear nodes having a shunt inductance and capacitance to ground, with a series capacitance between nodes. Check out "Electronics of Solids" by Walter R. Beam, chapter 2 for more information.
In microwave engineering, a slow wave structure is basically any periodic structure. For example, a transission line with small stubs equally spaced down the length of the line would count as a slow wave structure. In this case, the transmission line would have capacitively loaded stubs placed at periodic points down the line. David Pozar in "Microwave Engineering", chapter 9, derives equations for this type of structure treating the capacitively loaded transmission line as a periodic structure of transmission lines and shunt susceptances.
Periodic structures using microstrip transmission lines are frequently used to create bandpass/bandstop filters. Check out Pozar for more information.
Best example is the TW (travelling Wave ) tube: with spiral
or other periodic structure slows down the HC current to
match its speed to the one of the E-beam.
The Slow-wave structure is basically many serial L-parallel C units serially connected i.e. large # of low-pass
(with much higher corner fr. than the operating one so
each adds a small delay but no significant freq. dependency)
