  Plasma photonic crystals designed in this paper are composed of gas discharge
tubes to control the flow of electromagnetic waves. The band structures
calculated by the finite element method are consistent with the experimental
results in that two distinct attenuation peaks appear in the ranges of 1 GHz to
2.5 GHz and 5 GHz to 6 GHz. Electromagnetic parameters of the plasma are
extracted by the Nicolson-Ross-Weir method and effective medium theory. The
measured electron density is between 1e11 cm-3 and 1e12 cm-3, which verifies
the correctness of the parameter used in the simulation, and the collision
frequency is near 1.5e10 Hz. As the band structures are corroborated by the
measured scattering parameters, we introduce the concept of photonic
topological insulator based on the quantum Valley Hall effect into the plasma
photonic crystal. A valley-dependent plasma photonic crystal with hexagonal
lattice is constructed, and the phase transition of the valley K(K') occurs by
breaking the spatial inversion symmetry. Valley-spin locked topological edge
states are generated and excited by chiral sources. The frequency of the
non-bulk state can be dynamically regulated by the electron density. This
concept paves the way for novel, tunable topological edge states. More
interestingly, the Dirac cone is broken when the electron density increases to
3.1e12 cm-3, which distinguishes from the methods of applying a magnetic field
and changing the symmetry of the point group.
