  We report on the observation of a circular photogalvanic current excited by
terahertz (THz) laser radiation in helical edge channels of HgTe-based 2D
topological insulators (TIs). The direction of the photocurrent reverses by
switching the radiation polarization from right-handed to left-handed one and,
for fixed photon helicity, is opposite for the opposite edges. The photocurrent
is detected in a wide range of gate voltages. With decreasing the Fermi level
below the conduction band bottom, the current emerges, reaches a maximum,
decreases, changes its sign close to the charge neutrality point (CNP), and
again rises. Conductance measured over a 7 $\mu$m distance at CNP approaches
2e2/h, the value characteristic for ballistic transport in 2D TIs. The data
reveal that the photocurrent is caused by photoionization of helical edge
electrons to the conduction band. We discuss the microscopic model of this
phenomenon and compare calculations with the experimental data.
