  The Thick Gaseous Electron Multiplier (THGEM) is a simple and robust
electrode suitable for large area detectors. In this work the results of
extensive comparative studies of the physical properties of different
THGEM-based structures are reviewed. The focus is on newly suggested THGEM-like
WELL configurations as well as on recently developed characterization methods.
The WELL structures are single-sided THGEM electrodes directly coupled to
different anode readout electrodes. The structures differ by the coupling
concept of the bottom THGEM electrode to the metallic readout pads: a Thick
WELL (THWELL) with direct coupling; the Resistive WELL (RWELL) and the
Segmented Resistive WELL (SRWELL) coupled through thin resistive films
deposited on insulating sheets and a Resistive-Plate WELL (RPWELL) coupled
through a plate of high bulk resistivity. The results are compared to that of
traditional double-sided THGEM electrodes followed by induction gaps - in some
cases with moderate additional multiplication within the gap. We compare the
different configurations in terms of gain, avalanche extension, discharge-rate
and magnitude as well as rate capabilities over a broad dynamic range -
exploiting a method that mimics highly ionizing particles in the laboratory. We
report on recent studies of avalanche distribution in THGEM holes using optical
readout.
