  We describe a new micro-pattern gas detector structure comprising a
multi-layer hole-type multiplier (M-THGEM) combined with two in-built electrode
meshes: the Multi-Mesh THGEM-type multiplier (MM-THGEM). Suitable potential
differences applied between the various electrodes provide an efficient
collection of ionization electrons within the MM-THGEM holes and a large charge
avalanche multiplication between the meshes. Different from conventional
hole-type multipliers (e.g. Gas Electron Multipliers - GEMs, Thick Gas Electron
Multipliers - THGEMs, etc.), which are characterized by a variable
(dipole-like) field strength inside the avalanche gap, electrons in MM-THGEMs
are largely multiplied by a strong uniform field established between the two
meshes, like in the parallel-plate avalanche geometry. The presence of the two
meshes within the holes allows for the trapping of a large fraction of the
positive ions that stream back to the drift region. A gas gain above 10^5 has
been achieved for single photo-electron detection with a single MM-THGEM in
Ar/(10%)CH4 and He/(10%)CO2, at standard conditions for temperature and
pressure. When the MM-THGEM is coupled to a conventional THGEM and used as
first cascade element, the maximum achievable gains reach values above 10^6 in
He/(10%)CO2, while the IBF approaches of 1.5% in the case of optimum
detector-bias configuration. This IBF value is several times lower compared to
the one obtained by a double GEM/THGEM detector (5-10%), and equivalent to the
performance attained by a Micromegas detector.
