  A novel concept for ion blocking in gas-avalanche detectors was developed,
comprising cascaded micro-hole electron multipliers with patterned electrodes
for ion defocusing. This leads to ion blocking at the 10^{-4} level, in DC
mode, in operation conditions adequate for TPCs and for gaseous
photomultipliers. The concept was validated in a cascaded visible-sensitive gas
avalanche photomultiplier operating at atmospheric pressure of Ar/CH_{4} (95/5)
with a bi-alkali photocathode. While in previous works high gain, in excess of
10^{5}, was reached only in a pulse-gated cascaded-GEM gaseous photomultiplier,
the present device yielded, for the first time, similar gain in DC mode. We
describe shortly the physical processes involved in the charge transport within
gaseous photomultipliers and the ion blocking method. We present results of ion
backflow fraction and of electron multiplication in cascaded
patterned-electrode gaseous photomultiplier with K-Cs-Sb, Na-K-Sb and Cs-Sb
visible-sensitive photocathodes, operated in DC mode.

Instrumentation and Detectors