  We successfully operated a novel kind of LAr Time Projection Chamber based on
a Large Electron Multiplier (LEM) readout system. The prototype, of about 3
liters active volume, is operated in liquid-vapour (double) phase pure Ar. The
ionization electrons, after drifting in the LAr volume, are extracted by a set
of grids into the gas phase and driven into the holes of a double stage LEM,
where charge amplification occurs. Each LEM is a thick macroscopic hole
multiplier of 10x10 cm$^2$ manufactured with standard PCB techniques. The
electrons signal is readout via two orthogonal coordinates, one using the
induced signal on the segmented upper electrode of the LEM itself and the other
by collecting the electrons on a segmented anode. Custom-made preamplifiers
have been especially developed for this purpose. Cosmic ray tracks have been
successfully observed in pure gas at room temperature and in double phase Ar
operation. We believe that this proof of principle represents an important
milestone in the realization of very large, long drift (cost-effective) LAr
detectors for next generation neutrino physics and proton decay experiments, as
well as for direct search of Dark Matter with imaging devices.
