  Accessing, controlling and understanding nanoscale friction and dissipation
is a crucial issue in nanotechnology, where moving elements are central.
Recently, ultra-sensitive noncontact pendulum Atomic Force Microscope (AFM)
succeeded in detecting the electronic friction drop caused by the onset of
superconductivity in Nb, raising hopes that a wider variety of mechanisms of
mechanical dissipation arising from electron organization into different
collective phenomena will become accessible through this unconventional surface
probe. Among them, the driven phase dynamics of charge-density-waves (CDWs)
represents an outstanding challenge as a source of dissipation. Here we report
a striking multiplet of AFM dissipation peaks arising at nanometer distances
above the surface of NbSe$_2$ - a layered compound exhibiting an incommensurate
CDW. Each peak appears at a well defined tip-surface interaction force of the
order of a nN, and persists until T=70K where CDW short-range order is known to
disappear. A theoretical model is presented showing that the peaks are
connected to tip-induced local 2$\pi$ CDW phase slips. Under the attractive
potential of the approaching tip, the local CDW surface phase landscape deforms
continuously until a series of 2$\pi$ jumps occur between different values of
the local phase. As the tip oscillates to and fro, each slip gives rise to a
hysteresis cycle, appearing at a selected distance, the dissipation
corresponding to "pumping" in and out a local slip in the surface CDW phase of
NbSe$_2$.
