They are commonly used in the geological well measurements.
It is not really a "cold" fusion, since the appropriate voltage is
applied , so the target is being hit at a speed equivalent millions of
grades of temperature.
The design apparently does not scale up easily, and there is a problem
of converting the energy produced into heat or electricity, since that
energy comes out as a high end gamma radiation and neutrons.
One of the ways to convert neutrones and gamma readiation into the heat
is to use nuclear fission conversion in the depleted uranium.
Here is a tentative way, of how it can be done:
> http://mywebpages.comcast.net/mycommon/FUSION.JPG
Applying 5000 volt is equal to applying temperature 58,000,000 K
grades, which is sufficient in order to produce fusion reaction.
Botzmann k= 8.617385 x 10-5 eV/K
T=58 x 10**6
U=kT = 5000 volt
ES