  Recent experimental results on granular gas in Knudsen regime excited by a
vibrating piston in micro-gravity have measured distribution p(I) of impacts I
with a fix target. They give p(I) scaling as exp(-I/Io). This distribution
leads to a probability distribution function of speed v along z varying
approximately as f(v) scaling as (1/v) exp(-v/vo); hence it diverges as 1/v at
small speed and it is quite non Boltzmannian at large speed. Here, a model is
proposed, which explains these experimental impact distributions; it takes
account of the true role of the boundaries and of the dissipation in the gas.
This validates the experimental data. Different approximations are discussed.
Roles of boundaries and of 0-g condition are investigated theoretically. It is
argued that the piston plays the role of an impact generator or a "velostat"
for the Knudsen gas. These results cast a doubt on the efficiency of the notion
of Boltzmann temperature and on the necessity to refer to Boltzman distribution
in such dilute systems. The model shows also that the medium has to be
considered as a whole, in global equilibrium: each part of the system is
exchanging with the whole (at least in the direction of vibration); this is
quite different from classic approach of dissipative systems based on local
exchange and equilibrium, which leads to a "diffusive" Boltzmann equation; here
the distribution f(z,t) is mainly propagative, i.e. f(z,v,t+dt)=f(z-vdt,v,t)
instead of diffusive.
  Pacs # : 05.45.-a, 45.50.-j, 45.70.-n, 81.70.Bt, 81.70.Ha, 83.10.Pp
