  Helium (He) retention in tungsten (W) is a concern in fusion reactors since
it could be detrimental to plasma facing components performance and influence
the fusion fuel balance. He being not soluble in W, it tends to agglomerate on
preexisting defects (vacancy, grain boundary), but it could in theory also
self-trap (be immobilized on a non-preexisting vacancy) through the emission of
a vacancy/self-interstitial W pair in the vicinity of a Hen interstitial
cluster. In the present study, we prepared a pure single crystal W(110) sample
with a clean surface in order to evidence the self-trapping of He in the W bulk
at a sample temperature of 300 K and for a constant fluence of 2.0x10$^{21}$
He$^+$.m$^{-2}$. At a He$^+$ kinetic energy of 130 eV and a flux of
0.3x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$, we only observed a small He desorption
peak below 600 K. Rising the ion flux to 0.7x10$^{17}$
He$^+$.m$^{-2}$.s$^{-1}$, we observed the sudden appearance of two desorption
peaks at 950 K and 1700 K. For the highest flux studied in this work,
5.0x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$, an additional desorption peak at 1800 K
and a desorption shoulder at 1900 K are observed. The temperature position of
these He desorption peaks are consistent with the density functional theory
literature and points to the occurrence of self-trapping once the 0.7x10$^{17}$
He$^+$.m$^{-2}$.s$^{-1}$ flux is attained at 300 K and to the possible
realization of trap-mutation for the flux of 5.0x10$^{17}$
He$^+$.m$^{-2}$.s$^{-1}$. The present set of results should be used to
constrain the development of He retention and He bubbles growth models based on
ab initio quantities.
