Temperature dependence of the beam-foil interaction

The beam energy dependence between 50 and 200 keV of the linear polarization fraction (M/I) of the 2s 1S - 3p 1p, 5016 A transition in He I on temperature has been measured. The thin carbon exciter foils were heated externally by nichrome resistance elements. The energy and current dependences are the same assuming corresponding between beam heating and external heating. It is observed that γ, the number of slow secondary electrons produced per incident ion decreases, with increasing foil temperature. These two effects, in conjunction, offer a plausible explanation for the variation of polarization with beam current density