Measurement of the refractive index of silica aerogel in vacuum

The photo injector test facility at DESY Zeuthen (PITZ) uses Cherenkov radiation of electrons inside aerogel to measure the bunch length of an electron beam. To evaluate the results the refractive index of the aerogel has to be precisely known. The aerogel is used either at atmospheric pressure or in vacuum. The pressure dependence of the refractive index of two aerogel samples is investigated in the range between high vacuum (10−5 mbar) and atmospheric pressure.