Turner, M. Gschwendtner, E. Lotov, K.V. Petrenko, A.V. Sosedkin, A. JACoW Geneva, Switzerland 978-3-95450-180-9 10.18429/JACoW-NAPAC2016-WEPOA09 English 707-709 WEPOA09 proton ion plasma wakefield focusing Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-NAPAC2016-WEPOA09 https://jacow.org/napac2016/papers/wepoa09.pdf The AWAKE experiment will use a 400 GeV/c proton beam with a longitudinal bunch length of sigmq_{z} = 12 cm to create and sustain GV/m plasma wakefields over 10 meters. A 12 cm long bunch can only drive strong wakefields in a plasma with n_{p}e = 7 x 10¹⁴ electrons/cm³ after the self-modulation instability (SMI) developed and microbunches formed, spaced at the plasma wavelength. The fields present during SMI focus and defocus the protons in the transverse plane. We show that by inserting two imaging screens downstream the plasma, we can measure the maximum defocusing angle of the defocused protons for plasma densities above n_{p}e = 5 x10¹⁴ electrons/cm³. Measuring maximum defocusing angles around 1 mrad indirectly proves that SMI developed successfully and that GV/m plasma wakefields were created. In this paper we present numerical studies on how and when the wakefields defocus protons in plasma, the expected measurement results of the two screen diagnostics and the physics we can deduce from it.