Observation of synchrotron radiation from electrons accelerated in a petawatt-laser-generated plasma cavity - Kneip, S; Nagel, SR; Bellei, C; Bourgeois, N; Dangor, AE; Gopal, A; Heathcote, R; Mangles, SPD; Marques, JR; Maksimchuk, A; Nilson, PM; Phuoc, KT; et al. (PHYSICAL REVIEW LETTERS, 2008)