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)