Laser wakefield is generated inside capillary tubes in order to study the conditions for self-injection of plasma electrons and their acceleration inside a large domain of parameters. Dielectric capillary tubes are employed to guide the laser pulse and collect laser energy around the central focal spot to favor laser propagation. Electrons are observed to be self-injected and accelerated to the 200 MeV range when a peak laser intensity as low as 5×1017 W/cm2 is used. X-rays emitted by betatron radiation constitute a precise diagnostic of the electron acceleration process. Furthermore, the peak brightness of X-rays is increased to 1021 ph/s/mm2/ mrad2/0.1%BW when the laser pulse is focused to 5×10 18 W/cm2, which is about 30 times higher than the value obtained by using a 2 mm gas jet.