Measurement of the Betatron Radiation Spectrum Coming From a Laser Wakefield Accelerator

A Laser Wakefield Accelerator (LWFA) is under development at Lawrence Livermore National Laboratory (LLNL) to produce electron bunches with GeV class energy and energy spreads of a few-percent. The ultimate goal is to provide a bright and compact photon source for high energy density physics. The interaction of a high power (200 TW), short pulse (50 fs) laser with neutral He gas can generate quasi-monoenergetic electron beams at energies up to 1 GeV. The laser pulse can be self-guided over a dephasing length of 1 cm (for a plasma density of 1.5 x 10{sup 18} cm{sup -3}) overcoming the limitation of vacuum diffraction. Betatron radiation is emitted while the accelerated electrons undergo oscillations in the wakefield electrostatic field. Here we present electron spectra measurements with a two screen spectrometer allowing to fix the ambiguities due to electron deflections at the plasma exit. They have measured monoenergetic electron beams above 300 MeV. Furthermore a forward directed x-ray beam is observed. The measured betatron spectrum agrees well with the calculated spectrum in the synchrotron asymptotic limit (SAL) using the measured electron beam parameters.