A High Resolution, Broad Energy Acceptance Spectrometer for Laser Wake-field Acceleration Experiments

Laser wake¯eld experiments present a unique challenge in measuring the resulting electron energy properties due to the large energy range of interest, typically several 100 MeV, and the large electron beam divergence and pointing jitter >1 mrad. In many experiments the energy resolution and accuracy are limited by the convolved transverse spot size and pointing jitter of the beam. In this paper we present an electron energy spectrometer consisting of two magnets designed speci¯cally for laser wake¯eld experiments. In the primary magnet the ¯eld is produced by permanent magnets while a second electromagnet can be used for electron energies above 75 MeV. The spectrometer has an acceptance of 2.5-400 MeV (Emax=Emin > 100) with a resolution of better than 1% rms for electron energies above 25 MeV. This high resolution is achieved by refocusing electrons in the energy plane and without any post-processing image deconvolution. Finally, the spectrometer employs two complimentary detection mechanisms: 1) absolutely calibrated scintillation screens imaged by cameras outside the vacuum chamber, and 2) an array of scintillating ¯bers coupled to a low-noise CCD.