Space charge and coherent synchrotron radiation may deteriorate electron beam quality when the beam passes through a magnetic bunch compressor. This paper presents the transverse phase-space tomographic measurements for a compressed beam at 60 MeV, around which energy the first stage of magnetic bunch compression takes place in most advanced linacs. Transverse phase-space bifurcation of a compressed beam is observed at that energy, but the degree of the space charge-induced bifurcation is appreciably lower than the one observed at 12 MeV.Magnetic bunch compressors are widely used in advanced linacs to shorten the length of electron bunches [1][2][3]. One of the major challenges for these machines is brightness preservation of the electron beam through magnetic bunch compressors. Space charge, nonlinear correlated energy spread, coherent synchrotron radiation (CSR), and microbunching instability are the main concerns in preserving beam quality through compressors. In order to select a suitable energy for the bunch compression process-in particular for the first stage of compression-two effects must be considered: nonlinear correlated energy spread and space-charge effects. When the compression is chosen at a high energy, the bunch traveling through the linac accumulates too much nonlinear correlated energy spread from the accelerating radio frequency (rf) field. Compressors with nonlinear dependence of path length and energy spread will introduce nonlinear correlated energy spread, giving rise to other nonlinear terms like higher-order dispersion diluting the emittance [4]. When the compression is chosen to take place at a low energy, space-charge effects -which scale with 1= 2 , where is the Lorenz factor -will counteract compression to very short bunches and may also distort 6-dimensional phasespace distribution of the compressed beam. In addition, CSR enhances radiated power loss when the bunch passes through a compressor, which results in severe longitudinal effects, such as energy loss and increase of energy spread. Since these effects occur in a dispersive region, they will eventually create emittance dilution. These detrimental effects had been investigated before [5][6][7][8], but the collective parameters measured in these experiments, making a priori assumptions on the beam's phase-space distribution, will not represent all information about the beam if the beam's phase space is distorted. More information can be gained if the beam's phase-space distribution is reconstructed using tomographic methods. A group at the University of California at Los Angeles (UCLA) [9] had previously reconstructed the transverse phase-space distribution of a compressed beam with an energy of 12 MeV, using multi-slit diagnostics, and made the first observation of significant phase-space bifurcation. Simulations indicated that the space charge drives the effect. Our present experiment, carried out at the Brookhaven Accelerator Test Facility (ATF), investigates transverse phase-space distribution of a compressed beam with a h...