Progress and future directions in brightness electron beam sources

The progress of various high-brightness electron source technologies, specially laser based electron sources, such as photocathode RF gun, DC and pulsed photo-gun, and laser plasma electron source, will be reviewed. The physics processes of those sources to realize best performance are clarified according to three basic ideas. They are transverse space charge emittance compensation, space charge compensation and longitudinal emittance compensation. The major progresses and issues of photoinjector are briefly reviewed. The performance of photocathode RF gun at the Brookhaven Accelerator Test Facility (ATF) is presented.