Reduction of transverse emittance in electron injectors caused by space charge effects

We report that transverse emittance can be reduced by the space charge effects in electron injectors. The reduction of emittance in an ideal DC-accelerated continuous beam is numerically calculated and the mechanisms of the reduction are analytically discussed. Beam edge and image charge effects at a cathode produce a particular nonlinearity in r-r′ phase space, which is discussed in the paper, and they contribute to reducing the emittance immediately after the cathode. We also show examples of reduced emittance for practical electron injectors such as that in the SCSS thermionic gun with a continuous beam, and those in the SPring-8 rf photoinjector and the BNL-type rf photoinjector with pulsed beams. For pulsed beams, the focusing effects caused by an rf cavity also contribute to reducing the emittance. These reductions only appear when the beam charge is low.