EXPERIMENTAL-OBSERVATION OF HIGH-BRIGHTNESS MICROBUNCHING IN A PHOTOCATHODE RF ELECTRON-GUN

We report the measurement of very short, high-brightness bunches of el ectrons produced in a photocathode rf gun with no magnetic compression . The electron beam bunch length and the charge distribution along the bunch were measured by passing the energy chirped the electron beam t hrough a momentum selection slit while varying the phase of the rf lin ac. The bunch compression as a function of rf gun phase and electric f ield at the cathode were investigated. The shortest measured bunch is 370+/-100 fs (at 95% of the charge) with 2.5X10(8) electrons (170 A pe ak current); the normalized rms emittance of this beam was measured to be 0.5 pi mm mrad and the energy spread is 0.15%.