EFFECT OF DARK CURRENTS ON THE ACCELERATED BEAM IN AN X-BAND LINAC

X-band accelerating structures operate at surface gradients up to 120-180 MV/m. At these gradients, electron currents are emitted spontaneously from the structure walls (''dark currents'') and generate additional electromagnetic fields inside the structure. The authors estimate the effect of these fields on the accelerated beam in a linac using two methods: a particle-in-cell simulation code MAGIC and a particle tracking code. They use the Fowler-Nordheim dependence of the emitted current on surface electric field with field enhancement factor beta. In simulations they consider geometries of travelling wave structures that have actually been built for the Next Linear Collider project.