Abstract
Multiple electron impacting (multipacting) can take place in rf fields when the rf components are composed of materials with a secondary electron yield greater than one. In rf gun cavities, multipacting may change the properties of the vacuum components or even damage them. First systematic measurements of the multipacting occurring in a photocathode rf gun were made at the Fermilab/NICADD Photoinjector Laboratory in 2000. The multipacting properties were found to depend on the cathode material and the solenoid field configuration. In this study, we measure the multipacting properties in more detail and model the secondary electron generation for numerical simulation. Measurements and simulations for the photoinjectors at Fermilab and DESY are compared. The multipacting takes place at the photocathode in rf guns and is categorized as single-side multipacting. In a low rf field, the electrons emitted from the cathode area do not leave the gun cavity within one rf cycle and have an opportunity to travel back and hit the cathode. The solenoid field distribution in the vicinity of the cathode changes the probability of electron bombardment of the cathode and makes a major contribution to the multipacting behavior.
Highlights
Multiple electron impacting is an explosive increase of the number of free electrons in an rf component such as a cathode, rf cavity, or rf coupler
A type of single-side multipacting is introduced in which the electrons hit the same surface repeatedly, but not always at the same rf phase, with a number of rf cycles between impacts
Multipacting depends on the field configuration, the cavity geometry, and the secondary electron yield (SEY)
Summary
Multiple electron impacting (multipacting) is an explosive increase of the number of free electrons in an rf component such as a cathode, rf cavity, or rf coupler. Multipacting depends on the field configuration, the cavity geometry, and the secondary electron yield (SEY). To produce a low beam emittance, the solenoidal field at the cathode surface must be zero; otherwise there is a contribution to the emittance from the angular momentum generated by the solenoid. A nonzero solenoid field at the cathode is required in the so-called flat beam configuration [14 –16]. In this case, the angular momentum of the beam is used in a transformation to produce a beam with large emittance ratio "x="y. A dependence of the dark current generation and the photocathode QE on the solenoid configuration was measured during flat beam experiments at FNPL in 2000. Simulation results for different solenoid configurations, including the FNPL configurations, are presented
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