Emittance measurement for 200keV-electron beam extracted from NEA-GaAs photocathode using a pepper pot method

摘要

Recently it becomes important to generate an electron beam with very low emittance not only the for future 1TeV linear colliders but also for the next generation accelerators to produce ultra-high brightness X-rays. Especially, the energy recovery linac (ERL) project planned at Cornell, KEK and other labs, demands the emittance less than 0.5pi mm.mrad at exit of the electron gun. To achieve such a low emittance, the GaAs photocathode with NEA (negative electron affinity) surface is considered to have a significant advantage compared with other PEA photocathodes, because it gives the smallest initial emittance such as 0. 1pi mm.mrad (1mm radius). However, in order to demonstrate this advantage by experiment, the high field gradient DC-gun must be developed not only to suppress the beam divergence due to space charge effect but also to assure the long lifetime of NEA surface. As the first step to this direction, we plan to measure the emittance of electron beam produced by a 200keV polarized electron gun developed at Nagoya University. An apparatus developed for the emittance measurement of the KEK electron source by pepper-pot method has been modified to match with an ultra-high-vacuum condition (< 3x10~(-9)Pa) of the 200keV gun. Present status of preparation of the experiment will be reported at the conference.