Study Of Field Emission Cathode RF Gun

For the successful operation of low gain free electron lasers(FEL), electron bunches with relatively high brightness and relatively high average current are required. Metal photo-cathode rf guns can provide electron bunches with high brightness, but the average current is low due to the low repetition rate of drive laser, while the performance of thermionic cathode rf guns is a?ected by the electron back-bombardment, which introduces a large energy spread between bunches, though the average current is relatively high. Therefore, field emission rf guns, which combine the continuous emission of field emission cathodes and the high accelerating gradient of rf fields, are proposed as a potential candidate to drive the low gain FEL. In this thesis, the characteristics of such a gun is investigated, as well as experimental studies of various field emission cathodes.The distributions of field emission current within a rf cycle and during a rf pulse are described first. And taking the building of rf field in a gun into account, the average current as a function of time is obtained, from which the rms width of field emission can be derived. For a uniform and smooth field emission cathode, an analytical formula of the thermal emittance is given and compared with a Monte-Carlo model. The relationship between the thermal emittance and the temperature or electric field on the cathode surface is also discussed.RF guns operating in a single frequency and in two frequencies are studied in order to e?ectively accelerate the field emitted electrons. For a single frequency rf gun with one and half cells, the emittance of electron bunches can be minimized by choosing proper lengths of the cells and the longitudinal phase space can be optimized by adding a drift between cells. By doing so, electron bunches with low emittance and linear energy chirp can be obtained. For a double frequency rf gun, the field emission properties as a function of the harmonic field are numerically solved. Using particle tracking it shows that the rf e?ect of the harmonic field is superimposed with that of the fundamental field and thus degrades the bunch quality. Since it has a similar longitudinal distribution with the fundamental field, the rf e?ect of TM030 mode is smaller. To suppress the harmonic induced rf e?ect further, a 1.5 cell rf gun with only the first cell operating in two frequencies is proposed.Thermal e?ect is known as one of the major causes of failure of field emission cathodes. For carbon nanotubes(CNT) field emitters, we built a 1D heat conduction model.The threshold emission current density of CNT emitters grown on various substrates can be obtained from the model. And it has shown that the threshold is mainly dependent on the heat conduction in the substrates, which implies that the emission properties could be enhanced by choosing substrates with higher melting point and large thermal conductivity.In order to measure the field emission properties of various cathodes, we built a test stand which consists a S band single-cell rf gun and a solenoid. An average current of about 5.5 mA was measured from nitrogen-doped diamond films, with a rms width of about 21 degrees. And an average current of about 72 mA was observed from CNT films,with a rms width of about 16 degrees. The thermal e?ect of CNT films was discussed and it showed that the increase of cathode surface temperature didn’t contribute to the emission current. The beam spot was measured by a YAG screen downstream of the solenoid and the transverse distribution was found uniform. The emission current was measured back and forth and the emission was found to be stable.