Key Technology Researches Of Submilimeter-wave FEL Pre-injector

To meet strict quality demands of electron bunches for high performance submillimeter wave Free Electron Lasers (FELs), the high brightness electron bunch pre-injector has been focused on recently. Though photocathode injector has been rapidly developed as brilliant electron sources, the thermionic injector is expected to have potentials to generate short-pulse beams of high qualities, not to speak of its simple and compact structures which are useful for miniaturization and application tends of submillimeter wave FELs.As a new brilliant FEL pre-injector, the independently tunable cell (ITC) RF gun composes of thermionic cathode for electron emitting, and multi standing wave resonance cavities for beam micro-pulsing and energy spread compensating. Owing to independences between the two cells of ITC RF gun, bunches of excellent characteristics could be achieved by choosing appropriate feeding power and phase of the two cells. Additionally, the application of a-magnet and laser system could be avoided, which lead to more compact layout. Based on these, by adopting external cathode (EC) instead of embed one, the output micro-pulse current could be enlarged while energy spread could be reduced, the backbombard effects were eliminated as well. This paper has enhanced the performances of EC-ITC RF gun by means of changing electron source into double anode gridded electron gun, and adjusting structure parameters. At last, by tuning electric strengths and phases of the two cells, bunches of～ps order length,～103order energy spread could be obtained together with～2.6MeV kinetic energy,10πmm mrad normalized transverse emittance and over200pC charge quantity, whilea-magnet and compensating coils are avoided.Based on elaborating key technologies of the FEL pre-injector, enhanced EC-ITC RF gun has been studied deeply:first of all, one dimension model has been built to estimate bunching performance of EC-ITC RF gun, approximated lowest value of first cavity’s electric strength has been derivated. And each fraction of radial force expression of the charged particle has been analyzed separately for exploring it’s physical significance, then beam dynamics calculations have been performed to test and verify above researches, and the results meet theoretical calculations well. In addition, in order to analyze and study normalized transverse emittance variations of effective bunch head extracted from the first cavity of EC-ITC gun, the concepts of emittance growth and compensation are introduced from photocathode injectors. Beam dynamics results show that the emittance of bunch head has been compensated due to the combined actions of equivalent focusing lens and bunch reforming of the second cavity of EC-ITC RF gun. Furthermore, unstable states of EC-ITC RF gun, such as multi-pulse working states, unstabilitis of RF field and input beam parameters, alignment biases, have been estimated detailedly with adjustable ranges given.The physical design scheme of high performance compact submillimeter wave FEL injector facility has been proposed on the basis of enhanced EC-ITC RF gun. Under integral targets, preliminary designs of main equipments, such as travelling wave accelerators and focusing coil system have been performed, as well as start-to-end beam dynamics calculations. The results indicate that high brightness electron bunches could be generated stably by this kind of FEL injector.Furthermore, deeply researches indicate that enhanced EC-ITC RF gun still has improvement spaces. So that a new task of improving beam use ratio has been proposed here, aiming at reducing design stress of electron sources while guaranteeing to meet performance targets of submillimeter wave FEL facility. The design rules of ITC RF gun has been summarized, and a kind of novel ITC (TN-ITC) RF gun has been explored. On the basis of analysis and comparisons of different RF fields, as well as previous research experiences, the physical design results of TN-ITC RF gun have been presented, which demonstrate that only2.5A input beam needed for the high performance submillimeter wave FEL facility, meaning beam use ratio has been doubled.