Theoretical And Initial Experimental Research On Low Emittance Photoinjector

RF photoinjector is a kind of high-brightness electron source which can fulfill the needsof fourth generation light sources, such as X-ray FEL facilities, ultra-fast X-ray sourcesbased on Thomson scattering and so on. The normalized emittance of RF photoinjector hassignificant impact both on the length of total length of undulator for XFEL facility and onthe peak-brilliance of ultra-fast X-ray source based on Thomson scattering. One main partof this dissertation is dedicated to lower the normalized emittance of the beam at the exit ofRF photoinjector in theory. An ultra-fast hard X-ray source based on Thomson scatteringis being developed in accelerator laboratory of Tsinghua University. The primary step ofthis project is to develop a S-band 1.6 Cell photocathode gun. So the other main part of thisdissertation is concentrated on the design, fabrication and conditioning of a BNL/KEK/SHItype 1.6Cell S-band photocathode RF gun.The normalnized transverse emittance at photoinjector exit consists of two part: onepart is the thermal emittance which is determined by the photoemission process at cathodesurface, another part is the correlated emittance, which is induced by space charge effectsand RF effects during acceleration and propagating in photoinjectors. The correlated emit-tance can be reduced by the so-called emittance compensation process.The thermal emittance is the basic part of the emittance of the electron beam producedby photoinjectors, so it is very important in the design of photoinjectors. The reported mea-surement results of thermal emittance of copper and magnesium cathode are significantlylarger than theoretical prediction. This fact may imply that surface roughness effect mayhave significant contribution to thermal emittance. We established a model, and gave ananalytical expression to compute the thermal emittance caused by random transverse elec-tric field in the vicinity of the cathode surface due to roughness effect. We made surfaceprofile measurements on the samples after diamond cutting or diamond polishing. With theanalytical expression we get in the thesis, we estimated the dedication of roughness effectto the thermal emittance for our samples is about 0.3mm·mrad per mm laser radius.Emittance compensation process can be used to reduce the correlated emittance, so it isvery important in photoinjector. The correlated emittance within a slice of electron bunch,which is induced by nonlinear fields, is called correlated slice emittance. The correlatedemittance due to the correlation between the phase space angle and longitudinal positionof slices is called correlated projected emittance. We carried out first order analysis bothon correlated slice emittance and correlated projected emittance of space-charge dominatedrelativistic beams, and got the quantitative dependence of the two kinds of correlated emit-tances on initial phase space. Analytical expressions for the evolution of the two kinds ofemittances in drift space are also obtained. The expressions show two zero-emittance pointsexist if some constraints on the initial phase space are fulfilled, which may correspond to thedouble emittance minimum phenomenon often observed in simulations of photoinjectors.With the understanding of emittance terms and emittance compensation, we give twomethods to lower the emittance of 1nC, 10ps RF photoinjector based on S-band 1.6Cell RFgun. The first method is to shape the drive laser to be a uniformly distributed ellipsoid.The other method is to increase the field strength of RF gun as high as 250MV/m. Parmelasimulation shows, by use the two methods, the emittance for 1nC, 10ps RF photoinjectorcan be lowered to be 0.67μm and 0.65μm respectively.We finished design, fabrication and conditioning of a 1.6Cell RF gun. The RF gun isslightly different from the original design of BNL one. The tuner at the full cell is canceledto lower the risk of RF breakdown, and the coupling hole between full cell and waveguideis lengthened to realize critical coupling without inserting an inductive or capacitive pluginto the waveguide. For keeping symmetry, the dummy coupling hole at vacuum port isalso lengthened. A code, Mafia T3 is employed to compute the residual multipole RF fieldinside full cell and the power ?ow exist in full cell. The computation shows that, whenthe coupling hole is lengthened to have a length of 24.5mm, the power ?ow existing in fullcell contributes a normalized transverse emittance less that 0.1mm·mrad, and the residualmultipole RF fields contribute a normalized transverse emittance about 0.5mm·mrad. Aftertuning, the field balance of the 1.6cell cavity is 1.0, the frequency ofπmode under vacuumstate and at 45 centigrade is 2856.22MHz. After about 200 hours conditioning, the inputpower reached 4.5MW. Under this power level, the measured dark current is about 5mAand total charge of dark current is about 2.5nC. Between power level 2.5MW to 4.5MW, thefield enhancement factor is measured to be about 140.