Cutaway half-model of 3.5 cell RF photoinjector with RF waveguide power feed. Electrons are produced by a UV laser pulse striking the cathode plate. The short ~100 fs electron bunch is accelerated to relativistic energy within 1 cm by strong 150 MV/m fields at the cathode and through the 3 cells, Modeled by V. Dolgashev and E. Nanni of SLAC.
The photoinjector produces the electron beam and provides initial acceleration to relativistic energy. Its critical job is to accelerate a short bunch of electrons from rest at the cathode to a few MeV while maintaining the small beam emittance, low energy spread, and a short bunch length. To accomplish this it requires high RF fields of 100 MV/m or more with low focusing aberrations and high stability. Thermal loading of the copper structure sets the maximum field strength in the high repetition rate regime. We have investigated x-band structures having 1.5 cells, 2.5 cells, and 3.5 cells for their ability to produce a beam of several MeV with high cathode gradient, moderate thermal loading, and low RF power demand. The 3.5 cell photoinjector shown outperforms the shorter structures in thermal loading while maintaining a high gradient.
Engineering model of coolant flow through RF photoinjector at full power. Cool blue lines are input, and warm red lines are output pipes. Cathode is at bottom and beam exit at top. Each RF cell iris carries a high heat load so is directly water-cooled. Modeled by J. Bessuille, MIT.
The cell layout differs in several respects from other RF photoinjectors. The half-cell containing the cathode is significantly shorter than lRF/4 with the iris center just 5 mm from the cathode. This is to reduce the electron transit time across the cell allowing cathode emission at phases closer to the peak RF field. We designed the field balance among the cells to give the highest fields (140 MV/m peak) at the cathode and in cell 1 with the peak field falling to 90% of that in cell 2 and to 45% in cell 3, which acts as the coupling cell. The higher fields are important in the early cells to accelerate to relativistic energy to overcome space charge effects. Cell 3 is a coupling cell with a race track shape to cancel the quadrupole moment of the RF fields and dual waveguide feeds to cancel the dipole moment. The lower field in this coupling cell also helps avoid distorting the beam distribution and lowers pulsed heating on the coupling slots. The four cells can support four different modes. SUPERFISH studies show the nearest mode is 18 MHz from the desired pi mode, well outside the 2.2 MHz resonance width given by the cavity loaded quality factor of 4200.
Due to the large thermal load in the cavity, targeted water channel placement is required to minimize temperature rise and thermal stress. Water channels with a total flow rate of 0.21 liter per second will operate with a pressure drop of 0.45 atm and a temperature rise of 3-4 degrees C between the inlet and outlet. The RF cavity temperature will be regulated within 0.1 degrees C in order to keep the photoinjector cavity on resonance. The assembly consists of five separate sub-assemblies. Each section has integral water cooling channels with no joints between water and vacuum.