Beam Dynamics Simulations for the Laser Proton Injector Transport Line

The recent development in the field of "laser acceleration of protons and ions" has initiated several investigations of this concept of an innovative and compact accelerator. Unique features of the Laser Ion Source (LIS) are extremely high beam current and very low transverse and longitudinal beam emittances. A general disadvantage of the LIS is a huge spectrum of the proton energies. A simulation of proton collimation and transport, based on output data from the PHELIX experiment [1], is necessary to study chromatic and geometric aberrations of the first collimator as an interface between the LIS and the adjacent accelerator structure [2]. The multiparticle code DYNAMION [3], dedicated to simulate beam dynamics in linacs, was used as an advanced tool to perform investigations for the laser proton injector beam transport line. Non-linear effects and high order aberrations are included in this code automatically. Space charge effects are also taken into account, while the simulations for the "zero" current case might be treated as the most optimistic case. Diversifications of the input beam parameters leads to additional emittance growth. The following proton beam parameters were fixed for the investigations: energy 10 MeV; transverse size ± 0.03 mm; transverse divergence up to ± 172 mrad; total unnormalized emittance up to 5 mm·mrad; phase spread ∆φ= ± 0.75° (related to 108 MHz); energy spread ∆W/W up to ± 64%; current up to 560 mA. Calculations for different input beam parameters were done for a transport line with quadrupole (Q-line) and solenoidal (S-lines) focusing. A realistic distribution of the solenoidal field [4] was introduced into DYNAMION simulations as a field mapping. The results of simulation for the quadrupole transport line show a much higher emittance growth (up to a factor of five), than for the Sline. The non-paraxial effect and the chromatic aberrations are weaker due to the symmetric solenoid focusing strength, suppressing large transverse deviation of the beam. Therefore we concentrate on the S-lines. An RF re-buncher is placed at the end of the transport line at a distance of about 240 cm from the LIS. It provides for longitudinal beam focusing and decreases the energy spread of the core particles (energy spread inside ± 4%) to less than ± 0.5%. It is planed to make use of an already existing 108 MHz 3-gap re-buncher. Realistic 3D electric field of the re-buncher is calculated by the DYNAMION code as well, solving the Laplace equation on the base of the real geometry of drift tubes and gaps. Figure 1: Zero current particle trajectories for different energy spread. Transport line includes focusing solenoid and RF re-buncher.