Physics design for the compression and focusing of heavy ion beams

Summary form only given. Heavy ion beams are suitable for heating matter in the studies of high energy density physics and inertial fusion. To obtain the required rapid target heating and high beam intensity, the beam has to be longitudinally compressed and radially focused. The former can be achieved by neutralized drift compression which has been demonstrated by experiments [P. K. Roy et al., Phys. Rev. Lett. 95, 234801 (2005)]. After imparting a head-to-tail velocity gradient to the beam, the beam compresses as it drifts in a dense background plasma where the effects of space charge are neutralized. For low energy beams, the beam velocity is ramped by induction modules with tailored voltage waveforms. This method is, however, technically difficult and highly expensive in the high energy regime. A physics design is introduced to generate a velocity tilt for high energy beams at a low cost. By sweeping beam ions transversely with a time-dependent dipole upstream of a wedge, different slices of the beam pass through matter of variable thickness. The energy loss creates a velocity tilt for the drifting beam which subsequently compresses within a dense background plasma. Using stopping power models and particle codes, once the target position is given, the shape of the wedge can be designed accordingly to give maximum compression at the target. Simulations have been conducted using typical beam parameters of FAIR at GSI and HIAF at IMP Lanzhou. The results show that energy straggling has negligible effect on the bunching factor, whereas beam ion scattering and fragmentation within the wedge depend on the type of beam ion, beam energy and wedge material. A decisive factor affecting the current amplification at the target is the ratio between the head-to-tail sweep length and the spot size on the wedge. Based on these results, an experiment that demonstrates longitudinal compression using a wedge is proposed. The wedge is also incorporated into the design of a full sys- em that delivers both transverse focusing and temporal compression. Numerical studies are employed to determine the time-dependent parameters of the dipole and quadrupole elements. Simulation results are presented and the constraints and trade-offs in the design are discussed.