Synchrotron operation with intermediate charge state heavy ion beams

Increasing the intensity of heavy ion beams in synchrotrons, especially in the low and intermediate energy range, requires a decrease of the charge state. The FAIR project [1,2] at GSI is aiming for highest heavy ion beam intensities with an increase of two orders of magnitude compared to the present intensity levels. Space charge limits and significant beam loss in stripper stages disable a continuation of the present high charge state operation. The presently achieved level of heavy ion beam intensities is in the order of 10 and 10 heavy ions per cycle (e.g. with Uor Ta ions). The FAIR intensities of 10 to 10 heavy ions per cycle can only be reached by reducing the charge states, e.g. with acceleration of U-ions instead of U-ions. As an extreme example of the general tendency to lower charge states with increasing intensity, the Heavy Ion Fusion (HIF) driver studies HIBALL I/II and HIDIF can be taken, which have been conducted at GSI. For reaching ignition of the fusion pellet, about 10 Bismut ions with charge state 1+ are required. With high rigidity heavy ion synchrotrons, like the planned FAIR synchrotron SIS300 and low charge state heavy ions like U, intensity values in the middle of the presently achieved and the fusion driver intensities may be generated. DYNAMIC VACUUM AND CHARGE CHANGING PROCESSES Figure 1: Electron capture or ionization processes at collisions with residual gas atoms and molecules change the charge state of the beam ions. Gas desorption drives a pressure bump at the impact position after dispersive elements. Completely neglected in the fusion driver studies, however the main loss mechanism at heavy ion operation with low charge states in circular accelerators is ionization (or at low energies electron capture) due to collisions of the projectiles with residual gas atoms (see Figure 1). Ions with changed charge states and thereby large deviations form the reference rigidity are lost at positions with sufficient dispersion. Local pressure bumps at these impact positions, generated by ion desorption processes, drive a strong vacuum dynamics. Thus, a very low static, initial pressure (in the range of <10 mbar) is important but not sufficient and suitable to prevent a strong residual gas pressure dynamics and consequently major beam loss. In the collision process, the charge state of an ion tends to approach the equilibrium charge state at the relevant beam energy. Depending on the energy range, the dominating processes may even face a transition from electron capture to ionization during the acceleration process (Table 1). The problem arises especially since the injectors linacs are most often not well matched to the equilibrium charge state at the injection energy. The charge state of the injected beam is typically determined by the stripper system of the linac and depends on the equilibrium charge state at the energy of the stripping process. Thus, in existing facilities the charge state for injection can not easily be changed. Table 1: Injected and equilibrium charge states in SIS18 at injection and extraction energy for different ions. With respect to ionization beam loss 59+ would be the best charge state for injection into SIS18. Ion Injected charge with/without stripper Equilibrium charge state at injection Equilibrium charge state at extraction