Reduction of Transverse Emittance Growth along the UNILAC

The preservation of beam quality along the UNILAC Alvarez DTL section is a major concern with respect to the availability of the design beam parameters for FAIR. The DTL comprises 177 accelerating gaps and 184 quadrupoles for transverse beam focusing. They form a periodic structure of linear elements. The high beam currents needed for FAIR arise non-linear space forces leading to emittance growth. The growth depends on the beam envelope shape inside the DTL which can be varied by the quadrupole strengths. Integrating the focusing strength over one period gives the transverse phase advance σ. In case of low currents the so-called zero current phase advance σo is calculated analytically. The beam envelope also depends on the focusing prior to DTL injection, i.e. the matching. A beam is matched if its envelope inhabits the same periodicity as the focusing structure. Mismatched beam envelopes "breath" around the periodic (i.e. matched) solution with a mismatch M, quantifying the mismatch. Experimental campaigns aimed for the dependency of emittance growth on σo and M for the Alvarez DTL [1]. The design beam for FAIR is 15 emA of 238 U 28+ being space charge equivalent to 7.1 emA of 40 Ar 10+ . Since 40 Ar 10+ is available with higher intensity and allows for a larger spectrum of focusing strengths, 40 Ar 10+ was used.