These studies attempt to increase the coherence and the brightness of the MAX IV 3 GeV storage ring within the boundary constraints of the installed magnetic lattice. This calls for an improved optics while, however, respecting limitations of the available magnetic gradient strengths as well as lifetime and injection efficiency requirements.

brightness_vs_coupling
In a first step we propose to adjust the emittance coupling in the MAX IV 3 GeV storage ring [1]. The emittance coupling that results from errors is expected to be on the order of 0.4% [2,3] and during initial design studies it was assumed that this would be adjusted to the 2.5% level (1 Å diffraction limit). By instead choosing a lower emittance coupling we can, however, increase both the brightness and coherence at the typical 1 Å wavelength of a typical MAX IV IVU. It has been shown that this does not lead to too severe lifetime or dynamic aperture restrictions. But in fact, lifetime can be recovered despite operating at low emittance coupling, by employing successive vertical dispersion bumps as detailed in another project [4].

 

In a second step we study an improved optics that allows us to lower the emittance without exchange of magnets and/or power supplies, thereby increasing typical photon brightness and coherence in the MAX IV 3 GeV storage ring. A first candidate lattice has been developed [5] M4_achr_upgradewhere both the optics in the IDs and in the arc have been optimized for higher brightness. Specifically, the beta functions in the ID straights have been better matched to the intrinsic photon beam as well as the dispersion in the arcs further reduced. M4_optics_upgradeStudies indicate these measures render a bare lattice emittance reduction by 18% and an increase of brightness at 1 Å by 33%; meanwhile lifetime and dynamic aperture remain sufficient. M4_brightness_upgradeAgain, vertical dispersion bumps [4] could be employed to increase the lifetime in this optics.

 

Ongoing studies involving further enhanced lattices, reverse bends, and optimization with genetic algorithms aim at further pushing the optics into the vicinity of a bare lattice emittance of 200 pm rad. This will, however, necessitate the exchange of some power supplies. At some point the dynamic aperture will also become so constrained, that we plan on giving up our existing off-axis injection scheme in favor of an on-axis injection scheme based on a single dipole kicker which can act on individual bunches therefore allowing transparent top-up injection into a dynamic aperture of only a few mm. Factoring in the strong effect of IBS at 500 mA, as well as harmonic bunch lengthening cavities and high radiated losses from the operation of many IVUs, we believe we will ultimately reach the roughly 150 pm rad emittance region. At this point a further brightness increase will likely call for an exchange of the magnetic lattice.

 

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Last updated: March 26, 2016

 

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