Diffraction-limited storage-ring vacuum technology

Abstract

Some of the characteristics of recent ultralow-emittance storage-ring designs and possibly future diffraction-limited storage rings are a compact lattice combined with small magnet apertures. Such requirements present a challenge for the design and performance of the vacuum system. The vacuum system should provide the required vacuum pressure for machine operation and be able to handle the heat load from synchrotron radiation. Small magnet apertures result in the conductance of the chamber being low, and lumped pumps are ineffective. One way to provide the required vacuum level is by distributed pumping, which can be realised by the use of a non-evaporable getter (NEG) coating of the chamber walls. It may not be possible to use crotch absorbers to absorb the heat from the synchrotron radiation because an antechamber is difficult to realise with such a compact lattice. To solve this, the chamber walls can work as distributed absorbers if they are made of a material with good thermal conductivity, and distributed cooling is used at the location where the synchrotron radiation hits the wall. The vacuum system of the 3 GeV storage ring of MAX IV is used as an example of possible solutions for vacuum technologies for diffraction-limited storage rings.

Commissioning and first-year operational results of the MAX IV 3 GeV ring

“…The compact magnet design leads to narrow low-conductance vacuum chambers (Al-Dmour et al, 2014) requiring distributed pumping and distributed absorption of the synchrotron radiation heat load. The choice of copper for the chamber material helps to alleviate the heat-load problem with chamber cooling being provided along the extended region over which the synchrotron radiation heat is deposited, whereas distributed pumping is provided by non-evaporable getter (NEG) coating of the chamber's inner surface.…”

Design Study of an MBA Lattice for the Advanced Photon Source

“…Chambers for the L-bends, which are "C" magnets open on one side, will look quite similar to the insertion device chambers now commonly in use at the APS: machined aluminum extrusions with non-evaporable getter strips mounted in an antechamber. The difficult FODO section chamber must withstand significant synchrotron radiation heating while holding good vacuum with limited conductance, so a cooled NEG-coated copper chamber is planned similar to that being implemented for MAX-IV [5]. This chamber will have dedicated valves so that it can be installed in the ring already activated and under vacuum, to save installation complexity and time.…”

The MAX IV storage ring project

“…Finite-element analysis has been extensively used (Al-Dmour et al, 2011) to understand the effects of the deposited heat and ensuing stresses on the mechanical integrity of the cambers as well as on the positional stability of the BPM blocks, a critical issue due to the tight requirements on beam position stability that result from the machine's ultralow emittance.…”

“…The compact magnet design leads to narrow low-conductance vacuum chambers (Al-Dmour et al, 2011), which necessitate distributed pumping and distributed absorption of the heat load from synchrotron radiation. The heat load problem is dealt with by choosing copper as the chamber material and providing water cooling along the extended region over which the synchrotron radiation heat is deposited, whereas distributed pumping is provided by non-evaporable getter (NEG) coating of the chamber's inner surface.…”