The vertical beam halo is the main limitation for very low gap operation of in-vacuum undulators at the ESRF EBS. The vertical halo is due to Touschek electrons with large energy deviation crossing some betatron resonances. The crossing of the resonances can transfer horizontal momentum to vertical momentum. The beam halo has been characterized and measured and different low halo optics have been studied and tested to allow the operation of the machine with lower in-vacuum undulator gaps.

The European Synchrotron Radiation Facility - Extremely Brilliant Source (ESRF-EBS) is a facility upgrade allowing its scientific users to take advantage of the first high-energy 4th generation storage ring light source. In December 2018, after 30 years of operation, the beam stopped for a 12-month shutdown to dismantle the old storage ring and to install the new X-ray source. On 25th August 2020, the user program restarted with beam parameters very close to nominal values. Since then beam is back for the users at full operation performance and with an excellent reliability. This paper reports on the present operation performance of the source, highlighting the ongoing and planned developments and the sustainability efforts.

The European Synchrotron Radiation Facility (ESRF) presently operates with the Hybrid Multi-Bend Achromat (HMBA) lattice that features 𝛽-functions of 6.9 m and 2.7 m in the horizontal and vertical planes at the center of the straight sections. New optics were designed to increase the brilliance of beam lines with a single undulator placed at the center of the straight section. The reduction of the in-vacuum undulator gap and of the beta-functions both contribute to this increase. This paper reports on the optics beam commissioning results and experimental observation with the reduced in-vacuum undulator gap.