(Part of) the team involved in XAIRA installation, gathered in the beamline Control hutch to see the first beam going through the optics.
Cerdanyola del Vallès, 25th October 2022 On July 22nd, the front end – an element located inside the tunnel, which connects the storage ring with the beamline – was opened to transfer the synchrotron light to the beamline optics, which filters the X rays in the synchrotron light and transport and focus them onto the sample. XAIRA uses an insertion device as a light source, a 2.3 m long in vacuum undulator.
The first element in the optics, the horizontal prefocusing mirror (HPM), located inside the optical hutch, is used to absorb about half of the power of the synchrotron light and provides a first horizontal focus of the beam.
Commissioning started at 7 am, with the current of the storage ring initially set to very low values (1 mA), to test the system with a low power. Upon illumination with the photon beam, the optical elements take in the power and start to outgas the chemical species adsorbed on its surface (water, nitrogen, …), thus degrading the pressure in the vacuum chamber. Following the commissioning plan, storage ring current was slowly raised, while constantly checking that the temperature of the optical elements and the pressure inside the vacuum vessels remained within acceptable limits, until the nominal ring current of 250 mA was reached.
Once this first goal had been achieved, it was the time to set the insertion device to the nominal values of the beamline operation. For this, with the ring current kept at 250mA, the power was further increased by closing the gap between the jaws of the undulator, that is, the magnetic arrays that wiggle the trajectory of the electron beam. The undulator was gradually and carefully reduced from an initial gap of 30 mm to a final one of 5.5 mm, close to the instrument limit. At that point the power was about 4 kW in a few square millimetres. Reducing the undulator gap also increases the number of photons, especially in the range of X rays aimed for experiments at XAIRA. This task took most of the two days allocated to the commissioning, as the increase in power had an immediate effect on the degassing of the optic elements and led to a degradation of the vacuum levels, which takes some time to be compensated by the vacuum pumps installed for that purpose at the beamline optics vessels.
The beam reflected by the HPM mirror was detected by a fluorescence screen monitor located right downstream. This beam diagnostic consists in a camera imaging a screen made of diamond, the only material that can withstand the power, which glows with radiation. Thus, the beam appeared on the screen as a large luminescent spot. The size and shape of the beam spot were delimited by the beam-defining slits located between the front end and the HPM mirror.
Beam at the first fluorescent screen monitor. (Left) First observation of the beam, at ring current of 250 mA and with the undulator gap totally opened. (Right) Beam observed with a gap of 25 mm after centring the slits to the beam axis.
Using the intensity measurements obtained from radiation monitors, the slits were then centred with respect to the photon beam axis. This ensured that the central part of the power distribution, which is the part used in the experiments, was not blocked by the slits and was reaching the mirror and the downstream optics.
As a result of the data gathered in these two first days of commissioning, all the optic elements in the optics hutch were realigned to better match the real beam path.
XAIRA team is now working on the alignment of the beam on the monochromator with the help of a second fluorescence screen monitor located downstream. Following this successful first beam achievement, the next milestone will be bringing the photon beam at the Experimental Hutch. XAIRA will be the 10th beamline of ALBA and it is planned to receive its first users at the end of 2023.
This project is co-funded by the European Regional Development Fund (ERDF) within the Spanish Pluri-Regional Operative Programme (former Smart Growth Operative Programme) 2014-2020.
