Dr. E. Duke
CLRC Daresbury Laboratory, Warrington, Cheshire. WA4 4AD
Beamline 14 is a new beamline being built on the Synchrotron Radiation Source (SRS) at Daresbury Laboratory. The beamline will have 2 stations for protein crystallography. One of the stations will be of high intensity and will be able to access the selenium k-edge at 0.979Å. The other station will operate at higher wavelengths in order to gain the most benefit from the multipole wiggler insertion device.
The SRS at Daresbury was the first ever storage ring dedicated to the production of synchrotron radiation, it is what is termed a second-generation source. In reality what this means is that there are no long straight sections in the machine where insertion devices can be positioned with ease. However there was a desire to gain experience with insertion devices both from the machine point of view and the beamlines, as working with insertion devices requires different techniques especially when it comes to dealing with the high heatloads produced by the photon beams. This desire fitted neatly with a demand for more high intensity protein crystallography stations on the SRS; thus the concept of Beamline 14 came into being.
Given the parameters of the SRS and the desire for high flux in the hard X-ray region a multipole wiggler (MPW) was best suited to our requirements. The accelerator physicists carried out various experiments on the SRS to determine the minimum electron beam height before the lifetime of the stored beam was seriously compromised. This then dictated the minimum gap of the multipole wiggler. It was found that a gap of 20mm was possible. Calculations were then carried out to determine the magnet parameters. Two stations on the beamline were planned so good flux off-axis was an important criterion as well as that in the centre. For this reason a 2.0T magnet with 9 full poles plus two half-poles was chosen. The other possibility was a 1.8T magnet with 11 poles which gives 4.3x1013 photon/s/mrad at 300mA into 0.1% bandwidth at the centre of the fan. However there is a significant deterioration in flux going out to 6.5mrad. The 2.0T, 9 pole magnet gives marginally less flux at the centre (4.2x1013 photon/s/mrad at 300mA into 0.1% bandwidth) but the reduction in flux out towards 6.5mrad horizontally is much less.
Having established the magnet design the next stage was to design the beamline starting with the position that the 2 stations would take on the fan of radiation from the MPW. As the flux from the MPW drops by roughly an order of magnitude between the centre of the fan and 6.5mrad it is important to position the two stations as close to the centre as possible. This poses a problem: there needs to be space between the stations so that, for example, the monochromator mounting assembly does not put a shadow onto the beam for the adjacent station. A novel solution was found: have the beams for the two stations cross over each other. This allows the dead space on the outside of the radiation fan to be used for all the cooling and bending mechanisms. Calculations were done to establish the best horizontal ranges for the two stations. The best beam is at the centre of the fan. However if one station were set symmetrically about the centre then the second station would have significantly less flux. A compromise was found: one station was slightly offset from the centre of the fan bringing the second stations closer to the centre. The gain in beam quality for the second station more than compensates for the slight loss in flux for the first station. Therefore Station 14.2, the most intense station, takes beam from +3mrad to –1mrad and Station 14.1 from –1.5mrad to –4.5mrad.
In order for the beamline to be built quickly and have it commissioned for users as soon as possible simplicity of design was important. Therefore the decision was made to use an optical configuration similar to that used on 2 existing protein crystallography stations on the SRS, Stations 9.6 and 7.2. Thus the two stations on Beamline 14 have vertical focussing by a cylindrical mirror and monochromation and horizontal focussing with a single bounce monochromator. The effect of the single bounce monochromator, to deflect the beam out sideways, also allows the two stations to be fitted into the limited space available for the beamline.
The optical configuration chosen for the stations tends to lead to operation at a single wavelength - both Station 9.6 and Station 7.2 operate as fixed wavelength stations (0.87Å and 1.488Å respectively). However it is possible to provide a degree of tunability around certain pre-determined wavelengths and this is what is planned on Station 14.2. Both stations will operate at two wavelengths given by 2 different monochromators. For Station 14.2 these wavelengths will be 0.97Å and 1.2Å and for Station 14.1, 1.2Å and 1.5Å. The flux output from the multipole wiggler increases with wavelength in this wavelength region. However given the current demand for data collection facilities at the Se k-edge (0.979Å) it seemed sensible to provide this facility on Station 14.2. Even with slightly less flux we estimate that the flux provided at 0.97Å will be in the region of 20 times greater than what is available on Station 9.5. It should also be possible to provide tunability around other absorption edges. It is intended to look at the operational issues pertaining to this during the commissioning of the beamline.
In order to allow the wavelength to be changed between the 2 wavelengths of operation (eg. 1.2Å and 1.5Å on Station 14.1) a new mount has been developed which allows the 2 monochromators to be stacked in a "double-decker" arrangement. This means that the wavelength change can be effected by a vertical translation rather than the lengthy process of letting the monochromator vessel up to atmosphere, changing the monochromator followed by pumping back down again.
The heat-load within the X-ray beam on beamline 14 is great. Therefore all of the beamline components which could be struck by the beam will be water-cooled. This includes the mirror and monochromators and all the slits.
The two stations will be equipped with state-of-the-art detectors. Station 14.2 will have a PX210 made by Oxford Instruments and an ADSC Quantum 4R will be mounted on Station 14.1. The PX210 is a 3x3 CCD array with an active area of 210mm x 210mm. The full-frame readout time is 1.8s; this is reduced to 0.5s if 2x2 binning is used. This fast readout time will be of benefit for data collection in fine phi-slicing mode. The Quantum 4R is an updated version of the detector currently on Station 9.6. Here the active area of the 2x2 CCD array is 180mm x 180mm and the full frame "slow" readout is 9s. This time drops to 3s with the full frame fast readout. Initially the ADSC Quantum 4R will be on Station 14.2 as delivery of the PX210 is not expected until January 2000. Once the PX210 is commissioned on Station 14.2 the ADSC Quantum 4 will be installed on Station 14.1. The rotation axes on both stations will be an updated version of the rotation axis currently on Station 7.2 that was designed at the EMBL in Hamburg.
Commissioning has started on Beamline 14 and is progressing well. The first users are scheduled on Station 14.2 in late November. However prior to that users will be invited to come and collect data on Station 14.2. Station 14.1 will follow on after Station 14.2 with first scheduled users in early February.