November 2005

Beam Stability Tests at X18B

One important parameter in the success of experiments at the NSLS is the stability of the x-ray beam on the sample. Properties such as intensity, beam position, energy stability, and in a few cases degree of polarization play crucial rules in this success, and some can be neglected based on the experiment. Ideally, these properties, when normalized by their dependence on ring current, do not change over the course of an experiment. Change from the ideal performance can be caused by motions of the electron beam in the storage ring or by motions of the beamline optics. The latter are often thermally driven, their effect is not proportional to beam current, and they can cause any combination of beam position changes, intensity changes, or energy changes. Ideally, these motions are eliminated by stable supports (beamlines and storage ring), feedback systems (electron beam and, in some cases, beamline optics), and efficient cooling schemes. In practice, however, both the electron beam and the beamline optics move slightly over time, requiring regular realignment.

During a beam studies period, we measured the effect of beam motion on the intensity and energy of beamline X18B, which is equipped with a channel-cut Si(111) monochromator located at a distance of 18 meters from the source. X18B accepts a vertical (horizontal) divergence of 50µrad (0.5-1mrad). Since X18B is optimized for x-ray absorption spectroscopy, we do not care too much about position stability. In contrast, we care a lot about source angular stability, since this directly affects the photon energy selected by our monochromator.

Figure 1. Intensity and energy shift for changes of the vertical angle. The stability of the beam is better than 5µrad.

Based on the geometry of the X18B beamline (slit sizes and positions), we calculate that vertical offsets of the source (electron beam) by 60µm or angular changes by 5µrad cannot be observed in a typical experiment. In order to test these calculations, we aligned the beamline slits such that the beamline becomes sensitive to beam motions in one direction (angle or position), but not the other. We then moved the beam (angle and position) around the nominal values and measured the intensity and energy stability. We did not study the effects of orbit motion (position or angle) in the horizontal plane, since X18B is not sensitive to motion in this plane.

Figure 2. Intensity and energy shift for changes of the vertical position. The stability of the beam is better than 50µm.

The results of the orbit studies described above on the intensity and energy of beamline X18B are shown in figures 1 and 2. It is apparent that vertical angular changes have only a small effect on the energy calibration, independent of the slit position, but have a relatively large effect on intensity. Theoretically, the energy should remain constant as a function of vertical source angle, which demonstrates that the X18B beamline saw a positional shift in addition to an angular change during these studies. The photon energy shifts significantly more for changes in the vertical source position, about 0.1eV for 100µm, as expected. The intensity profile in Figure 2 is an asymmetric function of vertical source position relative to the standard orbit value. The reason for this is that we aligned our three beamline slits/apertures (5mm vertical Be-window, 1mm vertical white beam slit, 0.5mm vertical hutch slit) such that the lower edges of their upper jaws were aligned, as viewed from the source point. In this situation, if the electron beam moves up, one of the slits intercepts the beam and the intensity is reduced immediately. Beam motions in the other direction, however, move the beam more towards the center of the three slits, so the intensity is much less strongly reduced, as can be observed in the data.

In summary, we confirmed our calculations for the sensitivity of the X18B beamline to electron beam motions and demonstrated that the intensity can vary significantly if the beamline is not aligned appropriately. For normal operations, we align the slits symmetrically about the nominal beam axis in order to maximize intensity and to maximize the intensity stability. During normal operations, the intensity varies less than 0.4%, and the energy is stable within 0.05eV.