Only 10% of the X-rays generated in the bending magnets are delivered to the beamline'™s front-end. The other 90% are mainly dissipated inside the crotch absorber (Figure 150). However, ~ 2 ppm of this power (i.e. 300 uW/mRad horizontal) is not absorbed and traverses the complete structure (of ~ 40 mm Cu) into the air behind. This leakage power is carried by the very high energy photons. The spectrum of this signal presents a peak at ~ 170 KeV. At this high energy the natural photon beam's vertical divergence is small. The in-air X-ray (IAX) detector is located at ~ 1 m from the source point in the bending magnet and the observed vertical spot size essentially results from the projected natural divergence of the synchrotron radiation convoluted by the vertical size of the electron beam at the source. The imaging IAX detector uses a High-Z scintillator, such as CdWO4 or LuAg:Ce, with a simple optical system to focus an image onto a CCD camera (Figure 151). The assembly is compact since space is limited between the crotch chamber and the flanges, just a few cm further downstream. Lead shielding (not shown) was applied to protect the components against degradation from the hostile environment at this location.

Fig. 150: Position of the IAX-detector just behind a crotch absorber.


Fig. 151: Schematic side-view of the imaging IAX detector.

The image that the X-rays project onto the screen is essentially a wide horizontal line with a vertical profile of small dimensions. (Figure 152). The electron beam size is obtained from the measured beam size following a deconvolution with the natural divergence of the synchrotron radiation.

Fig. 152: Example of image and profile-plot obtained with the imaging IAX detector.


Fig. 153: Results of vertical emittance measurements with the imaging IAX and the two X-ray pinhole camera systems versus varying current in a skew-quadrupole.

The relative precision of the electron beam size measurement is estimated at better than 2%, taking into account the precision with distance and photon divergence. Comparative measurements (Figure 153) of the ring’s vertical emittance were carried-out with this device together with the two independent emittance measurement systems based on X-ray pinhole cameras. The current in a single skew-quadrupole was varied over a certain range and at each point the resulting vertical emittance measurements of the three devices were recorded. The apparent discrepancy between the three results (with each device at a different physical location in the ESRF ring) is expected with the applied method. The high resolution (estimated at < 0.05 pm for 1 second measurement & averaging time) is also shown in Figure 154 when the currents in the skew quadrupoles are switched a few times between two different settings (nominal and ultra-small emittance).

Fig. 154: Vertical emittances measured by the imaging IAX camera C5 for two different settings of the skew quadrupoles.

In 2006, five IAX Imaging devices were commissioned. Such detectors will later be used to develop a vertical emittance stabilisation system that will maintain a small unchanged vertical emittance over long periods for any gap settings of the insertion devices.