Nano Beam Position Monitor

Transcription

1 Introduction Transparent X-ray beam monitoring and imaging is a new enabling technology that will become the gold standard tool for beam characterisation at synchrotron radiation facilities. It allows highly focussed X-ray beams to be fully characterised in situ, allowing researchers to monitor beam delivery during measurements and create active feedback in order to stabilise beam position and shape. As beam size and sample size are ever decreasing, unavoidable vibrations and ground settlement will cause more and more issues with co-location between beam and sample. Furthermore, micro-focusing of the intense X-ray beams often needs re-adjustments either because the focusing optics is chromatic or the focal spot must be relocated. Therefore, a single device that is able to measure beam position and shape transparently represents an important forward step in technology. Issue 2.0 FMB Oxford Ltd 2012 Page 1 of 7

2 Principle of operation At the core of the Nano-BPM is a thin foil of an amorphous low-z type material that is placed in the X-ray beam (figure 2). A negligible amount of radiation is elastically scattered and detected by an X-ray camera system that is placed at right angles with the beam. The camera system is constructed is such a way that a magnified image of the beam is recorded and the magnification factor can be changed easily. An innovative aperture design allows sufficient flux to be recorded by the sensor for high-resolution beam sensing even in the case of bending magnet beam lines. This method was awarded patent protection on 24 th through UK patent GB , "Beam Sensing". For the characterisation of so-called white or polychromatic beams as produced by the SR source, thin Beryllium or diamond foils are appropriate. This capability is significant because it will allow high-resolution monitoring of the incident X-ray beam close to the source. The key innovation of the Nano-BPM technology is its capability to magnify any beam movements at the sensor position as shown in figure 3. Small changes (δz) in the centroid position of the incident X-ray beam produces a magnified position movement at the detector of Δy. Performance of the FMB Oxford Nano-BPM The performance of the device was verified experimentally at several synchrotron radiation facilities including the ESRF and Diamond Light Source (DLS). Figure 4 shows the recording of the vertical beam position at a small angle scattering beam line taken with a 50 μm mica foil. During the recording a series of 10 μm steps of the beam were made. The insets shows the full extend of the recording and highlights the large linear range of the sensor. Figure 5 shows data taken at an unfocused ESRF undulator beam line at an energy of 20 kev. In this experiment we make a direct comparison between the readings from the Nano-BPM with those taken with a quadrupole ion chamber and a directly exposed quad silicon diode. The silicon quadrant photodiode was placed at the end of the experiments hutch and proved to be the worst performer. In general, it is not straightforward to distinguish noise in position measurements from actual beam position fluctuations. For an estimate of precision by which the position is measured, we use a procedure that calculates the RMS value of deviations of individual measurements with a smoothed or multi-point averaged data set. Using this procedure we find that the position is measured with a precision better than 200 nm at this particular beam line. Feedback control One of the important features of the device is the fact that it is capable to provide feedback control signals to maintain beam position removing any beam drifts. The device offers several ways of doing this. For most applications a single analogue signal that controls a monochromator actuator (such as a piezo) is sufficient. The Nano-BPM can be configured with four 16-bit DAC outputs for this purpose. Shown below (figure 6) is an example of a typical beam line using a double crystal monochromator where the Bragg rotation axis is located at the surface of the first crystal. Upon energy changes the exit beam will change height that is proportional to the gap between the crystals. The measurements shown in figure 9 are for a 10 mm gap and show how the beam height changes during an energy scan around the copper K-edge of about 1 kev wide (a typical EXAFS scan). The left hand graph shows the situation where at the beginning the monochromator moves quickly to the low energy position and then slowly changes energy to complete the full 1 kev scan. During the scan both vertical beam position and width of the incident beam are recorded. The left hand graph shows the same scan but now the XBI s feedback signals are used to stabilise the beam position to within 2 μm (set by the mechanical limitations of the actuators used). During the measurement the system was Issue 2.0 FMB Oxford Ltd 2012 Page 2 of 7

3 using a relatively long time constant in the PID control loop to avoid quick mirror actuator changes and hence at the beginning of the scan there is a large (>100 μm) excursion after which the system catches up. Besides the beam centroid position information, we have also plotted changes in beam width as the energy is scanned. A further important important application of feedback control is to remove any beam drifts. Drifts caused by thermal effects on the highly thermally loaded wiggler monochromator - in which position changes of more that 150 μm where recorded - where reduced through feedback control on the second crystal pitch to below 1 micrometer fluctuation. Beam imaging An example of the imaging performance of the device is shown in figure 7. These images were obtained with two identical Nano-BPM devices simultaneously at a bending magnet beam line. The two devices separated by about 4 metres; one close to the sample position (focal spot) and one close to the focusing toroidal mirror. This configuration allowed us to distinguish between angular and parallel motions of the beam and also to show how the beam shape is influenced by the settings of the toroidal focusing mirror. Software for the FMB Oxford Nano-BPM The Nano-BPM comes standard with a precompiled Windows application for configuration and operation of the device. Additionally drivers and Graphical User Interface (GUI) applications for EPICS, SPEC and LabView have been developed. Figure 8 shows a screen shot of a GUI that provides full control over all parameters and allows plotting of beam position, profile and images. License The technology behind the Nano-BPM is exclusively licensed by FMB Oxford from the University of Manchester. The novel X-ray beam characterisation technique was developed by Dr Roelof van Silfhout. The data shown in this document was collected by Dr van Silfhout in collaboration with colleagues at the ESRF and DLS. The technology is covered by an international patent. Figure 2. Schematic of a Nano-BPM Issue 2.0 FMB Oxford Ltd 2012 Page 3 of 7

4 Figure 3. Schematic of beam position magnification at the sensor Figure 4. Recording a large series of 10 μm (inset) showing the large linear range. Issue 2.0 FMB Oxford Ltd 2012 Page 4 of 7

5 Figure 5. Comparison of Nano-BPM with quadrupole ion chamber readings taken simultaneously during series of steps of 200 nm. Issue 2.0 FMB Oxford Ltd 2012 Page 5 of 7

6 Figure 6. EXAFS scans with a double crystal monochromator, lower trace represents vertical beam position whereas the top trace indicates the full width at half maximum value of the vertical beam profile. The upper graph shows the measurements before stabilization, whereas the right hand graph shows the result after feedback was switched on. Issue 2.0 FMB Oxford Ltd 2012 Page 6 of 7

7 Figure 7. Beam imaging pictures taken with the XBI device at two positions along the beam path. Left is the image as measured close the toroidal mirror whereas the right image was taken close to the sample position. Figure 8. GUI for use with EPICS drivers. Issue 2.0 FMB Oxford Ltd 2012 Page 7 of 7