HARD X-RAY SYNCHROTRON RADIATION MEASUREMENTS AT THE APS WITH VIBRATING WIRE MONITORS

A 5-wire vibrating wire monitor (VWM005) was developed and tested at the Advanced Photon Source (APS). The sensor was mounted on the outboard side of a bending-magnet synchrotron radiation terminating flange in sector 37 of the APS storage ring. The parallel wires were separated vertically by 0.5 mm; however, due to the possibility of rotation about a horizontal axis, the effective distance between the wires was reduced by a significant factor. To increase the response speed, the sensor was installed in air, resulting in a step response time of less than one second. Due to the extreme sensitivity of the detector, the very hard x-ray component of synchrotron radiation was successfully measured after its passage through the terminating flange. BACKGROUND At the APS, insertion device photon beam position monitoring is integrated into DC orbit correction algorithms, providing submicroradian levels of pointing stability over periods greater than 24 hours. Existing photon beam position monitors (BPMs) make use of ultraviolet (UV) and soft x-ray photoemission from metallized diamond blades arranged in sets of four and positioned edge-on at the periphery of the photon beam. These monitors suffer from a number of insertion device gap-dependent systematic errors associated with unavoidable soft background radiation. This radiation is associated with strong steering correctors located immediately upstream and downstream from the undulator source; they are part of the so-called Decker distortion scheme for reducing stray radiation overall (1). In an effort to further improve upon photon beam position monitoring, an effort was initiated in 2005 to develop a BPM that is completely insensitive to the UV and soft x-ray components of undulator beams. One active area of research is the development of PIN diodes arranged to sense copper x-ray fluorescence in the backward direction (2-4). The present work is the culmination of a number of informal meetings that took place at DIPAC 2005, and is in fact a hybrid of Arutunian et al.'s vibrating wire monitor (VWM) concept (5) and Scheidt's work with very hard x-rays at ESRF (6). First experiments with the VWM at the APS were conducted in vacuum with an undulator beam in early 2007 (7) using the same facility where the PIN diode measurements were made. The main conclusions from that work were that the VWM is sensitive to extremely low levels of x-ray flux, but the detector's time response is quite slow since the primary means of achieving thermal equilibrium in vacuum is through radiation. Nevertheless, the measurments and subsequent analysis have provided an interesting quantitative validation of the theory of undulator radiation. Shown in Fig. 1 are data collected for the two-wire detector installed in vacuum at APS beamline 19-ID during a vertical angle scan of the undulator source, which was a standard APS undulator A operating at a large gap of 60 mm. The principle for extracting temperature changes from individual wire acoustic resonance frequencies is described in reference (7) and in detail in reference (8), which also describes the operating principle of the device.