IMPROVEMENT OF X-RAY BEAM POSITION STABILITY FOR THE BL4-2 SAXS FACILITY

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 我们继续缓解去年在新的BL4-2位置发现的光束位置不稳定问题。今年早些时候，我们确定了这种不稳定的主要原因，这种不稳定的程度通常是24小时内全宽垂直梁尺寸的1.5倍，很可能是几块厚混凝土地板随着每天的温度波动以不同的速度移动而逐渐产生的温度漂移。我们开始采用镜面螺距反馈系统，与在其他SSRL插入设备站上成功运行的系统非常相似。第一个版本直接复制了标准的束位监测器，在氦气气氛下工作时，铍信号叶片上涂有一层薄薄的钛。这种方法改善了光束稳定性，在16小时内实现了10微米的稳定性，低于同期典型的0.2毫米总漂移。然而，由于钛涂层在小散射角下具有较高的本底水平，它的使用仅限于较短的样品到探测器的距离。我们最近修改了这个设计，以最大限度地减少散射背景。建造了一种新的真空兼容外壳，并对不同的叶片材料进行了测试。研究发现，未镀、抛光的铍叶片为反馈系统的工作提供了适当的信号水平，同时将不需要的寄生散射保持在可容忍的水平。新的叶片现在正在制造中，当新的叶片可用时，该设备的最终版本将被安装和测试。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We continue to mitigate the beam positional instability issue, which were found last year at the new BL4-2 location. Earlier this year, we identified the major cause of the instability, whose magnitude is often as much as 1.5 times the full width half maximum vertical beam size in 24 hours, to be most likely the gradual thermal drifts of a few pieces of thick concrete floor moving at different rates in response to diurnal temperature fluctuation. We began adopting a mirror pitch feedback system very similar to those operating successfully on other SSRL insertion device stations. The first version involved a straight copy of the standard beam position monitor with beryllium signal blades coated with a thin layer of Ti working under helium atmosphere. This approach resulted in an improvement in beam stability, achieving ten micron stability over a 16 hour period, down from typical 0.2 mm overall drifts over the same period. Its use is, however, limited to shorter sample-to-detector distances due to the high back ground level given by the Ti coating at small scattering angles. We have recently modified this design to minimize scattering background. A new vacuum compatible housing has been built and different blade materials have been tested. It was found that un-plated, polished beryllium blades give an appropriate level of signal for the feedback system to work while keeping unwanted parasitic scattering at a tolerable level. The new blades are now being fabricated and finial version of the device will be installed and tested as the new blades become available.