Evaluation of the alignment for long linear accelerators using a level

We have studied to adopt a level, which is a gravity referenced precise inclinometer, for evaluating alignment of large linear accelerators, which is several hundred meters or larger. It has advantages for evaluating large objects because it is hardly affected by shape references, which becomes difficult to be defined enough accurate as the objects becomes large. We had already evaluated the vertical aligning straightness of the reference plates for the 70-m-long part of the KEK injector linear accelerator (KEK linac) with the standard deviations of less than 49 μm by using a level on a straight bar. The results are fairly reliable having good agreement within sub-mm range with those by the other methods; however, the evaluation distances were limited by obstacles which block the measurement path. Here, we devised new method which adopts two offset bars for avoiding the obstacles. Their one ends are placed on the measurement points with their axis directed perpendicular to the measurement path. One can avoid the obstacles by measuring the slope angles between the far ends of each offset bars instead of measuring directly those between the measurement points. Error arises from the offset bars can be eliminated by reversal measurement, which considers slope angles of the offset bars. As a result, straightness for the 206-m-long part of the linac, which corresponds to the three successive accelerator sectors of the linac, could be evaluated with our new method. The reproducibility expressed by the standard deviation of the slope angles for the arbitrarily sampled measurement point was 15 μrad, which is comparable with the average of our former measurements of 10 μrad. Moreover, the result agrees with those by the alignment telescope and our laser-based alignment system partially within sub-mm range. They indicate that our new method can be applicable for evaluating alignment of large accelerators in spite of its complexity caused by the offset bars. (author)