Hall D Photon Beam Line Alignment

Richard Jones, University of Connecticut
draft 1.0, January 31, 2006
Photon Tagger and Beam Line Working Group, GlueX Collaboration

The above figure shows the principal elements that must be aligned relative to the beam axis. The nominal beam axis is represented by a horizontal dashed line through the center of the figure. The heavy black lines that lie on top of the nominal axis are supposed to symbolize the tolerance with which beam line elements must be located in the transverse plane relative to the nominal axis. The numerical values for these tolerances are given below. One of the uses for these numbers is to specify how accurately the site survey must be able to locate the nominal beam axis for alignment purposes. It may be useful for other reasons to know the absolute coordinates to better accuracy than required for the photon beam alignment; these numbers represent an upper bound on the survey uncertainty.

Coordinate systems

The axis orientation for this discussion is as follows: the y axis points vertically up, the z axis points along the nominal beam direction, and the x axis is y x z. For the purposes of the Hall D photon beam line, the sensitive coordinates are x and y. Tolerances in z are on the order of cm, except for relative placement of tagging spectrometer components. Alignment of the tagger components is concerned only with relative coordinates within the tagger hall, whereas photon beam alignment involves the coupling of coordinate systems between the tagger hall and those in the experimental Hall D. Thus tagger alignment is considered separately from the photon beam, which is the subject of this note. To facilitate discussion of the absolute alignment of the photon beam line elements, the beam line has been divided into three regions, as shown in the figure. For each region the requirements on internal relative alignment is considered separately from external alignment, how well the absolute coordinates must be known relative to the other regions.

Results

Numerical tolerances on the transverse alignment of all systems shown in the above figure are given in the table below. Only simple translations are considered in relating the coordinate systems in different regions. Of course angular misalignments between the two systems are also very important. For an explanation of how angles are effectively included in the way the numbers are specified in the columns labelled "external tolerances" in the table, see note [1] below.

Note that these alignment requirements are allowed in some cases to be less restrictive than the requirements on beam positioning accuracy during experimental running. The most critical parameter is the position of the photon beam spot on the face of the primary collimator relative to the collimator axis, which must be 0 ± 0.2 mm. That requirement is met by use of an active photon beam collimator that senses the photon beam position and feeds back that information in real time to electron beam correctors upstream of the radiator position. The range of the correctors will be sufficient to steer the photon spot anywhere within a circular zone of radius no less than 20 mm of the null electron beam axis determined by the zeros of the beam position monitors (BPM's). If the BPM's and collimator are misaligned at the outer limits permitted in the table below, the null electron beam axis will miss the collimator center by 8 mm, well within the range of the correctors. The BPM's are assumed to be 10 m apart.

The effects of these allowances on the positioning accuracy of the electron beam spot at the radiator depends on how far upstream of the radiator the correctors are located. They must be upstream of the BPM's, which sets a lower bound of 10 m. Assuming a distance of 15 m, a displacement of 8 mm at the collimator becomes 1.3 mm at the radiator. This is about the maximum amount of freedom that one has in the beam position at the radiator in the y direction relative to the mid-plane of the tagging spectrometer. Therefore it is the restricted range of allowed motion of the electron beam in y at the radiator that sets the bound for the region III tolerances. The maximum displacement of the photon beam spot on the experimental hydrogen target allowed by the tolerances in the table is 1.2 mm in x and y, which is acceptable.

internal tolerance external tolerance external reference
x (mm) y (mm) x (mm) y (mm)
Region I 0.2 0.2 ? ? accelerator
Region II 20 20 25 25 Region I
Region III 0.5 0.5 2 2 Region I

The tolerances on Region II alignment have deliberately been relaxed to allow for a beam pipe with loose positioning requirements. It assumes a beam pipe of 7.5 cm radius (6-inch pipe), large enough to accomodate 45 mm of misalignment of sections of the beam pipe (relative to the axis defined in Region I) and a circular keep-out zone of diameter 6 cm for freedom to steer the photon beam during the beam alignment process.

  1. The last two columns in the table list the tolerences of the coordinate system origin relative to a reference system. How this is interpreted depends on where the two origins are chosen to be because the transformation involves rotations (three angles) in addition to translation (three offsets). This ambiguity is resolved by defining the tolerance in the relative coordinate as the maximum allowed transverse offset of the region's z axis relative to the reference z axis anywhere along the length where the region's components are located. For example, the region III specification for external tolerance in x and y of 5 mm defines a square tube of width and height 10 mm around the region-III z axis that must enclose the region-I z axis (extended into region III) everywhere between the primary collimator and the experimental target. How those two axes diverge outside of region III is irrelevant for the purposes of the experiment.


     
     

    This material is based upon work supported by the National Science Foundation under Grant No. 0901016.

    This page is maintained by Richard Jones.