Moving the Tagger Microscope

=Moving the Tagger Microscope Along the Focal Plane=

Tagger Microscope


Moving the TAGM to a new energy range not only involves the physical relocation of the microscope and its shielding along the tagger magnet's focal plane, but also requires internal realignment of the fiber, which is the subject of this page. The realignment of the scintillating fibers' longitudinal axes requires all 17 bundle supports to have new, individual crossing angles (&beta; angle) with respect to the focal plane. These angles are derived from a table generated based on a TOSCA map of the post-bremsstrahlung electrons' crossing angles as they pass through the magnet's focal plane. Since the crossing angle changes with energy (e.g. displacement along the focal plane) each bundle support will have a slight "kick" or "tow" from the adjacent bundle support. The tow is typically on the order of 0.1o.

Since the bundle supports have varying &beta; angles, the parallel railing system that holds these supports in place are no longer representative of their namesake. That is, the front and rear rails of the parallel railing system are not parallel to each other and have a pitch with respect to one another. Figure 7 is a pictorial explanation of this and shows that for two equal length rails (front & rear), the length of the upstream and downstream alignment bars differ by 0.039 &plusmn; 0.001 in. at a starting photon energy (E&gamma;) with a required tow of 0.09o between bundle supports. For each individual photon energy range covered by the microscope, or more simply stated for each energy that the microscope starts at, a new pitch for the parallel rails must be determined and three components (upstream alignment bar, downstream alignment bar, and upstream guide plate) unique to that starting energy must be fabricated. For this reason CAD drawings, an Excel spreadsheet, and this wiki page were created to provide the information needed to realign and move the TAGM to the following starting photon energies: 11, 10, 9, 8, 7, 6.5, 6, 5.5, and 5 GeV. The turnaround time for the machining components is typically two to five days, while the realignment of fibers will take no more than two days with inexperienced workers, and that's being conservative.



Coordinate System
There are three coordinate systems often referred to inside the GlueX Tagger Hall. These coordinate systems have origins associated with the Goniometer center (room coordinates), Tagger Magnet center (map coordinates), and the focal plane (FP coordinates). The room coordinate system has the positive z-coordinate along the beam axis with the positive y direction pointing to the ceiling of the tagger hall from the coordinates origin placed at the Goniometer's center, where the beamline radiator is located. Figure 1 shown below, taken from a presentation by Dan Sober, shows the three coordinate systems relative to one another. The map coordinate system (magnetic mapping) places the y-coordinate along the long axis of the magnet with the positive x-axis parallel to the tagger hall floor in the direction of the focal plane. the positive z-axis of the map coordinate system points toward the tagger hall ceiling. When viewed from above the map y-coordinate is offset from the room coordinate z-axis by 6.5 degrees towards the focal plane (clockwise). The focal plane coordinate system places its x-axis along the electron focal plane with zero being at an electron energy equivalent to a photon energy around 11.7 GeV. The positive x-axis points toward increasing electron energy (decreasing photon energy). The positive y-axis of the focal plane coordinate system points towards the tagger magnet and is parallel to the tagger hall floor. This leaves the focal plane positive z-axis pointing towards the tagger hall ceiling.

In Spring 2017 a survey of the various beamline components saw a small change in the Tagger Microscope position. The Microscope (upstream center) position derived from this survey is shown below and includes a magnet center shift from previous measurements. X room =  - 1.16666 m         Z room =  7.41009 m          Angle room =  - 8.05220o

X map = 0.73488 m         Y map = 1.22645 m          X FP = 3.31245 m          Y FP = -0.00932 m

Bundle Support (aka Popsicle Stick)
The Tagger Microscope (TAGM) contains 17 optical fiber bundle, each consists of a 5x6 array of 30 fibers for a total of 510 fibers (5 rows, 102 columns). Figure 2 below shows the fiber array as viewed by the electrons passing through the tagger magnet focal plane. Each optical fiber consists of a 2 mm2 x 2 cm BCF-20 scintillating fiber (SciFi) thermally fused to 2 mm2 x 165 cm BCF-98 light guide, see Figures 3 & 4. The SciFi end of each fiber is thermally bent into an "S" shape to remove the fiber from the electrons' path soon after passing through the SciFi, see Figure 5. The fiber begins its bend out of the electrons' path after only 4.5 cm. This length provides enough material past the fused joint to minimize strain on the joint resulting from the bend and fiber mounting straps, while also reducing the material in the line-of-fire that could result in backscatter.

Calculating Position of Each Bundle Support Mounting Rod
Up to 17 bundle supports can be mounted on the parallel railing system and instrumented in the TAGM. As the range of photon energies being tagged decreases (e.g. electron energy increases) the crossing angle (&beta; angle) of the electrons with the focal plane also decreases. The smaller the &beta; angle the more space along the XFP axis a bundle support will occupy. Below an E&gamma; of 6.5 GeV (&beta; = 9.5o) the length of the parallel railing system can only mount a maximum of 16 bundle supports.

Since the &beta; angle changes across the TAGM energy range, a universal bundle support &beta; angle cannot be used. Starting with the most upstream bundle support (e.g. highest energy tagged photons) an initial bundle support angle is selected by taking the average of the crossing angles at the tagger magnet's focal plane for the electrons that pass through the center of the first and sixth fiber columns of that bundle. Recall that each bundle support holds an array of 5x6 fibers split into two 5x3 bundle halves. These halves are offset so that the bundle "pivot point" and the front center of each bundle half will all sit on the focal plane for a &beta; angle = 12o. The pivot point is located at the midpoint of the bundle halves offset and the boundary point of the bundle halves along the bundle support long axis. The long axis that lies along the bundle support centerline passes through the pivot point, front mounting rod, and rear mounting rod. This fact is exploited during the calculations to follow. The pivot point is solely determined from the bundle support's design (e.g. offset distance) and is essential for fiber alignment on the focal plane. Regardless of the bundle &beta; angle, if the focal plane passes through the bundle support's pivot point then the SciFi's will be at their optimal location with maximum fiber extension &plusmn; YFP being the same.

Once the &beta; angle for the first (upstream) bundle support is set, derived from the the starting photon tag energy, each subsequent bundle support has a &beta; angle offset from the previous one by a tow angle. Thus, as we look downstream from one bundle support to the next, the &beta; angle differ by -(tow angle). Adjacent bundle supports will come in contact with one another at the SciFi end and form a triangular gap along their adjacent sides based on the tow angle.

An Excel spreadsheet(placeholder) has been created to calculate the location of the bundle supports' mounting rods with respect to the focal plane coordinate system. This spreadsheet also calculates the length of the parallel railing end supports which need to be fabricated anew for each unique TAGM location on the focal plane. One last, but very important thing that the spreadsheet calculates is the shim size needed during TAGM realignment in order to achieve the proper tow angle between bundle supports during mounting. In addition to the spreadsheet, AutoCAD drawings corresponding to a tagging energy spectrum starting at: 11 GeV, 10 GeV, 9 GeV, 8 GeV, 7 GeV, 6.5 GeV, 6 GeV, 5.5 GeV, and 5 GeV have been created. These CAD files contain the parallel railing system setup arranged for the corresponding photon energy range. These files are in US standard units (inches) and to scale with a tolerance of &plusmn; 0.001 inch.

A summary of the spreadsheet calculations is a follows:


 Select a starting energy for the photon tagging array (highest &gamma; energy to tag)</li> <li>Using hodoscope energy bin bounds interpolate the crossing angle with respect to the focal plane (&beta;1) of an electron associated the highest energy to be tagged (E&gamma; o )</li> <ul> <li> Interpolate the location on the XFP axis at which this electron crosses (X1)</li> <li style="padding-bottom: 16px;"> These electrons will pass through the center of the first column of SciFi fibers</li> </ul>

<li>Using &beta;1 calculate the X-displacement along the focal plane (&Delta;x) from the center of the first fiber column to the center of the sixth fiber column of the first bundle support</li> <ul> <li>Add &Delta;x and X1 to get X6, then interpolate the value of &beta;6</li> <li>Using the average value of &beta;1 and &beta;6 (&beta;avg.), recalculate the XFP displacement (&Delta;x) from the center of the first to sixth fiber column (X6)</li> <li>Repeat the above two steps until the bundle support angle &beta; (e.g. average between &beta;1 & &beta;6) does not change appreciably</li> </ul>

</ul>

Now we know the focal plane crossing locations for the center of the first and sixth SciFi columns in our first bundle, as depicted in Figure 16 by the endpoints of &Delta;x. Additionally, we know the &beta; angle of the first bundle (noted as &beta;avg above), which gives us the optimal alignment for each fiber in the bundle to their respective electron's path. The &beta; angles for the first and sixth columns will be off by the same magnitude, but with opposite signs.



The 5x6 fiber bundle supports were designed with two 5x3 bundle halves offset such that the center of the front face of the middle column in each bundle half would sit on the magnetic focal plane for a &beta; angle of 12.0o. This angle was selected as a compromise that would allow coverage through the photon energy range of 10 - 5.6 GeV.

'' As a side note - If required and finances permit, the 17 bundle supports can be easily redesigned for a different &beta; angle. This redesign would take less than an hour of CAD work, with a manufacturing turn-around time in as little as two days. Costs are estimated to be around $2k. A CAD drawing of a new bundle support design already exists, which incorporates updated locations of the threaded holes for mounting the clamps that keep the bundle straps in place. The best time to replace/modify the bundle supports, if so desired, would be during fiber replacement. This way the new fibers can be mounted to the new bundle support outside the tagger hall, before ever making it to JLab. A conservative time estimate for changing the TAGM fiber configuration would be approximately two days (16 hours).''

Each bundle has a "pivot point" that when placed on the focal plane, provides the optimal y-displacement from the focal plane for each fiber column in that bundle without encroaching too close to the tagger magnet window. If the bundle &beta; angle &ne; 12o, then the 1st & 6th, 2nd & 5th, and 3rd & 4th fiber column pairs will have the same magnitude offset as one another from the focal plane in Y, but with opposite signs, see Figure 17. This is all provided that when the bundle support is mounted on the parallel railing system the tagger magnet's focal plane passes through the midpoint between the front and rear bundle halves (the so called pivot point).



As shown in Figure 18 by the red and green lines, if &beta; &ne; 12o then the front face of the first column of SciFi no longer sits on the focal plane. This offset must be accounted for. The Excel spreadsheet starts by finding the optimal first bundle support crossing angle (&beta;avg) and places the front center of the first column of SciFi on the XFP location corresponding to E&gamma; o. Depending on &beta;'s departure from 12o the pivot point will be displaced from the focal plane. The spreadsheet calculates the (&Delta;x, &Delta;y) needed to return the pivot point to the focal plane and keep the first fiber column's longitudinal axis on the E&gamma; o electron's path. The spreadsheet's title for this is "Pivot Point Move" and the description is detailed below.

'' NOTE: &beta;avg is used to determine this displacement. While the E&gamma; o electron's &beta; angle differs slightly &beta;avg and would keep the column's centerline on the proper electron path, this discrepancy is so small that it does not come close to the TAGM machining parts' tolerance. Additionally, the sixth column's &beta; angle has the same magnitude difference from the bundle support's &beta;, but with a different sign. For these reasons &beta;avg was used.''

 For the next part we assuming the center of the first fiber column's front face is on the focal plane where an E&gamma; o post-bremsstrahlung electron will cross and the bundle support is placed at an angle &beta;avg 



<ul> <li>Calculate the YFP displacement required to place the focal plane back on the bundle support's pivot point, Figure 20</li> <ul> <li>The angle made by the first fiber column's longitudinal axis and a line from (X1, 0)FP to the pivot point is 19.5072o, see Figure 19</li> <li>Use the difference between &beta;avg and 19.5072o</li> <li>The distance of the center of the first fiber column's front face to the bundle support's pivot point (in the (x, y)FP plane) is 0.5895 inches</li> <li style="padding-bottom: 16px;">Note that if &beta; > ~19.5o (e.g. > 10.86 GeV photons being tagged), then the pivot point will be below the focal plane; therefore, &Delta;y will be positive (bundle shifts towards the magnet, positive YFP direction)</li> </ul> <li style="padding-bottom: 16px;">Utilizing the y-displacement value found above and the tangent of &beta;avg, calculate the associated x-displacement along the focal plane to keep column #1 aligned to the electrons associated with E&gamma; o </li> <li>Next determine the locations of the bundle support mounting rods in focal plane coordinates</li> <ul> <li>Figure 21 shows the required dimensions for these calculations</li> <li>For x locations, include x1 (for E&gamma; o electrons) in addition to the rods' displacement from the front center of the first fiber column</li> <li>For y locations, we assume y1 (for E&gamma; o electrons) lies on the focal plane y-axis</li> </ul>

<b> Since the post-bremsstrahlung electron's crossing angle changes with energy (e.g. displacement along the focal plane) each bundle support will have a slight "kick" or "tow" from the adjacent bundle support. Simply put, going from upstream [highest E&gamma;] to downstream [lowest E&gamma;], subsequent bundle supports will have smaller and smaller &beta;avg's. </b> <li style="padding-bottom: 16px;"></li> <li style="padding-bottom: 16px;"></li>

</ul>









Final
xxxxxxxxxxxxx

Popsicle Stick Limitations
Placing the "Pivot Point" on the focal plane and only allowing +0.5 cm towards the tagger magnet the maximum crossing angle for the current bundle support design is &beta;max = 19.51o, while the minimum angle is &beta;min = 4.05o. This does not account for the most forward bundle mounting strap clamp and bolts, but they are lower in ZFP than the fibers and should not extend significantly past the 0.5 cm limit even at lower &beta; angles.

xxxxxxxxxxxxx

Table with coordinates here ...


 * AAAA.C: Text here.