ISCOOL Re-Alignment

During the successful career of ISCOOL, certain problems have been encountered that can be traced back to an internal misalignment of the multiple electrodes that make up the injection and extraction systems of the machine. Since the first and second electrodes (as labelled in Fig. 5.9) for both the injection and extraction side are fixed to each other, and then mounted on the inside of the vacuum chamber, while the cylinder of the RFQCB is mounted to the cover of the vacuum chamber, there is no guarantee that they are adequately aligned. The problem lies in the multiple components (many of them welded) found between the first and last electrode. This produces an accumulation of error, so it is difficult to ensure the alignment of the electrodes even though the cover has alignment pins to confirm that it is well placed. If we treat the first and second electrodes as a whole (they are bolted together), then the problem becomes the alignment of this electrode group, on either the injection or extraction side, to the (injection or extraction) plate.

The misalignment of ISCOOL was first diagnosed via the observation that extreme beam steering was required in order to transmit any beam. The steering was so severe that the beam missed the faraday cup at the exit of the machine entirely. ISCOOL was also visibly misaligned with respect to the beam line - it could be seen that the bellows on the injection side were stretched to their limit.

To begin the process of alignment, the deviation between the plate electrode (fixed to the cover) and the electrode group (fixed to the vacuum tank) was measured. Fig. 5.10 shows the result of this measurement.

In total, it was shown that while the first and second electrodes are well aligned, the alignment of these electrodes to the plate at the entrance/exit of the RFQCB was

5.5. OPTICAL PUMPING WITH ISCOOL 89 Extraction Ground 2nd Extraction 1st Extraction Extraction Plate Injection Plate 2nd Injection 1st Injection Injection Ground

Figure 5.9: Mechanical drawing of ISCOOL with the injection and extraction elec- trodes colour-coded.

off by 0.75 mm. In order to fix this problem, the system supporting the RFQCB cylin- der was modified to create supports that were horizontally and vertically adjustable (no adjustment in the position along the axis of the cylinder was necessary). Fig. 5.11 shows the changes made. An adapter piece was added with two screws pushing on either side of the RFQCB support piece (for horizontal movement) and the two nuts holding the support up are adjusted to provide vertical movement.

Alignment Method and Results

The method used to align the apertures was to take a high resolution photograph (using a Canon EOS 5D Mark III on a Manfrotto adjustable tripod) of the injection or extraction plate, then compare it with a photograph taken from the same spot

Figure 5.10: Measurement of the misalignment of the injection side (left) and extrac- tion side (right). The three circles show the positions of the apertures in the first, second and plate electrodes on each side.

with the electrode group (first and second electrodes) attached. This allowed an accurate determination of the centres of the apertures and their radial distances from one another, as was shown in Fig. 5.10. This technique is useful only because it has been verified that the position of the electrode group does not change with each new instance of mounting and dismounting - every time the electrodes are bolted to the tank, they take up the same position, to within our accuracy limit. This means that an iterative process can be used in which the RFQCB cylinder is moved, and then the results are checked against the position of the electrode group. This is done several times to achieve the desired accuracy.

The first step in this process is to align the camera so that it is looking straight down the axis of the RFQCB cylinder. This is done by aligning the injection and extraction plate apertures on the RFQCB itself, as demonstrated in Fig. 5.12.

Once the camera was aligned, a picture was taken first of the injection/extraction plate aperture, then of the same spot with the electrode group attached. Since the

5.5. OPTICAL PUMPING WITH ISCOOL 91

Figure 5.11: The left figure shows the original support system used in ISCOOL and the right figure shows the modified system, with the adaptor piece attached above the support.

camera was not moved or adjusted between these two pictures, they could be directly compared to determine if the centres of the apertures lined up. Once the offset of the plate aperture from the electrode group apertures was determined, the adjustment screws on the supports were used to adjust the RFQCB cylinder (and thus the plate) to match the electrode group, and then another round of photos was taken to deter- mine the outcome. This system of move-and-check was applied until the apertures were aligned on both sides, to within 0.1 mm. It was finally possible to align both sides of the RFQCB cylinder with their respective electrode groups. The injection and extraction sides are shown in Fig. 5.13

Since ISCOOL had to be removed from ISOLDE to perform these operations, it had to be re-aligned to the ISOLDE beam line during re-installation. This was done by passing a RILIS laser through the machine and adjusting the RFQCB support

Figure 5.12: A photo showing the injection and extraction apertures aligned. The extraction aperture is the far one, shown by the white circle of light.

such that the laser passed through the centre of the static quadrupole triplets both before and after ISCOOL. An estimation of the accuracy of this process shows errors of approximately 0.256 mm in the vertical direction and 0.47 mm in the horizontal direction.

The results of the re-alignment were immediately apparent in the operation of the machine. Beam steering was reduced to a normal level and 100% transmission was easier to achieve. While this improved significantly the day-to-day operation of the machine, the real improvement in terms of physics was the ability to get lasers into the bunching region of ISCOOL and so facilitate optical pumping experiments.