20100531 Pre-shift Notes

Have 3 shifts this week. First two are combined with SLAC OTR experiments - four shifts in total - two for Laser-wire and two for OTR. Plan is for OTR 2 x Laser-wire OTR. Third Laser-wire shift is on its own.

Detector Shielding

Figure 1 shows lead bricks built up around the detector to the height of the entrance aperture. This hopefully should minimise any further background.

Figure 1: Photo of detector setup.

Additionally, Figure 2 shows a photo of the line of sight from the detector to the beam pipe, through the quadrupole. The blue circle is the 10 micron thick aluminium window covered in blue plastic to protect it.

Figure 2: Photo of compton photon beam line through quadrupole.

Laser & Chamber Alignment

Vertical Alignment

The chamber height was a known vertical position (from the actuator) which it was left at from the last scan on shift. At that position, the screen was moved to intercept the OTR alignment laser which is known to be parallel and overlapped with the electron beam by Alex when he was using the OTR laser. Today, the screen was moved vertically until it intercepted the middle of the laser-wire alignment laser beam. The chamber (with screen attached) was then moved back by what the screen had been moved by. Since the lens is fixed to the chamber, the laser-wire alignment laser focus moves with it. This meant both the alignment laser and the OTR alignment laser were overlapped representing the electron beam and the main laser. Inspecting the alignment laser on the back of the laser-wire lens, the beam was very high. The two mirrors before the final lens were used along with the alignment laser and the back reflector with iris to centre the alignment laser beam on the lens / chamber at this position. In the very initial alignment in March, Laura and Laurie simply aligned the laser to the chamber where it was given it's range of travel wasn't that far (5mm). It must have been at one end of its travel. The back reflection was used to make sure the angle of the laser impinging on the lens was correct.

The screen was rotated to be parallel to the electron beam (perpendicular to the main laser beam). It was already known that the screen in this position was not at the focus of the main laser beam but the screen had not been moved as this would mean the OTR alignment would be lost. By scanning the screen across the laser beam at the initial position, the laser beam was found to be around 300 microns where the screen was. With the screen set halfway through the laser beam, the screen / manipulator was then moved in the X direction as shown by the coordinate system in Figure 3 with the manual micrometer on the side. In this regime a shadow cast by the screen on the top of the output image means that the screen is intercepting the beam after the focus whereas a shadow on the bottom half of the post-IP image means the screen is intercepting the top of the laser beam before the focus. The image is inverted after the focus. By moving the screen / manipulator until the crossover was observed, the focus position could be found. This was verified by fully obscuring the beam with a 5 micron step of the screen vertically. A good confirmation that we are at the focus of the laser beam.

Figure 3: Coordinate system of Laser-wire.

The screen was moved from a micrometer setting of 11.695 initially to 13.300. This means the screen was 1.605mm from the focus of the laser.

Screen Rotational Alignment

It was already known that the rotation axis of the screen, whilst parallel to Y, was not located over the interaction point between the X axis of the laser and the Z axis of the electron beam / OTR alignment laser. This would affect alignment both temporally (given not so much) and horizontally (X axis and more so). The screen was therefore set vertically to be obscuring the beam by 50% and then rotated to be parallel to the X axis and the alignment laser beam. The screen / manipulator was then moved in Z with the manual micrometer. Figure 4 shows the images of the alignment laser beam post-IP with the screen parallel to that laser beam and the various images found when aligning it. With this alignment, when the screen rotates, it is still at the focus of the laser beam on the X axis.

Figure 4: From left to right, the screen parallel to the X axis intercepting the laser from one side, secondly the other side and thirdly midway i.e. centred.

The screen was moved from a micrometer setting of 9.865 initially to 12.700. This shows that the rotation axis was 2.835mm from the laser focus.

Chamber Horizontal Alignment

The screen was then rotated to be parallel to the OTR alignment laser in the Z axis and perpendicular to the laser-wire alignment laser. The screen was now at the focus of the alignment laser beam in the X axis and with 2.5 microns of the centre of the focus vertically in the Y axis. The chamber was then scanned (moving both the laser focus and the screen with it) to find in a similar fashion where the screen would intercept the OTR alignment laser. The OTR alignment laser enters the electron beam pipe about 15 metres before the laser-wire IP and exits around 15 - 20 metres afterwards. In both cases a mirror on a pneumatic insertion device is used. This alignment was unsuccessful as a clear image of the beam at the OTR alignment laser exit could not be observed even with the screen removed entirely. The OTR alignment laser was initially aligned to be in roughly the centre of the beam pipe in that it both entered and exited the beam pipe (around 3 cm wide) over 30 metres apart. The insertion angle was then adjusted to coincide with the electron beam image (OTR) on the CCD for the OTR. This means that at the laser-wire IP the OTR alignment laser is coincidental with the electron beam, but doesn't necessarily mean it is at its entrance and exit. It could be at a slight angle which would explain why it isn't exiting properly. Additionally, other objects in the beam line could be in the way (wire scanners etc.). The screen was set to 45 degrees to both laser beams such that the OTR alignment laser top half (as the screen is 50% obscuring both). The beams from each laser beam appeared to be reasonably collinear overlapping immediately at the chamber window and about 40cm away along the OTR beam line. This would seem to indicate that the beams are both hitting the screen very close to each other. If the screen were misaligned (as the angle is only known to within 2 degrees) the beams would appear to overlap at one point and not others. If the laser focus / chamber were offset vertically, the spots would be horizontally displaced (in the Z axis) but be parallel. This is however, not very precise as the roughly recollimated alignment laser was over 1 cm wide.

It will be more practical to keep the screen in this rotation (parallel to the electron beam) and align it to that. This will however affect the timing, but even if we scanned 1mm horizontally, this would only correspond to 10ps in time. Our temporal alignment will be within 300ps so this is not too much of an issue. Perhaps alignment / searching will go in the order of timing, vertical, horizontal, timing.

Main Laser Alignment

This alignment process was briefly repeated for the main laser. The back reflection was checked at box 1 at the bottom of the periscope, the vertical position found and the chamber corrected so that both the main laser and the OTR alignment laser were aligned with each other. The screen position in the X axis and Z axis were checked and tweaked slightly. This is the best we can do with the current setup.

Final Parameters