PPM (Power and Profile Monitor)
If you prefer to listen instead of Reading below is a link to a video explanation of imagers/PPM. Furthermore, for quick information the times and concepts of the video are given below.
https://drive.google.com/drive/u/1/folders/1Gr-6FmxK1F9tREUs7z3wuE-vIlGCMKaQ
If you would like to learn more about PPM controls and product names, click the link below to another confluence page.
https://confluence.slac.stanford.edu/display/L2SI/PPM+Controls
What A Profile Monitor Is Made Of
Time (2:33): Profile monitors contain a Scintillator screen and a Yttrium Aluminum Garnet(YAG) crystal doped with a few 10% of cerium. The cerium allows the beam to glow green when hit with a x-ray beam. The YAG crystal are 25x25 mm rectangles and the Profile monitors contain two of them. Moreover, inside a profile monitor is an aluminum mirror that sits behind the YAG crystal and reflects the image of the beam out of the vacuum chamber. A microscope and camera are then used to focus on the YAG screen.
Information On YAG Screens
Time (10:55): YAG screens are sent off to a sputtering company to sputter a little pattern of chromium on the screens. Also, on each corner of the YAG screens there is a pattern that is viewed in to see how focused the microscope is.
Profile Monitor Purpose
Time (11:32): Profile monitors define beam trajectory and are used to precisely to align the beam and how much it needs to be focused. PPMs are LCLS main source for aligning the beam. Typically, there should be one profile monitor after each optic, however two would be ideal.
Information On PPM Microscope and Camera
Time (12:39): No motorization for the focusing of the lens it’s all manual use. Furthermore, the lens has a fixed focal length and fixed field of view so you can mark a pixel on the software of where the beam is always supposed to be and that never changes. Although beam size does change it is not that bad of a compromise. H1.1 has a 4 megapixel camera and can digitally zoom in. The camera also runs around 10z or 20hz which is okay for alignment. Cameras that run at a high rate tend to have smaller pixels.
What Effects a PPM
Time (13:38): Depending on the location of the PPM the beam size changes because you have focusing optics that focus on the beam and the beam diverges. Usually want a profile monitor between every single optic, although two would be ideal.
How To Align A Microscope Setup
Three different field of views 20mm, 10mm, and 5mm screens all the same size, but different patterns.
Time (14:20): Matt Seaberg wrote software where it automatically locks into 4 fiducial markers on a YAG screen, which allows you to focus the beam. By reading out an image you can read the pixel number and find the distance between the fiducials, which you can than convert the pixel number to a distance unit.
How To Align YAG Screen
Time (22:10): Using a microscope they can measure the center of the 4 fiducials and map it to a tooling ball socket. Then using a laser tracker, they put it in the beam line and know where they have to adjust the screen so that it is in center of the trajectory. All 4 of the fiducials are numbered so they can do mapping.
Furthermore, plotting the contrast of the images and how sharp the transition is from dark to light is needed when adjusting the tip of the tilt of the microscope and the focus. You have 4 squiggly lines that are running, and you are trying to get them as high as possible to increase the contrast as high as possible. If you can get a good contrast in the four corners that means the whole screen is in focus.
About Power Meters
Time (24:30): Power meters have sensors in the back side of its surface so when the temperature changes it spits out an electrical signal. Power meters are used in optical lasers all over LCLS and typically have black paint. LCLS does all the mountain and painting. Each Power meter cost about $300 and contain a small electronic board amplifier.
Power Meter Purpose
Time (25:55): With power meters one can Monitor the reflect activity of every single optic. Without power meters you would not be able to see the degradation of the beam line lenses. Before when there was no power meters the signal of the beam line was degrading and nobody knew why.