Table of Contents
About Me
My name is Travis Nichols and I am an undergraduate physics major and math minor from California Polytechnic State University - San Luis Obispo, interested in lasers, plasmas, and accelerator physics. I am an experimentalist to my core, and I love being in the lab, pressing buttons, flipping switches, and turning knobs. There is something I find uniquely satisfying about the troubleshooting process of experimental physics, and being at SLAC was a wonderful opportunity for me to gain valuable experience and confirm my passion for this field. I was fortunate enough to be an intern at SLAC in the summer of 2023 where I worked primarily with Robert Ariniello and Spencer Gessner on alignment diagnostics and characterizations for the lithium Ionizing Bessel laser used in sector 20 of the linac for PWFA experiments.
Contact Information
Primary Email: tnicho02@calpoly.edu | Phone Number: (818)281-7723
Please feel free to reach out to me for questions or if my GUI starts bugging out
Background For My Project
At FACET-II we focus a laser pulse onto an oven of lithium gas to create the plasmas used in PWFA. Lithium is great because its first ionization state is relatively easy to reach, needing only 5.4eV, but the second ionization state is very difficult to reach, needing 75.6eV. This makes it easy to create a uniformly ionized plasma medium where all the lithium is singly ionized, but none of it is doubly ionized. The challenge comes from making these plasmas on a meter scale, requiring a uniform but high-intensity laser pulse, which can be quite difficult to create. Through chirped pulse amplification we can create a 10TW Ti:Sapphire laser in the Sector 20 laser room. To make the laser pulse uniform in intensity, we pass it through a masked axicon lens, creating a non-diffractive Bessel beam. This uniform high-intensity laser pulse will allow us to create long-length plasmas, but only if the laser is behaving as intended; my project was to develop alignment diagnostics and characterization systems to ensure the Bessel beam is forming as desired.
Laser Alignment GUI
The laser beam travels through quite the journey between its creation and the lithium oven. As you can imagine, this means the laser passes through dozens of mirrors, and beam samplers, and lenses, and nd filters, and so on and so forth. All of these instruments have to be perfectly aligned in order to create our nice plasma. Thankfully, we have installed a bunch of cameras in the beam path so we can check on our laser at different places. Moreover, we installed motors on most of the optics so that we can adjust them remotely. This is the basis for my GUI.
Essentially the GUI grabs the output from a camera and the PV’s for the motors on the corresponding instrument allowing the user to adjust the beam remotely until its aligned. On top of this however, I added some extra functionality. There is a current position finder function that projects the image onto each the X and Y axis and then fits a gaussian over the projected values to locate the maximum coordinates for each. This value is then displayed and a crosshair is drawn where the computer believes to be the center of the beam. This method of finding the maximum coordinates seems to run much faster than the gaussian filter I found In other places in the SLAC GitHub and certainly runs faster than simply fitting a 2d gaussian. Additionaly, now that the program knows the position of the beam, and given a target position, I was able to include a suggested correction button that calculates how to adjust the motors based on the current beam position. This is accomplished by finidng the difference in pixels between the current and target value for each the x and y components; the difference is the converted into motor movement using a predetermined conversion factor specific to each camera. After an adjustment is made the computer recalculates the suggested correction. This function works pretty well, but it only gets the beam approximately aligned, which is why I have still included the manual adjustment buttons with the fine, medium, and coarse, adjustment scales. The Final Function worth noting is the ROI setter. This button will set the display of the camera to a desired region of interested centered on the target position. This is particularly nice for when the beam is only 50 pixels across but the camera has a 1200x900 pixel display.
This widget is then made into a class that which a user can make instances of in a larger display class, giving the possibility to view and align multiple cameras at once, the current display below shows the beam transport cameras. This is very useful when alligning a beam since you can monitor the beam position on downstream cameras while adjusting upstream cameras to ensure the beam has not disapeared into oblivion forever. As of August 2023 the GUI is working for the Sector 20 laser room cameras, the transport cameras, and many cameras down in the tunnel, but the GUI is designed to work for any camera as long as you feed it the right PVs from a csv file.
Once again, if you discover any bugs in my GUI, please contact me and I will try to fix it as soon as possible.
Phase and the Deformable Mirror
Since the axicon is flat on the upstream side, it imparts no phase on the emerging beam, thus we can look at the images after the axicon to clearly observe phase issues of the beam. Ideally the laser would reach the axicon as a perfectly flat plane like shown on the left below. But in reality, all kinds of phase aberrations can occur as shown on the right, most commonly we get these saddle or bowl shaped pulses that wont produce clean Bessel beams.
There is a camera installed in the beam path that views the laser as it emerges from the axicon. If the wavefront is nice and flat we will see the clean bullseye pattern below on the left. But if the incoming beam looks like a saddle or trefoil when the beam reaches the axicon, we get patterns that look like these on the right. We can solve these phase issue with a deformable mirror. Essentially the deformable mirror is made of a somewhat flexible glass and has a bunch of tiny pistons allowing us to change the shape of the mirror surface to impart a corrected phase on an aberrated wavefront .
Rail Camera
Downstream Holed Mirrors (Near and Far)
