Andy and Courtney have completed their REUs with the group this summer and presented their results at the REU poster session.
Andy developed a machine vision system for characterizing the HV electrodes used for the Ra EDM experiment. This optical method is high resolution (10 microns), high precision (repeatable), real time (can be performed while the E-field is on), and noninvasive (performed in vacuum without directly contacting the delicate electrode surfaces). This involved forming images of the electrodes onto a CCD camera using a collimated light source and telecentric lens. A program, written in LabVIEW, then analyzes the image using an edge detection algorithm in order to determine the distance between the electrodes across the gap. This information is fed into COMSOL which allows us to more realistically model the electric field between the electrodes accounting for non-ideal conditions:
- electrodes are not the same size
- electrode surfaces are curved
- one electrode is tilted with respect to the other
- one electrode is offset with respect to the other
Andy studied the effect of the CCD saturation as well as the choice of algorithm on determining the edges of the electrodes. Specifically, he determined the sensitivity of the calculated electrode gap to the exposure time, light intensity, and parameters associated with the edge detection algorithm.
Our goal is to eventually adopt this technique for use on the Ra EDM apparatus in order to more accurately calibrate and monitor the electric field for the experiment. Knowledge of the degree of reversibility when the E-field changes sign as well as the relative orientation of the E-field with respect to the B-field is critical in controlling and minimizing systematic effects during an EDM measurement. Andy’s poster is here.
Courtney developed an optical method to determine the thickness of a thin film mounted to the substrate based on the thin film interference of polarized monochromatic light from a HeNe laser. Her setup will be incorporated into the prototype Single Atom Microscope (pSAM) being build this Fall and will be used a monitor & calibrate the Ne thin film growth rate. A measurement of the thickness of the thin neon film is needed to determine both the number of Yb atoms as well as the number of residual gas molecules that are deposited into the film. This information is needed to more realistically estimate the expected signal to background for a single atom detection experiment.
Following this paper, Courtney built a test setup using a 100 micron thick microscope slide mounted to a substrate as a surrogate for the neon film. After several iterations, she reached the following conclusions:
- care has to be taken in determining the appropriate interference pattern to study as well as choosing the appropriate reflected beam spot
- the power of the laser has to be monitored and accounted for when determining the reflectance or transmission because the power fluctuates over the course of the measurement
- the film thickness can be measured by varying the angle of incidence of the laser beam, the wavelength of the light, or the thickness of the film
- the reflectance measurement has a much higher SNR than a transmission measurement
- determining the thickness from the distance from interference maximum to maximum is probably more reliable than extracting the thickness from a nonlinear least squares fit when the data have low SNR
Courtney’s poster is here.
Thanks Andy and Courtney! -JTS