On Friday, Rana and I measured the scatter coming from the 35W beam dumps.
(These are the ones with big aluminum heat sinks on the back that kind of look like little robots with 2 legs...inside the horn is a piece of polished silicon at Brewster's Angle.)
For the measurement, we used the Scatterometer setup at the 40m on the small optical table near MC2.
We used a frequency of 1743 Hz for the Chopper, and this was also used as the reference frequency for the SR830 Lock-In Amplifier.
The settings on the Lock-In were as follows:
Time Constant = 1sec
'Scope reading Output A, Output A set to 'Display', and A's display set to "R" (as in magnitude).
Sensitivity changed throughout the experiment, so that's quoted for each measurement.
White Paper Calibration - white paper placed just in front of Beam Dump. Sensitivity = 500microVolts. Reading on 'scope = 7V
Laser Shuttered. Sensitivity = 500microVolts. 'scope reading = 9mV.
Black Glass at Beam Dump location. Sensitivity = 500microVolts. Reading on 'scope = 142mV. (DON'T touch the glass....measure the same setup with different sensitivity)
Black Glass at Beam Dump location (Not Touched since prev. measurement). Sensitivity = 10microVolts. Reading on 'scope = 6.8V
Laser Shuttered. Sensitivity = 10microVolts. 'scope Reading = 14mV +/- 10mV (lots of fluctuation).
Black Glass Wedge Dump at Beam Dump location. Sensitivity = 10microVolts. 'scope = 100mV.
Beam Dump with original shiny front plate. Sensitivity = 10microVolts. 'scope railing at 11V
Beam Dump with front plate removed. Sensitivity = 10microVolts. 'scope reading = 770mV
Beam Dump, no front plate, but horn's opening surrounded by 2 pieces of Black Glass (one per side ~1cm opening), BG is NOT flush with the opening...it's at an angle relative to where the front plate was. Sensitivity = 10microV. 'scope = 160mV +/- 20mV.
Beam Dump, no front plate, only 1 piece of Black Glass. Sensitivity = 10microV. 'scope reading = 260mV.
Beam Dump, no front plate, 2 pieces of Black Glass, normal incidence (the BG is flush with where the front plate would have been). Sensitivity = 10microV. 'Scope reading = ~600mV
Using our calibration numbers (Black Glass measured at 2 different sensitivities, not touching the setup between the measurements), we can find the calibration between our 2 different sets of measurements (at 500microV and 10microV), to compare our Beam Dump with regular white paper.
BG at 500uV was 142mV. BG at 10uV was 6.8V. 6.8V/0.142V = 47.9
So the white paper, which was measured at 500uV sensitivity, would have been (7V * 47.9) = 335 V in 10uV sensitivity units.
This is compared to the BG wedge dump at 10uV sensitivity of 100mV, and the Beam Dump reading of 770mV, and the Beam Dump with-black-glass-at-the-opening reading of 160mV.
So our Silicon/Steel horn dump is ~8x worse than a Black Glass wedge and (335 / 0.77) = 435x better than white paper.
We used regular white paper as a calibration because it has a Lambertian reflectance. For some general idea of how to do these kinds of scatter measurements, you can look at this MZ doc.
Assuming that our white paper had a BRDF of (1/pi)/steradian, we can estimate some numbers for our setup:
Sensitivity (signal with the laser shuttered) = (0.02 / 335 / pi) = 2 x 10^-5 / sr. This is ~3x worse than the best black glass surfaces.
Our wedge = (0.1 / 335 / pi) = 1 x 10^-4 / sr. Needs a wipe.
Our Silicon-Steel Horn = (0.75 / 335 / pi) = 7 x 10^-4 / steradian.
Our measurements were all made at a small angle since we are interested in scatter back along the incoming beam. We were using a 1" lens to collect the scatter onto a PDA55. The distance from the beam to the center of the lens was ~2" and the detector's lens was ~20" from the front of the horn. So that's an incident angle of ~3 deg.
* It seems that any front plate other than Black Glass is probably worse than just having no front plate at all.
* If we put in a front plate, it shouldn't be normal to the incident beam. Black Glass at normal incidence was almost at the same level as having no front plate. So if we're going to bother with a front plate, it should be about 30deg or 40deg from where the original front plate was.
* No front plate on the Dump is about 7x a Black Glass wedge dump.
* The silicon looks like it might have some dust on it (as well as the rest of the inside of the horn). We should clean everything. (Maybe with deionized nitrogen?)
* We should remeasure the Beam Dump using polished steel at a small (30-40deg) angle as the front plate.
* Photos taken with the Olympus camera, which has its IR blocker removed.
* In the photo you can see that we have a lot of reflection off of the horn on the side opposite from the silicon.
* The 2nd picture is of the scatterometer setup.