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Entry  Mon Apr 16 16:23:09 2018, awade, DailyProgress, scatter, Beam size on trans BN detector IMG_2792.JPG20180416_RazorBladeProfileVerticalTransBNDector.pdf20180416_BeamProfileAtPD.ipynb.zip
    Reply  Thu Apr 19 18:28:11 2018, rana, DailyProgress, scatter, Beam size on trans BN detector 
       Reply  Sun Apr 22 22:00:15 2018, awade, DailyProgress, scatter, Switching out NF1811 detector for KA25MHz 
Message ID: 2172     Entry time: Mon Apr 16 16:23:09 2018     Reply to this: 2174
Author: awade 
Type: DailyProgress 
Category: scatter 
Subject: Beam size on trans BN detector 

I thought I'd have a look at how big the beam is on the current 1811 New Focus detector. Over focusing here might be a source of scatter so this is a number we should probably know.

Razor blade measurement of beam on NF1811 Trans BN detector

I borrowed one of the translation mounts mounted with razor blades from the 40m and did a quick measurement this afternoon.  

Because of the tightness of space on the transmission beat breadboard and the shape of the mount, the closest I could get the blade to the PD was about 1.0 cm.  I took a series of measurements cutting the beam and noting the transmitted DC power (in units of Volts). 

# Data: vertical sweep of razor blade 1 cm in front of post cav BN detector
ypos = np.array([6.,7.,8.,9.,10.,11.,12.,13.,14.,15.,16.,17.,18.,19.,20.]) / 1000. *25.4e-3  # In units of 1/1000s of inch converted to [m]
yPDVolt = np.array([1.74,1.86,2.64,5.10,12.9,28.2,53.4,82.8,112,132,143,148,149,150,150])  # [mV]

I fitted the integral of the Gaussian profile and plotted (see plot below).  This is a quick diagnostic measurement. Iused least squares fit, so no error analyses. Here are the fitted values:

Fitted beam center relative to zero of measurement 0.3240 mm
Fitted peak power 148.2308 mV
Fitted detector dark DC reading 1.6333 mV
Fitted beam width wz 97.3314 um

Time to make a switch?

This beam is quite small although the NF1811 detector diameter is only 0.3 mm.  Not sure how scatter scales with beam size here, is there a good reference I can look up on this?

Now might be a good time to switch to Koji's new PD.  I've managed to stabilize the beat note to 20 MHz it seems to stay within a <1 kHz (3.2 µK) range over a periods of sometime more than 6 hours.  Although, it can take 12 hours to settle down over night after a large disturbance. 

Attachment 1: IMG_2792.JPG  2.240 MB  Uploaded Mon Apr 16 17:40:58 2018  | Hide | Hide all
IMG_2792.JPG
Attachment 2: 20180416_RazorBladeProfileVerticalTransBNDector.pdf  22 kB  Uploaded Mon Apr 16 17:41:17 2018  | Hide | Hide all
20180416_RazorBladeProfileVerticalTransBNDector.pdf
Attachment 3: 20180416_BeamProfileAtPD.ipynb.zip  2 kB  Uploaded Mon Apr 16 17:42:30 2018
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