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I was trying to participate in the conversation on apertures, but appear to have been muted at top level.</div>
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As Joe said, A1/A2 were an emittance limiter, expected to be flooded by the thermionic gun, but in practice, underfilled. The apertures were opened up for photoinjector use, largely because of asymmetric scraping from parity swap via Pockels cell —unintentional
deflections resulting in greater loss for one helicity setting than the other. They are now very oversized with respect to original purpose.</div>
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A3/A4 were sized originally to deny beam to the walls of the cryounit, an attempt to limit damage to the essentially irreplaceable cryounit in case of beam steering malfunctions. They are now also oversized, and their purpose is likely totally obsolete.</div>
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That is my experience, recollection, and rationale.</div>
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Michael</div>
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<div id="divRplyFwdMsg" dir="ltr"><span style="font-family: Calibri, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>From:</b> BTeam <bteam-bounces@jlab.org> on behalf of Michael Tiefenback via BTeam <bteam@jlab.org><br>
<b>Sent:</b> Tuesday, May 27, 2025 15:30<br>
<b>To:</b> bteam@jlab.org <bteam@jlab.org><br>
<b>Subject:</b> [BTeam] Chopper details</span>
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What I put into the bteam chat but am unsure it went anywhere:</div>
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Unless it was changed at some time long ago, since the thermionic gun days the rotation was counter-clockwise, and the slit order in time was B-A-C.</div>
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I have not been involved in chopper setup in decades. The RF system systematically is set up so that adding phase is the same as adding cable between source and cavity: positive PSET means RF arrives later. In such a system, adding 90 degrees to one channel
will result in a straight line. The question of circular rotation is answered by which orientation the straight line ends up. Either 1:30 to 7:30 or 4:30 to 10:30. Clean answer.</div>
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Michael</div>
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