Helicity for 89033

Helicity Overview

The electron beam is generated polarized by using circularly polarized laser light; the polarization is controlled by a Pockel cell.

In the initial implementation of the helicity scheme, there were two signals, hereafter called A and B , used to indicate the polarization state. For all labelling of cables, documentation, etc., the helicity scheme was defined to be:

These signals were generated at the source and coupled to the Hall A counting house, via optical fiber. For FPP test run, at the sub-ns level, one source signal would be logical true and the other would be logical false. The length of each helicity state was about 33 ms, the polarization in the Hall required about 100 microseconds to stabilize after the start of the change of the helicity, and + and - pulses alternated.

Important note: in replay in checking the phases, Ed Brash discovered that the phase convention listed above was backward. Hall C Moller confirmed. Accelerator stated that the phase convention was intended to be arbitrary, not A = + as we had believed.

For E89-033 running, it is planned to adopt a more uncorrelated scheme with a superperiod signal to allow parity parasite measurements. The convention is:

At this point it is not clear that the scheme reflects the actual helicities from the source, or the reversed, misunderstood helicities, from before.

Some operational notes:

• The easiest way to check the helicity signals is to look at the hadron arm scalers, and to check that both helicity gated scalers are incrementing, ans the scalers counting helicity gates are incrementing. More detailed checks require running espace and examining the signals and / or examining the sign of the asymmetries.
• The logic to blank off the first 100 microseconds of each pulse is setup in the hadron arm detector stack. The modules used must be adjusted locally, are are set to expect 33 ms pulses. If the accelerator pulse are not 33 ms, call and have them changed ; the logic circuit downstairs is not certain to work optimally / correctly. The pulses may be checked where they fan out from the Phillips level shifter, in the central aisle of the Hall A counting house. Click here for text copies of two emails giving more details on the circuitry.

Created July 7, 1996 Ronald Gilman