History: Helicity

GeN experiment utilized a delayed G0 helicity mode. Timing diagram for main signals
used to obtain helicity information presented on following figure.

::{img src=http://hallaweb.jlab.org/experiment/E02-013/wiki/show_image.php?id=77}::

Main signals used for obtaining helicity information in G0 mode are:
;:1. __MPS__ - 30 HZ master pulse, used as gate for helicity signal.
;:2. __QRT__ - Showing the beginning of new helicity quad.
;:3. __Helicity__ - signal showing helicity sign.
In addition we used another signal which is not directly related to helicity information that is
;:4. __105 kHz clock__ - signal used to reconstruct MPS or QRT signals missed due to some reasons(eg DAQ deadtime) and recover our position in the helicity sequence as well as the position in quad.

There is only one copy of QRT, MPS and Helicity signals in GeN experiment, that could be used to got the helicity information on event-by-event bases. And we have 3 redundant copies of 105 kHz clock signal. Helicity signal comes in quad structure either ~043~ ~045~ ~045~ ~043~ or ~045~ ~043~ ~043~ ~045~. Each helicity cycle (time between 2 helicity switches) is 33.3msec. There is 0.5msec blank-off period for each cycle which is necessary for Pockel cell changing and settling. Due to that dead period we have about 1.5% events with unknown helicity, which is shown in data as helicity = 0. Except that we have unknown helicity period at the start of each run, because of prediction algorithm need to collect enough information before it starts work.