Helicity

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



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 + - - + or - + + -. 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.

Helicity information (MPS, QRT and Helicity signals) comes from TIR (i/o registers) in ROC 28. They appear in in the hexadecimal dump in the 3-rd word (firs word is a header: 0xfadcb0b9, second word is event number): 7-th bit contains MPS(Gate), 6-th bit contains QRT and 5-th bit contains helicity itself.

Software code responsible for decoding helicity information is in the AGEN library: GenHelicity?.cxx Three functions in the code - ReadData(), QuadCalib() and QuadHelicity() are handling three main steps of decoding: reading helicity signals as well as 105 kHz clock signals, initializing of prediction algorithm, using of prediction algorithm to got helicity information.

1. On the first step of decoding program reads 3 bits from the data-file: QRT, MPS and Helicity and also the 105kHz clock information. 3 main helicity signals has only one read out source in GeN experiment. However 105 kHz clock information read out from 3 different scalers. Due to high radiation sometimes one of the scalers give wrong information. As final 105 kHz clock read out program takes the read out from at least two scalers that match each other.

2. On the second step code collecting enough helicity data to figure out where we are in the sequence and make helicity prediction for following events. For that part of run helicity is unknown.

3. As soon as prediction algorithm initialized, program starts use it to get helicity information. Because of we are working in delayed helicity mode, the predicted helicity doesn't correspond to current helicity reading (helicity information from electronics): the predicted value is delayed by 8 quads. However using 105 kHz clock signal and known duration of each quad one we can made the correspondence between helicity reading and predicted helicity and compare them to make additional checking of prediction algorithm. Another situation when code using 105 kHz clock together with the quad duration value is missing QRT or MPS signals due to DAQ dead time or beam trip. In the case of such missing program try to reconstruct lost quads using last available information. Therefore knowledge of the duration of one quad is very important for proper helicity prediction. That value in 105 kHz clock cycles is a constant number which can be found from data using Beam.H.timestamp variable. Duration of one helicity quad is difference between values of Beam.H.timestamp for two sequential events, with QRT=1 and presented T9 trigger(software inserted trigger which corresponds to MPS signal). If for some reason predicted helicity does not match helicity reading or if reconstruction of missing quads is impossible, decoding starts from the initialization step.