Memo on an Optimal Helicity Scheme for Jefferson Lab experiments E94010, eg1 and E93026
April 15, 1998
In the second half of 1998, there are presently 3 experiments scheduled to run concurrently, each requiring high polarization (>70%) and moderate-to-low intensity beam. This memo proposes a scheme for the sequence and timing of the beam helicity for these experiments, trying to optimize the situation for all three. In this present (DRAFT) version, I am outlining a proposal which can serve as basis for discussion within the three experimental collaborations. Since E93026 will begin data taking rather soon, we should attempt to come to a consensus within the next few weeks by direct negotiation between the spokespersons involved.
Proposal: Beam is delivered to all 3 halls with a fixed helicity for a period of 500 ms (or 30 cycles of standard AC line frequency), then with reversed helicity for the next 500 ms. After this "helicity pair" (of 1 second duration) the next helicity is chosen at random, again followed by the opposite helicity and so on. At any moment, the actual helicity state is transmitted instantaneously via the optical fiber link (no encoding as for the parity experiment).
Rationale: The longer period at every given helicity allows a more precise measurement of the integrated charge for each helicity under low-current conditions, which is crucial for proper normalization. The random choice of every second helicity state minimizes the influence of long-term drifts and low-frequency noise (in the 1 Hz range). The transmission of the actual helicity state allows the use of scalers etc. to display online asymmetries.