Log entry time 10:50:54 on November 07, 2005

Entry number 159394

This entry is a followup to: 159347

Below I've attached a figure generated from Run 6225, which was taken this morning with Hall B off. Position differences in the 100 keV injector of 3-6 times larger than our best configuration at the startup of hydrogen.

We are still seeing terrifically small position differences in the Hall, something <20 nm measured at the slug level. We must be seeing great benefit from damping, at least on the level of factor of 30.

It isn't so surprising to see changes in the position differences, given that there has been _no_ effort to actively maintain the optimal source configuration (since there has been no problem in the Hall). There are at least a couple of possibile causes for the degradation of the position differences in the injector.

- It is possible that there are slow drifts in the source configuration. However, I don't think that we measured any sources of position differences with a large sensitivity that did not also create significant charge asymmetry, and the necessary PITA offset voltage has remained reasonably low, so it is tough to come up with a specific drifting parameter to explain an increase in position differences.

- Tweaks to the laser to restore lock may cause slight steering. (We have no measurements to directly suggest this is a problem, but we haven't checked in the injector when these tweaks were done).

- A Q.E. hole has been dug at the beam spot. This amplifies gradient and steering sources of position differences. (An interesting point: the larger position differences appear to cancel well, although in our original setup, steering was not dominant. Does this suggest an amplification of the steering effect, which does cancel well?)

- Is it possible that a gradient in the Q.E. also changes with time? This seems like a parameter that could change in time as the cathode is damaged by use, and would certainly create position differences.

We should continue to flag any data with Hall B off in our logbook. If we are not getting data every couple of days with Hall B off, we should ask interested CASA folks if they would support (politically, not technically) an occasional 10 minute B-off measurement, in the same spirit as the daily PZT studies. This is not at all important for the present hydrogen measurement, but I think it is very important for future parity-violation studies at JLab (Lead and QWeak).

A copy of this log entry has been emailed to: snyder@jlab.org,chao@jlab.org, poelker@jlab.org, merminga@jlab.org

Position differences, injector through Hall, Run 6225