[Hallc_running] Suggestion to shorten NPS readout interval from 400 to 200 nsec

[Peter Bosted]

Background: we have been generally running considerably less beam current
than in the NPS and SIDIS proposals (mostly planned for 30 muA). As a
result, we are getting anywhere from 2 to 10 times less events than
we hoped for.

There are several factors that limit which current we use:
a) keep anode current in average of NPS columns 0 and 1 below 30 muA
b) keep data rate low enough that no crate exceeds 80 MB/sec. Since the
crates all have roughly the same rates, we need to be well below
400 MB/sec to avoid this happening to any of the 4 and a half VME crates
(one of the 5 is only half-populated). Last night we did a 30 minute
run at 300 MB/sec with no trips. I think there is general agreement
that keeping the rate below 200 MB/sec is acceptable.
c) keeping the trigger rate low eneough to avoid significant computer dead
time corrections, as well as exceeding the maximum transfer rate of data
from the hall to the mass storage system.

For most of the settings for the rest of the experiment, factor c) will
be the limit that we reach before factors a) and b). Since the event size
(and hence computer live time and transfer rates) are largely determined
by writing out the NPS FADC data, we can gain up to a factor of two
in what current we run by reducing the readout time for the FADC.

At present, we have a readout window of 400 nsec.

We only analyze events in a 100 nsec time window.

But we need to readout over an interval longer than 100 nsec in order
to catch the long tails of some pulses.

A resonable compromise as far as I can tell would be to shorten the
readout interval from 400 to 200 nsec.

To keep the window centered on the coincidence time peak, we would
reduce the time by 75 nsec on the back end, and 125 on the front end,
as far as I understand.

I propose that the experts get together on Tuesday after the Moller run
and implement this change, after taking one run with the 400 nsec
window, so we can make sure thaat no good data is being lost. As
far as I understand, the experts include Alex, Wassim, Ben, and Sanguang.

As I understand, only the config file(s) need to be changed: no knobs
on the crates need to be turned.


Prof. Peter Bosted
email: bosted at jlab.org
phone: (808) 315-1297 (cell)
P.O. Box 6254, Ocean View, HI 96737