This assumes the
Compton Polarimeter has been already
setup and
your job is to
If Compton setup is not up and running, first set up CODA and EPICS.
More
about the hallA Compton polarimeter.
In Desktop:
CODA: runcontrol -> Start Run
confirm
"transition Go suceeded" from roncontrol
If not go to recovery
from CODA crash
CODA: While taking
data check Compton rates:
The
display below can be started from any terminal window by typing: ComptonMonitor
-Purple line: Laser power accumulated
inside the optical cavity. An automatic procedure turns the laser On
and Off.
The laser polarization is reversed for each On
period, right polarization has usually more power than the left one.
Average
power should be about 500W or more, if power
drops below 400W, contact a Compton expert .
-Red line: photon rate/uA of
beam. Should be below 100 (400 for GEn) with laser Off,
the difference between laser On and laser Off should be above 200.
-Red line: electron rate/uA of beam. Should
be below 100 (400 for
GEn) with
laser Off, the difference between laser On and Off should be above 150.
-Orange line: Beam
current in uA. The
lower the beam current the higher the fluctuations of the above rates.
If no cavity
power, or no detector rates, or rates are too
high.
EPICS: While taking data check BPM position:
Vertical
position of the electron beam in the Compton chicane is critical to
maximize the compton luminosity.
The
overlap between the laser and the electron beam is optimal if the
vertical position in BPM 1PM102B
is at -0.70 +- 0.03 mm.
Check it!
AFTER 2hrs, end run.
Everytime when Beam IHWP status
changes (inserted or retracted), end and restart a run.
CODA: runcontrol
-> End Run
Logbook Pop-up window appears...
If the run
is good
-Enter your name as Lead Operator
- Select run type -> data
- Put comments in Run title (Production)
- Make sure "Log Run to Compton Database" and
"Analyze Run Automatically" are enable (red squares).
- click Submit
If the run is
bad
- click Exit.
Note:
Runend fails 50% of
the
time due to a coda bug, if runend fails, do
runcontrol
-> reset before starting next run
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Check
Results of Last Run.
Runs are analyzed automatically. It takes about 10% of
acquisition time to complete the analysis.
Results are posted in the Compton Logbook.
Click on the html link
next to run entry in the Logbook.
A list of plots will appear in a separate window. Scroll down and check
the following 4 figures for every runs.
Report any anomaly to compton expert, take actions listed below if
necessay. The quality of the data
depends on it.
Beam
tune:
-
If the overlap of the laser and electron beam is optimal our
sensitivity to vertical beam fluctuation is zero. This sensitivity is
displayed in the lower left plot below as the "Slope" parameter. It
should be compatible with zero at the 3 sigma level, or at least A_F
should be negligible w.r.t. our analyzing power (1.9% for this run). If not.
The
following plot shows the distributions of Y position at the center of
the optical cavity (top left graph), Y vs time (top right),
Compton rate vs Y-Y_mean (bottom left) and position differences
Y_right-Y_left (bottom right).Check the slope of the Compton
rate fit (red line). We want to keep this slope always small because it
corresponds to the maximal luminosity and minimal sensitivity to
position differences
correlated with the helicity.
Photon Data:
-
Check that the Compton Photon spectrum
looks similar to show below and
signal/background S/B > 10
- Laser On/Off and Laser polarization reversals
Green: laser Off periods.
Red:
Laser On, Right polarization
Blue: Laser
On, Left polarization
-Upper plot: photon rate asymmetry versus time.
-Lower left plot: laser On asymmetries. Left and Right laser states
should produce asymmetries of same amplitude but opposite sign.
-Lower right plot: background
asymmetry. Should be compatible with zero.
Electron
Data:
- Check
that the Compton Electron
spectra looks similar to show below
showing no significant beam halo
The electron detector consists of 4 planes of 48 micro-strips,
standing ~5mm above the beam axis. This plot shows the electron
counting rate versus strip number for each plane. The detector is
located behind the 3rd dipole of the compton chicane and the strip
number give the position of the scattered electron along the dispersive
axis with a resolution of 200 microns. Hence the horizontal axis
is proportional to the energy lost by the electon (and given to the
photon).
Green points
are signal+background rates, laser ON.
Red points
are background rates, laser OFF.
The
compton energy spectrum shows up as a flat rate on the first strips.
The backgrond spectrum has a 1/E shape, like bremsstrahlung.
- Beam halo would show up as:
Beam halo shows up on the first strip only, close to the beam
axis.
It usually has an exponential shape, a lot steeper than the
bremsstrahlung shape of the background at higher energy.
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Compton CODA is controlled by the script
$ coda {start | stop | status | reboot}
1) To begin a new
CODA setup do:
$ coda start
a) Wait for runcontrol
screen to
appear. Then...
runcontrol -> connect
->
configure (choose runtype "Production")
-> download
This completes CODA setup and you are reasy to
take data
2) Recovering from
CODA crashes:
CODA
ROC's sometimes crash and you get a lot of error messages on ROC not
responding...
or Download
or
Start Run fails, or run starts and there are no triggers
(heartbeat) then...
$ coda reboot
This will kill all
coda processes, restart servers, reboot all ROC's, rebooting takes
about 1 minute.
After 1 minute,
do step 1a).
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Compton EPICS system is accessed by the script
$ epics {start | stop | status | reboot |
init}
$ epics start
Starts Compton polarimeter
main control panel
Rebooting EPICS
IOC (iochacp):
Generally EPICS IOC reboot is not necessary.
If Compton EPICS control
freezes IOC reboot might be necessary
Typical symptoms are
-White panels on epics screens
-Cavity doesn't lock
-Cavity power reading is meaningless ( much
higher that 1.2 kW)
-Laser doesn't respond to epics commands...
$ epics reboot
It takes about 5 mins for the iochacp to reboot. After sucessful reboot
do:
$ epics init
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High quality data
Vertical
electron beam position:
Click on "BPM" in the Comtpon Polarimeter main control panel to get
this window
Check the vertical position in BPM IPM102B is at nominal
value. This norminal value could depend on beam energy and # of passes
and is usually determined from a vertical scan measurement. The lastest
vertical scan gives a position for 1PM102B of -0.7 +/-
0.03 mm.
If
not, call MCC x7047.
Make sure the "Compton lock" is still ON, ask MCC to turn it back ON if
necessary.
This is a slow
feed-back on the vertical beam position in the compton chicane.
Make sure the set point is at the nominal value (optimal vertical
position on BPM
1PM102B).
No detector rate:
From the epics main control
panel check the following:
- Photon
detector HV is at 1450 V (Calorimeter -> HV
-> Card 13, channel 0)
- Electron
Detector -> Status BEAM
Low Voltage ON
High Voltage 80
all thresholds 60
- Cavity
is locking properly: OPTICS -> Optic Table -> Cavity Power (upper
right corner)
Very high detector rate:
-turn
off the photon detector HV if the rates are higher than 100kHz/uA (Calorimeter
-> HV
-> Card 13, channel 0)
-Ask
MCC about the any change in the bam tune to help diagnose the
problem. Potential sources of backgrounds are:
*bad
pre-buncher setting
at the injector
*bad setting
of a quadrupole upstream the compton
chicane
*bad
path-length correction inducing energy tail...
-report to compton expert.
Low cavity power:
-Make sure the laser is On: Optics -> mini
optics
-> laser -> ON
-Optimize the position of the
remote controlled steering mirrors following instructions given by
Compton expert (to be updated soon);
-Before the optimization instruction is
available, contact a Compton expert to tune up the power.
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The Compton Interaction Point (CIP) is the beam position in the
vertical where the Compton photon counting rate in the photon
detector is maximized. The CIP is determined by scanning the beam in
the vertical direction by changing the field in the compton
chicane dipoles MCP1P01 to search for the CIP. It requires MCC operator
assistance.
A new CIP scan is necessary:
- After a new beam tune in the Compton chicane
- After shutdown periods of a shift or longer.
- If the photon rates fall significantly during steady state production
running
CIP Scan Procedutre
With steady beam conditions, start a run.
Bring up CIP scan strip chart by entering the command ComptonCIPScan
Terminate cavity aoutomatic flip and lock the cavity in Right
Polarization state:
compton->cavity->noflip
compton->cavity->right
compton->cavity->on
Call MCC and request to setup for Compton chicane vertical scan.
Ask to turn off "compton vertical orbit lock"
Choose a step size of 100 for the Bdl in MCP1P01
Chose a direction and step through while waiting for 5 seconds between
steps
Note the Photon rate in the stripchart, If rates fall, ask for changin
the direction
Step through until you have clearly gone past a maximum in the photon
rate.
Determine the value of IPM102B(V) corresponding to the maximum rate
from the stripchart.
Give the new value to MCC as the set point for the Compton orbit lock
and request to turn on the lock.
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The laser
polarization is measured at the exit of the cavity. Knowledge of this
quantity is one of the main systematic errors of Compton polarimetry.
The goal of the quarter-wave plate scans is to have an accurate
monitoring of the laser polarization at the exit of the cavity. It
should be measured once a day and results posted in the halog.
The procedure is the following:
1- Check the cavity is locked on the main control panel:
OPTICS -> Optic
Table -> Cavity Power (upper right corner)
2- Run the script: qwp_scan.
3-
The scan takes ~20 minutes. Stop taking Compton data during the scan.
4-
Re-initialize the position of the quarter-wave plate: type
qw2init in
any terminal window (2 minutes).
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The trigger configuration should be the following: type
"spyconf" after coda has started to get this window.
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Call List:
Sue Witherspoon 7579
EPICS
Xiaochao
Zheng
5433 584-5446 (pager)
General
Sirish Nanda
7176
General
last updated: March 22, 2006 by
Xiaochao Zheng.
