Beam Tune and Beam-position Monitoring

Beam position monitoring task list:

Check averaged BPM readings every 15 minutes ( this does not show 60 Hz noise).

IPM1H03B

Both vertical and horizontal readings from this BPM should be below +/- 0.2 mm

IPM1HO3A

<0.3 mm

BPM strip chart ( spikes ) (Example) - expect to get a better tune than this.

Find the Accelerator main menu (monticello screen) on the HAC x-terminal. If it is not there, you have to call the MCC and request it.
Go to BPM >> BPM overview >> Hall A >> Hall A Absolute

BPM position summary window (Example)

as above
Go to BPM >> BPM position summary >> BSY-experiemntal halls >> Hall A

BPM x-y plots (Example)

as above
Go to Hall A >> Hall A BPM position plots

When the beam is turned on and at the beginning of every shift, request a Bscope from MCC to check the 60 Hz noise. Check both the horizontal and vertical noise corresponding to IPM1H03B BPM. Both offset and the amplitude should be < 0.2 mm.

To get the Bscope (Example):

Call the Accelerator and request a Bscope for HAll A BPM's.
After 10 minutes, results from Bscope appear on the Web at http://devserve.cebaf.gov/internal/ops/daily_activity_log/html/logdir.html

Beam Tune

This is done by the accelerator. Currently, we have no control over the tune. The accelerator division will nominally check the tune once in every shift. At these times , we should verify that the beam spot is centered on the cross wires , and have the operators look fon any Halo.

The nominal tune calls for:

Point focus at the target.
Zero dispersion.
The Emitance is ~10E-7 m.rad so that the nominal beam spot size is very small.
A description of the beam line is available in the counting house.

Beam Position Monitoring

There is a four wire stripline antenna system to monitor the beam motion .This is documented in one of the beam line documents. What is important is to monitor the shifts in x and y positions of the beam. IPM1H03B, at 1.1m from the Hall A target, is the closest one to the HAll A target. X and Y absolute positions measured by this BPM are good approximations to the x , y positions of the beam at the target. To make sure their is no angular dispersion of the beam, should look at the IPM1HO3A.

Snapshot of Hall A BPM window

How to get the BPM window :
1) Find the Accelerator main menu (montecello screen) on the HAC x-terminal. If it is not there , you have to call the MCC and request it.
2) Go to BPM >> BPM position Summary >> BSY/Experimental Halls >> Hall A

Important Information about the Beam position:

1) Accelerator BPM's give a coordinate system z along the beam ,y vertically upwards, x horizontal making an orthogonal system. In the spectrometer coordinate system , at the target, z along the beam, x is the dispersive direction ( vertically down) , y is horizontal making a right handed system.

2) Magnification factor in the spectrometer along the x (dispersive) direction = 2.5
(A distance of 1 mm at the target in x correspond to 2.5 mm at the focal plane).

3) Dispersive constant of the spectrometer = 8 x10-2 m-1.
(1m of x at the focal plane correspond to dp/p =8 x 10-2 = 8 %)

Both x_acc, and y_acc beam positions are important to us because :

1) Y_acc < 0.5 mm (remember 0.2mm sigma corespond to a fwhm of 0.5mm) ( Momentum Resolution)
X_target < 0.5 mm , X_fp < 0.5 x 2.5 = 1.25 mm
dp/p < 12.5 x 10-4 x 8 x 10-2 = 1x 10-4

2) X_acc < 0.5 mm
Angular resolution in phi if the beam has just an offset or shift in X_acc
dx_acc < 0.5 mm = 5x 10-4 m
Distance between the target and the spectrometer ~ 1.1 m
dphi < 5x 10-4 / 1.1 = 0.5 mrad

3) Differencd between IPM1HO3B and IPM1HO3A beam positions < 3 mm
Distance between the Two BPms = 6 m.
d (Theta-note) = 3 x 10-3 / 6 = 0.5 mr

Determination of the Scattering angle Theta (if the beam has a slope in X_acc)

1) Instantaneous values of monitors should be read out in the data stream. This can be used in the offline analysis to correct the data.

2) As far as the visual monitoring is concerned, we should keep the relative offsets of the xy monitors to below +-0.2 mm( This will ensure fwhm. of dp/p < 10-4 and fwhm. of dphi < 0.5 mrad.

3) Since the visual monitors are locked at 60 Hz , we do not see the 60 Hz motion of the beam . Up to now the accelerator does not know the source of this 60 Hz noise. We should run Bscope once in every shift to make sure that this 60 Hz noise in not large . Peak to peak < 200micrometers.

To get the Bscope :

Call the Accelerator and request a Bscope for HAll A BPM's.
After 10 minuites, results from Bscope appears on the Web at http://devserve.cebaf.gov/internal/ops/daily_activity_log/html/logdir.html

4) Beam halo should normally be small . If it is too large , it will hit the magnet poles or other objects like iron-chamber , and will automatically shut the beam off. The accelerator is installing a halo monitor at the point of maximum despersion in the arc , and this is the best place to check for the halo, since it is likely to be energy dependent. When the accelerator is doing system checks once every shift , they can check whether the Crest of the wave coinsides with the RF kick.