Log entry time 20:40:59 on September 5,2009

Entry number 288174

This entry is a followup to: 288171

Followups:

• 288175 09/05/09 20:45 paschke repeating plot of dithering cycle

Below shows a typical beam modulation cycle.  in order, the beamline components 
dithered are listed, along with color in the plot and the amplitude of the 
triangle wave modulation.  This cycle is from run #12035.

1) 1C08 H  - red - 100 mA current
2) 1C08 V  - blue - 250 mA current
3) 1C10 H  - red  - 250 mA current 
4) 1C10 V  - blue - 100 mA current
5) SL Zone 20  - green - 50 keV / pass set

The plots show bpms vs. 30Hz sample number, starting with the top right, of 
bpmx 12x, 4ax, 4ay, 4bx, 4by.

Some conclusions:
1) The energy setpoint of 50 keV /pass should correspond to 150 keV / 3484 MeV 
~ 4.3e-5.  But the 0.5mm modulation amplitude at bpm 12x (which has, I believe, 
a dispersion of 4 meters) suggests that the energy modulation is 500e-6/4.0 = 
1.25e-5.  This suggests that the energy kick is three times larger than the 
setpoint would suggest.  Something to keep in mind.

2) There is an apparent vertical dispersion at the target which is very large.  
The modulation amplitude in response to energy was about 150 um, which (in 
comparison the 500 um at 4meter dispersion in 12x) implies that the vertical 
dispersion at the target is about 1.2 meters!

3) The horizontal coils completely fail to span the beam phase space.  They 
look as redundant as can be, which is pretty weird.  In fact, the amplitude is 
similar in each bpm, which suggests that both coils produce position (but not 
angle) deviations.  I can't figure this out: the 1C08 and 1C10 coils are 10 
meters apart, with no active optics between them (or so I was told... I haven't 
verified).  Thinking of an analogy to an optical system: it is as if each point 
is at the focus, and winds up being well collimated by the following lenses, 
even though the two points are 10 meters apart.  I'm no expert, but I find this 
very bizarre.

4) I might believe that the vertical is a little more independent, but it isn't 
very convincing. 

So, I'm surprised by the setpoint calibration (I remembered this setpoint 
actually being close to the observed energy kick, in the past), the vertical 
dispersion, and the lack of orthogonality in dithering magnets.  I believe that 
the vertical dispersion is a sign of a potentially important deviation from 
optical lattice design (and is notable given recent results suggesting a 
mis-steer of the scattered photon beam from the Compton).  

We will definitely require help from opticians to achieve orthogonality of the 
horizontal and vertical correctors.