Log entry time 22:36:32 on December 07, 2008

Entry number 252459

This entry is a followup to: 252411

Followups:

• 252467 12/07/08 23:57 Yi Qiang Field Drifting due to Polarity Change

Figure 1 shows polarization history since end of last month, which is a 
single spin flip session with transverse - holding field and 3x 25W 
Comet laser. We started production at end of Nov 28, then the 
polarization has been generally dropping.

There are a few 1/4 wave plate angle calibration during this period, 
which is shown as green line in figure 1. However, it's not likely to 
be responsible for the polarization drop, since it's not correlated to 
polarization change. Besides, the plate angle is not more than 2 degree 
away from setting. Otherwise it will generate limit-switch fail alarm.

Second, the peak amplitude of laser spectrum after the cell is 
dropping, which is generally agree with the trend of polarization drop. 
The reason of laser drop could be 
Case 1. Laser power or spectrum change
Case 2. Optics problem, dust problem, fiber problem
Case 3. 1/4 Wave plate angle is off
Case 4. Radiation Damage on Cell
Case 5. instability of spectrum analyzer

Comparing to laser spectrum of Nov 28 and Dec 07, main laser peak (@ 
795.1nm) dropped 25% and secondary peak (@ 795.9nm, on main peak tail) 
dropped 15%. Besides, there are two extra peak @ 796.4nm and 796.1nm. 
Case 2, 4, 5 should not introduce extra peaks, which is less likely 
here. For case 3, all 1/4 plate change is not likely related to laser 
spectrum drift (see fig 1).

Comparing fig 2 and 3, I would guess the main reason for polarization 
drift is due to that one of our COMET laser could be shifting its 
spectrum to longer wave length (and introduced the two extra peaks). 
However, to confirm, we need more test during non-production time. 

Besides above discussion, there are also some related facts: 
1. 1C rise in upstream pumping chamber RTD
   downstream RTD remain similar level (Figure 1, last plot)
2. 0.5% Polarization rising during day of 12/01 
3. 2% Polarization drop after LHRS polarity change 
   : Could related to Q1 field gradient.
4. The laser current, diode/fiber temperature are checked to be normal.

Figure 1 : Polarization, Laser Peak Amplitude and Pumping Chamber Temperature vs Time

Figure 2 : Laser Absorption Spectrum at 23:45 Nov 28, 2008

Figure 3 : Laser Absorption Spectrum at 20:13 on Dec 07, 2008