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    User name paschke

    Log entry time 14:31:49 on March 01, 2010

    Entry number 310319

    keyword=Summary of Injector work Feb 28, 2010




    Our understanding of the laser optics have been confirmed by our studies on the cathode. We
    understand very well where our sources of linear polarization are, and we can discriminate
    where we are getting position differences in the electron beam. It also seems we can find
    some reasonable optimum, with position differences small and potentially tunable.

    There are two serious problems which complicate this work: 1) the rotating waveplate
    birefringence error is about 3x larger than one would hope. 2) the vacuum window
    birefringence is at least 8 times larger than expectation.

    A bulleted list of details, drawn primarily from the studies on Sunday 2/28:

    http://www.jlab.org/~adaq/halog/html/1002_archive/100228152047.html
    http://www.jlab.org/~adaq/halog/html/1002_archive/100228153011.html
    http://www.jlab.org/~adaq/halog/html/1002_archive/100228155900.html
    http://www.jlab.org/~adaq/halog/html/1002_archive/100228162118.html
    http://www.jlab.org/~adaq/halog/html/1002_archive/100228185144.html
    http://www.jlab.org/~adaq/halog/html/1002_archive/100228211036.html
    http://www.jlab.org/~adaq/halog/html/1002_archive/100228221448.html
    http://www.jlab.org/~adaq/halog/html/1003_archive/100301001408.html

    - We measure the cathode analyzing power two different ways: by the total intensity variation
    observed in the bpm wiresum when scanning the RHWP with no voltage on the Pockels cell,
    and independently by the relationship between charge asymmetry and "delta phase" changes
    in Pockels cell voltages. Both of these introduce known amounts of linear polarization to the
    cathode.
    * With Pockels cell off, the intensity of the beam varies about about +/-6.5% under RHWP
    rotation.
    * The maximum PITA slope (measured through RHWP scans of different voltages) is
    approximately 42 ppm/volt, while 100% polarizer has a maximum PITA slope of ~660
    ppm/volt. This implies an analyzing power of about 6.4%.

    - This cathode analyzing power (about 6.5%) is larger than we would have anticipated from a
    superlattice cathode. We had expected supression of >25x from polarization effects measured
    on the laser table with a 100% polarizer, but the higher-than-expected analyzing power is
    more like 15x suppression.

    - The positions differences due to gradients in the Pockels cell (4-theta terms at PITA=0
    voltages) are small but measureable. We see approximate 150-200 nm 4-theta amplitude in
    the horizontal at BPM1I02. This is less than might be expected from laser table studies. On
    the table with an LP analyzer, we see 10 microns (X) and 4 microns (Y) position differences out
    of phase by 45 degrees, so one might expect as much as 0.6 microns (peak) with this cathode
    analyzing power. This discrepancy is welcome but not understood.

    - It appears that the position differences get large, especially in Y, for large applied linear
    polarization (DY 4-theta term in PITA=120 runs). (For Y, the position differences go from
    unmeasurably small at PITA=0 to 2.2 micron at PITA=120. For X, the position differences go
    from 0.1-02 to 0.5 micron at PITA 120.) This implies a very large gradient in the cathode
    analyzing power for that dimension. This effect is 5 or more times greater for the Y coordinate
    than the X coordinate.
    This is the most challenging effect to demonstrate without ambiguity, because it the
    signature is a porportionality between charge asymmetry and position difference, which is
    exactly the signature for pedestal error in the bpm readout. There are three potential handles
    on this: the first is to identify the same relationships with other bpms, since it is unlikely that
    the same pedestal error should affect all instruments in the same manned. A second
    approach comes from runs with a rotated cathode, where the gradients should rotate in x/y
    along with the cathode. The third approach is to compare studies done on the old cathode,
    since presumably a cathode gradient would be unlikely to be the same for two wafers whereas
    the bpm pedestal error would be the same for all measurements. We'll attempt to clarify this
    later, because an unambiguous signal from cathode gradients would be useful for interpreting
    the potential spot-size asymmetry issues.

    - The rotating waveplate birefringence error is evident in a 2-theta term in the RHWP scans.
    On the table, the asymmetry of the 2 theta term was as large as that from a PITA=50 volt PC
    voltage offset. This scales precisely to observations in the RHWP scans, where the 2-theta
    amplitude is 40% as large as the charge asymmetry 4-theta amplitude for a 120V PITA offset.
    Thus, the 2-theta term is well understood as a birefringence error in the RHWP. This
    contribution dominates the charge asymmetry variation in the RHWP scan.

    - The linear polarization from RHWP birefringence error also dominates the measured position
    differences, through the apparent cathode gradient. This further restricts the orientation of
    the RHWP such that the Pockels cell can be used to counter-act the RHWP phase error.

    - The vacuum window shows a very large birefringence, at least twice as large as the RHWP
    birefringence error. We can reduce this problem by rotating the photocathode, but it is
    difficult to do so with sufficient precision to make this term small. We started in a lucky
    orienation which supresses these effeects by a factor of >10, and managed to get back to
    comparable (or even better position) after the study. Still, this effect is potentially very
    dangerous and is certainly the largest effect in the entire system.

    - We observed a significant, non-polarization senstive position-difference offset in the RHWP
    scans. This was corrected via translation of the PC of approximately 1 mm in horizontal, and
    0.5 mm in the vertical.

    - the plots posted from injector studies on Saturday, Feb 27, are incorrect, due to DAQ
    problem. These can be corrected with some minor analysis tweaks, and will be reposted when
    this is done.. Accounting for the known effects of the DAQ error, supports conclusions from
    the current study:
    * the analyzing power on that cathode is somewhat smaller: about 4.7%, and is
    (conicidentally?) roughly at the same angle. This is measured by the scaling of the 4-theta
    amplitude with PC voltage.
    * the 2-theta charge asymmetry scales with this analyzing power, compared to the new
    cathode, consistent with the model of RHWP phase error.
    * vacuum window contribution was also comparable to the small values seen in the original
    orientation of the new cathode. This emphasizes that the analyzing powers were extremely
    similar in orientation. Is there a reason for that to happen (like, a known crystal axis mounted
    relative to some fidicual on the wafer holder)?

    Conclusions:

    - We can't fix the vacuum window, so we had best keep the the cathode oriented to suppress
    its importance.

    - We can fix the waveplate, and we have enough problems to deal with without that, so
    changing it is a priority.

    - If we can get a waveplate with a phase error of <1% (and phase gradients as small as the
    present device) we would be able to use the PC voltages to counteract the vacuum window
    birefringence.

    - We need to verify or refute the important cathode analyzing power gradient suggested by
    the RHWP scans measured at bpm1I02.