Dear Edu,

The quadrupole role affects to the linear optics (coupling and vertical dispersion).
And if the roll error is increased twice to the evaluated tolerance,
the luminosity reduction is increased from 1% to 2% for single parameter.
The total luminosity reduction also changed by 1% or 2% only for the error difference.
Therefore, I think the quadrupole roll is not the reason.

The most plausible source is imperfection of the optics optimization.
The linear optics (the balance of the beta function and dispersion at sextupoles and quadrupoles)
should be optimized carefully to minimize the higher order aberration.

If the optimization is not enough, 
not only 2nd order, but also 3rd order aberrations are generated.
i.e.) U3246, U3466 etc.

Since the octupoles affect not only U3222, but also affect to other 3rd order components,
I think the octupoles field is effective to reduce the IP beam size, 
when the optimization of the optics is not good.

regards,

Toshiyuki OKUGI, KEK



 

----- Original Message -----
Dear Okugi-san,

further investigation of sources of IP aberration U3222-like, allowed me 
to find that the tilt of quadrupole magnets are the main responsible.
Moreover, the impact of distributing these errors assuming a Gaussian 
distribution of width 100 urad or 300 urad changes the tuning results 
substantially.
In the attached plot, I compare the confidence level (sigma_y/sigma_y0) 
for 3 different cases of tilt error: 300, 200 and 100 urad
(Note that sigma_y represents the core of the beam at the IP after 
removing all 1st and 2nd order correlations).

Clearly if the alignment rotation is within 100 urad, we do not need the 
3rd order knob as you have also concluded from your study.

Nevertheless we can relax the tilt tolerance as expenses of using OCM0 
for tuning the system, if required.

Best regards,

Edu

On 04/12/2015 12:50 AM, Okugi, Toshiyuki wrote:
> Dear Glen,
>
> I communicated with Brett Parker the following issues.
> The requirement for the correction coils for SD0 or QD0 are as follows.
>
> When we don't have a SD0 mover,
> we can consider two methods of IP tuning instead of sextupole mover.
>
> 1) [by using the qudrupole and skew quadrupole correction magnet around SD0]
>   The requirement of the strength of the correction magnet are
>   the maximum strength of the correction magnet is GL=+/-2.6T, and the precision is at least 5e-5
>   in order to compensate the SD0 position movement by +/-1mm for ECM=250-500GeV operation.
>
> 2) [by using the combination of the cryo-mover and dipole correction coil around QD0]
>   The requirement of the strength of the correction magnet are
>   the maximum strength of the correction magnet is BL=0.15Tm, and the precision is also 5e-5.
>
> The requirement of SF1 is a little bit different, but I expect to be same order to SD0.
> I think the required precision is very hard to other magnets ( > 5e-4 ) for both cases
> (but not impossible??).
>
> We should decide whether will we put the SD0, SF1 mover or compensate by using correction coil.
>
> Sincerely,
>
> Toshiyuki OKUGI, KEK
>
>
>
>

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