Fringe field effect on Ct

[nnaka]

Dear Michael,

I would like to check change of the path length and the travel time of an electron in a bending magnet with a fringe field.
First I tracked an electron beam using each of magnet elements, CSRCSBEND and CSBEND, which has 1-m bending radius and
45-degree bending angle and lies between drift spaces and then checked Ct-values including the two drift spaces. But there
was no difference between two Ct-values of hard edge(no fringe field, HGAP=0) and soft edge(ex. HGAP=0.03,FINT=0.3),
though betatron function was significantly changed. I then used NIBEND element with linear and cubic-spline fringe models,
but the Ct-value was the same as those of CSRBEND and CSBEND. Elegant does not have a function to calculate change of the
travel time in a bending magnet with and without a fringe field ? If it has such function, please tell me about it.

Best Regards,
Norio Nakamura
KEK

[michael_borland]

Norio,

For SBEND, CSBEND, and CSRCSBEND, I'm not surprised, since the fringe effects are implemented in the impulse approximation.

For NIBEND, elegant automatically suppresses the extra (or missing) path length in order to obviate the need to change the rf frequency. In retrospect, that probably wasn't the best approach.

I can add an option to turn this feature off in the next release.

--Michael

[nnaka]

Michael

Thank you for your quick reply.
I treat an ERL and path length adjustment is a common issue to ERLs.
I would be grateful to you if you could add an option not to suppress
the extra (or missing) path length due to fringe field in the new release.
I am looking forward to the new release.

Best regards,
Norio Nakamura

[michael_borland]

Norio,

I've added the feature already and it will appear in the next release. Thanks for bring it to my attention.

--Michael

[nnaka]

Dear Michael,

I have a few questions about Ct.

(1) If a bunch initially has a finite length, initial longitudinal positions of electrons are different.
What is the definition of Ct for such bunched beam ?

(2) In calculation of Ct in elegant, we would like to use an electron or a single particle beam
that is initially on the center of the 6-D phase spaces in place of multi-particle tracking, because
the tracking time is saved. Is it possible by elegant ?

Best regards,
Norio Nakamura

[michael_borland]

Norio,

(1) If a bunch initially has a finite length, initial longitudinal positions of electrons are different.
What is the definition of Ct for such bunched beam ?

Ct = <t>, where t = s/(beta*c). Hence, initially, Ct will be the average time taken for the particles to reach the entrance of the first element, given their various velocities, assuming they originally started from the same upstream location at the same time.

(2) In calculation of Ct in elegant, we would like to use an electron or a single particle beam
that is initially on the center of the 6-D phase spaces in place of multi-particle tracking, because
the tracking time is saved. Is it possible by elegant ?

Yes, this should be possible, as follows. Assume the beam you are tracking is in a file bunch.sdds
Use sddsanalyzebeam to compute Ct, then compute Cs with sddsprocess.
% sddsanalyzebeam bunch.sdds bunch.ana
% sddsprocess bunch.ana -define=col,Cs,"Ct pAverage beta.p c_mks * *"

In your .ele file, use
&bunched_beam
n_particles_per_bunch=1
centroid[4] = "{sdds2stream -column=Cs bunch.ana}"
&end

I haven't tried this, but it should work. However, note that all rf systems will be phased to this single particle, so the effect will presumably only be seen in the t coordinate, not the dynamics.

By the way, there is no guarantee that a single particle at the centroid location will continue to be at that location as the beam travels through the beamline. This is because of the presence of higher-order effects in the beam transport.

--Michael