LSRMDLTR reference phase

[imartin]

Hi,
I am trying to understand better the LASER_PHASE parameter for the LSRMDLTR element. I can use this parameter to shift the phase of the laser, but have not been able to work out how the zero-phase is defined. Also, how does this relate to the TIME_OFFSET parameter for the external time-dependent modulation of the laser beam?
Thanks,
Ian

[michael_borland]

Ian,

The phase of the laser field is calculated as phi=k*(z-L/2) - omega*t + phi0, where L is the length of the undulator, t is the time coordinate of the particle, phi0 is the phase specified. Unfortunately, I didn't use (t-tc), so it a bit confusing to set phi0. I'd suggest using LASER_PHASE of omega*L/(2*c) + dphi, where dphi is the desired phase.

For the profile offset, the time coordinate used for the lookup table is t - tfid + tOffset, there tfid is the arrival time of the fiducial particle at z=L/2 and tOffset is the TIME_OFFSET parameter.

Hope this helps.

--Michael

[imartin]

Michael,

So if I understand correctly, the phase of the laser is not necessarily fixed with respect to the time envelope? Is this the case? ie if I set the LASER_PHASE and TIME_OFFSET to particular values and then track a different bunch through the element, this could give a different phase wrt to the peak of the time envolope due to the different value for tfid?

thanks,
Ian

[michael_borland]

Ian,

Yes, that's correct.

--Michael

[imartin]

MIchael,

I have been trying to carry out jitter simulations using the LSRMDLTR element, fixing the phase of the laser with respect to the time envelope. To do this I was tracking 2 bunches, the first to act as the fiducial bunch and then apply errors to the linac for the second bunch. When I try this the time envelope remains constant between the two bunches, but the phase of the laser changes. Is there a way to fix the phase? I've attached the .ele and error files I've used as reference, and also a plot showing the longitudinal phase space for the two bunches at the end of the linac.

thanks again,
Ian