Word of warning about using CHANGE_T=1 in RFCA or RFCW elements
Posted: 11 Nov 2020, 13:56
Hi,
I just spent a lot of time tracking down a mistake that involved setting CHANGE_T=1 on a 3rd-harmonic RF cavity. Even though it's a harmonic of the fundamental RF, the time that is subtracted from particles can easily be 1/3 or 2/3 of the period of the main RF, giving big phase errors. And it's one of those things that can lead to behavior that is only sporadically abnormal, and looks ok the rest of the time. For example, sometimes particle tracking would look fine, but the momentum aperture calculation would be way off.
I would recommend changing the wording in the manual to avoid setting CHANGE_T=1 on harmonic cavities, or in general to only use it on RF cavities that are at the lowest common denominator frequency. It's easy to set it on all cavities without thinking, and the current wording is a little too soft about using it on harmonic cavities. I think the intention was to say it's okay to use on the fundamental RF even if there are also higher harmonic cavities, but it doesn't quite read that way.
Thanks,
Gregg
I just spent a lot of time tracking down a mistake that involved setting CHANGE_T=1 on a 3rd-harmonic RF cavity. Even though it's a harmonic of the fundamental RF, the time that is subtracted from particles can easily be 1/3 or 2/3 of the period of the main RF, giving big phase errors. And it's one of those things that can lead to behavior that is only sporadically abnormal, and looks ok the rest of the time. For example, sometimes particle tracking would look fine, but the momentum aperture calculation would be way off.
I would recommend changing the wording in the manual to avoid setting CHANGE_T=1 on harmonic cavities, or in general to only use it on RF cavities that are at the lowest common denominator frequency. It's easy to set it on all cavities without thinking, and the current wording is a little too soft about using it on harmonic cavities. I think the intention was to say it's okay to use on the fundamental RF even if there are also higher harmonic cavities, but it doesn't quite read that way.
Thanks,
Gregg