local momentum aperture search: 5D vs 6D

[gzhsyw]

Dear Michael,

I have run the local momentum aperture searches with 5D (without rf cavity and radiation) and 6D (with rf cavity and radiation). My understanding is that if the height of rf bucket is large enough, rf cavity will not affect the result much. But my 6D result is much worse. Can you help me check if my setting of rf is correct?

[gzhsyw]

Here is my 6D result.

Thank you.
Zhenghao Gu

[michael_borland]

Zhenghao,

I'm looking into this and will get back to you in a few days.

--Michael

[michael_borland]

Zhenghao,

The problem appears to be the large horizontal chromaticity (16.8) coupled with nonlinear synchro-betatron coupling. I suggest reducing the chromaticity significantly and also paying attention to the higher-order chromaticity and higher-order dispersion in the rf cavities when you adjust the sextupoles.

To look at this, I tracked a series of single particles with various initial energy offsets, with and without the rf system. With the rf on, the horizontal tune spectrum (see attached graph) is very broad and soon touches the integer resonance.

Actually, the tune crosses the integer resonance in the case without rf as well, but since you don't have errors and have a large scan interval in the momentum_aperture command, you don't see this. My second suggestion is thus to reduce the step size and use the step-back feature in the momentum aperture scans. (See second attachment.) Perhaps more importantly, you should also add errors whenever you look at momentum aperture. Otherwise, the beam can cross resonances (particulary the half-integer resonance) without being lost.

--Michael