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Eric Prebys, FNAL

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 2 Equations of motion Define uncoupled frequencies: Try a solution of he form: Multiply the top by the bottom:

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 3 Weak Coupling Degenerate Case: Resonance splitting

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i.e. ω 2 are the eigenvalues of M and (a,b) are the linear combinations of x and y which undergo simple harmonic motion. USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 4 General coupled equationGeneral solution

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 5 Introduce skew-quadrupole term Planes coupled x and y motion not independent General Transfer Matrix Normal Quad

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 6 Skew quad So the transfer matrix for a skew quad would be: For a normal quad rotated by ϕ it would be (homework)

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 7 In Floquet Coordinate (lecture 10) Motion given by

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 8

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 9 Assume one perturbation per turn. Evolution of amplitude Evolution of phase

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 10 Recall

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 11 Focus on case when The sum terms will oscillate quickly, so we focus in the difference terms Note that Sum of emittances in transverse planes stays constant!

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 12 Transform into a rotating frameIn one (unperturbed) rotation Equations Become

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 13 Try Hey, this (finally!) looks like simple coupled harmonic motion

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 14 Rearrange Apply usual coupled oscillation formalism. Define normal modes

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 15 SolutionsSolve for eigenvalues of M Recall

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 16 If there’s no coupling, then If there’s coupling, then there will always be a tune split

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 17 The normal modes are given by (homework) Restrict ourselves to u ±0 real Emmittance oscillates between planes. Period = turns

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 18 Returning to the original equations, but examining the sum terms Same sign! Following the same math as before, we get In other words, emittances can grow in both planes simultaneously.

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 19 We won’t re-do the entire analysis for the sum resonances, but we find that the eigenmodes are integer “Stop band width”. The stronger the coupling, the further away you have to keep the tune from a sum resonance.

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USPAS, Knoxville, TN, January 20-31, 2014 Lecture 12 - Coupled Resonances 20 Although we won’t derive it in detail, it’s clear that if motion is coupled, we can analyze the system in terms of the normal coordinates, and repeat the analysis in the last chapter. In this case, the normal tunes will be linear combinations of the tunes in the two planes, and so the general condition for resonance becomes. This appears as a set of crossing lines in the nx,ny “tune space”. The width of individual lines depends on the details of the machine, and one tries to pick a “working point” to avoid the strongest resonances.

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