Efficient Computation of Matched Solutions of the KV ...lund/uspas/sbp_2018/lec_intro/06.env... ·...
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Efficient Computation of Matched Solutions of the KV Envelope Equations for Periodic
Focusing Lattices*
Sven H. Chilton,1 Steven M. Lund2 and Edward P. Lee1
1Lawrence Berkeley National Laboratory (LBNL)2Lawrence Livermore National Laboratory (LLNL)
Heavy Ion Fusion Group PresentationBerkeley, CA
18 January, 2006
* Research supported by the US Dept. of Energy at LBNL and LLNL under contract Nos. DEAC0376SF00098 and W7405Eng48
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Conventional rootfinding methods of solving the KV envelope equations often require a priori knowledge of initial conditions
Actual initial conditions:Incorrect IC's leading to nonmatched solutions
Syncopated Quadrupole Lattice
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New iterative numerical method converges rapidly to matched solution without prior knowledge of initial conditions
Iteration 0 Iteration 1
Iteration 2
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Introduction: New Iterative Numerical Method to construct matched solutions to the KV envelope equations
Based on consistency between particle orbits and the matched beam envelopeUses betatron formulation
Method works over entire parameter space Works for all parameterizations of matched solutionsValid for all linear lattices without skew couplingRapidly convergent and robust, even where envelope is unstable
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Outline
Introduction (Already Done)Theoretical ModelMatched Envelope PropertiesNumerical Iterative Method Example ApplicationsConclusions
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Theoretical Model: Definition of the KV Equations and Relevant Parameters
rms/KV envelope Equations:
define applied focusing forces of the lattice
Periodicity:
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Undepressed particle phase advance σ0x
measures the strength of
the applied focusing function κx(s) of periodic lattices
Undepressed particle phase advance:
Singleparticle orbit without spacecharge:
2 x 2 Transfer Matrix from
to
Tr
The same applies to
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Undepressed Principal Orbit Equations
Transfer Matrix:
Cosinelike Principal Orbit Equation:
Note that stability requires:
Tr
[Courant and Snyder, Annals of Physics 3, 1 (1958)]
Initial Conditions:Sinelike case analogousyplane analogous
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Depressed Principal Orbit Equations
Cosinelike Principal Orbit Equation:
Depression:
Maintain same basic formulation as before except:
Notation: drop 0 subscript to indicate depression
Applied focusing Spacecharge defocusing
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Depressed singleparticle phase advance in the presence of uniform spacecharge for a particle moving in the matched beam envelope:
Normalized space charge strength or “ depressed tune” :
The depressed particle phase advance provides a convenient measure of spacecharge strength
Cold Beam(spacecharge dominated)
Warm Beam(kinetic dominated)
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Parameterization Classes
Possible parameterizations of matched envelope solutions:
Examples from here on assume a symmetric system:
“ Normal” parameterization
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Typical principal orbit functions and corresponding matched envelope functions
Syncopated Quadrupole Lattice
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The betatron consistency condition allows us to construct matched solutions of the KV equations
Consistency Condition:
Used to formulate iterative numerical method for matched envelope soutions
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The continuous limit is employed to seed the numerical method
Period Averages:
Continuous Limit Replacements:
Continuous Limit KV Envelope Equation (xplane):
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Form of solution of continuous limit envelope equations depends on parameters specified
Symmetric System:
Q, ε parameterization:
Q, σ parameterization:
ε, σ parameterization:
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Numerical Iterative Method uses connection between principal orbits and envelope to generate a correction closer to actual matched solution
Notation: denote iteration order with superscript
For iterations , we calculate refinements of the principal orbit functions in terms of the envelope calculated at the previous iteration from
calculated from using consistency condition
Cosinelike initial conditions: Sinelike case analogous
and
Spacecharge defocusing from previous iteration
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Unspecified parameters may be calculated with one or more of the constraint equations below
Depressed Phase Advance:
Period Averaged Envelope Equation:
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Seed Iteration and Cutoff
Seed iteration:
Cutoff: Terminate iterations when
calculated depending on parameterization case
Continuous focusing spacecharge
Note: seed iteration is more accurate than continuous focusing limit
Actual applied focus
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Occupancy η
Syncopation Factor α
Lattice Period
Solenoid descriptioncarried out implicitly inLarmor frame [see Lund and Bukh, PRST AB7, 024801 (2004)]
Example applications solenoid and quadrupole lattices, treating the focusing functions as piecewise constant
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Typical Matched Solutions
Syncopated Quadrupole Lattice (α = 0.1)
Solenoidal Lattice FODO Quadrupole Lattice (α = 0.5)
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Iterative numerical method converges rapidly to matched solution for all parameterizations with specified σ
Iteration 0 Iteration 1
Iteration 2
Syncopated Quadrupole Lattice
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Parameter space plots illustrating the number of iterations necessary to achieve a fractional tolerance of 106
Syncopated Quadrupole Lattice
Envelope Instability Bands
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Tolerance decreases rapidly toward numerical precision
Syncopated Quadrupole Lattice
Tolerance decreases more slowly with:Increasing undepressed phase advanceIncreasing lattice complexity
Machine precision
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Problem: Simplest implementation of Q, ε parameterization fails over approximately half of the parameter space
x = Failure point due to complex
Solenoidal Lattice FODO Quadrupole Lattice:
in iterations: Beam squeezed too hard for given Q; principal orbits overcompensate, grow too large, and yield complex phase advances
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We attempted to implement the Q, ε parameterization in the entire parameter space through several methods
Raise Q until method fails, lower until method works, then increase adaptivelyFound this only works for very slow increases in Q, leading to many iterations
1) Calculate the depressed phase advances via previous iteration integral formula
Converges systematically to unphysical solutions
2) Vary perveance adaptively
3) Hybrid Method
Assume trial σx, σ
y values and find consistent values with specified Q and/or
εx, ε
y using numerical rootfinding
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Fortunately, the Q, ε parameterization can be extended to the entire parameter space by employing hybrid methods
(Q, ε)/(Q, σ) Hybrid
Find satisfying
Then employ Q, σ method
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Conclusions
Has a large basin of attractionConverges rapidlyWorks over entire parameter space, even in regions of strong instabilityApplicable to all linear lattices without skew couplingStraightforward to code
A new iterative method for generating matched envelope solutions to the KV equations has been developed
However, the Q, ε method can be implemented with hybrids
Downside: Direct application of Q, ε parameterization fails in about half of the parameter space
Manuscript submitted to PRSTABPrograms and presentation slides (soon) available online
Extra: