Transcript TRANSITION-FREE LATTICES
TRANSITION-FREE LATTICES
WG1 July 4 NuFact02 Imperial College, London July 1-6, 2002 C Johnstone and B. Eredelyi Fermilab NuFact02 Imperial College, London July1-6, 2002 *special thanks to A. Thiessan
TOPICS
1. What is transition in an accelerator definition and reality 2. Transitionless Lattices: types 3. Performance: Resonance and Dynamic Aperture (DA) studies of Transitionless Lattices 4. Recent results on Proton Driver Lattices; comparison with standard FODO 5. Preliminary Conclusions
Transition:
Transition is defined as the point during acceleration where there is no deviation in the revolution period as a function of momentum: D T/T 0 = (1/ g t 2 –1/ g 2 ) D p/p 0 = 0 where g is the Lorentz relativistic factor for the synchronous on-momentum particle and g t is a property of the lattice optics: 1/ g t 2 = 1/C 0 D(s)/ r (s) ds where D is the momentum-dispersion function, s is the longitudinal coordinate, and r is the radius of curvature (in dipoles only) So, transition in a lattice is changed by controlling the dispersion function location of dipoles
In general, transition decreases with cell length , keeping the phase advance constant; I.e. dispersion function decreases.
BUT: A standard FODO cell lattice for the Proton Driver would require ultra short cells with inter-quad spacing of only ~4m .
Clearly this is not an option, If only from beam injection/extraction standpoint
Types of Transitionless Lattices
Missing-Dipole FODO ** Based on the standard FODO module--but dipoles are removed from high dispersion regions Transition increases both from the “missing” dipoles, but also from the decrease in the dispersion function which occurs.
Strong-focussing Control the dispersion function by increasing the horizontal focussing strength over and above the FODO through a horizontal low-beta insertion Regions of negative dispersion can created, often driving transition imaginary (historically referred to as Flexible Momentum Compaction or FMC module).
**a more recent doublet lattice by G. Reese is not studied in this work
Comparative Features: Missing Dipole and Strong Focussing Lattices
Missing Dipole FODO Strong Focussing (FMC) Simplest Structure Lowered Dipole packing : requires spaces in arcs Limited range in g t Dispersion suppression: standard or phase-induced Shortened utility straights Low Beta Insertion Standard Dipole packing: comparable to standard FODO Large range in g t (real-imaginary) Dispersion suppression: generally efficient More generous utility straights
Example: FODO-based Missing Dipole Arc Module
Example: Strong Focussing Arc module, low
b
x insert (original FMC)
Example: Strong Focussing Arc module with added low
b
y insert
for increased g
t
Example: Strong Focussing Arc for 8GeV Ring
g
t
ranges from 11i to 14i for this design
Arrangements of Sextupoles in the three configurations: standard FODO vs. the PD
Horz. Sextupole locations Vert. Sextupole locations
Performance: FODO-based arc module Total arc module phase advance of 270° **
First, look at 3 x standard FODO cells with standard chromatic correction with x , y = 0.750000 (x 2 ) DA is almost nonexistent due to 4th and other HO order resonances For the Proton Driver module tune 0.75 the map is not as clear, but later tracking results showed an unacceptable sensitivity to any changes in its nonlinear composition.
**there is an enhancement in the DA at phase advances which are odd multiples of 90 °, 0.25, and 0.75, for example
Arc module tune How far from 0.75?
Answer: > ±0.03
0.75 coefficients order exp. New coefficients 19 -15.37743243630632 4 4 0 0 0 -4.685819636401535 20 875.6921990923597 4 3 1 0 0 4846.570946996038 21 -152602.0766201559 4 2 2 0 0 4915.864249415151 22 -1551378.006500757 4 1 3 0 0 -340048.9047066236 23 -2949880.021197357 4 0 4 0 0 -19231593.78153443 24 -203.7740240642712 4 2 0 2 0 -64.21627192997873 25 -11429.99602178208 4 1 1 2 0 -7178.460145068667 26 -216025.9940977090 4 0 2 2 0 -199617.5814816107 27 -3322.702237255702 4 2 0 1 1 -1227.186498329944 28 -55510.77100820898 4 1 1 1 1 11255.74739937733 29 -1407927.049676028 4 0 2 1 1 -320886.3452778102 30 -8596.828047477644 4 2 0 0 2 -3998.749005011314 31 10069.93733752568 4 1 1 0 2 126330.6797013642 32 -2069862.948423631 4 0 2 0 2 -570823.2944196387 33 -607.0883626214262 4 0 0 4 0 -377.6830629790347 34 -3767.383725082195 4 0 0 3 1 -1831.664889008636 35 -35265.17142457649 4 0 0 2 2 -5245.639853101801 36 -60166.60709072684 4 0 0 1 3 42101.58355180608 37 -2342.472689445537 4 0 0 0 4 19035.25442672397
Tracking Performance: Impact of Tune Change from
x , y = 0.75 to 0.72 Standard FODO: displays about half the dynamic aperture of the Proton Driver module Proton Driver module: very slightly enhanced at new module tune The primary nonlinear components, the chromatic correction sextupoles comprise two orthogonal (90 °) families in both lattices, so Why the dramatic improvement in DA of the proton driver module over the standard FODO?
Hypothesis: If this enhancement is due solely to the chromatic correction sextupoles, then of their location and definition dictate the performance this lattice.
Test: Relocation of the sextupoles in a standard 3 cell FODO should reproduce this effect.
Performance: Impact of Chromatic Correction Sextupole Placement
To study the role of the chromatic correction sextupoles in the PD lattice, three modules were studied and compared: FODO 3-cell module with standard sextupole familes (two per plane) PD arc module, which,again, has two sextupole families per plane, but are fewer in number and have different relative phasing between planes Rearranged FODO 3 FODO cells with sextupoles placed in the same relative location (phase advance) as in the PD arc module.
Arrangements of Sextupoles in the three configurations: standard FODO, PD, and rearranged FODO
Horz. Sextupole locations Vert. Sextupole locations
Performance: Rearranged FODO
Rearranged FODO shows identical DA to the PD arc module: it is increased by a factor of 2 over the normal arrangement.
Confirmation that the large DA evidenced in the PD lattice is an artifact of the exact sextupole arrangement used.
How will such a lattice perform to a change in the nonlinear optics?
Nonlinear Performance: Sensitivity to the chromatic correction sextupoles
Turn off the horizontal sextupoles:
DA aperture in FODO increases in both planes, as expected, from the removal of a nonlinearity.
In the PD module, for
x ,
y = 0.75, the vertical DA drops by an order of magnitude (from 15 cm to 1.5 cm); I.e. removal of a strong nonlinearity causes a tremendous decrease in the acceptance of the machine.
In the PD module, for
x ,
y = 0.72
, the decrease is still unacceptable, but is now 30%.
The Rearranged FODO verifies the unexpected decrease in DA WHAT IS GOING ON?
Nonlinear Performance and DA: FODO
Standard sextupole placement in a 90 ° FODO lattice is relatively insensitive to sextupole cross correlations: The two planes are strongly independent of each other Removal of nonlinearities results in the expected enhancement of performance.
Nonlinear Performance and D A: PD
PD module tune: x , y = 0.750000
singular solution in a nonlinear optics regime:
DA relies entirely on a delicate cancellation of tuneshift contributions between sextupole families in different planes. This solution has no inherent stability.
In this particular module, the vertical DA shows an extreme dependence on the horizontal sextupoles, but the reverse is not true, the horizontal DA is not sensitive to the vertical sextupoles (the horizontal sextupole placement is more near the standard).
Nonlinear Performance and DA: PD
PD module tune: x , y = 0.72
more robust, but performance still dictated strongly by the nonlinear rather than the linear optics :
DA still relies on cancellation of nonlinear terms between sextupole families in different planes. These conclusions hold for the rearranged FODO.
How will this delicate balance of nonlinear behavior withstand the introduction of nonlinearities: such as unavoidable magnet field errors * ?
IT DOESN’T- Dramatic decrease in DA, up to an order of magnitude in PD module performance.
No significant tune dependence of DA in any implementation of the arc module. (0.75, or 0.25 is no longer a magic tune) No significant performance difference between the arc modules. *magnet field errors are taken from the MI design report
Preliminary Conclusions
Lattices which rely on the delicate cancellation of nonlinear terms do not survive.
The DA enhancement of such lattices is artificial.
All of the modules show identical performance after introducing MI magnet errors