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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 1 Micro bunch evolution and „turbulent beams“ Sabrina Appel, Oliver Boine-Frankenheim, Thomas Weiland Beam physics for FAIR Bastenhaus, 6 July 2010
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 2 Outline Introduction and Motivation Beam quality Multi-stream instability Longitudinal space charge Costing beam Bunch beam Micro bunch evolution Envelope equation Simulation Measurement of the micro bunch parameter Summary
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 3 Micro bunch evolution Measurement at SIS18 Time signal and frequency spectrum U 28+ SIS18 f SIS = 214 kHz T SIS = 4.67 μs ≈ 1 turn injection + ‘Schottky’ pickup (≈ 10-100 MHz) f UNILAC = 36 MHz BB12 ‘Micro bunch’ pickup (≈ 32-500 MHz) + Long. Emittance measurement E10 The long. beam quality is essential Limit by the rf bucket σ should be routinely measure Difficult: Mutli-stream instability The initial micro bunch parameter are imporant They depends on Injected longitudinal beam quality (form UNILAC) Space charge effects in the transfer channel Manipulated by two buncher cavities at the TK
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 4 Measurement results Micro bunch structure ~ 1 turns inj. U 73+ N ≈8×10 7 low current debunching time: theoretical:
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 5 Momentum spread dp/p: σ >0.5 ×10 -3 Jump of momentum spread at 3 mA Momentum variation with current Measurement results Momentum spread dependence on UNILAC current Momentum spread σ Momentum spread σ / 10 -3 UNILAC Current / mA FAIR design value August 09 Momentum spread σ / 10 -3 + Measured at 12 MHz ∆ Measured at 25 MHz ♢ Measured at 20 MHz Turns Experiment: Constant UNILAC currents, number of injected turns is varied Momentum spread increasing with the number of injected turns ⇒ parasitic loss in TK or broad band impedance in SIS18+ multi-stream instability? Ar 18+
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 6 Theoretical Model Filamentation of Phase Space Single turn injection Because the rf is off the debunching of bunches causes filamentation in phase space Space charge leads to multi-steam instabilities A turbulent coherent current fluctuation spectrum develops Length Δp/p
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 7 Multi-stream instability With critical number of filaments: Time to reach M thr -filaments: SimulationMeasurement
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 8 Longitudinal Space Charge For an arbitrary perturbation of costing beam : A. M. Al-khateeb, Analytical calculation of the longitudinal space charge and resistive wall impedances in a smooth cylindrical pipe, PRE 2001 Cut-off harmonic: Z 0 =337 Ω vacuum impedance
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 9 Stokes ‘ Law: With radial E-Field and azimuthal B-Field Final result: With geometry (integration) factor: Longitudinal Space Charge
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 10 Longitudinal Space Charge Longitudinal electric field for a ellipsoidal bunch Nonlinearity of E z for long bunches The g-Factor dependent on bunch length for dependent for short bunches Reiser, Theory and design of charged particle beams Some literature:
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 11 Geometry-factor for bunched beams 3 D calculation with EM-Studio Linear fit on E z to estimate the g-factor Short bunches the field is linear For long bunches became nonlinear Long bunch g=2.2 b/a=2 Short bunch g=0. 6 b/a=2
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 12 SIS18 region Geometry-factor for bunched beams b/a=2 z m /a=5 z m =6 m PIC-Code: Describe approximate the longitudinal E-Field Discrepancy at the end, but there less particles A constant geometry-factor can be used for SIS18 simulation
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 13 SIS18 region Geometry-factor for bunched beams Also on the 3D calculation with EM-Studio linear fit for g Bunches with offset: Dependency of g-factor for small offset is near to analytic solution and can be negligible We found for a costing beam a modified g-factor The dependency is logarithmic
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 14 Envelope equation for linear rf (Φ<90°) and space charge force: Longitudinal Simulations codes Simulation for linear space charge effects in TK 160 m E10B12 TK SIS18 UNILAC z m bunch length, RF is a focusing force Space charge force and emittance term have defocusing effects. The code is used to optimize the buncher voltages for low momentum spread. Bunchers are optimized for no current: Weak space charge Space charge dominant s / m ϕ /deg ( f=36 MHz) Linear rf (Φ<90°)
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 15 The integration of the Envelope equation give the longitudinal invariant With the Total momentum spread Total momentum spread after a long drift follow: Envelope equation Total momentum spread
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 16 Longitudinal emittance measurement To get small transversal plane the rhomb and quadrupoles is used The dispersion is needed to monitor the energy spread (normal compensated) To gain the phase distribution (or length) rf-chopper is used
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 17 Longitudinal emittance measurement l3l3 Linear transformation: D G : Dispersion on Profile grid F C : 47.93 °Vm/mm u A c : Chopper Amplitude f c =108 MHz: Copper frequency A: Mass of Ion in u
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 18 Longitudinal emittance measurement In the TK the max. momentum spread larger than the FAIR design value With buncher the max. momentum in SIS18 lower than the FAIR design value
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 19 Longitudinal emittance measurement Initial parameter for Code the measurement parameter of micro bunch Bunchers are optimized for I=3mA For lowest current in agreement For the other currents the measured max. momentum higher than as expected
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6 July 2010 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Sabrina Appel | 20 Summary ▪ Introduction and Motivation -Beam quality -„turbulent beams“ (Multi-stream instability) ▪ Longitudinal space charge -For costing beam -Bunch beam ▪ Micro bunch evolution -Envelope equation -Simulation -Measurement of the micro bunch parameter ▪ Summary
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