High intensity heavy-ion synchrotrons Jens Stadlmann / Primary Beams Div. Heraeus Seminar 16.10.2012
Contents • Overview and some historic machines. • Basic principle of synchrotrons, - Layout, - principle of acceleration -> RF, - transverse motion, - tune and tune-shift. • Going to highest intensities with heavy ions, - Major obstacle: vacuum control. • Conclusion.
Some synchrotron-history • Proposal of a pulsed magnet ring by Oliphant 1943. • Discovery of phase stability by Veksler 1944 and McMillan 1945. • Electron synchrotron demonstration by Goward and Barnes 1946 at Woolwich Aresanl, UK. • Two month later General Electric Laboratory’s 70 MeV electron machine at Schenectady, USA, by Elder, Gurewitsch, Langmuir and Pollock. The first proton machine were build in the early 50s. Alternate gradient focussing and colliders followed. (All dates taken from E.J.N. Wilson, “Fifty Years of Synchrotrons)
„Living history“: PS and AGS PS @ CERN AGS @ Brookhaven 28 GeV p, late 1959 33 GeV p, Summer 1960 CERN 1959, PS to the right Alternating Gradient Synchrotron under construction, c. 1957. Both machines are still operational today! They accelerate ions and protons. They feed beams into SPS/LHC (PS) and RHIC (AGS) and are documenting the success story of synchrotron accelerators.
Collider I: Brookhaven Nat’l Labs, Long Island, USA RHIC: Relativistic Heavy Ion Collider AGS: Alternating Gradient Synchrotron
Collider II; CERN Large Hadron Collider (LHC) Protons and heavy ions (Pb) Energy: now 3.5 TeV (up to 7 TeV later) Protons in the ring: 3E14 Current: 0.5 A Circumference: 27 km Beam energy: 3 MJ Magnetic dipole field: 8 T
Special synchrotrons: Collider If the incoming beam is simply slammed into a stationary target, much of the energy is taken up by the target's recoil.
Intensity I: Neutron Spallation Source, SNS in Oakridge, USA Ion: Protons Energy: 1 GeV (0.88c) ppp: 10E14 Rep. Rate: 60 Hz 100 m Beam power: 2 MW
Intensity II: J-PARC, Japan J-PARC was heavily affected by the earthquake in March 2011 ! Damage has been repaired and the facility is working again. 9
Working principle I: "Schwer Ionen Synchrotron": SIS RF station Revolution frequency: ω 0 = qB 0 γ m = v s R Design momentum: p s = γ mv s = qB 0 R 0 • Constant orbit radius • Variable magnetic fields • ‚Synchronous‘: h ω 0 = ω RF • Pulsed beams SIS: B ρ =18 Tm RF station ω RF
Working principle of a synchrotron II Synchrotron cycle Repetition rate (T rep ) -1 : Types of synchrotrons (a bit arbitrary): (time needed for one complete cycle) -1 - slow cycling synchrotron: < 1 Hz Total beam energy: W tot = NW - fast cycling synchrotron: 1-10 Hz kin beam = W - Rapid Cycling Synchrotron (RCS): > 10 Hz Beam power: tot P T rep 11
RF: Phase Stability and Longitudinal Focusing •dp/p=0: no change Result: •dp/p<0: more accelerated phase focusing, •dp/p>0: less accelerated and oscillation around (dp/p=0) so called synchrotron oscillations
RF special I: Dual harmonic rf buckets Dual rf systems are employed e.g. in: CERN PSB, ISIS, J-PARC RCS, GSI SIS-18.. Advantages: Example case SIS-18: V 0 =40/16 kV, h=2/4 (f min =430/860 kHz) - flattened bunches (lower peak current) SIS-18: Dual rf bucket with flattened bunch profile - larger bucket area B f = 0.35 profile Complication: bunch - control of the phase difference bucket boundary - ‘fully nonlinear synchrotron oscillations’ φ s =45 0 ω s ( φ ) -> dedicated RF talk by H. Klingbeil 13
RF special II: Fast bunch compression Bunch rotation in SIS-18 For applications e.g. in 8 ɺ φ f 1.0 nuclear physics a single, x10 -3 0.85 short bunch is extracted 0.75 4 -3 0.62 (p-p 0 )/p 0 , 10 to the production target. ∆ W 0.50 φ i 0 0.37 W 0.31 Bunch rotation: 0.25 -4 0.12 Sudden switch-on of an additional 0.070 rf voltage causes the bunch to 0 -8 -400 -200 0 200 400 rotate in the bucket. τ = ∆ φ t - t syn , ns [ns] ω rf The compression takes time extract pre- compression compression only a quarter of a -ion synchrotron period. Current, arb.u. 85 ns Final bunch length rot = T s T 4 < 1ms depends on the initial momentum spread ! -> (broadband) rf cavity with fast rise time needed ! -400 -200 0 200 400 t-t syn , ns 14
Transverse motion in dipole magnets l Horizontal particle offset: x x ' = dx θ Divergence: p = p 0 + ∆ p ds Path length: s = β 0 ct ⊗ B y x ∆ θ = θ ∆ p ∆ p x = 1 ⇒ ′′ s p R p 0 ideal particle θ ∆ p x + 1 R 2 x = 1 ′′ θ = q ds ≈ l s ∫ R p 0 2 B y p 0 R s ‘weak’ inhomogeneous 1 0 focusing part 15
Rapid/fast ramping dipole magnets Examples Ramping rates (Bdot): Fast ramping (3 Hz) SIS-18 dipoles Large apertures SIS-18 dipoles: 10 T/s SIS-18 dipoles: 20 cm x 8 cm J-PARC RCS dipoles: 40 T/s J-PARC RCS: 25 cm x 19 cm Max. B-Field SIS-18: 1.8 T J-PARC RCS: 1.1 T SIS-100 superferric dipole: 13 cm x 6 cm Fast ramping ‘cold’ magnet J-PARC RCS (25 Hz) dipole Bdot = 4 T/s of the nuclotron-type B max = 1.9 T cryostat pipe at 20 K (as cryo- pump) magnet 16
Quadrupole magnets and beam focusing Quadrupole magnets at GSI Magnetic field: x y Equations of motion: a , B y = B 0 B x = B 0 a x + κ ( s ) x = 0 ′′ (horizontal) Focusing gradient: ∂ B y ∂ B x κ = q ∂ y = q (vertical) y − κ ( s ) x = 0 ′′ ∂ x p 0 p 0 17
Alternating Gradient Focusing Focusing De-focusing Focusing Drift Drift quadrupole quadrupole quadrupole ���������������������������� ��������������������������� ��������������������������� ��������������������������� � � ������������������������������������������������ ����������������������������������������������� ����������������������������������������������� �����������������������������������������������
Particles on the run, have a look 3 Quadrupoles 2 Dipoles Beamdirection by P. Puppel Jens Stadlmann | GSI Summer Student Lecture 2011
Errors: The ions stray from the ideal path Betatron oscillation: The errors of the dipoles are additive resulting amplitude (GSI's SIS18): Number of betatron oscillations per turn is the "tune" (Q).
Tune and resonances Order of resonance: |n+m|
Space charge tune shift Beam in vacuum tube E r a: beam radius charge + current space charge- ‚diamond‘ Space charge tune spread (e.g. CAS, A. Hofmann): g f : Transverse profile (Gauss: 2, homogenous: 1) sc ∝ − q 2 g f N 2 ∆ Q y B f < 1: bunching factor 2 γ 0 3 m B f ε y β 0 ε y ε x 1 + ε x,y : transverse emittances N: number of particles in the ring q: particle charge ‘Space charge limit’: m: particle mass (text books) 22
(Slow) extraction Slow extraction examples: GSI SIS-18 and SIS-100, J-PARC MR, BNL AGS Sextupole: Fast extraction: in one turn using a kicker (e.g. after bunch compression.) Slow extraction: over many turns (up to seconds !). The horizontal tune is moved close to a third order resonance excited by sextupole magnets. Separatrix (third order resonance) The particles on the resonance are extracted using electrostatic and magnetic septa. Septum should be as thin as possible to avoid losses ! Septum wires: Ø 0.025 mm (W-Re alloy) wires are mounted under tension SIS-18 septum 23
FAIR - Facility for Antiproton and Ion Research Proton Linac Improvements SIS 100/300 • Primary intensities: factor 100 – 1000 • Secondary radiocative beams: up to factor 10 000 HESR • Ion energy: factor 34 Super- FRS Special properties • Intense cooled radioactive ion beams, Upgrade of the • cooled anti-proton beams up to 15 GeV, present accelerators • Internal high luminosity targets in FLAIR storage rings. New technology CR + RESR • Fast ramped superconducting magnets NESR • Electron cooling for high intensity and high-energy beams. • Fast stochastic cooling
SIS 18 as Injector for SIS 100 SIS 18 as Injector for SIS 100 � 1,5x10 11 SIS 18 � 2x10 10 U 28+ U 73+ U 28+ SIS 100 cycle Choice of U 28+ + Lower space charge 0.7Hz 5x10 11 U 28+ higher intensity N max ~A/Q 2 + No stripping losses 2.7Hz SIS 18 - Lower beam lifetime
Life Time and Beam Loss: Life Time and Beam Loss: XHV is the key to heavy ion acceleration XHV is the key to heavy ion acceleration Life time of U 28+ is significantly lower Ion induced gas desorption ( η≈ 10 000) than of U 73+ increases the local pressure Beam loss increases over propotional Life time of U 28+ depends strongly on the with intensity -> Dynamic vacuum residual gas pressure and composition
What happened? - Dynamic vacuum! No cure by good initial vacuum or pumping power alone! 2.Mai 2012 L.Bozyk 27 27
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