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Integrable Systems for Accelerators

Integrable Systems for Accelerators. Sergei Nagaitsev Feb 7, 2013. Challenges of modern accelerators (the LHC case). LHC: 27 km, 7 TeV per beam The total energy stored in the magnets is HUGE: 10 GJ (2,400 kilograms of TNT)

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Integrable Systems for Accelerators

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  1. Integrable Systems for Accelerators Sergei Nagaitsev Feb 7, 2013

  2. Challenges of modern accelerators (the LHC case) • LHC: 27 km, 7 TeV per beam • The total energy stored in the magnets is HUGE: 10 GJ (2,400 kilograms of TNT) • The total energy carried by the two beams reaches 700 MJ (173 kilograms of TNT) • Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet • LHC vacuum chamber diameter : ~40 mm • LHC average rms beam size (at 7 TeV): 0.14 mm • LHC average rms beam angle spread: 2 µrad • Very large ratio of forward to transverse momentum • LHC typical cycle duration: 10 hrs = 4x108 revolutions • Kinetic energy of a typical semi truck at 60 mph: ~7 MJ S. Nagaitsev, Feb 7, 2013

  3. What keeps particles stable in an accelerator? • Particles are confined (focused) by static magnetic fields in vacuum. • Magnetic fields conserve the total energy • An ideal focusing system in all modern accelerators is nearly integrable • There exist 3 conserved quantities (integrals of motion); the integrals are “simple” – polynomial in momentum. • The particle motion is confined by these integrals. -- particle’s action S. Nagaitsev, Feb 7, 2013

  4. Ideal Penning trap • An ideal Penning trap is a LINEAR and integrable system • It is a harmonic 3-d oscillator S. Nagaitsev, Feb 7, 2013

  5. Kepler problem – a nonlinear integrable system • Kepler problem: • In spherical coordinates: • Example of this system: the Solar system S. Nagaitsev, Feb 7, 2013

  6. Other famous examples of integrable systems • Examples below can be realized with electro-magnetic fields in vacuum • Two fixed Coulomb centers – Euler (~1760) • separable in prolate ellipsoidal coordinates • Starting point for Poincare’s three-body problem • Vinti potential (Phys. Rev. Lett., Vol. 3, No. 1, p. 8, 1959) • separable in oblate ellipsoidal coordinates • describes well the Earth geoid gravitational potential S. Nagaitsev, Feb 7, 2013

  7. Non-integrable systems • At the end of 19th century all dynamical systems were thought to be integrable. • 1885 math. prize for finding the solution of an n-body problem (n>2) • However, nonintegrable systems constitute the majority of all real-world systems (1st example, H. Poincare, 1895) • The phase space of a simple 3-body system is far from simple. This plot of velocity versus position is called a homoclinic tangle. S. Nagaitsev, Feb 7, 2013

  8. Henon-Heiles paper (1964) • First general paper on appearance of chaos in a Hamiltonian system. • There exists two conserved quantities • Need 3 for integrability • For energies E > 0.125 trajectories become chaotic • Same nature as Poincare’s “homoclinic tangle” Michel Henon (1988): S. Nagaitsev, Feb 7, 2013

  9. Particle motion in static magnetic fields • For accelerators, there are NO useful exactly integrable examples for axially symmetric magnetic fields in vacuum: • Example 1: Uniform magnetic field • Example 2: Field of a magnetic monopole • Until 1959, all circular accelerators relied on approximate (adiabatic) integrability. • These are the so-called weakly-focusing accelerators • Required large magnets and vacuum chambers to confine particles; S. Nagaitsev, Feb 7, 2013

  10. The race for highest beam energy • Cosmotron (BNL, 1953-66): 3.3 GeV • Produced “cosmic rays” in the lab • Diam: 22.5 m, 2,000 ton • Bevatron (Berkeley, 1954): 6.3 GeV • Discovery of antiprotons and antineutrons: 1955 • Magnet: 10,000 ton • Synchrophasatron(Dubna,1957): 10 GeV • Diam: 60 m, 36,000 ton • Highest beam energy until 1959 S. Nagaitsev, Feb 7, 2013

  11. Strong Focusing S. Nagaitsev, Feb 7, 2013

  12. CERN PS • In Nov 1959 a 28-GeV Proton Synchrotron started to operate at CERN • 3 times longer than the Synchrophasatron but its magnets (together) are 10 times smaller (by weight) S. Nagaitsev, Feb 7, 2013

  13. Strong Focusing – our standard method since 1952 Christofilos (1949); Courant, Livingston and Snyder (1952) Particle undergoes betatron oscillations -- piecewise constant alternating-sign functions s is “time” 13 S. Nagaitsev, Feb 7, 2013

  14. Strong focusing Specifics of accelerator focusing: • Focusing fields must satisfy Maxwell equations in vacuum • For stationary fields: focusing in one plane while defocusing in another • quadrupole: • However, alternating quadrupoles • results in effective focusing in both planes S. Nagaitsev, Feb 7, 2013

  15. Courant-Snyder invariant Equation of motion for betatron oscillations Invariant (integral) of motion, a conserved qty. S. Nagaitsev, Feb 7, 2013

  16. Simplest accelerator focusing elements • Drift space: L – length • Thin quadrupole lens: S. Nagaitsev, Feb 7, 2013

  17. Simple periodic focusing channel (FODO) • Thin alternating quadrupole lenses and drift spaces • Let’s launch a particle with initial conditions x and x’ • Question: • Is the particle motion stable (finite)? D F D F D D F L L L L L L s particle (x, x') F L L D …Equivalent to: S. Nagaitsev, Feb 7, 2013

  18. Particle stability in a simple channel • Possible answers: • Always stable • Stable only for some initial conditions • Stable only for certain L and F D F D F D D F L L L L L L s particle (x, x') S. Nagaitsev, Feb 7, 2013

  19. Particle stability in a simple channel • Correct answer: • Always stable • Stable only for some initial conditions • Stable only for certain L and F D F D F D D F L L L L L L s particle (x, x') Stability: S. Nagaitsev, Feb 7, 2013

  20. Phase space trajectories: x’ vs. x F = 0.51, L = 1 50 periods, stable traject. F = 0.49, L = 1 7 periods, unstable traject. 5 3 2 x’ x’ 1 4 6 x x When this simple focusing channel is stable, it is stable for ALL initial conditions ! …And, ALL particles are isochronous: they oscillate with the same frequency (betatron tune)! F = 1.2, L = 1 1000 periods stable traject. -- Courant-Snyder invariants describe phase-space ellipses S. Nagaitsev, Feb 7, 2013

  21. Landau damping • Landau damping – the beam’s “immune system”. It is related to the spread of betatron oscillation frequencies. The larger the spread, the more stable the beam is against collective instabilities. • The spread is achieved by adding special magnets -- octupoles • External damping (feed-back) system – presently the most commonly used mechanism to keep the beam stable. Report at HEAC 1971 S. Nagaitsev, Feb 7, 2013

  22. Most accelerators rely on both • LHC: • Has a transverse feedback system • Has 336 Landau Damping Octupoles • Octupoles (an 8-pole magnet): • Potential: • cubic nonlinearity (in force) S. Nagaitsev, Feb 7, 2013

  23. Let’s add a weak octupole element… D F D F D D F L L L L L L s particle (x, x') add a cubic nonlinearity S. Nagaitsev, Feb 7, 2013

  24. The result of a nonlinear perturbation: • Betatron oscillations are no longer isochronous: • The frequency depends on particle amplitude • Stability depends on initial conditions • Regular trajectories for small amplitudes • Resonant islands (for larger amplitudes) • Chaos and loss of stability (for large amplitudes) S. Nagaitsev, Feb 7, 2013

  25. Example 2: beam-beam effects • Beams are made of relativistic charged particles and represent an electromagnetic potential for other charges • Typically: • 0.001% (or less) of particles collide • 99.999% (or more) of particles are distorted S. Nagaitsev, Feb 7, 2013

  26. Beam-beam effects • One of most important limitations of all past, present and future colliders Luminosity Beam-beam Force beam-beam S. Nagaitsev, Feb 7, 2013

  27. Example 3: electron storage ring light sources • Low beam emittance (size) is vital to light sources • Requires Strong Focusing • Strong Focusing leads to strong chromatic aberrations • To correct Chromatic Aberrations special nonlinear magnets (sextupoles) are added dynamic aperture limitations lead to reduced beam lifetime S. Nagaitsev, Feb 7, 2013

  28. Summary so far… • The “Strong Focusing” principle, invented in 1952, allowed for a new class of accelerators to be built and for many discoveries to be made, e.g.: • Synchrotron light sources: structure of proteins • Proton synchrotrons: structure of nuclei • Colliders: structure of elementary particles • However, chaotic and unstable particle motion appears even in simplest examples of strong focusing systems with perturbations • The nonlinearity shifts the particle betatron frequency to a resonance (nω = k) • The same nonlinearity introduces a time-dependent resonant kick to a resonant particle, making it unstable. • The driving term and the source of resonances simulteneosly S. Nagaitsev, Feb 7, 2013

  29. Linear vs. nonlinear • Accelerators are linear systems by design (freq. is independent of amplitude). • In accelerators, nonlinearities are unavoidable (SC, beam-beam) and some are useful (Landau damping). • All nonlinearities (in present rings) lead to resonances and dynamic aperture limits. • Are there “magic” nonlinearities with zero resonance strength? • The answer is – yes (we call them “integrable”) 3D: S. Nagaitsev, Feb 7, 2013

  30. Our research goal • Our goal is to create practical nonlinear accelerator focusing systems with a large frequency spread and stable particle motion. • Benefits: • Increased Landau damping • Improved stability to perturbations • Resonance detuning S. Nagaitsev, Feb 7, 2013

  31. Nonlinear systems can be more stable! • 1D systems: non-linear (unharmonic) oscillations can remain stable under the influence of periodic external force perturbation. Example: • 2D: The resonant conditions are valid only for certain amplitudes. Nekhoroshev’s condition guaranties detuning from resonance and, thus, stability. S. Nagaitsev, Feb 7, 2013

  32. Tools we use • Analytical • Many examples of integrable systems exist; the problem is to find examples with constraints (specific to accelerators) • Help from UChicago welcome • Numerical • Brute-force particle tracking while varying focusing elements; • Evolution-based (genetic) optimization algorithms; • Spectral analysis, e.g. Frequency Map Analysis; • Lyapunov exponent • Experimental • Existing accelerators and colliders; • A proposed ring at Fermilab: IOTA (Integrable Optics Test Accelerator) S. Nagaitsev, Feb 7, 2013

  33. Frequency Map Analysis (FMA) LHC FMA b*=15cm q=590mrad Dp/p=0, sz=7.5cm LHC head-on collisions (D Shatilov, A.Valishev) S. Nagaitsev, Feb 7, 2013

  34. Advanced Superconductive Test Accelerator at Fermilab IOTA ILC Cryomodules Photo injector S. Nagaitsev, Feb 7, 2013

  35. Integrable Optics Test Accelerator Beam from linac S. Nagaitsev, Feb 7, 2013

  36. IOTA schematic • pc = 150 MeV, electrons (single bunch, 10^9) • ~36 m circumference • 50 quadrupoles, 8 dipoles, 50-mm diamvac chamber • hor and vert kickers, 16 BPMs Injection, rf cavity Nonlinear inserts S. Nagaitsev, Feb 7, 2013

  37. Why electrons? • Small size (~50 um), pencil beam • Reasonable damping time (~1 sec) • No space charge • In all experiments the electron bunch is kicked transversely to “sample” nonlinearities. We intend to measure the turn-by-turn beam positions as well as synch light to obtain information about phase space trajectories. S. Nagaitsev, Feb 7, 2013

  38. Experimental goals with nonlinear lenses • Overall goal is to demonstrate the possibility of implementing nonlinear integrable optics in a realistic accelerator design • Demonstrate a large tune shift of ~1 without degradation of dynamic aperture • Quantify effects of a non-ideal lens • Develop a practical lens design. S. Nagaitsev, Feb 7, 2013

  39. On the way to integrability: McMillan mapping • In 1967 E. McMillan published a paper • Final report in 1971. This is what later became known as the “McMillan mapping”: • Generalizations (Danilov-Perevedentsev, 1992-1995) If A = B = 0one obtains the Courant-Snyder invariant S. Nagaitsev, Feb 7, 2013

  40. McMillan 1D mapping • At small x: Linear matrix: Bare tune: • At large x: Linear matrix: Tune: 0.25 • Thus, a tune spread of 50-100% is possible! A=1, B = 0, C = 1, D = 2 S. Nagaitsev, Feb 7, 2013

  41. What about 2D? • How to extend McMillan mapping into 2-D? • Two 2-D examples exist: • Both are for Round Beam optics: xpy- ypx = const • Radial McMillan kick: r/(1 + r2) -- Can be realized with an “Electron lens” or in beam-beam head-on collisions • Radial McMillan kick: r/(1 - r2) -- Can be realized with solenoids • In general, the problem is that the Laplace equation couples x and y fields of the non-linear lens • The magnetic fields of a dipole and a quadrupole are the only uncoupled example S. Nagaitsev, Feb 7, 2013

  42. Accelerator integrable systems • Two types of integrable systems with nonlinear lenses • Based on the electron (charge column or colliding beam) lens • Based on electromagnets Major limiting factor! The only known exact integrable accelerator systems with Laplacian fields: Danilov and Nagaitsev, Phys. Rev. RSTAB 2010 S. Nagaitsev, Feb 7, 2013

  43. Experiments with an electron lens • 5-kG, ~1-m long solenoid • Electron beam: ~0.5 A, ~5 keV, ~1 mm radius solenoid 150 MeV beam Example: Tevatron electron lens Electron lens current density: S. Nagaitsev, Feb 7, 2013

  44. Experiment with a thin electron lens • The system consists of a thin (L < β) nonlinear lens (electron beam) and a linear focusing ring • Axially-symmetric thin McMillan lens: • Electron lens with a special density profile • The ring has the following transfer matrix electron lens S. Nagaitsev, Feb 7, 2013

  45. Electron lens (McMillan – type) • The system is integrable. Two integrals of motion (transverse): • Angular momentum: • McMillan-type integral, quadratic in momentum • For large amplitudes, the fractional tune is 0.25 • For small amplitude, the electron (defocusing) lens can give a tune shift of ~-0.3 • Potentially, can cross an integer resonance S. Nagaitsev, Feb 7, 2013

  46. Practical McMillan round lens e-lens (1 m long) is represented by 50 thin slises. Electron beam radius is 1 mm. The total lens strength (tune shift) is 0.3 FMA analysis All excited resonances have the form k ∙ (x + y) = m They do not cross each other, so there are no stochastic layers and diffusion S. Nagaitsev, Feb 7, 2013

  47. Recent example: integrable beam-beam A particle collides with a bunch (charge distribution) Integrable bunch distribution Gaussian non-integrable S. Nagaitsev, Feb 7, 2013

  48. FMA comparison Integrable Gaussian non-integrable S. Nagaitsev, Feb 7, 2013

  49. Main ideas Integrable systems with nonlinear magnets • Start with a time-dependent Hamiltonian: • Chose the potential to be time-independent in new variables • Find potentials U(x, y) with the second integral of motion and such that ΔU(x, y) = 0 See: Phys. Rev. ST Accel. Beams 13, 084002 (2010) S. Nagaitsev, Feb 7, 2013

  50. Integrable 2-D Hamiltonians • Look for second integrals quadratic in momentum • All such potentials are separable in some variables (cartesian, polar, elliptic, parabolic) • First comprehensive study by Gaston Darboux (1901) • So, we are looking for integrable potentials such that Second integral: S. Nagaitsev, Feb 7, 2013

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