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Particle Accelerators

Particle Accelerators

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Particle Accelerators

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  1. Particle Accelerators Eric Prebys FNAL Accelerator Physics Center

  2. Outline • History and movitation for accelerators • Basic accelerator physics concepts • Overview of major accelerators • emphasis on LHC • Other uses for accelerators • The future • Crazy ideas Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  3. Some pre-history • The first “particle physics experiment” told Ernest Rutherford the structure of the atom (1911) • In this case, the “accelerator” was a naturally decaying 235U nucleus • The first artificial acceleration of particles was done using “Crookes tubes”, in the latter half of the 19th century • These were used to produce the first X-rays (1875) Study the way radioactive particles “scatter” off of atoms Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  4. Discovery: It’s all about energy and collision rate • To probe smaller scales, we must go to higher energy • To discover new particles, we need enough energy available to create them • The rarer a process is, the more collisions (luminosity) we need to observe it. 1 fm = 10-15 m (Roughly the size of a proton) Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  5. Another way to look at it: a window back in time Accelerators allow us to probe down to a few picoseconds after the Big Bang! Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  6. Natural particle acceleration Max LHC energy • Radioactive sources produce maximum energies of a few million electron volts (MeV) • Cosmic rays reach energies of ~1,000,000,000 x LHC but the rates are too low to be useful as a study tool • Remember what I said about luminosity! • On the other hand, low energy cosmic rays are extremely useful • But that’s another talk Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  7. Man-made particle acceleration The simplest accelerators accelerate charged particles through a static electricfield. Example: vacuum tubes (or CRT TV’s) Cathode Anode • Limited by magnitude of static field: • - TV Picture tube ~keV- X-ray tube ~10’s of keV- Van de Graaf ~MeV’s • Solutions: • Alternate fields to keep particles in accelerating fields -> RF acceleration • Bend particles so they see the same accelerating field over and over -> cyclotrons, synchrotrons FNAL Cockroft-Walton = 750 kV

  8. Longitudinal motion: phase stability Particles are typically accelerated by radiofrequency (“RF”) electric fields. Stability depends on particle arrival time relative to RF phase  “bunched” beams If momentum (path length) dominates If velocity dominates “bunch” Particles with lower E arrive earlier and see greater V. Particles with lower E arrive later and see greater V. Nominal Energy Nominal Energy Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  9. Examples of RF structures ILC prototype elipical cell “p-cavity” (1.3 GHz): field alternates with each cell JLab compact “toaster cavity” (400MHz): low frequency in a limited space Fermilab Drift Tube Linac (200MHz): oscillating field uniform along length Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  10. Round we go: the first cyclotrons • 1930 (Berkeley) • Lawrence and Livingston • K=80KeV • 1935 - 60” Cyclotron • Lawrence, et al. (LBL) • ~19 MeV (D2) • Prototype for many Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  11. Things keep getting bigger • 60” cyclotron (1935) • Berkeley and elsewhere • Fermilab • Radius = 1km • Built ~1970 • Upgraded ~1985, ~1997 • Until recently, the most powerful accelerator in the world. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  12. Bigger still: The LHC • Tunnel originally dug for LEP • Built in 1980’s as an electron positron collider • Max 100 GeV/beam, but 27 km in circumference!! • Now we’ll talk a little about how these things work… My House (1990-1992) /LHC Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  13. Warning! • The next few slides contain a lot of mathematical detail. • They’re not meant to be fully absorbed real time by everyone. • I’ll follow them with a “glossary”, which will qualitatively summarize the key concepts. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  14. Basics: bending beams with magnetic fields top view side view “Thin lens” approximation:If the extent of the magnetic field is short compared to r, then the particle experience and angular “kick” • A charged particle in a uniform magnetic field will follow a circular path or radius • Typical Magnet Strength • Conventional: ~1 T • Latest superconducting: ~8T • Next generation superconducting (Nb3Sn): ~15T

  15. Focusing beams with quadrupole magnets Vertical Plane: Horizontal Plane: Luckily… …pairs give net focusing in both planes! -> “FODO cell” Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  16. Betatronmotion For a particular particle, the deviation from an idea orbit will undergo “pseudo-harmonic” oscillation as a function of the path along the orbit: x s Lateral deviation in one plane The “betatron function” b(s) is effectively the local wavenumber and also defines the beam envelope. Phase advance Closely spaced strong quads -> small b -> small aperture, lots of wiggles Sparsely spaced weak quads -> large b -> large aperture, few wiggles Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  17. fract. part of Y tune fract. part of X tune Tune and tune plane Particle trajectory Ideal orbit • Generally, we don’t want the tune in either plane or their combination to be a low order rational number • As particles go around a ring, they will oscillate around the ideal orbit a fixed number of times. This number is called the “tune” (usually n or Q) 6.7 Integer : magnet/aperture optimization Fraction: Beam Stability “small” integers Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  18. Emittance As a particle returns to the same point on subsequent revolutions, it will map out an ellipse in phase space, defined by Area = e Twiss Parameters An ensemble of particles will have a “bounding” e. This is referred to as the “emmitance” of the ensemble. Various definitions: Electron machines: Contains 39% of Gaussian particles Usually leave p as a unit, e.g. E=12 p-mm-mrad Proton machines: Contains 95% of Gaussian particles (FNAL) Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  19. Normalized emittance As the beam accelerates “adiabatic damping” will reduce the emittance as: The usual relativistic g and b* so we define the “normalized emittance” as: We can calculate the size of the beam at any time and position as: Example: Fermilab Booster Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  20. Cross section • When a particle hits another particle, the probability that a particular reaction will occur has units of area • Think about the probability of hitting a window while randomly throwing balls at a wall. • This is referred to as “cross-section” • The higher the cross-section, the more probable an interaction • For historical reasons, we often use the unit of “barn”, where 1 barn  1x10-24 cm2  total nuclear cross-section • The processes we are interested in today are generally measured in small fractions of a “barn” picobarn (pb), femtobarn (fb), etc. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  21. Luminosity The relationship of the beam to the rate of observed physics processes is given by the “Luminosity” Rate Cross-section (“physics”) “Luminosity” Standard unit for Luminosity is cm-2s-1 For fixed (thin) target: Target thickness Example: MiniBooNe primary target: Incident rate Target number density

  22. Colliding Beam Luminosity Circulating beams typically “bunched” (number of interactions) Cross-sectional area of beam Total Luminosity: Circumference of machine Number of bunches Record e+e- Luminosity (KEK-B): 1.71E34 cm-2s-1 Record Hadronic Luminosity (Tevatron): 4.03E32 cm-2s-1LHC Design Luminosity: 1.71E34 cm-2s-1 Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  23. Integrated Luminosity • The total number of interactions is given by the cross-section times the integral of the luminosity over time: • The integrated luminosity has units of cm-2, but for historical reasons it is almost always quoted in “inverse barns” (or more often “inverse picobarns” (pb-1), “inverse femtobarns” (fb-1), etc) • 1 b-1 = 1024 cm-2 • 1 fb-1 = 1039 cm-2 • The integrated luminosity is the ultimate measure of “what an accelerator has delivered”. • Example: the Fermilab Tevatron has delivered roughly 7 fb-1 of proton-antiproton collisions per experiment, so something with a 10 fb cross-section would have produced 7x10=70 events by now. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  24. Review: glossary of terms • “RF cavity”: resonant electromagnetic structure, used to accelerate or deflect the beam. • “Bunch”: a cluster of particles which is stable with respect to the accelerating RF • “Dipole”: magnet with a uniform magnetic field, used to bend particles • “Quadrupole”: magnet with a field that is ~linear near the center, used to focus particles • “Lattice”: the magnetic configuration of a ring or beam line • “Beta function (b)”: a function of the beam lattice used to characterize the beam size. • “Emittance (e)”: a measure of the spacial and angular spread of the beam • “Tune”: number of times the beam “wiggles” when it goes around a ring. Fractional part related to beam stability. • “Cross-section”: a measure of how likely a reaction is to occur. • “Luminosity”: a measure of the rate at which “particles hit each other”. You need a high luminosity to observe a rare process. • “Integrated Luminosity”: luminosity x time, the “bottom line” as to what an accelerator has delivered. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  25. A case study: the LHC • How were the choices made? • Colliding beams vs. fixed target • Protons vs. electrons • Proton-proton vs. proton anti-proton • Superconducting magnets • Energy and Luminosity Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  26. The Case for Colliding Beams • For a relativistic beam hitting a fixed target, the center of mass energy is: • On the other hand, for colliding beams (of equal mass and energy): • To get the 14 TeV CM design energy of the LHC with a single beam on a fixed target would require that beam to have an energy of 100,000 TeV! • Would require a ring 10 times the diameter of the Earth!!

  27. Electrons (leptons) vs. Protons (hadrons) • Electrons are point-like • Well-defined initial state • Full energy available to interaction • Can calculate from first principles • Can use energy/momentum conservation to find “invisible” particles. • Protons are made of quarks and gluons • Interaction take place between these consituents. • At high energies, virtual “sea” particles dominate • Only a small fraction of energy available, not well-defined. • Rest of particle fragments -> big mess! So why don’t we stick to electrons?? Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  28. Synchrotron Radiation: a blessing and a curse As the trajectory of a charged particle is deflected, it emits “synchrotron radiation” An electron will radiate about 1013 times more power than a proton of the same energy!!!! Radius of curvature • Protons: Synchrotron radiation does not affect kinematics very much • Electrons: Beyond a few MeV, synchrotron radiation becomes very important, and by a few GeV, it dominates kinematics - Good Effects: - Naturally “cools” beam in all dimensions - Basis for light sources, FEL’s, etc.- Bad Effects: - Beam pipe heating - Exacerbates beam-beam effects -Energy loss ultimately limits circular accelerators Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  29. Practical consequences of synchrotron radiation • Proton accelerators • Synchrotron radiation not an issue to first order • Energy limited by the maximum feasible size and magnetic field. • Electron accelerators • Recall • To keep power loss constant, radius must go up as the square of the energy (weak magnets, BIG rings): • The LHC tunnel was built for LEP, and e+e- collider which used the 27 km tunnel to contain 100 GeV beams (1/70th of the LHC energy!!) • Beyond LEP energy, circular synchrotrons have no advantage for e+e- • -> International Linear Collider (but that’s another talk) Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  30. Proton-Proton vs. Proton-antiproton • Beyond a few hundred GeV, most interactions take place between gluons and/or virtual “sea” quarks. • No real difference between proton-antiproton and proton-proton • Because of the symmetry properties of the magnetic field, a particle going in one direction will behave exactly the same as an antiparticle going in the other direction • Can put protons and antiprotons in the same ring • This is how the SppS (CERN) and the Tevatron (Fermilab) have done it. • The problem is that antiprotons are hard to make • Can get ~2 positrons for every electron on a production target • Can only get about 1 antiproton for every 50,000 protons on target! • Takes a day to make enough antiprotons for a “store” in the Fermilab Tevatron • Ultimately, the luminosity is limited by the antiproton current. • Thus, the LHC was designed as a proton-proton collider. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  31. Superconducting magnets • For a proton accelerator, we want the most powerful magnets we can get • Conventional electromagnets are limited by the resistivity of the conductor (usually copper) • The field of high duty factor conventional magnets is limited to about 1 Tesla • An LHC made out of such magnets would be 40 miles in diameter – approximately the size of Rhode Island. • The highest energy accelerators are only possible because of superconducting magnet technology. Square of the field Power lost Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  32. Issues with superconducting magnets • Conventional magnets operate at room temperature. The cooling required to dissipate heat is usually provided by fairly simple low conductivity water (LCW) heat exchange systems. • Superconducting magnets must be immersed in liquid (or superfluid) He, which requires complex infrastructure and cryostats • Any magnet represents stored energy • In a conventional magnet, this is dissipated during operation. • In a superconducting magnet, you have to worry about where it goes, particularly when something goes wrong. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  33. When is a superconductor not a superconductor? • Superconductor can change phase back to normal conductor by crossing the “critical surface” • When this happens, the conductor heats quickly, causing the surrounding conductor to go normal and dumping lots of heat into the liquid Helium • This is known as a “quench”. Can push the B field (current) too high Can increase the temp, through heat leaks, deposited energy or mechanical deformation Tc Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  34. Quench example: MRI magnet* *pulled off the web. We recover our Helium. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  35. W (MW=80 GeV) Z (MZ=91 GeV) Experimental reach of LHC vs. Tevatron • The rate of physical processes depends strongly on energy • For some of the most interesting searches, the rate at the LHC will be 10-100 times the rate at the Tevatron. • Nevertheless, still need about 30 times the luminosity of the Tevatron to study the most important physics Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  36. Nominal LHC parameters compared to Tevatron *2 MJ ~ “stick of dynamite” -> Very scary Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  37. The long road to discovery • Even with the higher rates, still need a lot of interactions to reach the discovery potential of the LHC Z’@6TeV ADD X-dim@9TeV Note: VERY outdated plot. Ignore horizontal scale. Would probably take until ~2030 to get 3000 fb-1 SUSY@3TeV 3000 Compositeness@40TeV H(120GeV)gg 300 Higgs@200GeV SUSY@1TeV 30 SHUTDOWN 200 fb-1/yr 10-20 fb-1/yr 100 fb-1/yr 1000 fb-1/yr 500 x Tevatron luminosity (will probably never happen) 50 x Tevatron luminosity Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  38. Some other important accelerators (past): • LEP (at CERN): • 27 km in circumference- e+e-- Primarily at 2E=MZ (90 GeV)- Pushed to ECM=200GeV- L = 2E31- Highest energy circular e+e- collider that will ever be built.- Tunnel now houses LHC • SLC (at SLAC): • 2 km long LINAC accelerated electrons AND positrons on opposite phases.- 2E=MZ (90 GeV)- polarized- L = 3E30- Proof of principle for linear collider Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  39. B-Factories - B-Factories collide e+e- at ECM = M((4S)).-Asymmetric beam energy (moving center of mass) allows for time-dependent measurement of B-decays to study CP violation. KEKB (Belle Experiment): - Located at KEK (Japan) - 8GeV e- x 3.5 GeV e+- Peak luminosity 1E34 PEP-II (BaBar Experiment) - Located at SLAC (USA) - 9GeV e- x 3.1 GeV e+- Peak luminosity 0.6E34 Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  40. Relativistic Heavy Ion Collider (RHIC) • - Located at Brookhaven: • Can collide protons (at 28.1 GeV) and many types of ions up to Gold (at 11 GeV/amu). • Luminosity: 2E26 for Gold • Goal: heavy ion physics, quark-gluon plasma, ?? Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  41. Continuous Electron Beam Accelerator Facility (CEBAF) • Locate at Jefferson Laboratory, Newport News, VA • 6GeV e- at 200 uA continuous current • Nuclear physics, precision spectroscopy, etc Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  42. Research machines: just the tip of the iceberg Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  43. Example: Spallation Neutron Source (Oak Ridge, TN) A 1 GeV Linac will load 1.5E14 protons into a non-accelerating synchrotron ring. These are fast extracted onto a Mercury target This happens at 60 Hz -> 1.4 MW Neutrons are used for biophysics, materials science, industry, etc… Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  44. Light sources: too many to count • Put circulating electron beam through an “undulator” to create synchrotron radiation (typically X-ray) • Many applications in biophysics, materials science, industry. • New proposed machines will use very short bunches to create coherent light. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  45. Other uses of accelerators • Radioisotope production • Medical treatment • Electron welding • Food sterilization • Catalyzed polymerization • Even art… In a “Lichtenberg figure”, a low energy electron linac is used to implant a layer of charge in a sheet of lucite. This charge can remain for weeks until it is discharged by a mechanical disruption. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  46. The future: International Linear Collider (ILC)? • LEP was the limit of circular e+e- colliders • Next step must be linear collider • Proposed ILC 30 km long, 250 x 250 GeV e+e- • BUT, we don’t yet know whether that’s high enough energy to be interesting • Need to wait for LHC results • What if we need more? Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  47. “Compact” (ha ha) Linear Collider (CLIC)? • Use low energy, high current electron beams to drive high energy accelerating structures • Up to 1.5 x 1.5 TeV, but VERY, VERY hard Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  48. Muon colliders? • Muons are pointlike, like electrons, but because they’re heavier, synchrotron radiation is much less of a problem. • Unfortunately, muons are unstable, so you have to produce them, cool them, and collide them, before they decay. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  49. Wakefield accelerators? • Many advances have been made in exploiting the huge fields that are produced in plasma oscillations. • Potential for accelerating gradients many orders of magnitude beyond RF cavities. • Still a long way to go for a practical accelerator. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU

  50. Summary • Lots has been done. • Lots more to do. Eric Prebys, "Particle Accelerators", QuarkNet Summer Institute at NIU