Eric Prebys FNAL Accelerator Physics Center June 7
Eric Prebys FNAL Accelerator Physics Center June 7 -17, 2010
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 2
The first “particle physics experiment” told Ernest Rutherford the structure of the atom (1911) Study the way radioactive particles “scatter” off of atoms In this case, the “accelerator” was a naturally decaying 235 U nucleus The first artificial acceleration of particles was done using “Crookes tubes”, in the latter half of the 19 th century These were used to produce the first X-rays (1875) Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 3
To probe smaller scales, we must go to higher energy 1 fm = 10 -15 m (Roughly the size of a proton) 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. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 4
Accelerators allow us to probe down to a few picoseconds after the Big Bang! Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 5
Radioactive sources produce maximum energies of a few million electron volts (Me. V) Cosmic rays reach energies of ~1, 000, 000 x LHC but the rates are too low to be useful as a study tool Max LHC energy 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 6
The simplest accelerators accelerate charged particles through a static electric field. Example: vacuum tubes (or CRT TV’s) Cathode Anode Limited by magnitude of static field: - TV Picture tube ~ke. V - X-ray tube ~10’s of ke. V - Van de Graaf ~Me. V’s Solutions: FNAL Cockroft- Alternate fields to keep particles in Walton = 750 k. V accelerating fields -> RF acceleration - Bend particles so they see the same accelerating field over and over -> cyclotrons, synchrotrons 7
Particles are typically accelerated by radiofrequency (“RF”) electric fields. Stability depends on particle arrival time relative to RF phase “bunched” beams If velocity dominates If momentum (path length) dominates “bunch” Particles with lower E arrive earlier and see greater V. Particles with lower E arrive later and see greater V. Nominal Energy Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU Nominal Energy June 7 -17, 2010 8
ILC prototype elipical cell “p-cavity” (1. 3 GHz): field alternates with each cell Fermilab Drift Tube Linac (200 MHz): oscillating field uniform along length Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU JLab compact “toaster cavity” (400 MHz): low frequency in a limited space June 7 -17, 2010 9
1930 (Berkeley) Lawrence and Livingston K=80 Ke. V § 1935 - 60” Cyclotron Ø Lawrence, et al. (LBL) Ø ~19 Me. V (D 2) Ø Prototype for many Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 10
§ 60” cyclotron (1935) Ø Berkeley and elsewhere § Fermilab Ø Ø Radius = 1 km Built ~1970 Upgraded ~1985, ~1997 Until recently, the most powerful accelerator in the world. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 11
My House (1990 -1992) /LHC Tunnel originally dug for LEP Built in 1980’s as an electron positron collider Max 100 Ge. V/beam, but 27 km in circumference!! Now we’ll talk a little about how these things work… Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 12
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 13
side view top 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: ~8 T Next generation superconducting (Nb 3 Sn): ~15 T 14
Vertical Plane: Horizontal Plane: Luckily… …pairs give net focusing in both planes! -> “FODO cell” Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 15
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 Phase advance The “betatron function” b(s) is effectively the local wavenumber and also defines the beam envelope. 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 16
Particle trajectory Ideal orbit 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) Generally, we don’t want the tune in either plane or their combination to be a low order rational number “small” integers Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU 6. 7 Fraction: Beam Stability fract. part of Y tune Integer : magnet/aperture optimization fract. part of X tune June 7 -17, 2010 17
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: Proton machines: (FNAL) Contains 39% of Gaussian particles Usually leave p as a unit, e. g. E=12 p-mmmrad Contains 95% of Gaussian particles Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 18
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 19
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 1 x 10 -24 cm 2 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 20
Rate The relationship of the beam to the rate of Cross-section observed physics (“physics”) processes is given by the “Luminosity ” “Luminosity” Standard unit for Luminosity is cm-2 s-1 For fixed (thin) target: Target thickness Incident rate Example: Mini. Boo. Ne primary target: Target number density 21
Circulating beams typically “bunched” (number of interactions) Cross-sectional area of beam Total Luminosity: Number of bunches Record e+e- Luminosity (KEK-B): Record Hadronic Luminosity (Tevatron): LHC Design Luminosity: Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU Circumference of machine 1. 71 E 34 cm-2 s-1 4. 03 E 32 cm-2 s-1 1. 71 E 34 cm-2 s-1 June 7 -17, 2010 22
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 7 x 10=70 events by now. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 23
“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 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 24
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 25
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 Te. V 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 Te. V! Would require a ring 10 times the diameter of the Earth!! 26
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 27
As the trajectory of a charged particle is deflected, it emits “synchrotron radiation” Radius of curvature An electron will radiate about 1013 times more power than a proton of the same energy!!!! • Protons: Synchrotron radiation does not affect kinematics very much • Electrons: Beyond a few Me. V, synchrotron radiation becomes very important, and by a few Ge. V, 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 28
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 Ge. V beams (1/70 th 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 29
Beyond a few hundred Ge. V, most interactions take place between gluons and/or virtual “sea” quarks. 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 No real difference between proton-antiproton and proton-proton Can put protons and antiprotons in the same ring This is how the Spp. S (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", Quark. Net Summer Institute at NIU June 7 -17, 2010 30
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) Power lost The field of high duty factor conventional magnets is limited to about 1 Tesla Square of the field 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. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 31
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 32
Superconductor can change phase back to normal conductor by crossing the “critical surface” Can push the B field (current) too high Can increase the temp, through heat leaks, deposited energy or mechanical deformation Tc 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”. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 33
*pulled off the web. We recover our Helium. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 34
The rate of physical processes depends strongly on energy W (MW=80 Ge. V) Z (MZ=91 Ge. V) Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU For some of the most interesting searches, the rate at the LHC will be 10100 times the rate at the Tevatron. Nevertheless, still need about 30 times the luminosity of the Tevatron to study the most important physics June 7 -17, 2010 35
Parameter Tevatron “nominal” LHC Circumference 6. 28 km (2*PI) 27 km Beam Energy 980 Ge. V Number of bunches 36 2808 Protons/bunch 275 x 109 115 x 109 p. Bar/bunch 80 x 109 - Stored beam energy 1. 6 +. 5 MJ 366+366 MJ* Initial luminosity 3. 3 x 1032 (cm-2 s-1) 1. 0 x 1034(cm-2 s-1) Main Dipoles 780 1232 Bend Field 4. 2 T 8. 3 T Main Quadrupoles ~200 ~600 Operating temperature 4. 2 K (liquid He) 1. 9 K (superfluid He) 7 Te. V *2 MJ ~ “stick of dynamite” -> Very scary Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 36
Even with the higher rates, still need a lot of interactions to reach the discovery potential of the LHC 3000 Z’@6 Te. V ADD X-dim@9 Te. V SUSY@3 Te. V Compositeness@40 Te. V H(120 Ge. V) gg SUSY@1 Te. V 10 -20 fb-1/yr 100 fb-1/yr 50 x Tevatron luminosity Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU Would probably take until ~2030 to get 3000 fb-1 200 fb-1/yr 30 Higgs@200 Ge. V SHUTDOWN 300 Note: VERY outdated plot. Ignore horizontal scale. 1000 fb-1/yr 500 x Tevatron luminosity (will probably never happen) June 7 -17, 2010 37
LEP (at CERN): - 27 km in circumference - e+e- Primarily at 2 E=MZ (90 Ge. V) - Pushed to ECM=200 Ge. V - L = 2 E 31 - 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. - 2 E=MZ (90 Ge. V) - polarized - L = 3 E 30 - Proof of principle for linear collider Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 38
- B-Factories collide e+e- at ECM = M( (4 S)). -Asymmetric beam energy (moving center of mass) allows for timedependent measurement of B-decays to study CP violation. KEKB (Belle Experiment): - Located at KEK (Japan) - 8 Ge. V e- x 3. 5 Ge. V e+ - Peak luminosity 1 E 34 PEP-II (Ba. Bar Experiment) - Located at SLAC (USA) - 9 Ge. V e- x 3. 1 Ge. V e+ - Peak luminosity 0. 6 E 34 Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 39
- Located at Brookhaven: - Can collide protons (at 28. 1 Ge. V) and many types of ions up to Gold (at 11 Ge. V/amu). - Luminosity: 2 E 26 for Gold - Goal: heavy ion physics, quark-gluon plasma, ? ? Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 40
Locate at Jefferson Laboratory, Newport News, VA 6 Ge. V e- at 200 u. A continuous current Nuclear physics, precision spectroscopy, etc Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 41
Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 42
A 1 Ge. V Linac will load 1. 5 E 14 protons into a nonaccelerating 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 43
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 44
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 45
LEP was the limit of circular e+e- colliders Next step must be linear collider Proposed ILC 30 km long, 250 x 250 Ge. V 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", Quark. Net Summer Institute at NIU June 7 -17, 2010 46
Use low energy, high current electron beams to drive high energy accelerating structures Up to 1. 5 x 1. 5 Te. V, but VERY, VERY hard Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 47
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 48
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", Quark. Net Summer Institute at NIU June 7 -17, 2010 49
Lots has been done. Lots more to do. Eric Prebys, "Particle Accelerators", Quark. Net Summer Institute at NIU June 7 -17, 2010 50
- Slides: 50