Few things about Accelerators - Uniudcobal/Lezione_II_trieste.pdf · Van-de Graaff II First large...
Transcript of Few things about Accelerators - Uniudcobal/Lezione_II_trieste.pdf · Van-de Graaff II First large...
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Few things about Accelerators
M. Cobal, University of Udine
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Contents
Introduction - Terms and Concepts Types of Accelerators Acceleration Techniques Current Machines
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Rutherford’s Scattering (1909)
Particle Beam Target Detector
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Results
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Sources of Particles Radioactive Decays
Modest Rates Low Energy
Cosmic Rays Low Rates High Energy
Accelerators High Rates
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Why High Energy?Resolution defined by wavelength
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Energy Scales
Particles are waves
Smaller scales = HE
1 GeV (109 eV) =1 fm (10-15m)
1 MV
1 MeV electron
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Roads to Discovery
High Energy
High Luminosity
Probe smaller scalesProduce new particles
Detect the presence of rare processesPrecision measurements of fundamental parameters
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Cross-section
Area of target
Measured in barns = 10-24 cm2
Cross-section depends upon process
Hard Sphere -
1 mbarn = 1 fm2 - size of proton
about 16 pb (others fb or less)
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Luminosity
Intensity or brightness of an accelerator
Events Seen = Luminosity x cross-section
In a storage ring
Rare processes (fb) need lots of luminosity (fb-1)
Current
Spot size
More particles through a smaller area means more collisions
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Accelerator Physics for Dummies
Electric Fields Aligned with field Typically need very high fields
Magnetic Fields Transverse to momentum Cannot change |p|
Lorentz Force
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M. Cobal, PIF 2005
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Types of Accelerators
Linear Accelerator (one-pass) Storage Ring (multi-turn)
electrons (e+e-) protons (pp or pp)
Fixed Target (one beam into target) Collider (two beams colliding)
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Circle or Line? Linear Accelerator
Electrostatic RF linac
Circular Accelerator Cyclotron Synchrotron Storage Ring
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Synchrotron Radiation
Linear Acceleration
Circular Acceleration10 MV/m -> 4 10-17 Watts
Radius must grow quadratically with
beam energy!
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LEP Accelerator (CERN 1990-2000) 27 km circumference 4 detectors e+e- collisions
LEPI: 91 GeV 125 MeV/turn 120 Cu RF cavities
LEPII: < 208 GeV ~3 GeV/turn
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Protons vs. Electrons
Can win by accelerating protons
But protons aren’t fundamental
Only small fraction at highest energy
Don’t know energy (or type) of colliding particles
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Electrons vs Protons
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History of accelerator energies
e+e- machines typicallymatch hadron machines with x10 nominal energy
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Fixed TargetSLAC End Station A 196850 GeV electons
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Colliding BeamsDESY HERA 1990s
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Center of Mass Energy
To produce a particle, you need enough energy to reach its rest mass.Usually, particles are produced in pairs from a neutral object.
To producerequires 2x175 GeV = 350 GeV of CM Energy
Head-on collisions:
One electron at rest:
Need 30,000,000 GeV electron...
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Secondary Beams
Fixed-target still useful for secondary beams
NuTeV Neutrino Production
protons
pions -> muonsneutrinos
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Accelerator Types
Static Accelerators Cockroft-Walton Van-de Graaff Linear Cyclotron Betatron Synchrotron Storage Ring
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Static E FieldParticle Source
Just like your TV set
Fields limited by Corona effectto few MV -> few MeV electrons
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Cockroft-Walton - 1930s
FNAL InjectorCascaded rectifier chain
Good for ~ 4 MV
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Van-de Graaff - 1930s
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Van-de Graaff II
First large Van-de Graaff
Tank allows ~10 MV voltagesTandem allows x2 from terminal voltage
20-30 MeV protons about the limitWill accelerate almost anything (isotopes)
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Linear Accelerators Proposed by Ising (1925) First built by Wideröe (1928)
Replace static fields by time-varying periodic fields
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Linear accelerators
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Linear Accelerator Timing
Fill copper cavity with RF powerPhase of RF voltage (GHz) keeps bunches together
Up to ~50 MV/meter possibleSLAC Linac: 2 miles, 50 GeV electrons
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Electron Linacs
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Cyclotron
Proposed 1930 by Lawrence (Berkeley)Built in Livingston in 1931
Avoided size problem of linear accelerators, early ones ~ few MeV4” 70 keV protons
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“Classic” CyclotronsChicago, Berkeley, and others had large Cyclotrons (e.g.: 60” at LBL) through the 1950s
Protons, deuterons, He to ~20 MeV
Typically very high currents, fixed frequency
Higher energies limited by shift in revolution frequency due to relativistic effects. Cyclotrons still used extensively in hospitals.
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Betatron
Variant to cyclotron, keep beam trajectory fixed,ramp magnetic fields instead. 25 MeV protons in 1940s.
First fixed circular orbit device...
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Synchrocyclotron Fixed “classic” cyclotron problem by
adjusting “Dee” frequency. No longer constant beams, but rather
injection+acceleration Up to 700 MeV eventually achieved
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SynchrotronsUse smaller magnets in a ring + accelerating station
3 GeV protonsBNL 1950s
Basis of all circularmachines built since
Fixed-target modeseverely limiting
energy reach
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Synchrotrons
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Storage Rings
Two beams counter-circulating in same beam-pipeCollisions occur at specially designed Interaction Points
RF station to replenish synchrotron losses
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Beamline ElementsDipole (bend) magnets
Quadrupole (focusing) magnets
Also Sextupoles and beyond
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• In cyclic accelerators, protons make typically 105 revolutions, receiving an RF kick of the order of a few Mev per turn
•To provide focussing, two types of magnets
bending magnets: produce a uniform vertical dipole field over the width of the beam pipe and constrain protons in a circular path focussing magnets: produce a quadrupole field. Used with alternatively reversed pole so that, both in vertical and horizontal directions one obtains alternate focussing and defocussing effects.
Like for a serie of diverging and converging lenses: net effect is focussing in both planes
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Largest HEP Accelerator LabsNuTev
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Fermilab Tevatron
Highest Energy collider: 1.96 TeVtop quark, Higgs search, new physics
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SLAC - SLC and PEPII
SLAC Linear Collider (1990-1998)Z-pole, EW physics, B-physics, polarized beams
PEPII Asymmetric Storage Ring (1999-present)
3 GeV e+ on 9 GeV e-
Very high luminosity, CP Violation, B-physics, rare decays
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CERN Large Hadron Collider
Will collide pp at 14 TeV (presently at 7 TeV)Higgs, EW symmetry breaking, new physics up to 1 TeV
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CERN Complex
Old rings still in useMany different programs
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Proposed 1 TeV e+e- collider
Similar energy reach as LHC, higher precision
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- Gaseous H2 is ionised to have H- ions. - H- accelerated first with a Cockroft Walton accelerator until They reach an energy of 750 GeV, and then with a linear accelerator (Linac) which brings them to 200 MeV
- After they are focused: sent against a thin carbon foil. Due to this interaction they loose 2 electrons, and become protons- Protons are transferred to a circular accelerator (the Booster, a synchrotron with 75 m radius) and brought to an energy of 8 GeV
- With an accelerating RF, protons are grouped in bunches, and bunches are injected in the Main Ring, synchrotron of the same dimension of the Tevatron (R = 1 Km), in the same tunnel- Conventional magnets drive bunches until 150 GeV, then p’s are transferred to the Tevatron
Proton beams production
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-A fraction of protons in the Main Ring , when they are at 120 GeV, are extracted and sent against a target to produce antiprotons- Goal: produce and accumulate large number of anti-protons, reducing momentum spread and angular divergency. In this way, can be transferred with high efficiency into the Main Ring, and after into the Tevatron- To this purpose, antiprotons are focalized through a parabolic magnetic lithium lens, and then transferred to the Debuncher, where the monocromaticity in longitudinal momentum is improved.- Antiprotons are then transferred to the Main Ring and stored there for thousands of pulses. A stochastic cooling system reduces the momentum spread in all 3 directions
- When about 6x1011 antiprotons are accumulated, 6 bunches of 4x1010 antiprotons are transferred to the Tevatron
Anti-Proton beams production
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- Made of several “pickups”, amplifiers and “kickers”
- Pickups detect locally a deviation of the Antiproton bunches from main orbit in the Accumulator
- Signal coming from the pickups is amplified and sent to kickers located at opposite azimuthal angles along the ring
- Kickers produce an electromagnetic field, which corrects the deviation detected by the pickups
Stochastic cooling
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