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Few things about Accelerators
M. Cobal, University of Udine
Contents
Introduction - Terms and Concepts Types of Accelerators Acceleration Techniques Current Machines
Rutherford’s Scattering (1909)
Particle Beam Target Detector
Results
Sources of Particles Radioactive Decays
Modest Rates Low Energy
Cosmic Rays Low Rates High Energy
Accelerators High Rates
Why High Energy?Resolution defined by wavelength
Energy Scales
Particles are waves
Smaller scales = HE
1 GeV (109 eV) =1 fm (10-15m)
1 MV
1 MeV electron
Roads to Discovery
High Energy
High Luminosity
Probe smaller scalesProduce new particles
Detect the presence of rare processesPrecision measurements of fundamental parameters
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)
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
Accelerator Physics for Dummies
Electric Fields Aligned with field Typically need very high fields
Magnetic Fields Transverse to momentum Cannot change |p|
Lorentz Force
M. Cobal, PIF 2005
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)
Circle or Line? Linear Accelerator
Electrostatic RF linac
Circular Accelerator Cyclotron Synchrotron Storage Ring
Synchrotron Radiation
Linear Acceleration
Circular Acceleration10 MV/m -> 4 10-17 Watts
Radius must grow quadratically with
beam energy!
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
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
Electrons vs Protons
History of accelerator energies
e+e- machines typicallymatch hadron machines with x10 nominal energy
Fixed TargetSLAC End Station A 196850 GeV electons
Colliding BeamsDESY HERA 1990s
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...
Secondary Beams
Fixed-target still useful for secondary beams
NuTeV Neutrino Production
protons
pions -> muonsneutrinos
Accelerator Types
Static Accelerators Cockroft-Walton Van-de Graaff Linear Cyclotron Betatron Synchrotron Storage Ring
Static E FieldParticle Source
Just like your TV set
Fields limited by Corona effectto few MV -> few MeV electrons
Cockroft-Walton - 1930s
FNAL InjectorCascaded rectifier chain
Good for ~ 4 MV
Van-de Graaff - 1930s
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)
Linear Accelerators Proposed by Ising (1925) First built by Wideröe (1928)
Replace static fields by time-varying periodic fields
Linear accelerators
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
Electron Linacs
Cyclotron
Proposed 1930 by Lawrence (Berkeley)Built in Livingston in 1931
Avoided size problem of linear accelerators, early ones ~ few MeV4” 70 keV protons
“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.
Betatron
Variant to cyclotron, keep beam trajectory fixed,ramp magnetic fields instead. 25 MeV protons in 1940s.
First fixed circular orbit device...
Synchrocyclotron Fixed “classic” cyclotron problem by
adjusting “Dee” frequency. No longer constant beams, but rather
injection+acceleration Up to 700 MeV eventually achieved
SynchrotronsUse smaller magnets in a ring + accelerating station
3 GeV protonsBNL 1950s
Basis of all circularmachines built since
Fixed-target modeseverely limiting
energy reach
Synchrotrons
Storage Rings
Two beams counter-circulating in same beam-pipeCollisions occur at specially designed Interaction Points
RF station to replenish synchrotron losses
Beamline ElementsDipole (bend) magnets
Quadrupole (focusing) magnets
Also Sextupoles and beyond
• 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
Largest HEP Accelerator LabsNuTev
Fermilab Tevatron
Highest Energy collider: 1.96 TeVtop quark, Higgs search, new physics
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
CERN Large Hadron Collider
Will collide pp at 14 TeV (presently at 7 TeV)Higgs, EW symmetry breaking, new physics up to 1 TeV
CERN Complex
Old rings still in useMany different programs
Proposed 1 TeV e+e- collider
Similar energy reach as LHC, higher precision
- 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
-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
- 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