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