Introduction to Accelerators

Eric Torrence

University of Oregon QuartNet 2005

Special Thanks to Bernd Surrowhttp://web.mit.edu/8.701/www/

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 High Energy

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)

technically infinite (E field)

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

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 Acceleration

10 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 288 SC RF cavities

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

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 EnergyTo produce a particle, you need enough energy to reach its rest mass.Usually, particles are produced in pairs from a neutral object.

To produce

requires 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 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

Cyclotron

Proposed 1930 by Lawrence (Berkeley)Built in Livingston in 1931

Avoided size problem of linear accelerators, early ones ~ few MeV

4” 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

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

Largest HEP Accelerator Labs

NuTev

Fermilab Tevatron

Highest Energy collider: 1.96 TeV

top 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

Under construction in old LEP tunnelWill collide pp at 14 TeV (mini-SSC)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