Physics of Particle Accelerators

Kalanand MishraDepartment of Physics

University of Cincinnati

How a Particle Accelerator Works

Speed up particle with E/M field Smash particles into target or other

particles Record collisions with detectors Able to identify product particles

Physics of a Particle Accelerator

Beam production Bunching Electron guns Beam focusing Colliding and Detecting

Beam production

Electron Beam

Thermoionic Emission

Ionizing Hydrogen

•Glow Discharge Column

•From H- Ion

Proton Beam

Other Beams

Secondary Beams:

• Proton

• Antiproton

• Other Particle Beams

Bunching

Bring the Particles in phase.

As spread out beam gives fewer collisions than a narrowly focused one, e- & e+ bunches are sent into

damping rings (e- to north, e+ to south).

Colliding

•Fixed target

E = (2mEp)

•Colliding beam

E = 2Ep

Beam Focusing

As spread out beam gives fewer collisions than a narrowly focused one, e- & e+ beams have to be focused.

This is done by bent magnets.

Two Types

L inear C ircu lar

Acce lera tor

•Linear Path

•Travel once

•Circular Path

•Travel several times

Linear Accelerator

LINAC Operation

Methods of Acceleration in Linear Accelerator

SLC Polarized Electron Gun

Methods of Acceleration in Linear Accelerator

•Basic idea

•Synchronization

•Length of the tube

•Shielding

LINAC cont’d

Klystron: Microwave generator

1. Electron gun produces a flow of electrons. 2. Bunching cavities regulate speed of electrons so that bunches arrive at the output cavity. 3. Bunches of electrons excite microwaves in output cavity of the klystron.4. Microwaves flow into the waveguide , which transports them to the accelerator. 5. Electrons are absorbed in beam stop.

Overall Operation of LINAC

Electrons are Accelerated in a Copper Structure

Bunches of electrons are accelerated in the copper structure of the linac in much the same way as a surfer is pushed along by a wave.

Changing Electric and Magnetic Fields:

E/M waves that push the electrons in the linac are created by higher energy versions of the microwaves used in the microwave ovens in our

kitchens.

The microwaves from the klystrons in the Klystron Gallery are fed into the accelerator via waveguides.

This creates a pattern of E&B fields, which form an E/M wave traveling down the accelerator.

Klystron Operation

The 2-mile SLAC linear accelerator (linac) is made from over 80,000 copper discs and cylinders brazed together.

LINAC Structure

Microwaves set up currents that cause E pointing along accelerator

and B in a circle around interior of accelerator.

Want e- and e+ to arrive in each cavity at right time to get max. push from E.

e+ needs to arrive when field polarity is opposite.

Circular Accelerator

Methods of Acceleration in Circular Accelerator

Cyclotron

•The Ds

•Electric field across the gap

•Circular orbit

•Increasing radius

Cyclotron

The maximum speed a proton could have

in a dee of radius R and strength B is given

by (ignoring relativistic effects.)

vm = BeR / mp

Synchrotron (synchro-cyclotron)

Methods of Acceleration in Circular Accelerator

• Electromagnetic resonant cavity

• Magnetic field for circular

orbit

• Field synchronization with increasing particle energy

• Synchrotron radiation

• Storage ring

Synchrotron

The radius of curvature of the path of particles of momentum p and charge q in a synchrotron is given by the formula

R = p / q B where B is the field strength.

If a synchrotron of radius R has 4 straight sections of length L each and period of the radio frequency oscillator corresponds to the time of one revolution then

(a) The speed of the particles is v = ( 2pR + 4L ) f

Synchrotron

(b) By considering the relativistic momentum of particles of mass M, the magnetic field strength of the synchrotron is given by

where f is the frequency.

Storage Rings

Similar to a synchrotron, but designed to keep particles circulating at const. energy not increase energy further

SPEAR : 3 GeV

PEP I : 9 GeV

PEP II : e- 9 GeV e+ 3.1 GeV

Detection

•Tracking bubble, radiation

•Tracking curvature (charged particle)