particle accelerators

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PARTICLE ACCELERATORS International Scientific Spring, 7-11 March 2016 F. Gerigk (CERN/BE/RF)

Some numbers

~200

research accelerators are in operation worldwide

~24000 ~11000

industrial accelerators medical accelerators

built over the last 60 years

for electrons, ions, protons, X-rays

~400B€ ~15000

of end products/year

sterilised, produced, examined by accelerators

patients worldwide treated with hadron therapy in 2015

OVERVIEW • Historical

developments

• Accelerator • Basics

types

of accelerator and beam physics

• Accelerators

at CERN

COCKROFT - WALTON (1932) voltage multiplier + proton accelerator (< 1 MeV)

typically used up to 750 kV

crucial technology: voltage multiplier

the original machine (200 keV)

CERN Linac2 pre-injector until 1993 (750 keV)

VAN DER GRAAFF GENERATOR (1931)• a DC voltage is connected to the lower electrode (7), • charges

are transported (4) to the dome (1), where they are collected by the upper electrode (2)

• until

a spark equalises the potentials

•1

MV for 90 $!

typically < 10 MeV crucial technology: charge separation and accumulation

TANDEM: VAN DER GRAAF WITH TWICE THE VOLTAGE 0V

belt

anode 5 MV

H-

0V p

stripping foil

Using ions with at least one extra electron and a stripping foil the particle can see twice the accelerating voltage. However, this “trick” can only be applied once.

20 MeV accelerator in 1981 (NSF, Daresbury, UK)

5 MV generator in 1933 (MIT, Round Hill, USA) •

one sphere contains an ion source, the other one a target,

•

beam through the air or later through vacuum,

From DC to RF acceleration

THE WIDERÖE LINAC (1927) energy gain: period length increases with velocity:

E-field particles

crucial technology: RF oscillators & synchronism

the RF phase changes by 180 deg, while the particles travel from one tube to the next

The use of RF enables to have ground potential on both sides of the accelerator. This allows a limitless cascade of accelerating gaps!!

THE ALVAREZ LINAC (1946) after WW2 high-power high-frequency RF sources became available (radar technology): most old linacs operate at 200 MHz! the RF field was enclosed in a box: RF resonator

crucial technology: high-freq. RF sources & RF resonators

While the electric fields point in the “wrong direction” the particles are shielded by the drift tubes.

LINEAR ACCELERATORS (LINACS) particle source

FQ

RF cavity

•

succession of focusing elements (e.g. FODO) and accelerating cavities in a straight line,

•

dominated by accelerating cavities and RF systems,

•

beam passes only once

DQ

RF cavity

FQ

RF cavity

Linac4 at CERN: CCDTL section

LINAC APPLICATIONS particle source

FQ

RF cavity

DQ

RF cavity

FQ

RF cavity

•

electron linacs: cancer treatments, electron beam welding, particle colliders, material studies, industrial radiography, cargo screening, sterilisation, free electron lasers (FEL), food irradiation, maximum energy limited by length…

•

proton linacs: cancer treatments, neutron scattering, material studies, particle physics, production of radioactive ion beams/antiprotons, isotopes, injectors for synchrotrons, irradiation, ADS

•

ion linacs: ion implantation of semiconductors, cancer treatment, nuclear physics

CYCLOTRONS (1932) top view

F = q(v ⇥ B)

Ernest Lawrence

Nobel price 1939 side view

•

Particles are bent onto a circular path by 2 large D-shaped dipoles.

•

An RF electric field between the “Dees” accelerates the particles.

•

The velocity and radius increase after each acceleration step.

BETATRONS (1935) iron core

coils

Max Steenbeck magnetic flux

orbit

air gap

vacuum chamber

electron acceleration

• The magnetic field rises from 0 to its maximum value, F = q(v ⇥ B) • The particles are kept in orbit by Lorentz Force: Z d • The particles are accelerated by induction: E · dl = = 2⇡rE dt • The orbit remains constant if the betatron condition is fulfilled (design of the iron pole): Bz (r) = 2⇡r2

SYNCHROTRONS RF cavity

ipo

po le

D

Di

le

DQ

FQ

FQ

DQ

Di

po le

DQ FQ

D

le o ip

Low-Energy Ion Ring (LEIR) at CERN

•

Dipoles keep the particles on a circular path, quadrupoles focus transversally,

•

few RF systems: energy increase turn by turn,

•

dipoles are ramped in synchronism with the particle energy,

•

dominated by magnets and their power supplies,

APPLICATIONS OF CIRCULAR MACHINES •

cyclotrons: CW proton/ion acceleration, high average beam power, limited energy reach (~500 MeV for protons)

•

electron synchrotrons: synchrotron radiation, maximum energy limited by energy loss through synchrotron radiation (e.g. 104.5 GeV for LEP)

•

proton synchrotrons: cancer treatments, neutron scattering, high-energy physics, fixed target physics, irradiation, medical isotopes, maximum energy limited by maximum dipole magnetic field (7 TeV for LHC)

•

ion synchrotrons: cancer treatments (carbon), nuclear physics, fixed target physics, colliders,…

•

colliders: first circular machine to study beam-beam collisions: ISR (Intersecting Storage Ring, CERN)

BASICS OF PARTICLE ACCELERATION

ENERGY GAIN energy gain of a particle with charge q:

RF phase

passing a gap with the electric field E:

gap -L/2

synchronous phase

this can be written as: average electric transit time field on axis factor

-L/2

cavity or cell length

(phase seen by the bunch in the center of the gap)

ENERGY GAIN energy gain of a particle with charge q:

RF phase

passing a gap with the electric field E:

gap -L/2

synchronous phase

this can be written as: average electric transit time field on axis factor

-L/2

cavity or cell length

ENERGY GAIN energy gain of a particle with charge q:

RF phase

passing a gap with the electric field E:

gap -L/2

synchronous phase

this can be written as: average electric transit time field on axis factor

-L/2

cavity or cell length

TRANSIT TIME FACTOR accounts for the difference in field seen by particles when they cross a DC or RF gap average electric field: transit time factor:

ignoring the velocity change in the cavity and assuming a constant field between -g/2 and g/2, T simplifies to: assuming:

RF CAVITIES CAN ONLY ACCELERATE BUNCHED BEAMS. WE NEED LONGITUDINAL FOCUSING TO KEEP PARTICLES TOGETHER.

LONGITUDINAL FOCUSING: NOT ALL PARTICLES OF A BUNCH “SEE” THE SAME FIELD

The synchronous particle “sees” the synchronous phase Φs at the centre of the accelerating gap.

Depending on their position with respect to the synchronous particle, the early and late particles see higher or lower accelerating field.

E0 early particle

late particle

synchronous particle, Φs=-x°

time RF phase

LONGITUDINAL FOCUSING: PHASE STABILITY The early particle sees less acceleration and the early particle late particle sees more acceleration:

E0 late particle

synchronous ➜ the outer particles particle, Φs=-x°

time RF phase

move towards the bunch centre and will oscillate around the synchronous phase

LONGITUDINAL FOCUSING: PHASE STABLE REGION The late particle oscillates in a stable motion as long as its energy gain is larger early than the one of the particle synchronous particle: ΔWlate > ΔWs Phase stability requires ⇡ < ⇡/2