July 24, 2019

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TRIUMF AcceleratorsOverview and current developments

Oliver Kester

ALD accelerator division

TRIUMF student lecture

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Outline

• Basics of particle accelerators

• Radio Frequency

(RF)-accelerators

• Overview TRIUMF accelerator

facility and rare isotope production

• Driver accelerators: Cyclotron and

e-linac

• Production and preparation of rare

isotope beams

• Post acceleration or rare isotopes

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What is a „particle accelerator“?

An accelerator is a device that uses electromagnetic forces to accelerate and guide charged particles.

THE ESSENTIALS:

• Particle source(electrons, protons, ions)

• Vacuum

• Electric field for acceleration

• Magnetic and/or electric fields for focusing and steering

• Controls

4

Acceleration of charged particlesAcceleration a with an electric field E

for a particle with mass m and charge q

Kinetic energy

The kinetic energy of charged particles is measured in electron volts (eV)

1 eV is the energy a singly charged particle acquires when it moves through a potential of

1 Volt. 1 eV = e * (1 Volt) = 1.6022*10-19 J

A convenient unit for heavy ion acceleration is energy/nucleon

Em

qaamF

d

UqEqF ==== ,

d

kin kin

qE q U q E d Joule E U Q U eV

e= = = =

/Q

W U eV uA

=

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Charged particles in electromagnetic fields

BvqEqF

+=Right hand

rule

In electromagnetic fields, the Lorentz force F acts on a particle with the charge q with

E is the electric field, B the magnetic field. The electric field causes acceleration in direction of the field vector,whereas the magnetic field causes an accelerationperpendicular to the direction the particle moves:

In the magnetic field, v(t) is constant!

Fmagnetic can be used forbeam manipulation(bending and focusing)!

vBvqFmagnetic

tois ⊥=

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Electrostatic accelerator

☺

☺

☺

+ -

Source

TargetHigh voltagegenerator

E-Field

example:

TV tube

vacuum tube

electron emitter

horizontal deflection

vertical deflection

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Van de Graaff accelerator

5 MV van de Graaf of HMI Berlin

Basics of RF-accelerators

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Linear and circular accelerators

Use multiple passes through a small

number of cavities

For ions:• Cyclotron, Synchrotron

For electrons:• Microtron, Betatron

Use a single pass through a large

number of cavities

Structures for ions:• Wideroe, Alvarez and H-type structures

Structures for electrons: • Elliptical cavities

Circular Accelerators Linear Accelerators

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RF-accelerator I

Multiple use of the same alternating voltage

→ Wideroe principle!

Proof of the rf- acceleration principle by Rolf Wideroe 1928 in Berlin.

Frequency: 1 MHz

Electric Field = 25000 V

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RF-accelerator IIThe crucial innovation was the field-freedrift tubes, shielding the ions from the electric field whenever it reversed direction.

Note that the beam is non-continuous –a stream of short pulses – separated by theradio frequency period Trf

Time to travel from center of gap i to gap i+1is half of the rf-cycle time Trf.

Wideroe condition: 2 2 2

i rf i rf i rf

i

v T cl

c

= = =

accU

2 acci i

i

q iUv c

m

l i

= =

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Example: Wideroe rf-Linac

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Resonator / cavity

C RpLUo

"pill box cavity„ - most common cavity in ring accelerators!!

Transformation from a resonance circuit to a cavity

1

2f

C L=

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RF Cavities

• RF cavities are specially designed structures

with electrically conductive walls

• The cavity is sized to resonate at a particular

rf frequency and with a shape such that an

electric field is produced along the path of the

charged particle as it passes through the

cavity

• A small driving rf signal couples electro-

magnetic energy into the cavity to establish

the accelerating field.

• A resonator can sustain an infinite number of

resonant electromagnetic modes but only one

mode is used for acceleration ( )tqEtqEF cos)( 0==

Overview TRIUMF accelerator facility and rare isotope production

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Primary beam driver:Cyclotron, 500 MeV, H-

Produces rare isotopes, neutrons and muons!

Isotope Separator and Accelerator facility -

ISACIsotope Separator Online (ISOL) facility

ISAC-I: Normal conducting-linac, 0.15-1.5 MeV/u

ISAC-II: Superconducting-linac, 5-15 MeV/u

Advanced Rare Isotope Laboratory - ARIELSuperconducting electron linac

30 MeV, 10 mA, cw

4 Cyclotrons for medical isotope production500 MeV

Cyclotron

ISAC-IIHigh energy

ISAC-ILow and medium energy

ARIEL

Cyclotronsfor medical

Isotopeproduction

TRIUMF accelerator complex

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TRIUMF will transition into ARIEL:

• Multi-user, multi-disciplinary RIB Facility

• Intense, clean RIB beams into ISAC

experiments:

– New 35 MeV superconducting electron linac

– New 100 kW electron beamline and target station

– New 50 kW proton beamline and target station

Cyclotron

ISAC

e-linac – 30MeV

Existing

ARIEL1.5

ARIEL 2

Advanced Rare Isotope Laboratory - ARIEL

Driver accelerators –cyclotron and e-linac

19• H- cyclotron as proton driver (multiple extraction at different

energies) for RIB production

• Proton at 500 MeV up to 100 mA (50 kW)

• Two production lines:

• ISAC BL2A existing

• ARIEL-II BL4N expected 2022/23

The 520 MeV H--cyclotron

Largest Cyclotron in the world:D = 18 m

Magnet weight 4000 t

Coil current:18500 A

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Visitors in the cyclotron

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The cyclotron - principle

Cyclotron frequency

Bm

q

r

vc ==

qvBr

mv=

2

RF-amplifier

Invented by Ernest O. Lawrence in 1932

The particles are held to a spiral trajectory by a static

magnetic field and accelerated by an RF-field.

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The cyclotron technical implementation• Ions are injected in the center of the cyclotron.

• The electrodes can be excited at a fixed rf frequency – the cyclotron frequency.

• The particles will remain in resonance throughout acceleration, running “isochronous” and a

new bunch can be accelerated on every rf voltage peak (like in a linac).

• “continuous-wave” (cw)

operation

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Injection and extraction from a cyclotron

H- extraction

B

v

Carbon FoilH- Ion

p

( )11 u om N VAR N

B Q e

+ =

-

N-3 N-2 N-1 N

septumshoe

Separation of turns:

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Cyclotron historyBelow the 27-inch cyclotron,

Berkeley (1932). The magnet

was originally part of the

resonant circuit of an RF current

generator used in

telecommunications.

In late 1930, Lawrence’s student, Stanley Livingston, built a

“4-inch” version in brass. Clear evidence of magnetic field

resonance was found in November, and in January 1931

they measured 80-keV protons. Ions were produced from

the residual gas by a heated filament at the centre.

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Limits of the classical cyclotron

.))((0

constrBm

qzc =

=

Relativistic mass effect require a stronger magnetic

field at the outside of the cyclotron that the particle

stay in sync with the RF → isocyclotron

An outwardly-decreasing (negative-gradient) field ⇒vertical focusing.

Positive axial focusing requires B decreasing with r

→ provided naturally by B fall-off towards pole edge.

Solution to this problem:

The use of edge focusing to allow vertical focusing

and stay isochronous.

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Edge focusing

When a particle crosses a magnet end at an angle κ to the

normal, longitudinal components of the fringe field By interact

with velocity

components vx

parallel to the

edge, giving a

vertical force!

Kerst (1956)

suggested using

spiral sectors to

increase the

axial focusing

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Superconducting electron linac

To reach high energies with normal-conducting rf

cavities requires:

- very high power and usually pulsed operation;

- very long machines, as field strength is limited.

Superconducting cavities have been pursued

since ~1960 in the hope of reducing the power

dissipation in the walls to zero

→ complex infrastructure

Success came in the 70s and 80s using niobium!

Much higher electric fields can be produced with

those cavities – up to 50 MV/m.

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Electron Linacs

As an electron’s speed v → c (=1), the

speed of light, at relatively low energies

(~500 keV), the gap and pillbox cavity size

can be kept constant for the higher energies.

E-linacs are then built from identical sections

and cavities.

For higher energies and cw-operation

superconducting elliptical cavities are used

like the 9-cell niobium cavity (TESLA cavity)

for the FLASH free electron laser linac and

the European XFEL at DESY in Hamburg.

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ARIEL – superconducting electron-Linac• E-gun delivers max. 10 mA at 300 keV beam

• The injector cryomodule accelerates to 5-10 MeV

• The accelerator cryomodule is equipped with

two cavities and reaches max. 30 MeV.

E-gun

Injection

cryomoduleAcceleration

cryomodule

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Required electron beam energy

• Converter made of high Z material, Au, W, Ta.

Thickness ~ 3.5 mm.

• Electrons MUST be stopped in low Z material Al.

• The number of fissions per second saturates

beyond 35 - 40 MeV beam energy.

Production, and preparation of rare isotope beams

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ISAC at TRIUMF

Isotope Separation and Acceleration facility

(ISAC)

• Isotope Separation On Line (ISOL)

facility for rare isotope beam

(RIB) production

• Highest power driver beam (50 kW)

• Extracted ions are mass separated

and either post-accelerated

or delivered to low energy

experiments directly.

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Target ion sourcesOberflächenionisation Plasmaquelle mit heißer Transferlinie

Plasmaquelle mit

kalter Transferlinie

Surface ionisation Plasma ion source

• Target and ion sources units,

common is surface ionisation, laser

ionisation and plasma ionisation

• Targets are heated up to high

temperatures to support diffusion of

isotopes into the ionisation region

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Isotope extraction

• Simulation of the path of one Ga atom

produced in a Ta-foil target towards the

ionizer (on the left)!

• Extraction times vary significantly

between elements. Driven by volatility

and in-target chemistry

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Production of RIBs with electron beams

10 mA of 30-50 MeV electrons from the

superconducting e-linac (via the photo fission

process) yielding a range of isotopes not available

from proton reactions and higher beam purity.

An electron-to-gamma converter is required because

the direct power deposition imposed by the 35 MeV

electrons in a target is unsustainable

500MeV protons 50MeV electrons

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• Two underground target stations with

extraction voltage up to 60 kV

Target module sits in a big vacuum

tank!

• Proton beam sent to one of the target

stations at the time

• Common pre-separator inside the

shielded area

• Mass separator on high voltage

platform (typical operation resolving

power 3000)

• Charge breeder (ECR type) for post

acceleration

ISAC target stations and mass separator

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Beam delivery: Mass separation and charge state breeding

A/q-

analyzer

charge

state breeder

Low energetic

1+ ions

Low energetic

q+ ions

Post accelerator

or experiment

Analyzing

magnet

Buffer gas

emittance cooler

Switch

yardIsotopes from 1+

ion source

Mass separation by a high resolution

separator

(resolution of ARIEL-HRS ~20000)

Charge breeding =

Generation of highly charged ions

from externally injected

singly charged ions

in a high charge state

ion source:

Electron beam ion source (EBIS)

or

Electron Cyclotron Resonance

Ion Source – (ECRIS).

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Charge state breeding? Why?

Post acceleration of radioactive ion beams (RIBs):

• Post accelerator compact and more efficient if charge state is n+ instead of

1+ → higher cavity frequencies (smaller cavities)

• Pulsed structures can be used (normal conducting)

• Beam matching (Time structure, injection energy, emittance cooling)

• A/q matching (isotopes from any region of the nuclear chart become

available)

• Beam purification (molecule break up)

Post acceleration of rare isotopes

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ISAC linacs overview IISAC-I:

• DTL normal conducting at 106.08 MHz:

– Separated functions

– Variable energy machine

– 150 keV/u ≤ E ≤ 1.8 MeV/u

– 2 ≤ A/q ≤ 7

• Radio Frequency Quadrupol (RFQ)

normal conducting at 35.36 MHz:

– 8m long split ring structure

– 153 keV/u, 3≤A/q≤30

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ISAC-I RFQ

Irf max

Vrf max

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ISAC-I drift tube linac (DTL)

• Mode produces transverse electric field that gets

transformed to longitudinal field through the drift tubes

supported alternately from two ridges

• Suitable for heavy ions from = 0.02 → 0.15

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H-type structure(TE110) Acceleration

• TE110 is a deflecting mode

(transverse E) but it can accelerate

by loading with drift tubes to create

on axis electric field

• We use the same mode in a rf

deflecting cavity for ARIEL (shown

below)

-

+ + + +- -- -

+

+ + + +- - - -

-

+

-

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ISAC linacs overview II

ISAC-II: Superconducting linac at

106.08 MHz:

– SC-Linac using quarter wave

resonators (QWR) with

= 0.057, 0.071, 0.11

– Max. energy range

6.5 MeV/u (A/q=6)

16.5 MeV/u (A/q=2)

– Cryomodules with 4, 6 and 8

QWR and one SC solenoid 9T

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ISAC linacs cavities and modules

Quarter Wave Resonators (QWR)

– TEM mode cavities can produce accelerating

voltages across a coaxial gap with variable gap

distance

– Inner conductor about the length of /4 (quarter

wavelength of the RF-el. magn. Wave)

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Beam physics at TRIUMF

Beam envelope

• Beam physics describes the behaviour

of charged particle in electromagnetic

fields in an accelerator or a beam

transport system.

• The special distribution and the

momenta of the particles are

summarized by the phase space

occupation or the so-called beam

emittance.

• For intense beams the effects of the

repelling forces between the charges

particles, so called space charge,

must be taken into account.

47Accelerators are no miracles, but require a profound know-how and technologies!

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Student education and research

• Ion sources Plasma ion sources, high charge state ion sources

(Charge state breeders)

• Beam physicsIntense, space charge dominated beams (HL-LHC

beam-beam effects, electron linac, cyclotrons)

High level computer applications, automatic tuning

• Target research and development –Material properties, ions source optimization

(plasma physics)

• Superconducting RF (SRF) and RFCavity design, cavity and cryo modules

Surface properties (-NMR and m-SR

investigations and processing),

Digital low level RF technology

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Dis

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Thanks for your attention!

Follow us @TRIUMFLab

www.triumf.ca

Additional slides

51

Masses or, Why do we use eV rather than kg?

According to Einstein mass is equivalent to energy E = m*c2

Therefore, we can calculate the equivalent energy of the mass of an electron or proton and express this energy in eV:

The mass of an electron is me = 9.109*10-31 kg, mec2/e = 0.511 MeV

The mass of the proton is mp = 1.672*10-27 kg, mpc2/e = 938 MeV

For relativistic calculations the mass in eV is much more convenient!

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Phase focusing

Phase

The electric fields in an RF-acceleratorare time dependent. The field strengthdepends on the time a particle entersthe acceleration gap.The energy gain in the gap depends onthe phase Y0 the particle arrives in the gap center.

Synchronous phase in front of thecrest (negative synchronous phase)→ longitudinal focusing

Synchronous (perfect) particle →perfect synchronism in the linac

max 0sinkinE qU = Y

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ARIEL – electron-Linac modules

• Elliptical cavities,

1.3 GHz

• 9 cell,

TESLA type

• 11 MV/m

demonstrated

Cryo modules

(FUTURE)Klystrons