accelerators at triumf 8_2019.pdf · the cyclotron technical implementation • ions are injected...
TRANSCRIPT
March 8, 2019
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Dis
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Accelerators at TRIUMF -Overview 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
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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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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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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
20• 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)
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Electron impact ionisation
electron impact
ionization
Aq+→ A(q+1)+
KL
∞
The probability for removing one electron and changing the
charge state of the ion from q→q+1 is determined by the
cross section sq→q+1 [cm2].Collision frequency and time between ionising collisions
1 1 1
1 1 1
1q
q q e q e q q q q
q q e e q q e q q
n en n v
n v j s
s s→ + → + → +
→ + → + → +
= = = =
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separation
of rest gas ions
singly
charged
ions
n+ ions
solenoid
coils
Magnetic field confinement
RF injection
Electron Cyclotron Resonance ion source (ECRIS) charge state breeder
Microwave
Injection
Hexapole (Radial Magnet ic Field)Beam
ExtractionfB= 28B[T] (GHz)
Axial
B fieldc= qB/mCyclotron Frequency
Beam born in Magnetic field !
Microwave
Injection
Hexapole (Radial Magnet ic Field)Beam
ExtractionfB= 28B[T] (GHz)
Axial
B fieldc= qB/mCyclotron Frequency
c= qB/mCyclotron Frequency
Beam born in Magnetic field !
• Resonant microwave plasma heating
• Electron energies – up to MeV via
electron cyclotron resonance
• Magnetic confinement
• Higher frequency (28 GHz)
Becr ~ f, Ion current ~ f2
1
2c
e
ef B
m=
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U(z)
Z
ionisation
extraction
drift tubes
solenoid
ion
beam
electron beam
anode
electron
collector
electron
repeller
barrier
electrode
• Electrostatic confinement
• Intense electron beam
(current density,
up to 104 A/cm2)
• Tunable electron beam
energy
• Storage capacity
~ trap length, electron
current
Electron Beam Ion Source/Trap (EBIS/T) charge state breeder
Charge development for
stepwise ionisation
1+
n+
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.
51Accelerators 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
co
very
,accele
rate
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Thanks for your attention!
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