cern accelerator school 2012
TRANSCRIPT
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T. Thuillier, CAS, Senec, 29/5-8/6 2012
CERN ACCELERATOR SCHOOL 2012:
ELECTRON CYCLOTRON RESONANCE ION SOURCES - I Thomas Thuillier LPSC, 53 rue des Martyrs 38026 Grenoble cedex France E-mail: [email protected]
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
OUTLINE
• Electron Cyclotron Resonance Ion Source - I • Introduction • Summary of the main microscopic processes occuring in an ECRIS • Electron Cyclotron Resonance Mechanism • Magnetic confinement and ECR plasma generation • Geller Scaling Law and ECRIS Standard Model
CAS 2012 – ECR Ion Sources I
2
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Ingredients of Electron Cyclotron Resonance Ion Source • An ECR ion source requires:
• A secondary vacuum level to allow multicharged ion production • A RF injection in a metallic cavity (usually multimode) • A sophisticated magnetic Field structure that enables to:
• Transfer RF power to electrons through the ECR mechanism • Confine long enough the (hot) electrons to ionize atoms • Confine long enough ions to allow multi-ionization ions • Generate a stable CW plasma
• An atom injection system (gas or condensables) to sustain the plasma density • An extraction system to accelerate ions from the plasma
• In the following, we will try to detail these points to provide an overview of ECR ion sources
3
Extracted Ion beam
Wall
vacuum
Atoms
RF power Magnetic Field lines
ECR ZONE
ECR PLASMA
Introduction
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Ion creation through Electron Impact Ionization (in gas or plasma) •
4
In
2-3×In
Electron kinetic Energy
High energy tail
Elec
tron
Im
pact
Cro
ss s
ecti
on
Summary of the main microscopic processes occuring in an ECR Ion Source
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Ion creation through Electron Impact Ionization (in gas or plasma)
•
5
In= nth Ionization Potential
Z In (eV)
1+ 7.2 ~2.4×10-16
22+ 159 ~4.9×10-19
54+ 939 ~1.4×10-20
72+ 3999 ~7.8×10-22
82+ 90526 ~1.5×10-24
Example for Bismuth
Bi
Summary of the main microscopic processes occuring in an ECR Ion Source
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Ion loss through Charge-Exchange (in gas or plasma) •
6
Z 1+ 22+ 54+ 72+ 82+
1.5×10-15 5.6×10-14 1.6×10-13 2.2×10-13 2.6×10-13
Example : Bismuth with O2
Summary of the main microscopic processes occuring in an ECR Ion Source
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Zero Dimension Modelization •
7
Summary of the main microscopic processes occuring in an ECR Ion Source
creation destruction
Losses (ion extraction,
wall…)
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Elastic Collision in an ECRIS plasma •
8
Summary of the main microscopic processes occuring in an ECR Ion Source
90°
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Motion of a charged particle in a constant magnetic field •
9
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The Electron Cyclotron Resonance (1/3) •
10
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The Electron Cyclotron Resonance (2/3) •
11
e- Electron trajectory
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The Electron Cyclotron Resonance (3/3) •
12
ECR Heating here
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Stochastic Heating (1/5) •
13
Electron accelerates
Electron decelerates!
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Stochastic Heating (2/5) •
14
Former solution (v0=0)
Initial condition (v0≠0)
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Stochastic Heating (3/5) •
15
Constant term
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Stochastic Heating (4/5) •
16
HEATING
COOLING!
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Stochastic Heating (5/5) •
17
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Heating in a Magnetic Gradient •
18
z
B(z) ECR
Heating zone
BECR
Electron Cyclotron Resonance Mechanism
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Properties of particle motion in a magnetic field (1/2) •
19
Magnetic Field line In a Tokamak
http://www-fusion-magnetique.cea.fr
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Properties of particle motion in a magnetic field (2/2) •
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Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The Magnetic Mirror Effect •
21
Axial mirror done with a set of 2 coils
http://www.astronomes.com/
Solar wind reflection by the
Earth magnetosphere
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Corrolary of Magnetic Mirroring: The Loss Cone •
22
VII
V┴
θmin confined Particles
Loss cone
In an ECRIS plasma, The loss cone is perpetually populated By particles interaction (Spitzer)
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Magnetic confinement: Minimum |B| structure • ECR ion sources features a sophisticated magnetic field
structure to optimize charged particle trapping • Superimposition of axial coils and hexapole coils • The ECR surface (|B|=BECR) is closed
23
Iso B lines
z
Bz Axial Mirror
BECR
BECR
Source LBNL
Hexapolar Field
Source RIKEN, Nakagawa
|B|(x,z)
z 2r Source D. Xie
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Axial Magnetic Confinement • The axial magnetic confinement in a
multicharged ECRIS is usually done with a set of 2 or 3 axial coils. • Either room temperature coils + iron to
boost the magnetic field • Or superconducting coils
• In the case of 3 coils, the current intensity in the middle one is opposed to the others so that it helps digging Bmed
• Usually Binj, Bext respectively stand for the magnetic field at injection (of RF, atoms…) and (beam) extraction
• Bext should be the smaller magnetic field in the ECR to favor Ion extraction there!
24
z
Bz
Axial Mirror
Binj
Bmed
Bext
z
cinj
cmed cext
r
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Radial Magnetic Confinement • The radial magnetic confinement
is usually built with a hexapole field
• Either with permanent magnets • Br Up to 1.6 T maximum possibly 2T
with some tricks • Advantage : economical • Inconvenient: not tunable
• Either with a set of superconducting coils • Br>1.6 T-2 T • Advantage: tunable online to
optimize a population of ion in the source.
• Inconvenient: expensive, complicated design and building
25
|Br|
r
(HallBach Hexapole With 36 permanent magnets
30° rotation/magnet)
r
Superconducting hexapolar coil
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
ECR Plasma build up
•
26
vacuum Atoms
RF
Extraction
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The Famous plasma shape in an ECR Ion Source • To understand why the ECR plasma ends with 3 lines only, one
needs to follow the heated electron through the ECR zone
27
Br
Bz
Near to the wall
Near to the axis: axial field only
HOT ECR ELECTRONS ⇒ High density Plasma ⇒ LIGHT emission ⇒ Tracks on walls
No hot electrons ⇒ Low density Plasma ⇒ No Light emission ⇒ No Tracks on walls
Plasma shape at injection (L) and Extraction (R)
Half unrolled Liner with plasma marks (3 poles= π only)
z
Photo of ECR plasma from injection (all thickness is superimoosed=> 6 lines) 10+14 GHz heating
Elevation view from an ECRIS chamber along 2 hexapole poles
Magnetic Confinement and ECR Plasma Generation
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
Plasma Oscillations – ECR cut off density •
28
Geller Scaling Law and ECRIS Standard Model
Above cut off: RF is reflected => no more ECR heating!
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The ECR Scaling law (R. Geller, 1987) •
29
fECR
[GHz] λECR [cm]
ne
[cm-3] Λ0->1+
[cm] τ0->1+
[µs] B
ECR [T]
2.45 ~12 7.4 ×1010 ~7 ~10 0.09
14 ~2 2.5×1012 0.2 3 0.5
28 ~1 ~1013 0.05 0.7 1
60 ~ 0.5 4.4×1013 0.01 0.17 2
Geller Scaling Law and ECRIS Standard Model
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T. Thuillier, CERN Accelerator School 2012, Senec, 29/5-8/6 2012
The ECR standard model • Optimum high charge state ion production and extraction have been
experimentally studied as a function of ECR frequency. • General Scaling laws for the magnetic field have been established
30
r
Br
Binj
Bmed
Bext
Axial Mirror Radial Mirror
~1990 2003 ?
fECR [GHz] 14 28 56
BECR [T] 0.5 1 2
Brad~2×BECR 1 2 4
Binj~3-4×BECR 2 3.5 7
Bmed~0.5-0.8×BECR 0.25 0.5 1
Bext≤Brad 1 2 4
VENUS
|B|(x,z)
z 2r Source D. Xie
Z
Geller Scaling Law and ECRIS Standard Model