CERN ACCELERATOR SCHOOL 2012

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: thuillier@lpsc.in2p3.fr

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

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

Extracted Ion beam

vacuum

RF power Magnetic Field lines

ECR ZONE

ECR PLASMA

Introduction

Ion creation through Electron Impact Ionization (in gas or plasma) • 

2-3×In

Electron kinetic Energy

High energy tail

Summary of the main microscopic processes occuring in an ECR Ion Source

Ion creation through Electron Impact Ionization (in gas or plasma)

• 

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

Ion loss through Charge-Exchange (in gas or plasma) • 

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

Zero Dimension Modelization • 

creation destruction

Losses (ion extraction,

wall…)

Elastic Collision in an ECRIS plasma • 

Motion of a charged particle in a constant magnetic field • 

Electron Cyclotron Resonance Mechanism

The Electron Cyclotron Resonance (1/3) • 

e- Electron trajectory

ECR Heating here

ECR Stochastic Heating (1/5) • 

Electron accelerates

Electron decelerates!

Former solution (v0=0)

Initial condition (v0≠0)

Constant term

HEATING

COOLING!

ECR Heating in a Magnetic Gradient • 

B(z) ECR

Heating zone

Properties of particle motion in a magnetic field (1/2) • 

Magnetic Field line In a Tokamak

http://www-fusion-magnetique.cea.fr

Magnetic Confinement and ECR Plasma Generation

Properties of particle motion in a magnetic field (2/2) • 

The Magnetic Mirror Effect • 

Axial mirror done with a set of 2 coils

http://www.astronomes.com/

Solar wind reflection by the

Earth magnetosphere

Corrolary of Magnetic Mirroring: The Loss Cone • 

θmin confined Particles

Loss cone

In an ECRIS plasma, The loss cone is perpetually populated By particles interaction (Spitzer)

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

Iso B lines

Bz Axial Mirror

Source LBNL

Hexapolar Field

Source RIKEN, Nakagawa

|B|(x,z)

z 2r Source D. Xie

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!

Axial Mirror

cmed cext

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

(HallBach Hexapole With 36 permanent magnets

30° rotation/magnet)

Superconducting hexapolar coil

ECR Plasma build up

• 

vacuum Atoms

Extraction

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

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)

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

Plasma Oscillations – ECR cut off density • 

Geller Scaling Law and ECRIS Standard Model

Above cut off: RF is reflected => no more ECR heating!

The ECR Scaling law (R. Geller, 1987) • 

[GHz] λECR [cm]

[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

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

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

|B|(x,z)

z 2r Source D. Xie

CERN ACCELERATOR SCHOOL 2012

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