Sources of intense beams of heavy ions

Transcription

1 Sources of intense beams of heavy ions 517. WE-Heraeus-Seminar Accelerator physics for intense ion beams Oliver Kester Institut für Angewandte Physik, Goethe-Universität Frankfurt and GSI Helmholtzzentrum für Schwerionenforschung

2 Heavy ion accelerator facility

3 Outline Introduction and Basics Ionisation, plasma generation confinement and extraction of ions High current ions sources Multi-cups ion sources rf-driven ion sources Metal vapor vacuum arc sources (MeVVA) High charge state ion sources The Electron Cyclotron Resonance Ion Sources (ECRIS) The Electron Beam Ion Source (EBIS)

4 Ion sources accelerator application Ion- Source LEBT accelerator Typical structure of an injection line: Ion implanter Penetration depth beam energy Intense ion source volume ion sources For semiconductor production

5 Introduction and Basics

6 Physics of ion sources? extraction system The physics of ion sources: plasma generator Ionisation of atoms (Electron impact ionisation, photoionization) Production of charged particles (Electrons, ions) Production of plasma Plasma physics Beam shaping and charged particle transport Electromagnetic fields (electric, magnetic, rf) Principle of a plasma ion source: plasma generation extraction

7 Plasma generation Make atoms available in the plasma generator via: Generate free electrons: Supply the ionisation energy: Gas injection Vapour, melting of solids Sputtering Thermionic emission Photo ionisation Discharge Electron acceleration (electrostatic e-gun) rf-heating E x B-drift

8 Atomic processs in a Plasma collisions with electrons Impact Ionization A + e Impact excitation A Three-Body-Recombination (TBR) A ( Z + 1) + e' + e'' Z + + A Z+ : Atom of species A Z + + e Impact de-excitation Z + ( A ) + e' with charge state Z e : electron changed energy Non-radiative transition collisions with photons Photo ionization Radiative Recombination (RR) A Excitation A + hυ A ( Z + ) Z Z + + hυ + e Spontaneous emission ( Z A + ) Line spectrum Photo absorption Z + A + hυ + e Bremsstrahlung Z + A + e' Continuous spectrum 2.8

9 Electron impact Ionisation The electron impact ionization is the most fundamental ionization process for the operation of ion sources. electron impact ionisation K L A q+ A (q+1)+ The probability for removing one electron (q q+1) is determined by the cross section σ q q+1 [cm 2 ]. τ n q q q+ 1 = = = ν q q+ 1 neveσ q q+ 1 1 e j σ e q q+ 1 The average ionization time in the charge state q depends only on the cross section and the current density of the electrons.

10 e-impact ionisation cross section Approximation of the cross section in quantum-mechanical calculations done by Bethe (1930) using the Born Approximation: Scattering of a matter wave at a central potential V(r) for E kin >> E ion (perturbation). σ nl i = const Z nl E 1 E kin nl E ln E kin nl All electrons in an atom or ion contribute with their individual σ ei to the total cross section σ, as long as the kinetic energy E kin of the projectile is larger then the ionization energy E nl of these electrons. σ N N q q+ 1 = σ i = qiσ ei i= 1 i= 1 Semi-empirical formula developed by Lotz 1967 for the energy dependence of the cross sections for the elements from H to Ca and for energies < 10 kev is commonly used. σ ln E kin N Pi q q+ = cm i= 1 Ekin Pi 2 [ ]

11 Ionisation potential 1E+6 ev Ionisation energies by C. Moore 1E+5 1E+4 1E+3 1E+2 1E+1 1E OXGAS2.XLS R. Becker, Z Ionization energies up to 100 kev (U 91+ U 92+ ) required Shell structure of the atomic shells is visible 2.11

12 Successive e-impact ionisation Multi electron removal or single electron successive ionization? Electron impact Single ionization Electron impact Double ionization Cross section Cross section electron energy electron energy Single step much higher cross section

13 Successive e-impact ionisation Why electron impact? The cross section for the impact ionization is orders of magnitudes higher than the cross section for photo ionization. The cross section depends on the mass of the colliding particle, since the energy transfer from a heavy projectile is reduced.

14 Plasma generation - discharge Fremdionisation hν 1. Generation 2. Generation The discharge depends on the gas pressure and the electric field strength d Paschen curve Kathode cathode Anode anode Helium U / V Argon Wa sserstoff p d / Pa mm

15 Plasma potential and potential distribution plasma anode (set = 0 V): (plasma chamber wall) U PW + H 2 + H 3 H + H 1 e H 2 j = env em I I potential cathode (-80 V): not in scale Plasma-wall interface: Position a) ideally reflected b) complete wall recombination plasma different mobilities of electrons and ions lead to a positive potential of the plasma towards the wall

16 Magnetic confinement If a charged particle moves into a zone with stronger magnetic field, the kinetic energy of the gyration increases! total kinetic energy is constant, therefore the particle is decelerated in B-field direction ω = c r q m m v q B B cyclotron frequency = gyration radius α v 1 v 1 magnetic mirror reflexion plane

17 Child-Langmuir-Law plasma electrode + d puller electrode - Plasma electrode Plasmaelectrode U d E=U/d Puller- U r Ion beam U A Plasmameniscus x I 4π = ε 0 9 2qe A m u r 2 U d 3/ 2 2 I = P U 3/ 2 P = Perveance d

18 Ion extraction Low PD S = r d aspect ratio I = 4π ε 9 2eq 3/ U S = m i q A S r 3/ 2 E 3/ 2 Matched Case Plasma electrode Screening Ground +U Ext -U Scr 0 V High PD r Ion trajectories Plasma meniscus d

19 Extraction Systems Triode extraction system most simple system for preservation of space charge compensation. Pentode extraction system Formation electrode (FE) + Einzellens set-up single and multi-aperture extraction to overcome the current limit for a single-aperture extraction system

20 Classification of ion sources The electron energy determines the maximum charge state that can be reached by electron impact ionisation Highly charged ions Low current The density*confinement time determines the time required to reach a certain charge state If the confinement time is too low, the ion species gets lost before reaching the required charge state! Singly charged ions High current Multiple charged ions Medium current Electron density*confinement time

21 High current ion sources

22 Overview - high current sources Filament driven Vacuum Arc driven High Duty factor MUCIS, MUCIS New, CHORDIS Working material: Gases MEVVA, VARIS Metalls and Gases PIG Metalls and Gases

23 Volume source multicusp source U PE cathode solenoid plasma ion e - e - filter magnet emission opening B filter gas inlet ~ 80 V U A anode typical operation parameters (hydrogen): U A : ~ 80 V, I A : < 80 A U PE : < -150 V, I PE : < 25 A P arc : < 10 kw, p s : Pa B sol : ~ 25 mt plasma electrode

24 Special type of volume sources rf-volume sources as ion propulsion device (ion thruster)

25 Plasmageneration via rf - injection Pressure region: hpa x f ν e,n λ e < L rf-power ~ kw f = MHz m E = E p sin( ωt) e dv dt ee p v = v0 + t m ω e = ee sin( ω t + φ) [ cosφ cos( ω + φ) ] Without collisions, the electrons do not take energy out of the rf-field! p HF The phase of the electron movement can be changed by collisions. New electrons are generated by ionization processes. If the production rate is bigger than the loss rate, a stable rf-discharge follows.

26 rf ion sources internal antenna external antenna

27 Vacuum arc driven sources R. Hollinger The MeVVa-ion source (Metal Vapor Vacuum Arc) produces intense current of multi-charges metallic ions. Discharge between the hot cathode and the cold trigger generates the Metal Vapor". The discharge between the cathode and the anode generates the plasma.

28 GSI MeVVA source in operation µs after µs after Swiching Ignition OFF Cathode: Ignition Pulse Ti Up = V Coil 1 IArc = 1000 ACoil 2 Cathode (desired Me) U=0 Anode Ua = +( ) V To Extraction System Trigger Finger Isolator Trigger Ring Trigger Pulse: τ ~ 20 µs Self Pinching Effect: U = 12 kv IArc > 700 A I = 30 A R. Hollinger

29 VARIS (Vacuum Arc Ion Source) 17 cathodes 2 solenoids (0,1 and 0,2 Tesla) Arc power: 50 kw (13,3 MW/cm 2 ) Arc current: ~1 ka Duty cycle: usually 1 Hz, 1 ms Optimized for Uranium (67% of 238 U 4+ ) Emission current density 170 ma/cm kv kv 20 ma in front of the RFQ Challenges: Improving the beam quality at plasma extraction Lifetime of cathodes 3 Hz operation

30 High charge state ion sources

31 Production of multi-charged ions Charge state stripper Stripper foil Fast ions Slow electrons MI Ion sources ECR plasma Slow ions Fast electrons MI EBIS beam Fast electrons Slow ions MI

32 Electron Cyclotron Resonance ion source (ECRIS) Resonant microwave plasma heating Magnetic confinement Higher frequency (28 GHz) higher Becr ~ f Challenges: Magnetic field Ion extraction and beam transport multipole field B solenoidal field ECR zone B ECR =eω/m ω = f c = e m e B eb 2π m e = B[ T ] Microwave magnetic field distribution

33 ECRIS basics Variation of the plasma potential Intense heating of electrons MeV energies The following magnetic-field relations are valid for high power ECRs: B inj /B ecr ~ 4, B ext /B ecr ~ 2, B min /B ecr ~ 0.8, B rad /B ecr >2, B ext /B rad <0.9

34 ECRIS scaling laws and ion intensities intensity (eµa) GHz SC-ECRIS (extrapolated) 28 GHz SERSE 18 GHz SERSE 18 GHz RT-ECRIS 14 GHz GSI-CAPRICE II Xe charge state ECRIS is usually running cw pulse operation in afterglow mode

35 Plasma boundary and profile Mismatched extraction Kr ma Kr ma Matched extraction Kr ma Kr ma

36 Electron Beam Ion singly solenoid barrier electrode electron repeller charged ions space charge potential U W U (z) electron beam anode drift tubes ionisation electron collector ion beam U(R) n +-ions U 0 U U W extraction R r W r Electrostatic confinement Intense electron beam (current density, up to 10 4 A/cm 2 ) Tunable electron beam energy Storage capacity ~ trap length, electron current Z N = m 2e e 3 l I e U Ie = electron beam current Ue= electron beam energy l = length of confinement region e

37 Charge state development in an EBIS/T Charge development for stepwise ionisation dn dt = Sources Drains example dn dt Electron impact ionisation z + 1 ne nz veσ z z+ 1 ne n z+ 1 Radiative recombination (RR) = σ z+ 1, RR

38 Extracted ion beams - example Ion current pa C 3+ O 4+ Ne Ne5+ Ar 9+ N Ne Magnetic field mt t = 18 ms Efficiency Cs 23+ Trap to RFQ=8.7% 18+ Ion current pa A/q=2 Ne 9+ Ne 8+ Ne 7+ Ne t = 158 ms Ne Magnetic field mt Efficiency Cs 32+ Trap to RFQ=7.3%

39 RHIC EBIS / BNL Intense pulses of HCIs Parameter RHIC EBIS Max. electron current I el = Electron energy E el = Electron density in a trap j el = Length of ion trap l trap = 10 A 20 kev 575 A/cm m Ion trap capacity Q el = 1.1x10 12 Ion yield (charges) Q ion = Yield of ions Au 32+ N 32+ Au = 5.5x10 11 (10 A) 3.4x10 9 Maximum B-field Warm ID Length of solenoid He refilling period 5.0 T 204 mm 1900 mm 30 days

40 Top-ten reasons for why it s great to be an ion source physicist 10. Spouses never need to worry about what their ion source physicist husband or wife is up to when they still aren t home by midnight...they know with great confidence that they re just at the lab changing a source! 9. Between the International Conference on Ion Sources, the Workshop on Ion Source Issues Relevant to a Pulsed Spallation Neutron Source, the ECR Ion Source Workshop, The International Conference on Negative Ions, Beams and Sources, the International Symposium on the Production and Neutralization of Negative Ions and Beams, the Workshop on Negative Ion Formation and Beam Handling, the International Conference on Sources of Highly Charged Ions, the ion source community has the greatest number of conferences and workshops of any scientific discipline measured per linear inch of subject matter 8. Endless hours can be devoted to the important philosophical meaning of pi as in the output emittance is 0.2 pi-mm-mrad 7. Believe it or not, sometimes the rf-ion source antenna picks up free Satellite Television! 6. High-voltage, hydrogen gas, antennas and power supplies: it s every little kids dream! 5. In what other field can you acquire a Ph.D., and then spend your professional life practicing the occult? 4. When the phone rings during dinner time, you always know who it is: no its not a tele-marketer, it just the lab...the ion source went down! 3. In what other field could you advertise a workshop on RF-driven, multicusp, Cesium-enhanced H-sources, and have 50 people actually show up? 2. Job security...when was the last time you heard a manager say, the ion source is running so well, we re going to let the entire group go. 1. It s a subject in which anyone can make a contribution, and everyone has an opinion! Courtesy of Stuart Stuart Henderson/ORNL