Accelerators and Ion Sources

1. Accelerators and Ion Sources CHARMS Basic Physics Topics series November 2nd, 2005

2. Outline • Accelerators • Ion Sources (This is logically reverse order, but it is easier to present things this way) Accelerators and Ion Sources

3. Accelerators – basic ideas • Charged particles can be accelerated in the electric field. • Examples from the nature – electrostatic discharge, α- and β-decays, cosmic rays. • Rutherford's experiments with α-particles • Discovery of the nucleus in 1911 • First artificial nuclear reactions • Inspiration for high-voltage particle accelerators • Muons and pions were discovered in cosmic-ray experiments with emulsions. • Everyday life: TV-set, X-ray tubes... Accelerators and Ion Sources

4. Types of Accelerators Used in Science • Electrostatic: Cockroft-Walton, Van de Graaff • Induction: Induction linac, betatron • Radio-frequency accelerators: LINAC, RFQ, Cyclotron, Isochronous cyclotron, Synchrocyclotron, Microtron, Synchrotron Accelerators and Ion Sources

5. Cockroft-Walton • High voltage source using rectifier units • Voltage multiplier ladder allows reaching up to ~1MeV (sparking). • First nuclear transmutation reaction achieved in 1932: p + 7Li → 2·4He • CW was widely used as injector until the invention of RFQ Fermilab 750 kV C-W preaccelerator Accelerators and Ion Sources

6. Van de Graaff • Voltage buildup by mechanical transport of charge using a conveyor belt. • Builds up to ~20 MV Accelerators and Ion Sources

7. Tandem Van de Graaff • Negative ions accelerated towards a positive HV terminal, then stripped of electrons and accelerated again away from it, doubling the energy. • Negative ion source required! • Examples: • VIVITRON @ IReS Strasbourg • 25 MV Tandem @ ORNL • 18 MV Tandem @ JAERI • 20 MV Tandem in Buenos Aires Accelerators and Ion Sources

8. Induction linac • Creation of electric field by magnetic induction in a longitudinal evacuated cavity in magnetic material • Very high intensity beams (up to thousands of Amperes) N. C. Christofilos et al., Rev. of Sci. Inst. 35 (1964) 886 Accelerators and Ion Sources

9. Betatron • Changes in the magnetic flux enclosed by the circular beam path induce a voltage along the path. • Name derived from its use to accelerate electrons • To the left: Donald Kerst with two of the first operational betatrons (2.3 and 25 MeV) Accelerators and Ion Sources

10. RF Accelerators • High voltage gaps are very difficult to maintain • Solution: Make the particles pass through the voltage gap many times! • First proposed by G. Ising in 1925 • First realization by R. Wiederöe in 1928 to produce 50 kV potassium ions • Many different types Accelerators and Ion Sources

11. RF LINAC – basic idea • Particles accelerated between the cavities • Cavity length increases to match the increasing speed of the particles • EM radiation power P = ωrfCVrf2 – • the drift tube placed in a cavity so that the EM energy is stored. • Resonant frequency of the cavity tuned to that of the accelerating field Accelerators and Ion Sources

12. RF LINAC – phase focusing • E. M. McMillan – V. Veksler 1945 • The field is synchronized so that the slower particles get more acceleration Accelerators and Ion Sources

13. LINAC – Examples • SLAC – 3 km, 50 GeV electrons, 2.856 GHz • UNILAC @ GSI – HI • GELINA @ IRMM Geel – 150 MeV electrons GELINA maquette Accelerators and Ion Sources

14. RF Quadrupole • Simultaneous generation of a longitudinal RF electric field and a transverse focusing quadrupole field • Low-energy, high-current beams • Compact • Replacing Cockroft-Walton as injectors 2 MeV RFQ @ Idaho State Univ. Accelerators and Ion Sources

15. Cyclotron • The cyclotron frequency of a non-relativistic particle is independent of the particle velocity:ω0 = eB0 / γm ≈ eB0 / m • E. O. Lawrence in 1929 • Limitations: relativistic effects break the isochronism → Epmax≈ 12 MeV Accelerators and Ion Sources

16. Isochronous Cyclotron • In order to restore the isochronism, the magnetic field needs to be shaped in function of the radius to match the change of the frequency with the particle energy. • However, such configuration leads to vertical orbit instability → restoration of the orbit stability using the Azimuthal Varying Field (AVF) L. H. Thomas (1938) Accelerators and Ion Sources

17. Synchrocyclotron • Instead of modifying the magnetic field, the radio frequency can be modulated → pulsed beams • Limit at ~1GeV • Example: SC in CERN (600 MeV) Accelerators and Ion Sources

18. Synchrotron • Use of the phase-focusing principle in a circular orbit with a constant radius • RF and magnetic fields are tuned to synchronize the particle revolution frequency and confine its orbit. • Examples: • PS, SPS, LHC @ CERN (28, 450, 7000 GeV) • SIS @ GSI Accelerators and Ion Sources

19. CERN Accelerator Complex Accelerators and Ion Sources

20. GSI The Present and the Future Accelerators and Ion Sources

21. Ion Sources

22. Ion Sources • Very broad field with many applications: • Material science and technology (e.g. ion implantation) • Food sterilization • Medical applications • Military applications • Accelerators • ... • Beams of nanoamperes to hundreds of amperes • Very thin to very broad beams (μm2 to m2) Accelerators and Ion Sources

23. Types of Ion Sources (selection) source: http://linac2.home.cern.ch/linac2/seminar/seminar.htm#intro Accelerators and Ion Sources

24. Plasma ion sources • Ionization is actually a process of creation of a plasma • Plasma ion source: Ionization mechanism: eˉ-eˉ collisions • Most widely used – many different types • Types differ according to plasma production and confinement mechanisms. Accelerators and Ion Sources

25. Metal Vapor Vacuum IS (MEVVA) • Electrostatic discharge between a cold anode and a hot cathode in a vacuum • Evaporation and ionization of cathode atoms Accelerators and Ion Sources

26. Penning Ion Sources • Arc discharge in a magnetic field – electrons confined radially by the magnetic field and axially by electrostatic potential well • In cyclotrons it is possible to use the magnetic field of the accelerator • One PIG is used @ GSI Penning Ion Gauge (PIG) Ion Source Accelerators and Ion Sources

27. Multi-Cusp Ion Source (MUCIS) • Cusp-like magnetic field lines • Most of the plasma volume in a relatively weak magnetic field • Large volume of uniform and dense plasma possible (2.5 cm – 1m size) MUCIS used @ GSI Accelerators and Ion Sources

28. Electron Cyclotron Resonance IS (ECRIS) • Vapor held in a cavity with high magnetic field • Microwaves with frequency that coincides with eˉ cyclotron frequency in the field heat the electrons (and only electrons). • No electrodes, no arc discharge – very reliable, high currents • 14 GHz, 0.5 T @ GSI, Dubna, LBNL, CERN http://www.casetechnology.com/source.html Accelerators and Ion Sources

29. Surface Ion Source • Hot surface of a metal with high work function ionizes elements with low ionization potential (like alkalis) • Negative surface ion source also in use EXTRACTION ELECTRODE Surface Ion-Source http://isolde.web.cern.ch/ISOLDE/ Accelerators and Ion Sources

30. Sputter Ion Source • Cesium vapor, hot anode, cooled cathode • Some of the vapor gets condensed on the cathode, some gets ionized on the anode and accelerated towards the cathode where it sputters atoms from the cathode • Produces negative ions of all elements that form stable negative ions Accelerators and Ion Sources

31. Laser Ion Source • Stepwise resonant excitation and photoionization of the atom • Chemically selective – wavelength tuned to the specific element • Pulsed http://isolde.web.cern.ch/ISOLDE/ Accelerators and Ion Sources

32. Electron Sources • Thermionic emission – escape of electrons from a heated surface. Condition: Ee > φ • High field emission (fine point cathode) • Photo emission: λ < hc/φ Accelerators and Ion Sources

33. The End Questions? Comments? Tea? Coffee?