Exploring Particle Accelerators: From Linacs to Innovation

1. Accelerators • We’ve seen a number of examples of technology transfer in particle detector development from HEP (basic science) to industry (medical, …) • Particle accelerators provide another such example • There are currently more than 30,000 particle accelerators in use throughout the world with only a small fraction being used in HEP/nuclear research

2. Accelerators • Circa 2000

3. Accelerators • A brief history

4. Accelerators • A brief history • Electrostatic (Cockcroft-Walton, van de Graaf) • Linac (linear accelerator) • Circular (cyclotron, betatron, synchrotron) • Development of strong focusing • Colliding beams (present day) • Plasma wakefield, ???

5. Accelerators • “Moore’s law” ~ e+t/C

6. Accelerators • “Moore’s law”

7. Linac • Linac = linear accelerator • Applications in both high energy physics and radiation therapy

8. Linac • Linacs are single pass accelerators for electrons, protons, or heavy ions • Thus the KE of the beam is limited by length of the accelerator • Medical (4-25 MeV) – 0.5-1.5 m • SLAC (50 GeV) – 3.2 km • ILC (250 GeV) - 11 km • Linac – static field, induction (time varying B field), RF • Operate in the microwave region • Typical RF for medical linacs ~ 2.8 GHz • Typical accelerating gradients are 1 MV/m – 100 MV/m

9. Linac • Brief history • Invented by Wideroe (Germany) in 1928 • Accelerated potassium ions to 50 keV using 1 MHz AC • First realization of a linac by Sloan (USA) in 1931 • No further progress until post-WWII when high power RF generators became available • Modern design of enclosing drift tubes in a cavity (resonator) developed by Alvarez (USA) • Accelerated 32 MeV protons in 1946 using 200 MHz 12 m long linac • Electron linac developed by Hansen and Ginzton (at Stanford) around the same period • Evolved into SLAC laboratory and led to the birth of medical linacs (Kaplan and Varian Medical Systems)

10. Linac • Wideroe’s linac

11. Linac • Alvarez drift tube linac • First stage of Fermilab linac

12. Linac • A linac uses an oscillating EM field in a resonant cavity or waveguide in order to accelerate particles • Why not just use EM field in free space to produce acceleration? • We need a metal cavity (boundary conditions) to produce a configuration of waves that is useful • Standing wave structures • Traveling wave structures

13. LINAC • Medical linacs can be either type

14. Waveguides

15. Waveguides • Cyclindrical wave guide

16. TM Modes TM01 mode

17. Waveguides

18. Waveguides • Phase and group velocity

19. Waveguides • Phase and group velocity

20. Waveguides • The phase velocity can be slowed by fitting the guide with conducting irises or discs • The derivation is complicated but alternatively think of the waveguide as a transmission line • Conducting irises in a waveguide in TM0,1 mode act as discrete capacitors with separation d in parallel with C0

21. Waveguides • Disc loaded waveguide

22. Traveling Wave Linac • Notes • Injection energy of electrons at 50 kV (v=0.4c) • The electrons become relativistic in the first portion of the waveguide • The first section of the waveguide is described as the buncher section where electrons are accelerated/deaccelerated • The final energy is determined by the length of the waveguide • In a traveling wave system, the microwaves must enter the waveguide at the electron gun end and must either pass out at the high energy end or be absorbed without reflection

23. Traveling Wave Linac

24. Standing Wave Linac • Notes • In this case one terminates the waveguide with a conducting disc thus causing a p/2 reflection • Standing waves form in the cavities (antinodes and nodes) • Particles will gain or receive zero energy in alternating cavities • Moreover, since the node cavities don’t contribute to the energy, these cavities can be moved off to the side (side coupling) • The RF power can be supplied to any cavity • Standing wave linacs are shorter than traveling wave linacs because of the side coupling and also because the electric field is not attenuated

25. Standing Wave Linac

26. Standing Wave Linac • Side coupled cavities

27. Electron Source • Based on thermionic emission • Cathode must be insulated because waveguide is at ground • Dose rate can be regulated controlling the cathode temperature • Direct or indirect heating • The latter does not allow quick changes of electron emission but has a longer lifetime

28. RF Generation • Magnetron • As seen in your microwave oven! • Operation • Central cathode that also serves as filament • Magnetic field causes electrons to spiral outward • As the electrons pass the cavity they induce a resonant, RF field in the cavity through the oscillation of charges around the cavity • The RF field can then be extracted with a short antenna attached to one of the spokes

29. RF Generation • Magnetron

30. RF Generation • Magnetron

31. RF Generation • Klystron • Used in HEP and > 6 MeV medical linacs • Operation – effectively an RF amplifier • DC beam produced at high voltage • Low power RF excites input cavity • Electrons are accelerated or deaccelerated in the input cavity • Velocity modulation becomes time modulation during drift • Bunched beam excites output cavity • Spent beam is stopped

32. RF Generation • Klystron

33. Electron source Bending magnet Accelerating structure Pulse modulator Klystron or magnetron Treatment head Medical Linac • Block diagram

34. Medical Linac

35. Medical Linac

36. Cyclotron • The first circular accelerator was the cyclotron • Developed by Lawrence in 1931 (for $25) • Grad student Livingston built it for his thesis • About 4 inches in diameter

37. Cyclotron • Principle of operation • Particle acceleration is achieved using an RF field between “dees” with a constant magnetic field to guide the particles

38. Cyclotron • Principle of operation

39. Cyclotron • Why don’t the particles hit the pole pieces? • The fringe field (gradient) provides vertical and (less obviously) horizontal focusing

40. Cyclotron • TRIUMF in Canada has the world’s largest cyclotron

41. Cyclotron • TRIUMF

42. Cyclotron • NSCL cyclotron at Michigan State

43. Cyclotron

44. Betatron • Since electrons quickly become relativistic they could not be accelerated in cyclotrons • Kerst and Serber invented the betatron for this purpose (1940) • Principle of operation • Electrons are accelerated with induced electric fields produced by changing magnetic fields (Faraday’s law) • The magnetic field also served to guide the particles and its gradients provided focusing

45. Betatron • Principle of operation Steel r Coil <B> B0 Vacuum chamber Bguide = 1/2 Baverage

46. Betatron • Principle of operation

47. TM Modes

48. TE Modes Dipole mode Quadrupole mode used in RFQ’s

49. Waveguides