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) – m
SLAC (50 GeV) – 3.2 km
ILC (250 GeV) 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. Accelerating structure
Medical Linac
Block diagram
Pulse modulator
Klystron or magnetron
Bending
magnet
Electron
source
Accelerating structure
Treatment
head

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