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

Accelerators Circa 2000

Accelerators A brief history

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, ???

Accelerators “Moore’s law” ~ e+t/C

Accelerators “Moore’s law”

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

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

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)

Linac Wideroe’s linac

Linac Alvarez drift tube linac First stage of Fermilab linac

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

LINAC Medical linacs can be either type

Waveguides

Waveguides Cyclindrical wave guide

TM Modes TM01 mode

Waveguides

Waveguides Phase and group velocity

Waveguides Phase and group velocity

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

Waveguides Disc loaded waveguide

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

Traveling Wave Linac

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

Standing Wave Linac

Standing Wave Linac Side coupled cavities

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

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

RF Generation Magnetron

RF Generation Magnetron

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

RF Generation Klystron

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

Medical Linac

Medical Linac

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

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

Cyclotron Principle of operation

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

Cyclotron TRIUMF in Canada has the world’s largest cyclotron

Cyclotron TRIUMF

Cyclotron NSCL cyclotron at Michigan State

Cyclotron

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

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

Betatron Principle of operation

TM Modes

TE Modes Dipole mode Quadrupole mode used in RFQ’s

Waveguides