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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

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AcceleratorsCirca 2000

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AcceleratorsA brief history

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Accelerators

A brief historyElectrostatic (Cockcroft-Walton, van de Graaf)Linac (linear accelerator)Circular (cyclotron, betatron, synchrotron)Development of strong focusingColliding beams (present day)Plasma wakefield, ???

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Accelerators“Moore’s law” ~ e+t/C

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Accelerators“Moore’s law”

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LinacLinac = linear acceleratorApplications in both high energy physics and radiation therapy

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LinacLinacs 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 mSLAC (50 GeV) – 3.2 kmILC (250 GeV) - 11 km

Linac – static field, induction (time varying B field), RF

Operate in the microwave regionTypical RF for medical linacs ~ 2.8 GHz Typical accelerating gradients are 1 MV/m – 100 MV/m

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LinacBrief history

Invented by Wideroe (Germany) in 1928Accelerated potassium ions to 50 keV using 1 MHz AC

First realization of a linac by Sloan (USA) in 1931No further progress until post-WWII when high power RF generators became availableModern 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)

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LinacWideroe’s linac

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LinacAlvarez drift tube linac

First stage of Fermilablinac

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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 structuresTraveling wave structures

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LINAC

Most medical linacs are standing wave type

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TM Modes

TM01 mode

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Waveguides

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WaveguidesPhase and group velocity

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WaveguidesThe phase velocity can be slowed by fitting the guide with conducting irises or discsThe derivation is complicated but alternatively think of the waveguide as a transmission line

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Waveguides

Disc loaded waveguide

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Traveling Wave LinacNotes

Injection energy of electrons at 50 kV (v=0.4c)The electrons become relativistic in the first portion of the waveguideThe first section of the waveguide is described as the buncher section where electrons are accelerated/deacceleratedThe final energy is determined by the length of the waveguideIn 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

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Traveling Wave Linac

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Standing Wave LinacNotes

In this case one terminates the waveguide with a conducting disc thus causing a π/2 reflectionStanding waves form in the cavities (antinodes and nodes)Particles will gain or receive zero energy in alternating cavitiesMoreover, 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 cavityStanding wave linacs are shorter than traveling wave linacs because of the side coupling and also because the electric field is not attenuated

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Standing Wave Linac

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Standing Wave Linac

Side coupled cavities

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Electron SourceBased on thermionic emissionCathode must be insulated because waveguide is at groundDose rate can be regulated controlling the cathode temperature

Direct or indirect heatingThe latter does not allow quick changes of electron emission but has a longer lifetime

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RF Generation

MagnetronAs seen in your microwave oven!Operation

Central cathode that also serves as filamentMagnetic field causes electrons to spiral outwardAs the electrons pass the cavity they induce a resonant, RF field in the cavity through the oscillation of charges around the cavityThe RF field can then be extracted with a short antenna attached to one of the spokes

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RF Generation

Magnetron

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RF Generation

Magnetron

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RF GenerationKlystron

Used in HEP and > 6 MeV medical linacsOperation – effectively an RF amplifier

DC beam produced at high voltageLow power RF excites input cavityElectrons are accelerated or deaccelerated in the input cavityVelocity modulation becomes time modulation during driftBunched beam excites output cavitySpent beam is stopped

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RF Generation

Klystron