Fundamental Concepts of Particle Accelerators I : Dawn of ... · Dawn of Particle Accelerator...

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Fundamental Concepts of Particle AcceleratorsI : Dawn of Particle Accelerator Technology

Koji TAKATA

KEK

[email protected]://research.kek.jp/people/takata/home.html

Accelerator Course, Sokendai

Second Term, JFY2012

Oct. 25, 2012

Contents

§1 Dawn of Particle Accelerator Technology

§2 High-Energy Beam Dynamics: (1)

§3 High-Energy Beam Dynamics: (2)

§4 RF Acceleration

§5 Future of the High Energy Accelerators

§6 References

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 2 / 20

Dawn of Particle Accelerator Technology

Contents

1 discovery of artificial nuclear disintegration（1919 - 1932）and birth of particle accelerators

2 various types of early accelerators

3 from DC acceleration to RF acceleration

4 problems in RF acceleration

5 Great Progress Just after World War II（1941 - 1945）


Discovery of artificial nuclear disintegration（1919 - 1932）and the birth of

particle accelerators (1)

Ernest Rutherford (Cavendish Lab, Cambridge, UK）discoverednuclear disintegration by the alpha (α) rays (1917 - 1919).

• He confirmed that protons were produced in a nitrogen-gas filledcontainer in which a radioactive source emitting alpha rays was placed.

α+ 147N → p+ 16

8O

This discovery provoked strong demands to artificially generate highenergy beams to study in more detail the nuclear disintegrationphenomena.

Thus started the race for developing high energy accelerators, andRutherford himself was a great advocator.


Discovery of artificial nuclear disintegration（1919 - 1932）and birth of particle accelerators (2)

The first disintegration of atomic nuclei with accelerator beams wasachieved at the Cavendish Laboratory in 1932 by John D. Cockcroftand Ernest T. S. Walton, who used 800 kV proton beams acceleratedby a DC voltage-multiplier.

p+ 73Li → α+ α

They revised the multiplier circuit first invented by H. Greinacher(1919).


DC HV Accelerators

DC Generators：two major methods

Cockcroft & Walton’s 800 kV voltage-multiplier circuit withcapacitors and rectifier tubes.

Van de Graaff’s 1.5 MV belt-charged generator (1931).

Electrostatic accelerators are still in use for the mass spectroscopy,because of their fine and stable tunability of the acceleration voltage.

• analysis of the ratio 14C/12C : an important tool for archaeology.

• the time after a creature stopped breathing is estimated in 14C’s halfdecay time 5, 730 years.


Cockcroft & Walton’s voltage-multiplier circuit

6V 0

V(3+cos ωt)V(1+cos ωt)V cos ωt

AC

V(5+cos ωt)

4V2V0


Cockcroft around 1932

Ref.:E. Segre, From X-rays to Quarks, page 227,

(W. H. Freeman and Company, 1980).


Van de Graaff’s 1.5MV Belt-charged Generator

Insulating Belt

High Voltage for Acceleration


HV Limits in Electrostatic Accelerators

DC acceleration is limited by high-voltage breakdown (BD).

• Typical BD voltages for a 1cm gap of parallel metal plates

Ambience Typical BD Voltages

Air (1 atm) ≈ 30 kV

SF6 gas (1 atm) ≈ 80 kV

SF6 gas (7 atm) ≈ 360 kV

Transformer oil ≈ 150 kV

Ultra High Vacuum ≈ 220 kV

• Wider gaps do not make drastic improvement in BD limits.


High Voltage Breakdown Demonstration for a Van de Graaff generator

Ref. :“van der graaf generator” in “ http://en.wikipedia.org/wiki/”


Intermediate stage towards RF Acceleration：D. W. Kerst’s betatron (1940)

Electric field due to time variation of the magnetic flux Φ.

• The AC transformers work on this principle.

• Faraday’s law in Maxwell’s equation:

∇×E = −∂B

∂t.

• Integrate the tangential component of the electric field Ealong a closed boundary C of an area S:∮

CE · dl = − ∂

∂t

∫∫SB · n dxdy = − ∂

∂tΦ,

where dl: line element of the curve C, andn: unit normal-vector of the area dS = dxdy.


Kerst’s First Publication of the Betatron


First Linear Accelerator (Linac) by Wideroe

Proposal by Gustaf Ising (Sweden, 1925).

Trial study by Rolf Wideroe (Norway/Germany, 1928).

VRF ∼ 25 kV (1MHz) per gap ×2 with a drift tube.

He convinced that the scheme can be repeated any number of timesto reach ever higher beam energies.

RF

BeamIon So urce

Drift Tube

This is the prototype of the present-day drift tube linacs (DTL).


Ernest Lawrence’s Cyclotron (1931)

Trial study of the multiple RF acceleration of charged particlesmoving on a circular orbit in a magnetic field.

• The first circular accelerator.

• Multiple acceleration at the cyclotron frequency

　　　　 ωc = eB⊥/m.


Early Cyclotrons

Lawrence with the first cyclotron

Ref. :Segre, E. From X-rays to Quarks,page 229 (W. H. Freeman and

Company, 1980)

A cyclotron at RIKEN, Japan, acceleratedprotons to 9 MeV and

deuterons to 14 MeV (1939).


Circular Motion of Particles in the Cyclotron

�

RF Generator

rn rn+1(> rn)

Electric FieldMagnetic Field

dee

dee

dee

dee

beam

Circular orbit of particles with charge eand mass m in magnetic field B（assuming β = v/c ≪ 1).

• orbit radius: r = mvc|e|B .

• revolution frequency: fc =|e|B2πm .

• f depends only on B and neither onr nor on v.

• cyclotron frequency: ωc = 2πfc.


Demonstration of the Circular Orbit of Electron Beams in a Magnetic Field

Ref. :“http://en.wikipedia.org/wiki/Cyclotron”


Problems in RF Acceleration

1 Linacs：

• poor RF power sources: electron tube technology was not yet matured.

2 Cyclotrons：

• relativistic increase of particle mass:

→ decrease of ωc,

→ asynchronism with RF.

3 Betatrons：

• It was very difficult to inject and trap electron beams correctly on thecircular orbit in the donut.

• Indispensable was the analysis of the transverse oscillations of particles.

• It led to the present-day theory of the betatron oscillations.


Great Progress Just after World War II

1 Discovery of the phase stability principle in RF acceleration.

• Vladimir Veksler (1944) and Edwin M. McMillan (1945).

• Cyclotrons.→ synchrocyclotron, and eventually

→ synchrotron.

2 Strong focusing: new idea for the transverse beam focusing.

• Christofilos (1950) and Courant-Livingston-Snyder (1952).

3 Radars in practical use quickened the development of high power

microwave tubes.

• magnetrons and klystrons.


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