Dawn of Particle Accelerator Technology

Home , Van de Graaff generator

. Fundamental Concepts of Particle Accelerators . I : Dawn of Particle Accelerator Technology . .. . . Koji TAKATA

KEK

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

Accelerator Course, Sokendai

Second Term, JFY2013

Oct. 24, 2013 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. 2013 2 / 1 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 along with World War II（1941 - 1945）

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 3 / 1 Discovery of artiﬁcial nuclear disintegration（1919 - 1932）and the birth of particle accelerators (1)

Ernest Rutherford (Cavendish Lab, Cambridge, UK）discovered nuclear disintegration by the alpha (α) rays (1917 - 1919). • He conﬁrmed that protons were produced in a nitrogen-gas ﬁlled box containing an alpha ray source.

14 → 16 α + 7N p + 8O

This discovery provoked strong demands to artiﬁcially generate high energy beams to study in more detail the nuclear disintegration phenomena.

Thus started the race for developing high energy accelerators, and, naturally enough, Rutherford himself was a great advocator.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 4 / 1 Discovery of artiﬁcial nuclear disintegration（1919 - 1932） and birth of particle accelerators (2)

The ﬁrst disintegration of atomic nuclei with accelerator beams was achieved at the Cavendish Laboratory in 1932 by John D. Cockcroft and Ernest T. S. Walton, who used proton beams accelerated by a 800 kV DC voltage-multiplier. 7 → p + 3Li α + α

The multiplier was of a type invented by H. Greinacher (1919), a Swiss engineer, but was greatly improved by them.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 5 / 1 DC HV Accelerators

DC Generators: two major methods

Voltage-multiplier circuits with capacitors and rectiﬁer tubes ﬁrst used by Cockcroft & Walton.

Belt-charged generators ﬁrst constructed by Van de Graaﬀ (1931).

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

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

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

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 6 / 1 Cockcroft & Walton’s voltage-multiplier circuit

V cos ωt V(1+cos ωt) V(3+cos ωt) V(5+cos ωt)

0 2V 4V 6V 0

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 7 / 1 Cockcroft-Walton Accelerator (1)

J. D. Cockcroft and E. T. S. Walton, Proc. Roy. Soc. (London) A137 (1932) p.229. Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 8 / 1 Cockcroft-Walton Accelerator (2)

J. D. Cockcroft and E. T. S. Walton, Proc. Roy. Soc. (London) A137 (1932) p.229.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 9 / 1 Cockcroft around 1932

Please see the ﬁgure in p.227 of ref.∗

∗E. Segr`e, From X-rays to Quarks, p.227, (W. H. Freeman and Company, 1980) Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 10 / 1 Van de Graaﬀ’s 1.5 MV Belt-charged Generator

Insulating Belt

High Voltage for Acceleration

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 11 / 1 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.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 12 / 1 High Voltage Breakdown Demonstration with a Van de Graaﬀ generator

∗

∗http://web.mit.edu/museum/exhibitions/mindhand.html Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 13 / 1 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: ∂B ∇ × E = − . ∂t

• Integrate the tangential component of the electric ﬁeld E along a closed boundary C of an area S: I ∫∫ ∂ ∂ E · dl = − B · n dxdy = − Φ, C ∂t S ∂t where dl: line element of the curve C, and n: unit normal-vector of the area dS = dxdy.

∗There had been an earlier proposal by Wider¨oe(1924). Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 14 / 1 Kerst’s First Publication of the Betatron

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 15 / 1 First Linear Accelerator (Linac) by Wider¨oe

Proposal by Gustaf Ising (Sweden, 1925). Trial study by Rolf Wider¨oe (Norway/Germany, 1928).

VRF ∼ 25 kV (1 MHz) per gap ×2 with a drift tube. He convinced that the scheme can be repeated any number of times to reach ever higher beam energies.

RF Ion Source Beam Drift Tube

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

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 16 / 1 Ernest Lawrence’s Cyclotron (1931)

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

• The ﬁrst circular accelerator.

• Multiple acceleration at the cyclotron frequency

ωc = eB⊥/m.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 17 / 1 Early Cyclotrons

Please see the ﬁgure in p.229 of ref.a

A cyclotron at RIKEN, Japan, accelerated aE. Segr`e, From X-rays to Quarks, p.227, protons to 9 MeV and (W. H. Freeman and Company, 1980) deuterons to 14 MeV (1939).

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 18 / 1 Circular Motion of Particles in the Cyclotron

RF Generator

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

mvc r r > r • n n+1( n) orbit radius: r = |e|B . • |e|B revolution frequency: fc = 2πm . • f depends only on B and neither on Magnetic Field Electric Field r nor on v.

• beam cyclotron frequency: ωc = 2πfc. dee dee

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 19 / 1 Demonstration of the Circular Orbit of Electron Beams in a Magnetic Field

∗

∗http://en.wikipedia.org/wiki/File:Cyclotron motion.jpg Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 20 / 1 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 the circular 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.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 21 / 1 Breakthrough Innovations 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.

Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2013 22 / 1