M1 accelerators

Accelerators

Layout of Presentation

Introduction

History

Types of Accelerator

LINAC

Pelletron

HCI

Types

Need

How

• To probe in to the structure of nuclei (matter) using different energy beams to study nature of matter.

Use of Accelerator

Research

Ion

implantation

Material Characterization

Industrial processing

Radiotherapy

Biomedical purposes

Accelerators @ IUAC, New Delhi

Atom beam sputtering system: ~1.0keV

Low energy ion beam facilities: Few tens of

keV – MeV

1.7 MeV Pelletron Accelerator

15UD Pelletron Accelerator

Superconducting LINAC

Accelerators

Particle Accelerator:-

Any device that accelerates charged

particles to very high speeds using electric

and/or magnetic fields.

They all have the same three basic parts:

• Source of elementary particles or ions

• Tube pumped to a partial vacuum in which the particles can travel freely

• Some means of speeding up the particles.

Electrostatic Accelerators-

Cockroft-Walton, Van de Graaff

Induction Accelerators

-

Betatron

Types

RF accelerators-

LINAC, Cyclotron,

Synchrotron

Development of Accelerators

1870 • William Crookes- Cathode rays

1895 • J.J. Thomson- Electrons.

1896 • Röngten- X-rays

1909 • Rutherford- Scattering of alpha particle on a gold foil

1929 • E. O. Lawrence- Cyclotron (circular device)

1931 • Robert Jemison Van de Graaff - Van de Graaff accelerator

1932 • John D. Cockcroft and Ernest Walton- Cockroft-Walton generator

1940 • Donald W. Kerst - Betatrons

Van de Graff Accelerator

Builds up a potential between two electrodes by transporting charges on a moving belt.

Charges are mechanically carried by a conveyor belt from a low potential source to a high potential collector.

Van de Graaff accelerators can accelerate particles to energies as high as 15 MeV.

Capable of producing particles of higher energy than the energies of radioactive decay.

PELLETRON ACCELERATOR

Pelletron - Electrostatic particle accelerator

Similar to a Van de Graaff generator.

Pelletrons have been built in many sizes,

Small units producing voltages- up to 500 kV)

Beam energies-up to 1 MeV of kinetic energy, to the

Largest system DC voltage- over 25 MV

Beam energies- over 900 MeV

The IUAC Pelletron

- ve Ion Source

Injector Magnet

Terminal

Analyzing Magnet Switching Magnet

Buncher

Ion

Source

Room

Tower

Vault

Beam Hall

Tank Radio-biology

Atomic Physics

HIRA

Materials Science

GDA

Scattering

Chamber

Tank ht: 26.5 m Diameter: 5.5 m Pressure: 86 PSI of SF6 gas Ions accelerated: H to Au beams Ion Currents: Typically 5 - 50 pnA Energy : 30 - 250 MeV

Generating electric charge is done by a mechanical

transportation system made of a chain of pellets (short

conductive tubes connected by links made of insulating

material), that is used to build-up high voltages on the

Pelletron terminal.

The system is enclosed by a pressure vessel filled with

insulating gas , such as SF6 (sulfur hexafluoride), and an

evacuated beamline. The potential difference between the

terminal and ground is used to accelerate several kinds of

particles, such as positron, electrons and negative and

positive ions.

Comparing with Van de Graaff Generator

Compared to the Van de Graaff generator, the pellet chain can

operate at a higher velocity than a rubber belt, and both the

voltage and currents that can be attained are far higher.

The chain is charged more uniformly than the belt of a Van de

Graaff, so the stability of the terminal voltage and the particle

energy is also higher.

Applications of Pelletron Accelerators

Pelletron accelerators are used as analytical tools in many

fields, including -

• Materials analysis,

• Nuclear physics,

• Semiconductor development and production,

• Pharmaceutical research, and as

• Ultra-sensitive mass spectrometers for carbon dating

and the

• Measurement of other rare isotopes.

Linear Accelerators

The Principle : Rolf Widroe

• In Linear accelerators the charged particle receives several

small energy kicks in acceleration gaps between drift tubes that are

powered by a radio frequency voltage source.

• The length of the drift tubes is so adjusted that the time taken by the

particle to traverse a drift tube is integral multiple of half RF period.

• The Drift tube length is thus particle velocity dependent and increases

with particle energy.

• Very high particle

energies can be achieved

through multiple

acceleration.

SLAC : 25 GeV electrons

ILC : 250 GeV electrons L=v/2f

L

Linear Accelerators

The Principle : Alvarez

The Widroe structure becomes inefficient at high frequencies due to

dissipation of electromagnetic energy.

Alvarez structure is enclosed in a metallic tank to form a resonant

cavity.

Unlike the Widroe structure the drift tubes are passive structures

and the accelerating field arises from the electromagnetic radiation

flooding the tank.

L

L=v/f

Linear Accelerators

Superconducting Linear Accelerators

Why Superconductivity ??

Q

EUω P

P

U Q

cycle RFper dissipatedEnergy

StoredEnergy 2π (Q)Factor Quality

2

a00

0ω

Suppose, stored energy = 0.1 J/(MV/m)2 & = 2 x 108 Hz

the resonator @ room temp, Q = 10000

to generate a field of 1 MV/m Power ~ 6 KW

the resonator @ LHe temp, Q = 108

to generate a field of 1 MV/m Power ~ 0.6 W

Linear Accelerators

Superconducting Linear Accelerators

Why Superconductivity ??

However

From the concept of efficiency of Carnot Cycle,

The electrical power required at R.T.(300K) to remove

1 watt heat at 4.2 K (Refrigeration)

= (300-4.2)/4.2 = 70 watts

Assuming refrigerator efficiency = 0.2 (i.e. 20%)

Power required to keep LHe @ 4.2K ~ 70 x 5 ~ 350 watts

So, power required to produce 1 MV/m field inside the

SC resonator = 350 x 0.6=210 Watts << 8 KW of power for

R.T. QWR

Linear Accelerators

Materials for Superconducting LINAC

Requirements • High critical temperature (Tc)

• High Critical magnetic field (Hc)

• Good machinability

Niobium Lead

Hc 1980G 803G

Tc 9.2 K 7.2 K

Rs 7x10-9 W 3x10-8 W

@4.5 K, 150 MHz

Linear Accelerators

Alternatives Materials to bulk Niobium

Nb3Sn

• Tc 18.2 K

• Hc 5350 G

Cavities are formed by vapor diffusion of tin into Niobium

Fields achieved are of the order of ~15 MV/m

Niobium thin film sputtered on copper

High Tc Superconductors

Performances deteriorate at high fields.

Fields achieved are of the order of ~15 MV/m

Linear Accelerators The Superconducting Quarter wave

Resonator at IUAC

Material - Niobium (inside)

Stainless Steel(outside)

Bath cooled by LHe (4.2 K)

Four important ports on QWR

to put power (coupler)

to take back small amount

of power (pick up)

to pass the beam thru QWR

A niobium bellow acts as tuner

Linear Accelerators The Superconducting Quarter wave

Resonator at IUAC

Frequency 97MHz

Length of central conductor

~l/4

TEM mode, EM wave

propagation along the length

High voltage at the open end,

high current at the shorted end.

bopt0.08, distance between

the mid points of the two gaps:

d boptl/2

Linear Accelerators The Superconducting Linac

Beam Acceleration through Linac: Bunching

•Beam is bunched before injection into Linac

•Bunch width ~ 200ps.

•Bunching in two steps:

Pre-tandem buncher ~ 1.5ns

Post tandem superbuncher ~ 200ps.

• Bunching : Velocity modulation, retard the early arriving particle

and accelerate the late arriving particle.

Time

Early

On Time

Late

Vo

ltag

e

+

-

+ ve particles

Beam Acceleration through Linac: Acceleration

• Acceleration is done @ 70° phase angle for phase stability

Time

Early

On Time Late

Vo

ltag

e

+

-

+ve particles

Beam Acceleration through Linac: Energy Gain

• For a single Resonator:

If gap voltage is Vg &

q is the charge

Maximum energy gain= 2Vgq

However, the voltage is varying with time and a charged particle

takes finite time to cross the Resonator

Hence the actual energy gain is less that the maximum value

Transit time effect

Thus actual energy gain (DE)2Vg qT(b) DEEa L q T(b) Ea=2Vg/L is the accelerating field

L is the active accelerating length

Beam Acceleration through Linac:Energy gain

T(b) is the transit time factor and a function of particle velocity

Beta Vs TTF Plot for the IUAC QWR

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32 0.36

Beta

TT

F(A

bs

olu

te)


Further since acceleration is done at 70°phase angle the

energy gain is still lesser

Energy gain from one Resonator(DE)Ea L q T(b) Sinf

If there are N Resonators in Linac the total energy gain

Etotal=N DE


For our linac, accl. Field Ea ~ 4 MV/m (say) @ 6 watts power

Accelerating length of QWR = 16 cm = 0.16 m

So if the linear accelerator contains contains 24 resonators,

total energy of the beam per charge state

Etotal /q = 24 x 0.64 x 0.8 x0.94 = 11.6 MV/q (assuming T(b)=0.8,

and f=70°)

Suppose from Tandem, O-beam of energy 100 MeV enters

into linear accelerator with a charge state of 8+ . Then

at the exit of the linac, the total energy will be

= 100 + 11.6 x 8 = 192.8 MeV

HCI

High Current Injector : Motivation

High Current More Species

Present : Typically few nA

Aim : few hundred of nA Noble Gases etc…

Major Components of HCI

HTS ECR Ion Source (PKDELIS)

Room Temperature RFQ,

48.5 MHz

Room Temperature DTL,

97 MHz

Eout = 8 keV/amu

Eout = 180 keV/amu

Eout = 1.8 MeV/amu*

SC Low Beta QWR

Module, 97 MHz b = 0.05

Design Value A/q 6

Bunching Technique

A dc/long bunch enters an RF cavity; the reference particle is the one which has no velocity change. The others get accelerated or decelerated. After a distance L bunch gets shorter while energies are spread: bunching effect.

L

Radio Frequency Quadrupoles

Radio Frequency Quadrupoles (RFQ) is a Linear Accelerator for

high current low velocity ion beams which focuses, bunches and

simultaneously accelerates with the help of RF quadrupole fields

These accelerators follow the ion source and serve as injectors to

Widroe or Alvarez type Linacs.

The principles of operation of the RFQ were first presented by

two scientists (Kapchinsky and Teplyakov) from ITEP Moscow

in the former Soviet Union (now Russia) in the late 1960’s.

First practical RFQ was realized by LANL (then LASL) in

1980. This 425 MHz RFQ accelerated a 100keV proton beam to

640 keV.


The Quadrupole Lens

+

-

+

-

N S

N S

Magnetic Quadrupole Lens

Electric Quadrupole Lens

Focusing in one plane, de-focusing in the other


F F

F F

F

D D D F

D D D Horizontal

Vertical

A focusing RFQ


RFQ with acceleration Vertical electrodes

Horizontal electrodes

Vertical electrode

Horizontal electrode

Quadrupole

focusing field

Axial

acceleration

field


Four Vane type RFQ

Capacitance between the vane tips, inductance in the intravane space.

Frequency depends on the cylinder dimensions. High frequency

structures (frequencies of the order of ~200MHz).

Each vane behaves as a resonator. Suitable for low energy protons.


Four Rod type RFQ

Capacitance between the rods, Inductance with the holding bars.

Frequency independent of the cavity dimensions.

Each cell is a resonator. Low frequency structures suitable for very

low velocity heavy ions.


Frequency: 48.5 MHz

Length : 4m

A/q : 7

RF power : ~100kW

8 keV/u 180 keV/u

The IUAC RFQ

Drift Tube Linac

Features

Tank Diameter 85 cm

Tank Length 38.5 cm

Operating

Frequency

97.000 MHz

A/q 6

Input Energy 180 keV/u

Output Energy 320 keV/u

No. of Cells 11

Prototype DTL Development

Chamber

Stem

Ridge

Transit Time Factor (TTF) Curve

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32

Beta (v/c) ->

TT

F(A

bs

olu

te)

Beta = 0.060 (TTF~80%)

E/A ~ 1.8 MeV/u

Need of low beta module…

Parameter Value

b 0.05

f 97 MHz

U0 26 mJ

Features

Low Beta Cavity

SC low b module located so that it can accelerate beams from

HCI as well as Pelletron accelerator.

Education

M1 accelerators