experimental tools in physics: electrostatic accelerators jan pallon€¦ · ·...

2012-11-28 Electrostatic Accelerators Experimental tools in Physics

Experimental tools in Physics:Electrostatic Accelerators

Jan Pallon

• High vacuum• Ion sources• High Voltage generation• Beam optical components• Radiation Protection

•Atmosphere: 1013 mbar (=760 torr [ mm/Hg] )

•Low vacuum: >10-2 – 10-3 mbar•High vacuum: between 10-3 and 10-8 mbar•Ultrahigh vacuum: <10-8 mbar

•Above 10-2 – 10-3 mbar: Viscous flow•Below 10-2 – 10-3 mbar: Molecular flow


Vacuum

Designing a vacuum system• Low vacuum: The gas behaves like a liquid. Narrow pipes OK!

Viscous conductivity for a pipe:CV=(180.D4/L).Pav CV: conductivity in l/s

D: Diameter of pipe in cmL: Length of pipe in cmPav: average pressure in torr

• High vacuum: Wide openings required!!!– Molecular conductivity for a pipe:

Cpipe=11.6.A/(1+L/D) Cpipe : conductivity in l/sA: crossection area of pipe in cm2

L: : Length of pipe in cmD: Diameter of pipe in cm


Why pump vacuum all the time?

Sources of gases in a vacuum system• Gas from initial atmosphere• Leakages• Out-gassing of material• Vapour pressure of material• Material permeability

– Absorption on high-pressure side– Diffusion through material– Desorption on low-pressure-side


Low vacuumRotary vane pump

10-2 – 10-4 mbar


High vacuum pumps

• Diffusion pump• (Cryopump)• Turbo-molecular pump


Diffusion pump


Turbo-molecular

pump


Electrostatic accelerators and ion sources


Two main types of electrostaticaccelerator


+ High Voltage U

Ion source, positive ions

1. Single-endedaccelerator, ion sourceinside tank

Ekin=q*U (eV)


+ High Voltage U

Ion source, negative ions

2. Tandem accelerator, ion source outside tank

Charge changer; stripper gas or foil

-q1 +q2

Ekin=(q2 + q1)*U (eV)


High voltage stabilizer

Kirchoff´s law: • Current up = • ion current + • column current + • tube current + • corona current.

Corona current is used for HV regulation, a thin needle points towards the terminal and a small discharge current flows through a pentode. Feedback voltage controls the discharge current.


Ion sources• The purpose of the ion source is to produce

either positive or negative ions from neutral atoms.

• Positive ion sources are placed inside the tank of a single-ended accelerator;

• negative ion sources inject the ion beam into the tank of a tandem accelerator.

• Different types of sources may be used depending on the mass and charge of the desired ion.


Ion sources – Duoplasmatronpositive (and negative)


The Source of Negative Ions by Cesium Sputtering (SNICS) produces a negative ion beam. Cesium vapor comes from the cesium oven into an enclosed area between the cooled cathode and the heated ionizing surface. Some of the cesium condenses on the front of the cathode and some of the cesium is ionized by the hot surface. The ionized cesium accelerates towards the cathode, sputtering particles from the cathode through the condensed cesium layer. Some materials will preferentially sputter neutral or positive particles which pick up electrons as they pass through the condensed cesium layer, producing negative ions.


RF – negative ions by charge exchanger

An RF (radio frequency) ion source produces positive ions. A gas or gas mixture is bled into a quartz bottle; an RF oscillator connected to the quartz bottle dissociates the neutral gas. A voltage difference (usually about 2-6 kV) is used to push the ions out of the chamber through the exit aperture, making a continuous beam. To produce a negative beam, the positive beam is immediately injected into a rubidium charge exchange cell.


Radio Frequency Ion Source (RF)H2

100 MHz


Radio Frequency Ion Source (RF)H2

100 MHz

20 µA p

+ 20 kV (extractor voltage)

Probe

1-2 kV


Ion source and lenses

RF-source Einzel lens

Velocity selector (crossed E/B fields)

+ 20 kV extractor


Single-ended electrostatic accelerator


Microbeam acceleratorin Lund


Charging system

• The purpose of the charging system is to generate the high voltage to be used for ion acceleration.

• Pelletron• Transformer/ rectifier system


The Pelletron

Pelletron chains are made of metal pellets connected by insulating nylon links and are charged by an induction scheme that does not use rubbing contacts or corona discharges.


For a positive terminal Pelletron, the negatively-charged inductor electrode pushes electrons off the pellets while they are in contact with the grounded drive pulley. Since the pellets are still inside the inductor field as they leave the pulley, they retain a net positive charge.


The chain then transports this charge to the high-voltage terminal, where the reverse process occurs. When it reaches the terminal, the chain passes through a negatively-biased suppressor electrode which prevents arcing as the pellets make contact with the terminal pulley. As the pellets leave the suppressor, charge flows smoothly onto the terminal pulley, giving the terminal a net positive charge. Most Pelletrons, as shown in the charging system diagram, employ "down-charging" as well as "up-charging."


Down-charging works identically to up-charging, except the inductor/suppressor polarities are reversed, and it effectively doubles the charging current capacity of the chain. For the configuration shown here, small, slightly-conductive "pickoff pulleys" provide the voltages for the terminal suppressor/inductor electrodes by drawing a tiny amount of charge from the chain, each pulley thus biasing the opposing electrode. This technique allows down-charging "for free," i.e. without the use of HV supplies in the terminal.


• Depending on the particular design options, the system delivers charging currents of 100 - 200 µA or more per chain to the high voltage terminal. The drive pulleys, typically 30 cm to 60 cm in diameter, and motors are supported on movable platforms which are

counterweighted, automatically providing proper chain tension.


Singletron (High Voltage Engineering)


CENBG, Bordeaux


Very good energy stability

iThemba labsCape Town


6 MV vertical VDG.


Beam transport

All movements of charged particles through an accelerator and beam handling system are along with • Maxwell's equations

and • Newton's second law.

The motion of charged particles in electrostatic and magnetic fields is described by the Lorentz equation

d(mv)/dt = qE + qv×B.


Simple case: Electrostatic field

Consider two parallel plates having an electrostatic field E between them.

Positively charged particle in a uniform electrostatic field.

0F

qEdt

ydmF

0F

z

y2

2

y

x


Electrostatic steerer

Angular deviation:dT2

Uq

q: charge of particlel: length of plated: distance between platesU: Potential of platesT: is the kinetic energy


Aperture

+

-

E

B

Velocity filter (or cross-field analyser, or Wien filter)

For the undeflected particles the two forces must be identicalFE = FB

orqE = qvB

Velocity selector!

Electrostatic field E applied at right angle to the beam and a magnetic field B orthogonal to both E and the beam.

FB

FE


Alternative velocity filter: The electrostatic analser (ESA)

Fig. 41 The electrostatic force on a positively charged particle is in the direction of the electrostatic field, i.e., from positive to negative potential. Thus, the potentials ±V0 at two concentric electrodes can be chosen such that a particle of energy K0 and charge (ze) can move along a circle in the middle between the two electrodes. In this case, the electrostatic force is always directed toward the centre of curvature of the electrodes and is thus perpendicular to the circular particle trajectory. Consequently, along this circle, charged particles move with constant velocity.


Electrostatic aperture lens

E1 E2

E2>E1: focussing

E2<E1: defocussing


Example: cylindrical gap lensCylindrical electrodes surrounding the beam creates the most elementaryelectrostatic lens

Equipotential surfaces are indicated.


Gap lens

Changes the energy of the beam!


Einzel lens: no energy change!


Electrostatic quadrupole lens


Magnetic fields

z

y

x

B

R=mv/qB


Dipole magnet,changes the beamdirection

Magnetic quadrupole lens


Q-pole field


Q-pole focusing


The Nuclear MicroprobeFocusing Principle

Quadrupole lenses

Chamber of analysis

Aperture slits

Object slits

Deflecting/ analyzing magnet

Accelerator


Ion optics tools


x

y

z

Ion Coordinates: x, y, zAngles: dx/dz, dy/dz

x (mm)

x’=dx/dz (mrad)

Phase space

y (mm)

y’ (mrad)

Transfer matricesfor rays of the beam

LTH-NANOBEAM-E

Horizontal /Vertical focus orderVH, HV, VH (-++--+)

H

V


Ellipses of phase space



Accelerator: X-rays and -rays are expected from the acceleration tubewhere electrons may be released and accelerated against the high voltageterminal.

Modern machines have permanent magnets along the tube to force electronsinto spiral orbits instead. Bad vacuum can quickly increase radiation due to collisions with molecules.For electrostatic accelerators running at energies below 3MeV, it will normallynot be a problem to stay close to the accelerator.For cyclotrons (and larger) – on the other hand, radiation is intense and the accelerator must be heavily shielded with thick concrete walls.

Target: , n may be produced and depends of the type of target, mass, energyand intensity of the beams. Deuterons produce easily neutrons when collidingwith matter.

Activation of beam line components: -rays. Declines with time after beam-off.

The potential radiation hazard at an accelerator laboratory has threemain sources.


Important units


Radiation and the DNA


MONITORS


END

Accelerator tasks 2012. Pairwise solutions are OK. Deadline: December 21

Mail answers to [email protected]

1a) One technique to generate high voltage for an electrostatic accelerator is to utilize a voltagemultiplier like in the early Cockroft‐Walton machine. Analyze the circuit on the left and explain how the voltage is multiplied and rising through a couple of voltage cycles. How is the charge moving?

1b) A similar type of voltage multiplier is used by the accelerator company High Voltage Engineering(www.highvolteng.com). Find out how it works.

Cyclotron energy

2a. Derive an expression for the kinetic energy of ions accelerated in a cyclotrongiven mass, charge, radius and magnetic field.

2b. When (at what energy) will relativistic mass changes start to limit the use of the cyclotron? Specify what percentage of mass change you assume (e.g, 1, 5 or 10%)

Ion source voltages

Biasvoltage Vb

Einzellens Ve

VelocityselectorVv

B fix

For H+ production the voltages on different elements are shown. Calculate the new settings if

3a) D+ (deuterium) and 3b) 4He+ is to be extracted!

Vb is kept unchanged.

Accelerator choiceAs a consultant responsible for the technical design of a new accelerator institute you shall propose two new accelerator types that are to be used for the following (analytical) tasks:

a. Production of radioactive isotopes to be used at medical investigations, e.g.15O for positron emission tomography (PET) scans, see figure for cross section versus energy.

b. Microprobe analysis with a proton beam focused to micrometer dimensions using magnetic quadropolelenses. In this technique the beam (2-3 MeV/nucleon) first passes a narrow collimator and then by the magnetic lenses is focused to a very small spot. A typical value of beam intensity is 0.5 nA. The beam is scanned across a sample and in each point e.g. characteristic X-rays, gamma rays etc are created and used for analysis. The technique requires a very stable energy and intense , constant beam.

c. Accelerator Mass Spectrometry (AMS) for counting of 14C atoms.

4a) What type of accelerator is the ideal for each of the three cases above; give a motivation for each case. (Energy that can be reached, pulsed/ continuous, energy stability,…).

4b) As only two accelerators can be installed, which two types will you advise and what will you then compromise?

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