Beam Current Optimization of Electron

Cyclotron Resonance Ion Accelerator

Submitted by

KARUKA SIDDARTH REDDY

Guided by

PROF. L C TRIBEDI

Tata Institute of Fundamental Research

Homi Bhabha Road, Mumbai 400005, India

Introduction Highly charged ions in low energy range are required in many of the research areas.

Electron Cyclotron Resonance (ECR) ion sources are well known in producing such ions. The

objective of this project work is to know about the functioning of ECR ion source, beam line,

factors that play role in the extraction of a specific ion beam. However, the main objective is to

optimize the beam current obtained from various gases such as Argon, Neon and Oxygen and of

various charge states.

Electron Cyclotron Resonance

A moving electron in the vicinity of a uniform and static magnetic field experiences a

force (Lorentz force) causing it to move in a cyclotron motion with a frequency ( ), given by

where e and m are the charge and mass of the electron respectively and B is the magnetic field

strength.

Such electrons can be further energized under the application of electromagnetic (EM) waves.

Energy is transferred via resonance when the frequency of the EM wave matches the cyclotron

frequency. As the energy of the electrons (in cyclotron motion) can be raised using the

phenomenon of resonance, hence the name Electron Cyclotron Resonance.

In ECR ion sources the ions are produced by the electron impact ionization of neutral

gas molecules which produces positive ions and sequence of this produces highly charged ions.

Ion source consists of a plasma chamber made of copper. But plasma cannot be restrained in

any material confinement as the positive ions tend to neutralize. Hence the plasma in confined

in a magnetic bottle. It is enclosed with a set of magnets to produce axial as well as radial

magnetic fields. The ECR ion source at TIFR is a permanent magnet (SmCo magnet) with dipolar

axial and hexapolar radial fields. The field maxima at the ends are 0.8 T and 1.1 T. Such a

configuration gives rise to a minimum B value along the axis and at the center of the chamber

and such a configuration is known as “min B” configuration.

Bias rod present inside the plasma chamber acts as an additional source of electrons

besides the available electrons in the plasma chamber. Electrons are then energized by the

application of MW (energy ranging from hundreds of eV to tens of keV) and their helical path

motion increases the number of collisions with the atoms, thus increasing the ionization

efficiency. A positive voltage of maximum 30kV, called the extraction voltage is applied so as to

extract and accelerate the ions obtained in the plasma chamber.

Details of ECR beam line

1. ECR Ion Source

The ECR Ion Source (ECRIS) present at TIFR is a “supernanogan”, designed and

manufactured by Ms. Pantachnik, France. It is a permanent magnet (SmCo magnet) Ion

Source with dipolar axial and hexapolar radial fields. Field maxima at the ends are 0.8T

and 1.1 T and the maximum applicable extraction voltage is 30kV. The plasma chamber

is made up of copper.

Figure 1. ECR ion source

2. RF Amplifier

Electrons present in the plasma chamber are energized by the MW and RF Amplifier is

the part of ECR Ion Source which plays a role in the increment of electrons energy for

production of higher charge states. This sector comprises of three units.

I. RF Oscillator

II. RF Amplifier

III. Power supply

RF Oscillator generates the RF frequency signals which are furthur amplified by the RF

Amplifier and the power supply is to provide the power to the RF Amplifier.

At TIFR ECR the RF frequency is 14.5 GHz with 500W maximum power output. The RF

power is transferred from the amplifier to the plasma chamber through the waveguides.

3. Einzel Lens

The extracted ions are focussed by means of an electrostatic lens known as Einzel lens.

This consists of three metallic (Stainless Steel) cylinders each 50mm long with 50mm

inner diameter and separated by a distance of 7mm. The first and third cylinders are

grounded while the middle one is given a positive voltage.

Figure 2(a). Einzel Lens

Figure 2(b). Schematic diagram of Einzel Lens

4. Faraday Cup

Three Faraday Cups (FC) are mounted in the beam line so as to measure the beam

current at varoius places. FC1 is located after Einzel lens, FC2 is between the

Quadrupole1 and Switching magnet, FC3 is at the end of beam line. All the FCs are

pneumatic.

Figure 3. Pneumatic valve controlling faraday cup and Farady cup

5. Analyzing Magnet

The focussed ion beam in then passed through the 90° analyzing magnet of bending

radius 40cm and pole gap 50mm. Maximum of 0.3Tesla can be generated by this

magnet. Particular charge state ions can be obtained by selecting a suitable magnetic

field. Lorentz force acting upon the particular charge state bends it by 90° and comes

out of the magnet uninterrupted. Rest of the charge states either bend more or less

than 90° and collide with the wall of the magnet.

Figure 4. Analyzing magnet

6. Accelerating Column

All the components in the beam line upto the analyzing magnet are placed on a deck

enabling us to raise the deck to a maximum voltage of 400kV. One end of the

accelerating column is joined to the analyzing magnet and the other end is joined to the

quadrupole which is at zero voltage or grounded. Thus the difference in voltage across

this column accelerates the ions furthur. This column is made up of several metal

electrodes joined by ceramics in between and the metal electrodes are connected via

resistors ensuring a uniform potential drop across the accelerating column.

Figure 5. Accelerating Column

7. Quadrupole

The ion beam obtained from the analysing magnet has different emittance in horizantal

and vertical planes. Einzel lens focusses the beam without changing the relative

emittance of horizantal and vertical planes. Hence a simple Einzel lens cannot be used

here.

An electrostatic quadrupole lens has four alternative positive and negative electrodes

varying by 90° each, i.e., the alternative electrodes have same polarity. This

arrangement gives rise to focussing in one plane and de-focussing in the other. Three

such arrangements are used and the second one is rotated by 90° to that of the first one

and the third one is the same as that of the first. Middle one is 100mm long where as

the first and the last one are 40mm long and are equally seperated from the middle one.

Thus quadrupole can focus in both the planes independently. There are two

quadrupoles in the beam line, one before the switching magnet and the other after it.

Figure 6. Schematic diagram of Quadrupole and Quadrupole

8. X-Y Deflectors

A set of X-Y deflectors are placed before and after the quadrupole. These help in

maintaining a better control over the beam. These plates deflect the beam in X-Y

direction based on the voltage applied to them. These are placed in the flanges of the

quadrupole.

9. Beam Profile Monitor

The shape and intensity of the ion beam can be measured using the Beam Profile

Monitor (BPM). It has a semi helix coil which sweeps X and Y planes completely in one

complete rotation. It thus measures the intensity distribution and the position of the ion

beam. There are two BPMs, one after the quadrupole 1 and the other before FC 3.

10. Switching Magnet

After the quadrupole a switching magnet is employed so as to deliver the beam in the

desired direction. The present switching magnet can deliver the beam in five different

beam lines (0°, ±25°, ±50°). The maximum field generated is 0.5 T and the bending

radius is 92.4 cm (±25°) and 47.3 cm (±50°).

11. Four-Jaw Slits

Two four-jaw slits are placed after the quadrupole-2, which enables to cut the beam as

per the requirement. These slits are controlled electrically by a motor.

Figure 7. Four Jaw Slit

12. Corona Rings

Since the ECR is operated at high voltages there is a chance of corona discharge. Corona

rings are employed at several places to control this corona discharge.

13. Gate Valves

Different secions of ECR beam line are connected by gate valves. There are three

manual gate valves (first one after the faraday cup 1, second after before the switching

magnet, third after the switching magnet) and one pnuematic gate valve after the

analyzing magnet. Also among the five beam lines after the switching magnet, both ±25°

lines have pnuematic gate valves. These valves help in the modification and

maintainance of beam line.

14. Lead Shielding

From the ion beam generated in the ECRIS a particular charge state is selected using the

analyzing magnet and the rest of the charge state beams collide with the magnet walls

thus emitting radiation. The ECR beam line upto the analyzing magnet is covered with

lead shielding so as to minimize the radiation.

Figure 8. Lead shielding near the analyzing magnet

15. Control Panel

LabView software has been installed to control all the remote units. RF Power is also

controlled remotely along with the gas inlet valves, extraction voltage, einzel lens

voltage, magnetic field, bias rod voltage and tuner position.

Figure 9. Screenshots of ECR LabView

Source and Beam Optimization

All the elements have to be properly adjusted to obtain a good beam current. Source

optimization is done using the bias rod and tuner position. Beam optimization is done using all

other parameters such as quadrupole, deflectors and magnetic field correction.

Two gases Argon and Neon are used to study the optimization of beam current. Oxygen

was used as mixing gas in case of Argon to study the effect of mixing gas. Higher charge states

were obtained with the use of mixing gas.

Experimental data

Current obtained for various charge states of Argon and Neon is shown below.

Figure 10. Current vs Charge state distribution

Maximum charge state of Ar11+ was observed with 0.24 µA current at 180 Watt power and at a

pressure of 4.4 x 10-6 Torr.

In case of Neon, maximum charge state of Ne8+ was observed with 0.32 µA current at 100 Watt

power and at a pressure of 1.3 x 10-6 Torr.

Effect of mixing gas is also observed as an improvement in the charge states.

Figure 11. Current distribution of Ar + O

Figure 12. Current distribution of Argon with and without mixing gas

It is also observed that the current shown by the charge states 5 and 10 are greater than usual

and it is due to the same mass to charge ratio of the main gas and mixing gas.

Power dependence of current of various charge states is also observed.

Figure 13. Power dependence of current for different charge states

The reason behind achievement of higher charge states with mixing gas is not yet confirmed.

However the possible reasons include:

Ion cooling resulting from the ion-ion collisions, the low mass or low charge ions of the

added gas drag energy from heavy ions and would efficiently carry out the plasma.

An increase of the electron density because of better ionization efficiency of the added

gas.

In case of Argon, current of Ar8+ beam is observed to be greater than Ar7+ and the case is yet

to be studied.

Acknowledgements

I am deeply thankful to Prof. L C Tribedi for providing me this opportunity. I would also

like to thank Desai Ji, Thulasiram, Fernandes, Aditya, Siddarth, Amarabh, Shubodeep, Arnab,

Neelesh and Saikat for their help and support.

References

1. A G Drentje, Techniques and mechanisms applied in electron cyclotron resonance

sources for highly charged ions, Review of scientific instruments V 74, N 5.

2. A N Agnihotri, An ECR ion source-based low-energy ion accelerator: development and

performance, IOP.

3. G Melin, Ion behaviour and gas mixing in electron cyclotron resonance plasmas as

sources of highly charged ions.