At Hughes Research Laboratories (HRL, LLC), we worked on ion sources from 6 inche x 2 foot rectangles for neutral beam injectors to sources smaller than a pinhead for a focused ion beam used for ion-beam lithography and microcircuit fabrication. Our major involvement was in the area of ion propulsion where trusters were built from 5 cm diameter to 30-cm diameter. So we were builders rather than users. I've been retired for a number of years, so I don't know the current state of affairs.

Dear Julius,

Thanks for your useful answer.

Beam diameter depends on the application, for example it is 0.01- 5 mm for ion implantation and ion-solid interaction studies, it is also few mm in case of neutron generators. But it can be few hundred mm in case of the ion sources developed for sputter coating and surface cleaning .

Dear ccoleages, in case of neutron generators, more ion beam diameter will result in more neutron yield. Beside, for a larger beam diameter, we can produce a determined neutron yield at a lower ion beam energy. Why the beam size is afew mm for this application?

Hello Mohsen,

first of all you need to clarify, wether you are using a single aperture or multiaperture (grid like) system.

Single aperture extraction systems usually have ab bore of of 5 to 15mm. This depends on the plasma density which you can provide from your plasma generator and what your extraction voltage (beam energy is).

Your ion current does not depend on the aperture! At 10kV you can reach a proton current of about 10mA. At 100kV it is about 300mA. I.e. the beam current is space charge limited. To obtain a good quality beam you have to match your plasma denstiy to this current.

Please tell us, what kind ouf ion source you are using

Best regards - Klaus Langbein

Dear Klaus,

Thanks for your comment,

We are working on Helicon ion source. Plasma density is approximately 10^18 m^-3 ( (for Argon operating gas) near to the extractor and the extraction voltage is 20 kV. Why the extracted ion beam current dose not depend on the aperture diameter? How do you estimate the amount of ion current that can be extracted from a determined ion source?

Kind regards- Mohsen

Hello Mohsen,

the current you can draw is space charge limited, i.e. the Child-Langmuir law determines the current. On the other hand the ion current depends on the plasma density. You can adjust this at your will, but you have to stay within a certain range. If you "push" too much current into the extraction system, the plasma meniscus will bulge out and you get a convex meniscus with a diverging beam wich will hit your extraction electrode - causing a breakdown. If your plasma density is too low, you will obtain a concave meniscus wich will overfocus the beam. So in the experiment you have to adjust the plasma density and produce a slightly concave meniscus, wich means that you have to actively obeye the space charge limit. If you don't you, get a poor beam quality or an electrical breakdown.

You may deviate slightly from the current given by Child's law, +-10% but not much more. You have to balance plasma density and space charge limit. You cannot exceed the space carge limit unless you use a multiaperture extraction system.

I assume you have to inject your ion beam into a linear accelerator (RFQ?). So you also have to provide a beam with a certain emittance (please correct me if my assumption is not correct).

If you have the possibility, please consult the books by Ian Brown (Ion Sources), by Forrester (Large Ion Beams) and Bernhard Wolf

Best regards, Klaus

Greeting Klaus,

Thanks for your useful comment.

We want to use the constructed ion source for low- energy ion implantation.

Beside the the Child- Langmuir law, can we use Bohm relationship for estimation of the ion current density that can be drawn from a plasma?

Regards- Mohsen

Bohm's criteria alpply only to processes inside the plasma, e.g. the formation of double layers. The process of extraction however depends on space charge limited flow (Child-Langmuir). The plasma boundary, i.e. the plasma meniscus is not subject to Bohm criteria. The plasma mensicus is a single layer.

Regards - Klaus

P.S. I found your article

https://accelconf.web.cern.ch/accelconf/a98/APAC98/6D061.PDF

Any more full publictions would be helpful.

Please feel free to contact me under [email protected]

I will also reply to any questions on

http://foren.activevb.de/forum/physik/

This is a German site but feel free to use English language.

Klaus Langbein is quite correct about matching plasma density to achieve a concave meniscus. However, for every plasma density, there is a corresponding extraction voltage that will maintain the same meniscus shape. Also, one can use a higher voltage than wanted for the final beam velocity to increase beam current and then employ a 3rd so-called decal electrode to reduce the beam velocity to the desired value. Of course, some defocusing results from the decal electrode.

Thanks dear colleges,

The extracted ion beam current is also a fraction of the current of the HV DC power supply which has been applied to the extraction system.

( please correct me if my assumption is not correct).

Regards

Thank you for the remark Julius.

In most high current ion sources the extraction system is a so called accel/decel-system. You have a voltage on the outlet aperture (on the whole ion source) which determines the beam energy. The next electrode is on a negative potential. This electrode serves mainly to prevent a backcurrent of secondary electrons. Its usually at -10% of the positive source potential. e.g. at 10kV on the source it will be at -1 or -2kV. You have a certain range within you can play with this parameter to influence the shape of the meniscus. However you should not exceed about -5kV. But keep in mind, this electrode is mainly there to prevent electrons beeing accelerated into the source. If you turn it on, you will usually see a decrease of the current drained from the positive HV-supply. The electron current can be as much as the total beam current. So dont be surprised if the Current on the HV supply drops by a factor of two. There sould be very little current on the negative HV-supply.

The final electrode is at ground potential.

All three electrodes have the same bore. Furtermore the distance between the outlet electrode and the negative electrode sould be approx. equal to the aperture of the outlet electrode. You may vary this a little, but this ratio has been found to be the best. So you cannot really deviate from this.

This results in the fact that your perveance is always in the range of 1-2*10-8. This value is to be divided by the sqare root of the charge to mass ratio. So for Ar1+ divide by the sqare root of 40.

I am agree with Klaus. One can also play with the shapes of the electrodes for low emmitance and highly focused high current low energy ion beam because space charge dominated (Low energy ) beams are not that much focused.