how does one measure the power of an ion beam?

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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UG!
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how does one measure the power of an ion beam?

Post by UG! » Wed Aug 02, 2006 1:15 pm

i am planning some experaments to investagate the power of various ion beams and i am wondering what the best way to go about this is.

the options would seem to be:
measurements of the heating power of the beam which would require a thermaly insulated target of low attomic weight and precicly known mass, but would sputter material evrywhere and may loose considerable energy in x-rays (and i have no precice balances)

or

electrical measurements of the current and speed of the beam.
which, short of measureing the charge on a conductor plased in the beam path, i have little idea about.

any comments or surgestions?

Oliver

Chris Trent
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Re: how does one measure the power of an ion beam?

Post by Chris Trent » Wed Aug 02, 2006 2:10 pm

Oliver,

I'm still learning here but sometimes the best way to learn is to teach, so here goes: As I understand it, the electrical method is the easiest for characterizing an Ion beam.

The voltage determines how fast the ions move. At ion beam speeds temperature, velocity and energy in electron volts become interchangable.

The current determines how many ions move. I don't have the exact equations handy but they're not hard to find.

The product of these two is the power.

The experimental method I see the most often is to place a probe in the path of the beam. This link should have some good information.

http://en.wikipedia.org/wiki/Langmuir_probe

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Carl Willis
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Re: how does one measure the power of an ion beam?

Post by Carl Willis » Wed Aug 02, 2006 4:01 pm

A most-frequently used method for determining beam current is to insert a Faraday cup into the beam. At its simplest, this is just a conducting solid cup that catches all the ions, and then you measure the current flowing off the cup. There are some caveats if accuracy is important. Most practical Faraday cups use at least one additional electrode that is negatively charged with respect to the "main" electrode and serves to prevent the escape of secondary electrons knocked out of the cup by ions. This effect would make the cup overestimate the true beam current. As I think you already said, the cup can be used as a calorimeter to determine the effective beam energy if it is effectively surrounded completely by vacuum. You just put a thermocouple in it, and from knowledge of the mass and the heat capacity, the rise in temperature, and the charge collected in a given period, you can determine the beam energy. The thermocouple, unless electrically well-isolated from the cup, should not be read while the beam is actually on.

Calibrated broadband current transformers, such as those made by Pearson, are a way to non-invasively measure the current in pulsed beams. You just put the transformer coaxial with the beam and treat the beam as if it were just a wire. Of course, you can't use a transformer with a DC beam.

Beam energy can be determined with an analyzing magnet and a Hall probe for calibrating the field. Lacking such apparatus, if the energy of the beam is high enough, you can put thin foils of aluminum or some other metal in the beam and measure the fraction of the beam current that passes through given thicknesses of foil. This will also give you a rough energy profile of your beam.

Good luck.

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Re: how does one measure the power of an ion beam?

Post by DaveC » Wed Aug 02, 2006 8:20 pm

Re: Beam current profile measurements.

With the same caveats that Carl has mentioned, regarding secondary electron emission you can often use a printed circuit approach to lay out 10 to 100 closely spaced parallel electrodes as part of your generalized "Faraday" cup detector. We have done this to profile 75 kV e beams in air, with fairly good results. One very convenient configuration is to use the flexible Kapton based printed circuit materials. You DO have to be aware of beam current density with the polymeric materials, so that your background pressure does not spike. But with a fairly good conductor coverage, the beam will generally be collected by the electrodes, and there will be minimal impact on the exposed polymer.

A way with higher resolution, is to use a scanning slit detector. This is a movable plate plate detector with a narrow slit in front of a second smaller flat plate detector. Both are maintained at the same potential, and currents are measured from both the slit and main detector.

the two detectors provide some self checking. With a slit, you get detectable currents over a small, element of the target. When summed, the slit currents will equal the currents measured by the main detector, if all things are going properly.

This method works more convenienly when the source is at high voltage and the target (Faraday Cup) is at ground potential. It also avoid most of the problems the Langmuir probe causes, by keeping all local potentials unchanged. With open air measurements, there are such large ion fluxes, that even a few tenths of a volt difference in target potential cause large changes in current pickup. In vacuum, of course, one does not have this problem.

Current measurements at high potential, whatever the method, do require some special attention to detail to account for and eliminate leakage currents and such.

I usually expect 1 to 10 G Ohms as a minimum leakage resistance as the sign of a "healthy" system. Gigohms at kV give microamps. So microamp levels of leakage may be reasonable depending on the design. The exact value depends on the specifics of your design.

Pulsed leakage currents - such as in partial discharges - can super-impose on existing DC currents to substantially add to the overall value. As Carl has said, the Pearson coil, or almost any torroidal current transformer - Digi-Key has some great low cost ones - will pickup the AC and pulsed currents.

A good test procedure is to see that input currents, power and voltages are all "in the ball park", before getting to serious experimental fun.

Dave Cooper

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Re: how does one measure the power of an ion beam?

Post by UG! » Thu Aug 03, 2006 1:48 pm

thanks, you have given me lots to think about as i suspected, building something accurate is a non-trivial matter.

Oliver

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Re: how does one measure the power of an ion beam?

Post by John Futter » Mon Oct 02, 2006 7:54 am

All comments - agree - As said by others

Xrays are not a problem with Ion beams unless acceleration voltage is above the elements characteristic xray emission spectra IE PIXE / RBS so for accelerations around 50 - 100Kev very little problem exists with most target materials and yield is very low ie very little energy loss so very small x-ray flux.
Sputtering is slow I've done 350nm depositions with a 1mA ion Argon beam at 12kV that have taken 8 hours to produce.
Sputtering yields vary enormously within the periodic table there are very good look up tables around (result of countless PhD's) that give yield with what ion beam at what acceleration and what target. Alot of elements peak in the 10-30kV acceleration area.

Hope this helps

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Re: how does one measure the power of an ion beam?

Post by Richard Hull » Tue Oct 03, 2006 8:34 pm

The very best amateur beam currents are on the order of no more that a few milliamps.

The current is usually measured with a faraday cup placed in the beam line with a sensitive electrometer to the return. Ion beam current measurements are usually taken in a very hard vacuum. The velocity is usually related to the extractor voltage (complex factors related to how hard the vacuum is).

Richard Hull
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