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RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 4:18 am
by Joe Gayo
I need someone to tell me why this doesn't work...

If I have an ion source generating low energy ions in a high vacuum environment, then if I remove the in-built ion source from an RGA (repeller cage and filament) and axially align the external source with the RGA, can't I perform a mass analysis of the incident ion beam?

I realize that ions produced by an external source will likely enter the quadrupole analyzer at energy different than the design point of the in-built ion source, but I assume dramatics peaks will appear coincident with the external ion source being powered.

Re: RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 4:53 am
by JoeBallantyne
Rather than destroy a functioning RGA, why not just connect the RGA to the source side of the gas going into the ion beam generator.

If you build a small chamber separate from the main chamber into which the feed gas goes, and then you put an RGA connected to that, and the ion source separates the source chamber from the fusor chamber, then you should be able to run the RGA normally, and determine the composition of the source gas going into the ion gun.

In fact, depending on how you plumb it, you could switch the RGA from being connected to the source gas chamber, to being connected to the main fusor chamber, and use it to measure either chamber.

If I owned a functioning RGA, I certainly wouldn't tear it to pieces. But would figure out how to use it unmodified to measure what I wanted.

RGAs are usually pretty pricey pieces of equipment.


Re: RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 4:58 am
by Joe Gayo
That doesn't tell me what I'm looking to know ... I obviously know the gas being supplied to the ion source but I don't know what fraction of the ions exiting the source is D+, D2+, or D3+.

At the moment, I'm using the ratiometric data of voltage applied to neutron output to determine a best fit fraction distribution.

Re: RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 6:26 am
by Joe Gayo

What exactly do you think will destroy the RGA? It will be operating within its acceptable background pressure range and I'll slow down the ion beam with a repeller ring so that they enter the monopole around 150eV.

I'm not caviler with my equipment and I appreciate what I have, but I have a goal and some risk is more than acceptable.

Re: RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 11:57 am
by Justin Fozzard
This might not be very much help solving your problem, Joe, but your post reminded me about this paper, a very simple design for a TOF Mass spectrometer:

Re: RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 4:42 pm
by Scott Moroch

As you pointed out, I would imagine this depends on the accelerating potential of the RGA (likely a few hundred volts, I would guess), compared to the accelerating potential of your ion source. What is the bias of your source?

For me personally, I likely would not alter an existing, working RGA. There are alternative options which are simple to implement. You could use an E cross B filter (also known as a Wien or velocity filter). The different charge states from your ion source will have a spread of energies (qV) and, therefore, a spread of velocities. You could use a set of permanent magnets and two parallel plates for the E field. Sweeping the applied voltage will reveal the desired peaks.

Hope this helps.


Re: RGA to Analyze Ion Beam Composition

Posted: Wed Jan 06, 2021 5:31 pm
by Joe Gayo

I would intentionally slow down the ion beam before it entered the RGA (actually with my setup this is quite straight forward without adding complexity). I actually fixed this RGA so I'm somewhat familiar with the inner workings.

However, the Wien filter seems easy to make.

P.S. If I'm not wrong, the deuterium ions are all the same charge state but the masses are different between the atomic and molecular ions, that's why the velocities are different.

Re: RGA to Analyze Ion Beam Composition

Posted: Thu Jan 07, 2021 4:18 am
by Frank Sanns
I can only say that plasma travels a long way in a vacuum. Even locating a device off axis, far away, or down the vacuum line does not guarantee that high voltage cannot get to your sensor. I am on my third thermocouple vacuum gauge because of it.

Some people have the same problem that I have had while other people have no issues. This one is a mystery to me but my solution initial was to disconnect the sensor once I was near the pressure I needed an operate only by the voltage and current. This is great for coarse operation but lacking that piece of data just doesn't fly when you are logging runs. So my next one was located far outside of my outer grid. The though was that the grounded outer grid would prevent the HV from getting to the sensor. Wrong. Lost the second one the same way.

These days, I have a fine SS mesh cup, grounded, and covering the sensor. This has been the way I have run for the last decade an have not lost a sensor.

Sorry for the long story but it might be prudent to measure voltage and current at the location you want to mount detector. A Langmuir probe might just save your RGA.

Re: RGA to Analyze Ion Beam Composition

Posted: Thu Jan 14, 2021 4:37 am
by Joe Gayo
I started running tests today to analyze the ion species from my source. One thing is for sure, there are some interesting pit-falls of RGAs like "zero-blast" and ions with no radial velocity on the asymptotes (x=y). I've been able to compensate/overcome these obstacles to perform several measurements that provided insight and answered some of my key questions.

Attached is a picture of the setup and some results.