Small and simple mass-spectrometer

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Rapp Instruments
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Small and simple mass-spectrometer

Post by Rapp Instruments » Sat Jan 23, 2016 10:41 pm

Hi,
I have build a small and simple mass spectrometer which could be valuable for testing vacuum conditions in a fusor. Although the resolution is very low the instrument easily differentiate between air and deuterium. It's also sensible for helium and so may be used as a leak detector, but I didn't try this.
The working principle is seperation by magnetic fields. I have tryed two ion sources, one thermic source for testing alkali and alkaline earth ions, and a electron ionisation source for gases. Both sources use opened light bulbs as an emitter The detector is a simple faraday cage with a homemade electrometer amplifier.
The spectrometer is mostly build from simple vacuum parts, the seperating magnet is made of NdFeB magnet disks with soft iron pol pieces extending in the vacuum region.
Image Image
<<< left thermic source ............................................................................... right electron ion source >>>
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By sweeping the accelerating voltage of the source ions with different masses find the way to the detector.

Image Image

more details are found one my homepage http://www.rapp-instruments.de/Radioakt ... sector.htm
sorry, still in german language.

Thomas

Jeroen Vriesman
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Re: Small and simple mass-spectrometer

Post by Jeroen Vriesman » Sun Jan 24, 2016 10:48 am

Great!

I also used a light bulb as an emitter for my ion source, but there was a lot of tungsten evaporating sputtering (at 20mA emission from a 5W 12V car lamp) on the anode.

I'm ordering some thoriated tungsten wire now (WT20, different sizes), way too much for me, so I'm selling some of it, you need some?

Roberto Ferrari
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Re: Small and simple mass-spectrometer

Post by Roberto Ferrari » Sun Jan 24, 2016 11:54 am

Hi Tom,

As always, your instruments are excellent!
Please, comment about the magnets.
Two sets of five slabs?
300 mT is the total field?
Thanks

Rapp Instruments
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Re: Small and simple mass-spectrometer

Post by Rapp Instruments » Sun Jan 24, 2016 9:48 pm

Hi Jeroen,

thanks for the offer. For this project I need only very low emission current ~ 100µA, so there no problem with sputtering or evaporating. But anyway, thoriated tungsten is always very interesting for high emission. Please tell me more about size and pricing.


Hi Roberto,

yes there are two sets of five discs, 5 mm thick, with a diameter of 20 mm, made of NdFeB grade N42. Simulation program >>FEMM<< showed a field of 300mT in the gap, but measuring with a calibrated hall sensor only 220 mT. Also the measured ion deflection radius proved the lower field.

best regards
Thomas

Roberto Ferrari
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Re: Small and simple mass-spectrometer

Post by Roberto Ferrari » Mon Jan 25, 2016 2:09 am

Thanks Thomas

Can you refer me to some detailed info about the DAC control of the sweep voltage?

Rapp Instruments
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Re: Small and simple mass-spectrometer

Post by Rapp Instruments » Mon Jan 25, 2016 8:46 pm

Hi Roberto.
I use a simple homemade high voltage amplifier with VMosFet BUZ 80. You can find the circuit http://www.rapp-instruments.de/Elektron ... /hvamp.htm.
The amplifier can be driven by a DAC LT1257 which is controlled by an Arduino. The output voltage depend on the used powersupply and can be some hundred volt.

Thomas

Jeroen Vriesman
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Re: Small and simple mass-spectrometer

Post by Jeroen Vriesman » Tue Jan 26, 2016 11:50 am

Thomas,

within about three weeks I will have 5 sizes of 2% thoriated tungsten wire, 0.05, 0.1, 0.15, 0.2 and 0.25 mm.
I will probably make some sets with all the sizes and put them on the trading post here, it's not about making profit so the price will be lower than anything you can find on ebay, but I don't know the exact price yet, have to see the amount of import tax first.

adelk
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Re: Small and simple mass-spectrometer

Post by adelk » Thu Feb 27, 2020 1:24 am

good job you did!
I followed your link to rapp-instruments. I found the 'poor mans mass spectrometer' required 1e-4mmHg vacuum (hope to find the full text soon).
can you tell how deep a vacuum your system needs in order to work properly?

John Futter
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Re: Small and simple mass-spectrometer

Post by John Futter » Thu Feb 27, 2020 5:13 am

Adelk
Please follow the site rules
You did not use your real full name
You did not introduce yourself in the appropriate subforum
please correct this before more posts


Ands it is a bit odd replying to a post that is 9 years old!!!

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