h1

RF Ion Source

February 8, 2009

I have been working on an ion source to support my next fusor and other small accelerator projects.  Criteria for this source were that it had to be easy and inexpensive to fabricate myself with common components from reliable sources.  My goals were to obtain high beam current and long service lifetime.  I settled on an RF ion source  concept with specific influences from Kiss and Koltay (1977).  Tests of the prototype indicate stable sub-milliampere currents of deuterium ions over hours of operation.  Cost (excluding RF and vacuum equipment): about $250.

_______________________________________________________________

ion_source_modRF ion sources function by extracting ions from radiofrequency electrodeless discharges.  These sources can deliver high-purity beams of atomic H+ ions.  The vessel supporting such a discharge can be as simple as a glass test tube surrounded by an inductive coupling to the RF supply, as my design at left illustrates.  The discharge is “enhanced” with the field from a strong magnet.  Some builders attempt to exploit specific enhancement effects, e.g. electron cyclotron resonance or helicon phenomena.  My goal with the magnet is just to promote generic electron trapping / heating, possibly by the above-mentioned modes if conditions are appropriate.  Components, with suppliers’ names and stock numbers, are provided in the drawing.

ion_source_2ion_source_3Construction techniques involve drilling, lathe turning, silver brazing, and soft silver soldering.  Photos at left show components of the source  (most prominent are the extraction electrodes) during assembly on a ConFlat cube for testing.

Extraction of ions is accomplished by a strong DC electric field imposed between the negatively-biased “nozzle” on the 5/16″ tube and the grounded septum on the 1/2″ tube.   I use up to -5 kV for extraction of ions.  The extraction nozzle also throttles neutral gas flow from the discharge region into the vacuum chamber.

am-6155_highpowerRF power is supplied by an FAA-surplus AM-6155 power amplifier operating at 200 MHz.  These amplifiers are a common hamfest bargain.  Circuit details and modifications for the amateur radio hobby are easy to find online.  To date I have not produced more than ~60W with this amplifier, driving it with signals below 1W.  Beam current depends am-6155-innardsvery strongly on RF power, and I plan to upgrade the driver for the AM-6155 to produce more.  The top photo shows this amplifier producing power (lighting a mercury-vapor discharge), and the bottom photo shows the tube compartment of the amplifier modified for shunt feed of plate current.

schemat_ltunerion_trap_3Inductive coupling of the 200-MHz power to the discharge plasma is effected with a single loop of heavy conductor that forms part of a resonant “L-match” circuit, providing an easy interface to 50-Ω cable.  This is illustrated schematically at left.  The right photo shows the ion source ready for testing, with the RF coupling loop visible along with other components including gas for the discharge (deuterium lecture bottle).

d2_is_2Photos from operation.  The top photo shows the RF deuterium discharge in a standard 19-mm (3/4″) Pyrex test tube, and below it a beam of extracted ions impinging on a graphite Faraday cup target.  RF power is about 50W, extraction voltage -3 kV, and target at -10 kV.  Background pressure has been raised into the millitorr range to enhance beam visibility.  Extracted current is 0.25 milliampere.  10kv_tBottom photo shows the exit aperture clearly, with deuteron beam passing through a ring electrode at -10 kV.  Here the extraction voltage was -5 kV.  It is not possible to accurately measure the beam current in this arrangement, but it is probably on the order of 0.5 mA.  Not surprisingly, a few neutrons from 2H(d,n) fusion reactions can be detected with higher potentials on the ring cathode.

_______________________________________________________________

More information about this ion source

7 comments

  1. While looking for modifications to the am6154 that I have I came across your website. I noticed you would like more power. The unit will run at 600watts with mods. There are a few sites that detail 220 mhz conversions. Just wanted to help out, don’t knopw if you knew they could be modified
    Be Safe


    • Hi, and thanks for your interest in my site.

      I should note that in the time since I put this post up I have built a new 10-watt driver, and it’s easy to push 550 watts into a 50-ohm load at 200 MHz with the modifications I did make to the unit. I would not do that for very long, though…my 100% duty usage brings to mind the original application, for which the unit was rated at 50 watts! The onboard plate supply seems likely to burn up if taxed much beyond 100W / 100%, not to mention the tube. My load is also an unstable gas discharge, and anybody trying to power a badly-matched load at full excitation WILL destroy the Kapton blocking cap and possibly the output loading cap (with dangerous results). I replaced both these dielectrics with Teflon in ample quantities to resist discharges.


  2. Hi Carl,
    I was curious if you have given any thought to trying any experimentation using RF to create active nitrogen which was worked on by Lord Raleigh I believe in the early 20th century (a bit of old somewhat forgotten science along w/ Lenard rays and other such findings-my favorite-Raleigh’s perpetual motion clock based on a grounded electroscope and a bit of radium, w/ a similar apparatus mentioned in the book Uncle Tungsten). I’d be fascinated if you could post some pictures if you were to get any results. Also, I would be interested in hearing your thoughts on atomic vapor laser isotope separation as a means of selectively enriching certain elements? Last mention, I got some interesting results using home-built canal ray tube to obtain photographic prints of the various isotope streams within the vacuum tube–have you tried any similar experimentation? Contact me anytime to compare notes-received an email a while back from you on some ebay sales of mine of UF4. Thanks for your time as always,
    Best Regards,
    James


    • Hi James, I played around with nitrogen afterglows about eight years ago. Nothing too involved, but I was interested in the chemical reactivity of the activated nitrogen and I found the glow effects to be aesthetically very pleasing. Some old photos here. I haven’t done anything with isotope separation (including AVLIS). If I ever get a trashed analytical mass spec I might see what it does with uranium…maybe turn out some SNM in micromolar quantities.


  3. Hi carl,
    I want to know the advantege of the rf ion source and where the rf ion source is applying on?
    Thank you

    sincerely
    willian


  4. Hi, Carl,
    I am interested in how to get an excess of electrons in the RF engines. My idea is to use RF engine – as the RF cathode neutralizer.
    What parameters should be changed in your experimental setup that would get in excess electrons? A working material do you recommend?


  5. I am putting together an experiment to follow up on the fusion successes you guys have had with the Farnsworth type fusors. Although the geometry is quite different, the basic idea is the same: forcing deuterons into the strong field through inertial energy. As you know, we’ll need something over 5 Kv of energy and I would like to have a few milliamps of current from the ion source, focused to about 4mm. Is this out of the question?
    Preston Hay



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 77 other followers

%d bloggers like this: