12 INCH CYCLOTRON ION SOURCE - PART II



The conclusion from analyzing the betatron motion data was that a significant vertical electric field was imparting an angle to the ion beam as it was being launched from the chimeny. Please see Ion Source Studies - Part II: Simulations and Measurements to get the fine details. Modeling with PSF and SIMION has shown that the vertical electric field came from the slight vertical asymetry in the design of the first copper chimney, previously described on the Early Ion Source web page.


Fig.1 A 3D mechanical drawing of the new chimney.

Great effort has gone into designing an electrically symmetric chimney, including metalizing the ceramic boat on which the chimney sits. During the design it was decided to thin the wall near the aperture to increase the extraction field at the plasma sheath.


Fig.2 The machined piece is beautifully done, but not as polished.

Additionally, we decided to incorperate "pullers" (extraction electrodes) mounted on the face of the DEE to further enhance the extraction field. This has proven to be very successful, but has brought about it's own troubles. The vertical alignment of the "pullers" with the chimney aperture is critical to the initial launch angle (see figures 6 & 7). A DEE positioner is being built so the vertical alignment can be adjusted during operation.


Fig.3 The left side (270-degree) view of the pullers installed (chamber lid removed).


Fig.4 Front (~180-degree) view of the pullers installed (chamber lid removed).


Fig.5 Side view of the pullers installed through the 90-degree view port.

During operation of the cyclotron, the internal pressure was brought quite high (1E-4 Torr) for short periods of time (~ 1 minute). Under the high pressure conditions the beam can be seen via recombination. In the following two images, the RF power was about 700 watts - creating 10kVp-p on the DEE. The inital first half of the ions' revolution can be seen spirallig as expected, but also downward (this comes from the poor vertical aligment of the pullers). The downward slope is so great, and at this early stage of the ions path the magnetic focusing effects are very weak, thus the ions are just lost to hitting the bottom of the chamber.


Fig.6 First half of the ions' revolution as seen through the 180-degree viewport.


Fig.7 First half of the ions' revolution as seen through the 90-degree viewport.


Fig.8 A long exposure shows the dramatic ion's line-of-sight spray.


Fig.9 After operating for a filament lifetime, one can see the wear and tear on the internals; note the "clean" spots near the aperture - guess which one is from electrons and the other from ions...

To increase the electrons available to ionize the plasma, and to protect the bottom of the macor boat, a small copper disk was placed beneath the filament. It was a crude test; preferably it would float, thus charge up and reflect downward electrons up, or at worst touch one of the Ti filament holders and be at the same potential as the filament. It seems to have worked, warranting a Ti version. It also took a beating (it was flat and polished to start).


Fig.10 A partially melted/ablated copper reflector.

Only after the new chimney was designed, did the author find the following images of existing ion sources. We were pleased to find that our own, independant, evolution has brought us to the same construction as the Cyclotron Pioneers (Cyclotroneers) had developed.


Fig.11 Scanned image of UCRL 60-inch ion source.

(scanned from E. Segre's Methods of Experimental Nuclear Physics, vol. III)


Fig. 12 Texas A&M cyclotron ion source.

(scanned from the proceedings of the Sixth International Cyclotron Conference, Vancouver, 1972.)


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