The invention relates to the generating of liquid films, particularly to a method and apparatus for generating thin, free-standing liquid films for use as an electron stripper for an ion beam, and more particularly to an improved liquid-film electron stripper for high intensity heavy ion beams.
Producing a copious ion current with more than a few charges per ion in a typical ion source is very difficult. Hence, heavy charged particles have a low charge-to-mass ratio when leaving an ion source. Electron removal from these ions with a "stripper" to increase the charge of the ion, is feasible at a higher energy. Less power is needed to accelerate particles with a large charge-to-mass ratio. Examples of commonly used stripping devices are carbon-foils. Also, gas strippers have been developed, causing less energy straggling and less multiple scattering but producing lower average charge states than carbon-foil strippers that produce higher average charge states but are easily damaged by high intensity, particularly high mass beams. The lifetime of one of these carbon-foils can be as low as a few minutes when exposed to high intensity, high mass beams.
The use of thin, free-standing liquid films has been more recently developed as another means of solving the stripper foil lifetime problem. This technique was first introduced by J. C. Cramer et al wherein oil is spun from a sharp edge of a rotating disc. The sharp-edged rotating disc touches the surface of an oil reservoir and spins a thin film from the edge of the disc, with a thin scraper mounted so that it is tangent to, and almost touching, the rotating disc at a point within the oil-disc contact region. The scraper's functions were to increase the gradient of the shear forces in the liquid, to reduce the amount of oil retained on the disc for a given immersion depth, and to support the lower edge of the generated film, thereby increasing film stability.
An improvement to the Cramer et al device is described and claimed in U.S. Pat. No. 4,453,077 issued June 5, 1984 to B. T. Leeman et al, and assigned to the assignee of this invention. This improvement was composed of a disc with a centrally located, hollow-ground, razor-sharp, outer edge, the lateral surface of the edge being rough in order to grab the oil composing the liquid film. Care had been taken to make the drive mechanism of the disc vibration less to achieve film stability. The oil-disc contact comprises a fine oil stream ejected from a nozzle mounted above the disc, and the oil flows downward, tangentially touching the edge of the rotating disc. The device utilizes a scrapper consisting of a brush apparatus that actually touches the rotating disc and removes excess oil on the disc, thereby preventing the formation of undesired small drops of oil which spin off the disc and damage or destroy the liquid film formed by the rotating disc. Films on the order of 0.25 to 0.85 micron thick have been generated with the Leeman et al device.
While the prior art electron strippers have been effective, a need has existed for an electron stripper which produces constantly regenerated, stable, free-standing films having a thickness on the order of 4-5 time thinner than those produced by Cramer et al and Leeman et al.
Therefore, it is an object of this invention to provide an improved liquid-film electron stripper.
A further object of the invention is to provide a liquid-film forming apparatus, which produces liquid-films stable enough to be tested with an ion beam from a working accelerator.
A further object of the invention is to provide an apparatus for producing a film of liquid having a thickness in the range of 0.3 to 0.05 microns.
Another object of the invention is to provide an apparatus which produces constantly regenerated, stable, free-standing films of liquid, particularly applicable for use in an electron stripper for high intensity heavy ion beams.
Another object of the invention is to provide an improved electron stripper utilizing a disc having a razor-sharp edge formed on one side of a flat disc via a tapered surface forming the edge and with the flat surface being smooth and highly polished.
Another object of the invention is to provide an electron stripper utilizing a pair of rotating discs, one being down beam with respect to the other.
Other objects of the invention will become apparent to those skilled in the art from the following description and accompanying drawings.
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